# A Tale of a Tail: A Tidally-Disrupting Ultra-Diffuse Galaxy in the M81 Group

Rokas Žemaitis1, Annette M. N. Ferguson1, Sakurako Okamoto2,3,4, Jean-Charles
Cuillandre5, Connor J. Stone6, Nobuo Arimoto3,7, Mike J. Irwin8
1Institute for Astronomy University of Edinburgh, Blackford Hill, Edinburgh
EH9 3HJ UK
2Subaru Telescope, National Astronomical Observatory of Japan, 650 North
A’ohoku Place, Hilo, HI 96720, USA
3National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo,
181-8588, Japan
4The Graduate University for Advanced Studies, Osawa 2-21-1, Mitaka, Tokyo
181-8588, Japan
5Université Paris-Saclay, Université Paris-Cité, CEA, CNRS, AIM, 91191, Gif-
sur-Yvette, France
6 Department of Physics, Engineering Physics & Astronomy, Queen’s University,
Kingston, ON K7L 3N6, Canada
7Astronomy Program, Department of Physics and Astronomy, Seoul National
University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea
8Institute of Astronomy, Madingley Road, Cambridge CB3 0HA UK E-mail:
<EMAIL_ADDRESS>(RZ)

(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present the discovery of a giant tidal tail of stars associated with F8D1,
the closest known example of an ultra-diffuse galaxy (UDG). F8D1 sits in a
region of the sky heavily contaminated by Galactic cirrus and has been poorly
studied since its discovery two decades ago. The tidal feature was revealed in
a deep map of resolved red giant branch stars constructed using data from our
Subaru Hyper Suprime-Cam survey of the M81 Group. It has an average surface
brightness of $\mu_{g}\sim 32$ mag arcsec-2 and can be traced for over a
degree on the sky (60 kpc at the distance of F8D1) with our current imagery.
We revisit the main body properties of F8D1 using deep multiband imagery
acquired with MegaCam on CFHT and measure effective radii of 1.7-1.9 kpc,
central surface brightnesses of $24.7-25.7$ mag and a stellar mass of $\sim
7\times 10^{7}M_{\odot}$. Assuming a symmetric feature on the other side of
the galaxy, we calculate that $30-36$% of F8D1’s present-day luminosity is
contained in the tail. We argue that the most likely origin of F8D1’s
disruption is a recent close passage to M81, which would have stripped its gas
and quenched its star formation. As the only UDG that has so far been studied
to such faint surface brightness depths, the unveiling of F8D1 ‘s tidal
disruption is important. It leaves open the possibility that many other UDGs
could be the result of similar processes, with the most telling signatures of
this lurking below current detection limits.

###### keywords:

galaxies: individual: F8D1 – galaxies: interactions – galaxies: groups:
individual: M81 Group – galaxies: structure – galaxies: stellar content

††pubyear: 2022††pagerange: A Tale of a Tail: A Tidally-Disrupting Ultra-
Diffuse Galaxy in the M81 Group–A

##  1 Introduction

Ultra-diffuse galaxies (UDGs) are distinguished by their low central surface
brightnesses and large sizes. A commonly-used, albeit rather arbitrary,
definition is that they have effective radii $R_{\rm eff}\geq$ 1.5 kpc and
central surface brightnesses fainter than $\mu_{g}(0)\gtrsim 24$ mag arcsec-2
(e.g., van Dokkum et al., 2015; Koda et al., 2015). Although systems with
these properties have been known to exist for decades (e.g., Sandage &
Binggeli, 1984; Caldwell & Bothun, 1987; Impey et al., 1988), there has been a
recent resurgence in interest in them due to the sheer abundance of such
objects being discovered in modern deep imaging surveys. They appear
particularly common in dense environments (e.g., van Dokkum et al., 2015; Koda
et al., 2015; Mihos et al., 2015; Janssens et al., 2017; Iodice et al., 2020),
but are also found in low density groups and in the field (e.g., Merritt et
al., 2016; Román & Trujillo, 2017; Greco et al., 2018; Müller et al., 2018).

Studies of UDGs in the last few years have yielded important new information
about the properties of these unusual galaxies. It has been established that
many UDGs have round isophotes and radial distributions of starlight that
follow an exponentially-declining or flatter profile (e.g., Yagi et al., 2016;
Cohen et al., 2018; Alabi et al., 2020). Their star formation properties are
observed to vary, with systems which ceased to form stars a long time ago
dominating in cluster environments while the field contains examples where
star formation has continued until recent epochs (e.g., Leisman et al., 2017;
Greco et al., 2018; Prole et al., 2019).

Much debate has centered on the dark matter content of UDGs and, again, this
seems to be a property with considerable variance. There have been extremely
dark matter-dominated UDGs reported as well as systems that appear to have
very little dark matter at all (e.g., Toloba et al., 2018; Lim et al., 2018;
Danieli et al., 2019; Forbes et al., 2020). A common way to infer the amount
of dark matter in a UDG is to exploit the relationship between the number of
globular clusters (GCs) in a system and the host halo mass (e.g., Spitler &
Forbes, 2009; Harris et al., 2013; Forbes et al., 2018). While this is in
principle a simple technique, results can vary considerably depending on how
it is applied in practice. For example, while van Dokkum et al. (2017) find
the Coma cluster UDG DF44 to have $\sim 75$ GCs associated with it, Saifollahi
et al. (2021) use the same dataset to argue that this number should instead be
$\sim 21$. Other studies support the idea that the GC systems of UDGs are
generally consistent with them being dwarf galaxies with normal dark matter
content (e.g., Amorisco et al., 2018; Lim et al., 2018).

Several explanations for the origin of UDGs have appeared in the literature.
Firstly, they may be rapidly rotating normal dwarf galaxies, resulting in a
very extended profile for their stellar mass (Amorisco & Loeb, 2016).
Alternately, their large sizes may result from tidal stripping and heating
experienced through interactions with massive neighbouring galaxies (Carleton
et al., 2019; Doppel et al., 2021) or through formation in tidal debris
(Bennet et al., 2018). Other possible scenarios for the origin of UDGs are
that they are puffed-up dwarf galaxies that have experienced gas loss due to
star formation-driven outflows (Di Cintio et al., 2017; Jiang et al., 2019) or
that they are ‘failed’ galaxies that did not manage to build-up their expected
stellar mass (van Dokkum et al., 2015). Ram-pressure stripping may play a
further role in transforming gas-rich blue UDGs into red ones (e.g., Junais et
al., 2021). While it is likely that the present-day UDG population results
from a variety of these formation channels, the question of which is the
dominant channel remains open.

Figure 1: Left: Our M81 Group Survey footprint (7 HSC pointings) overlaid on
an SDSS image. Each HSC pointing has a diameter of 1.5 degrees and the known
galaxies within this area are marked. The red circle indicates the pointing
containing F8D1, which is the focus of this paper. Right: A portion of a deep
i-band image taken with CFHT/MegaCam that shows a zoom-in on the central
region of the red-circled HSC pointing. Highly-structured cirrus is present
throughout this region of the sky which greatly complicates integrated light
studies of the low surface brightness emission of F8D1 and its neighbours.
F8D1 appears projected over a particularly bright ISM filament. The scalebar
in both panels reflects the physical scale at the distance at M81. North is up
and east is to the left.

Local universe examples of UDGs are particularly valuable since their
proximity allows for much greater insight into their properties compared to
more distant systems. Some very low surface brightness extended dwarf galaxies
have been uncovered in the nearby universe. Toloba et al. (2016) discuss the
NGC 253 satellite Scl-MM-Dw2 which has $R_{\rm eff}=$ 2.9 kpc,
$\mu_{V}(0)=27.7$ mag arcsec-2 and $M_{V}=-12$, while Martin et al. (2016)
find the M31 satellite And XIX to have $R_{\rm eff}=$ 3.1 kpc,
$\mu_{V}(0)=29.3$ mag arcsec-2 and $M_{V}=-10$. These may be extreme examples
of the UDG category but they are not direct analogues of the dominant
population of UDGs seen at greater distances. While most famously known for
its triplet of interacting galaxies (M81-M82-NGC3077), the nearby M81 Group (D
= 3.6 Mpc; Radburn-Smith et al. 2011) hosts a low luminosity galaxy with
properties that unequivocally place it in the UDG realm. F8D1 was discovered
more than 20 years ago by Caldwell et al. (1998, hereafter C98) during a CCD
survey for M81 Group analogues to Local Group dwarf elliptical galaxies. It
has a luminosity of $M_{V}\sim-14$ or $\sim 4\times 10^{7}$ L⊙ and lies
roughly 2 degrees in projection from M81 in a region that is heavily
contaminated by Galactic cirrus (Sandage, 1976). Although C98 remarked on the
fact that its low central surface brightness ($\mu_{V}(0)\sim 25.4$ mag
arcsec-2) and large effective radius ($R_{\rm eff}\sim 2.5$ kpc) were in stark
contrast to those of most galaxies known at that time, F8D1 has barely
received any attention beyond its initial discovery. Using Hubble Space
Telescope Wide Field Planetary Camera 2 (HST/WFPC2) data, C98 were able to
resolve the upper two magnitudes of the red giant branch (RGB) of F8D1 and use
the luminosity of the RGB tip to firmly place it at the distance of the M81
Group (see also Karachentsev et al., 2000). They also used the colour of the
RGB to measure the mean metallicity of the galaxy as [Fe/H] = $-1.0\pm 0.26$
dex, with no significant radial abundance gradient and showed that it hosts a
considerable population of luminous asymptotic giant branch (AGB) stars. Such
a population is a testament to an extended period of star formation; C98
argued that roughly 30% of the stellar population was of intermediate age,
with the youngest stars having formed only 3-4 Gyr ago. A single globular
cluster candidate was identified near the centre of the galaxy. This object
was selected on the basis of its semi-resolved morphology and integrated
colour. It is rather blue ($V-I\sim 0.76$) which could be indicative of a low
metallicity. On the other hand, Chiboucas et al. (2009) report a radial
velocity of $-125\pm 130$ km s-1 for this object which leaves open the
possibility it is a background galaxy. Unfortunately, there has been no
published radial velocity for F8D1 yet.

In this paper, we revisit the properties of F8D1 – in retrospect, arguably the
closest currently-known bona fide UDG to the Milky Way111Trujillo et al.
(2017) have previously reported the system UGC 2162, lying at 12.3 Mpc as the
closest UDG with a main body that is still largely intact. – using new
observations and report the discovery of an enormous ($\gtrsim 60$ kpc) tidal
stream emanating from this system. This provides direct evidence that the
unusual properties of F8D1 are a result of significant tidal stripping and
also highlights the thus far unappreciated role of this galaxy in the overall
dynamical history of the M81 Group. The primary data are drawn from a deep
survey of the M81 Group that we have conducted using the Hyper Suprime-Cam
(HSC) wide-field imager on the 8.2m Subaru telescope. Early findings from this
survey were presented in Okamoto et al. (2015) and Okamoto et al. (2019), and
results of the full survey, including stellar halo profiles and a new
satellite search, will be the subject of forthcoming papers (Žemaitis et al.,
in prep; Okamoto et al., in prep).

The paper is structured as follows: Section 2 describes the observational data
used in the paper together with its reduction and corrections. We present the
discovery of the tidal tail in Section 3 and characterise its properties as
well as those of the main body of F8D1. We interpret and discuss our results
in Section 4, and summarise our findings in Section 5.

##  2 The Data

###  2.1 HSC Observations and Data Reduction

Our primary dataset consists of observations obtained with HSC on the Subaru
Telescope. The HSC imager consists of a mosaic of 104 CCDs, sampling a field-
of-view (FOV) of 1.76 sq. deg with 0.17 arcsec pixels (Miyazaki et al., 2018).
Our M81 Group survey covers $\sim 12$ square degrees and reaches to roughly
two magnitudes below the tip of the RGB at the distance of M81. The survey
footprint is centered on M81 and comprises seven HSC pointings in total,
extending to a projected radius of $\sim 130$ kpc from the centre of M81 (see
Fig. 1, left panel). The data were obtained in the course of various classical
and queue-mode runs during the period 2015–2019. The survey uses the g- and
i-band filters to image to depths of g$\sim 27.5$ and i$\sim 26.5$ (5-sigma),
with seeing ranging from 0.6′′ to 1.0′′. This depth corresponds to more than
two magnitudes below the tip of the RGB at the distance of M81. In this work,
we focus on one particular HSC pointing from the survey, the South-West field
highlighted in the left panel of Fig. 1 which contains F8D1 near its South-
Western edge. The total exposure time for this field is 6160s ($28\times
220$s) and 6900s ($32\times 210$s) in the g\- and i-filters, respectively.
This field also includes two additional M81 Group members, the low mass spiral
NGC 2976 and the dwarf galaxy F12D1 (also known as [KK98] 077).

The HSC data were processed using the native image reduction pipeline hscPipe
8.4 (Bosch et al., 2019), developed as a precursor to the Rubin Observatory
data reduction pipeline (Axelrod et al., 2010; Jurić et al., 2017; Ivezić et
al., 2019). The images were corrected for bias, dark count and flat-fielding,
and the sky was modelled internally and subtracted from each frame. The frames
in each filter were then mosaicked and co-added after photometrically and
astrometrically calibrating against Pan-STARRS1 (Magnier et al., 2013). The
final photometry is in the HSC filter system and in AB magnitudes.

We also used hscPipe to perform PSF-fitting photometry on the co-added g- and
i-band frames, forcing the photometry on the i-band at the positions of the
g-band (primary) detections. Experimentation revealed that source detection
across the field was optimised when an aggressive 32-pixel (5.4′′) mesh was
used to subtract the sky from each frame. While this removed the local sky
very well and aided in point source detection, it rendered the co-adds
unusable for diffuse light analyses. The source catalogue was initially
cleaned by retaining only sources that have signal-to-noise ratio (SNR) $\geq$
5 in both bands. Extended sources were subsequently excluded by considering
the PSF to cmodel flux ratio. Following Pucha et al. (2019), we classify an
object as point-like if the $f_{PSF}/f_{cmodel}$ ratio is within $1\sigma$ of
unity at the respective magnitude.

###  2.2 Corrections to the Stellar Catalogue

To assess the completeness of our HSC catalogue, we conduct comparisons to
archival HST pointings that fall within our HSC FOV. We select three Advanced
Camera for Surveys (ACS) pointings from GO 16191222The particular datasets
used are: JEEA32010, JEEA32020, JEEA33010, JEEA33020, JEEA35010, JEEA35010.
which consist of exposures of 1103s and 1153s in the F606W and F814W
passbands, respectively. These HST/ACS datasets detect stars to roughly three
magnitudes below the TRGB and hence are about a magnitude deeper than our HSC
dataset. As a result, they can be considered to be 100% complete relative to
our ground-based RGB photometry.

The chosen HST/ACS fields sample a distance range of 6-14 arcmin from the
centre of F8D1. Two of the fields lie to the North-East of F8D1, along the
major axis and are roughly coincident with the feature we discuss in Sec. 3,
whereas the third field lies South-East of the centre along the minor axis.
After conducting photometry on the flc images using DOLPHOT (Dolphin, 2016),
we searched for positional matches between stellar sources in the HST/ACS
catalogue (selected using the criteria defined in Radburn-Smith et al., 2011)
and those in our HSC catalogue. We then convert the HST/ACS F606W and F814W
magnitudes of the sources onto the HSC g- and i-band filter system using
transformations that we derived. Specifically, we followed the procedure
outlined in Komiyama et al. (2018) in which the HSC filter transmission curves
were convolved with stellar spectra from the Gunn & Stryker (1983) and Pickles
(1998) spectral atlases. This allowed us to extract synthetic photometry and
derive transformations between the HSC and HST systems across a broad range of
colour (see Appendix A for more detail). Finally, we merge the catalogues of
stars in the three HST/ACS fields into one, thus ensuring that the
completeness curves do not suffer from statistical noise in less populated
magnitude bins.

We bin the stellar sources in varying g– and i–band magnitudes separately,
both within the colour limit of 1.0 < $({\it g-i})_{0}$ < 3.0 to encompass the
RGB stars. For each bin, we determine the fraction of the HSC sources which
appear in the HST catalogue and characterise the behaviour by fitting Equation
7 of Martin et al. (2016) to the recovered fraction as a function of
magnitude. We find that the i-band catalogue reaches the 50% completeness
limit at 25.35 mag, while the g-band catalogue is 50% complete until 27.06
mag. We use these completeness curves to produce a 2-D histogram that
represents the completeness fraction as a function of magnitude and colour and
in the subsequent analyses we correct our photometric catalogue where
appropriate. It is worth noting that all the analyses in this paper concern
RGB stars above the 50% completeness level, so these corrections are never
dominant.

The M81 Group lies in a part of the sky with significant and highly variable
extinction from foreground dust. In Fig. 2, we show the magnitude of
foreground extinction in the g-band towards the F8D1 field as derived from the
Schlegel et al. (1998) reddening map with corrections from Schlafly &
Finkbeiner (2011) and the HSC filter coefficients provided in Rodrigo & Solano
(2020). The values of $A_{g}$ reach up to 0.6 mag in some regions and so it
was necessary to do a star-by-star extinction correction. We use the
extinction-corrected PSF magnitudes for the remainder of this paper, which
will be referred to as g and i hereafter.

###  2.3 Ancillary Low Surface Brightness Data

As previously mentioned, the sky subtraction method we have adopted within
hscPipe optimises the detection of point sources but removes large portions of
diffuse galaxy light. We found that reprocessing the data using a global sky
subtraction (which corresponds to super-sky pixels of $1000\times 1000$ pixels
in hscPipe) still results in a visible oversubtraction of the galaxy light. To
facilitate integrated light analysis of the main body of F8D1, we therefore
acquired deep images in the g,r,i filters with the 1.1 square degree MegaCam
imager on the 3.6m Canada-France-Hawaii Telescope (CFHT) between Feb 6 and Mar
3 2022. The images were obtained using an observing technique that is
optimised for low surface brightness (LSB) surveys at CFHT (e.g., Ferrarese et
al., 2012; Duc et al., 2015). Using a 7-position large dither captured in a
contiguous time window, with relative offsets greater in scale than most
extended features encountered in the field such as galaxies (10′), a median
correction map is derived. This smoothed map with features starting at a scale
of $\sim 5^{\prime}$ integrates all systematic illumination effects and
restores, upon a properly scaled subtraction, the true night sky background in
each individual science image which is a flat pedestal upon which we now see
undisturbed compact and extended diffuse sources. Stacks at full depth are
obtained by first removing the specific true sky pedestal from each image and
scaling the astronomical flux according to a zero point adjustment (accounting
for variable atmospheric absorption, etc). Magnitudes are AB in the MegaCam
system (Regnault et al., 2009). Two sets of $g$-band exposures (7$\times$300s
each, total 4200s), 3 sets of $r$-band exposures (7$\times$300s each, total
6300s), and 1 set of $i$-band exposures (7$\times$300s, total 2100s each) were
acquired. The sky was dark to gray and photometric throughout but the seeing
was poor (between 1.1 and 1.5′′) although adequate for diffuse emission. The
final science stacks are produced with a scale of 0.561′′/pixel, still
sampling the seeing adequately while boosting the SNR in the diffuse emission.
The sky brightness in the $g,r,i$ bands is respectively 21.56, 21.25, 20.15
magnitude per square arcsecond. The limiting surface brightness is usually
derived by measuring the residual flatness in the sky background, commensurate
with the faintest measurable contrast from a diffuse emission against the flat
sky. This is however impossible here due to the prevalence of the bright
variable cirrus all over the field. Typical MegaCam LSB performance however
adds 7 magnitudes with respect to the sky background, i.e. $\sim$28.56
magnitude per square arcsecond in the $g$-band.

The right panel of Fig.1 shows the central high SNR portion of the CFHT
stacked $i-$band image (the sky area covered by all 7 exposures from the LSB
set), with the main galaxies in the field indicated. This deep image also
serves to beautifully illustrate the complex nature of the F8D1 field in terms
of galaxy locations and foreground cirrus. It demonstrates precisely why the
wide-field resolved star approach described in Sec. 2.1 is the only means to
study stellar features in the M81 Group at extremely low surface brightness
levels. As expected, the cirrus features in the right panel of Fig.1 generally
agree with the areas of high extinction seen in Fig. 2.

Figure 2: Foreground extinction in the F8D1 HSC field derived from the
Schlegel et al. (1998) map, calibrated with the HSC g-band coefficients from
Schlafly & Finkbeiner (2011). The large white circle shows the approximate
footprint of the HSC pointing analysed here, with the main galaxies marked.
The small white circle delineates 3 $R_{\rm eff}$ around F8D1 derived from our
results.

##  3 A Stream from F8D1

Fig. 3 shows the spatial density distribution of candidate RGB stars across
the HSC field in a standard coordinate projection anchored to the pointing
centre. The sources are selected from our stellar catalogue to lie within a
polygonal region on the colour-magnitude diagram (CMD). This region is defined
to isolate old metal-poor stars ([M/H]$=-2.0$ to $-1.0$ dex and $\sim$10 Gyr)
and extends down to $i=25.5$, where we have $\gtrsim$50% completeness (see
Fig. 4). As the three galaxies in our field are all low mass systems, this
metallicity range captures the bulk of the RGB stars in the field.

The most striking feature in this map is a giant stream of stars which extends
from F8D1 to the North-West, in the direction of NGC 2976 and M81. This
feature can be visibly traced for at least a degree on the sky (and $\approx$
two-thirds of the HSC field), and can be seen on both sides of NGC 2976. Along
with F8D1 and its tidal stream, NGC 2976 and F12D1 also appear as prominent
overdensities in this field, both of which have stellar extents that are
considerably larger than previously thought (Karachentsev et al., 2000; Simon
et al., 2003). To illustrate this, a DSS cutout of the main optical body of
NGC 2976 is superposed on the star count map of Fig. 3 (left); this is the
only one of the three galaxies that is of high enough surface brightness to be
seen in the DSS. All three galaxies have voids in their central regions where
the hscPipe failed to return good resolved star photometry due to crowding and
the high brightness background.

The right panel of Fig. 3 shows a deep false-colour image created from our
combined CFHT $g,r$ images with the contours from the RGB map overlaid. These
contours have been created by binning the star count map into 0.5 arcmin bins
and gaussian-smoothing with a kernel of 1 arcmin, and represent levels of 0.7,
1, 2 and 3$\sigma$ above the background. This visualisation underscores the
fact that the tidal tail we have uncovered via star counts is completely
impossible to discern on a deep integrated light image due to the bright
cirrus in the field. Indeed, while there is an apparent distortion of F8D1
towards the North-East in integrated light (also noted by C98), this feature
is complicated by Galactic cirrus since its position angle is offset somewhat
from that of the tidal stream. In contrast to F8D1, the outer star count
contours of NGC 2976 and F12D1 are well-behaved and show no evidence that
these systems are tidally-distorted in their outer regions.

Figure 3: Left: The RGB star count density across our HSC pointing. A giant
tidal stream can be seen emanating from F8D1, which is located at the South-
Western edge. The stream can be traced for more than a degree ($\gtrsim 60$
kpc) towards the North-East, in the direction of NGC 2976 and M81. A DSS image
of NGC 2976 is superposed for scale and the hollow centres of the galaxies
indicate regions where the pipeline failed to return photometry. Right: A CFHT
false-colour image created from our combined $g,r$-band observations with the
RGB stellar density contours representing 0.7, 1, 2 and 3 $\sigma$ from the
left panel overlaid. Diffuse emission from Galactic cirrus covers the whole
field.

Fig. 4 shows various Hess diagrams constructed from our stellar catalogue with
PARSEC isochrones (Bressan et al., 2012) with fixed age of 10 Gyr and varying
metallicity [M/H] of $-2.0$ to $-1.0$ overlaid. The 50% completeness level is
shown as a yellow line and the polygonal region used to select RGB stars is
delineated in red. The top left panel shows sources across the full HSC field.
In addition to the conspicuous RGB around 0.7$\lesssim(g-i)_{0}\lesssim$2.0,
the other obvious sequence that can be seen is due to unresolved background
galaxies which populate bluer colours, $-0.5\lesssim(g-i)_{0}\lesssim 0.7$.
The top right panel shows the Hess diagram for a circular region of radius 6
arcmin which is centered on F8D1, and bottom left panel shows a region
extending from 6 to 24 arcmin away from F8D1 along the tidal stream. Compared
to the Hess diagram of a similarly-sized reference field (bottom right) that
lies far from the main galaxies and tidal stream, these panels show prominent
RGBs. The Hess diagram of F8D1 shows no evidence of recent star formation in
the form of young MS stars, although it does reveal a significant population
of luminous AGB stars above the RGB tip, as earlier pointed out by C98.

Figure 4: (a): The Hess diagram of stellar sources across the full HSC field.
(b): The Hess diagram of stellar sources within a circular radius of 6 arcmin
of F8D1. (c): The Hess diagram of a region extending from 6 to 24 arcmin away
from F8D1, along the direction of the tidal stream. (d): The Hess diagram of a
reference field with circular radius 9 arcmin, located well away from the
tidal stream and main galaxies. The RGB star selection box (red) and 10 Gyr
isochrones of varying metallicity ([M/H] = $-2.0$, $-1.5$, 1.0) are overlaid
in each case. Note the RGB populations in the area of F8D1 and its stream are
not visible in the reference field. The solid yellow line shows the 50 percent
completeness threshold.

###  3.1 TRGB Distances

While the distances to F8D1 and NGC 2976 have previously been determined using
various HST datasets (e.g., Caldwell et al., 1998; Karachentsev et al., 2000;
Dalcanton et al., 2009), we decided to rederive these in a consistent manner
with our HSC dataset. We use the tip of the RGB (TRGB) method (e.g., Lee et
al., 1993; Sakai et al., 1996) and follow the procedure described in Okamoto
et al. (2019). We begin by defining regions around the galaxies that are to be
used for analysis. For F8D1, we select all stars lying within 6.6 arcmin of
our adopted center (see Sec. 3.2) and for NGC 2976 we use stars lying within a
radius of 10 arcmin. For each system, we select stars with colours in the
range 1.4 < $(g-i)_{0}$ < 2.25 and with photometric errors smaller than 0.5
mag. We then construct their luminosity functions which are convolved with a
Sobel filter. The same method is also applied to stellar sources in empty
reference fields of the same area so that we can subtract off the foreground
and background contamination. The highest peaks in the convolved luminosity
functions are determined to be $i_{\rm TRGB}=24.28\pm 0.03$ and $24.18\pm
0.02$ for F8D1 and NGC 2976, respectively.

Okamoto et al. (2019) used PARSEC v1.2 stellar models to explore how the
$i$-band absolute magnitude of the TRGB varies with $g-i$ colour assuming
metallicities of [M/H] < $-1.0$ and 10-13 Gyr ages. We measure a median colour
of $(g-i)_{0}=1.87\pm 0.12$ for TRGB stars in F8D1 which using Eqn. (1) of
Okamoto et al. (2019) translates into $M_{\rm iTRGB}$ = $-3.55\pm 0.02$.
Therefore, the distance modulus to F8D1 is $(m-M)_{0}$ = 27.82 $\pm$ 0.03,
corresponding to D = 3.67 Mpc $\pm$ 0.06 Mpc. This is in excellent agreement
with the distance moduli of 27.82–28.88 which have been derived from previous
HST studies (Karachentsev et al., 2000; Ferrarese et al., 2000; Dalcanton et
al., 2009).

Similarly, we measure a median colour of $(g-i)_{0}=1.89\pm 0.08$ for TRGB
stars in NGC 2976 which translates to $M_{\rm iTRGB}=-3.55\pm 0.02$ and a
distance modulus of NGC 2976 is $(m-M)_{0}$ = 27.72 $\pm$ 0.02, or D = 3.50
Mpc $\pm$ 0.04 Mpc. This is also in very good agreement with the literature
value of $27.76$ derived by Dalcanton et al. (2009). The uncertainties on our
measurements include photometric errors, the $M_{\rm iTRGB}$ calibration
uncertainty and the reddening uncertainty. We do not include uncertainties
arising from the assumption of a single fixed age, or the variance in TRGB
luminosity according to different stellar models, which are likely to be far
larger.

These measurements confirm that F8D1 and NGC 2976 lie at different distances,
separated along the line-of-sight by $\sim 170$ kpc. The angular distance
between F8D1 and NGC 2976 is 0.53 degrees, which corresponds to a projected
separation of 34 kpc and a 3D distance of 173 kpc. There are insufficient
stars near the TRGB for us to be able to measure a distance gradient along the
stream itself but it seems safe to assume that this feature lies well behind
NGC 2976 along the line-of-sight.

###  3.2 Structural Analysis of F8D1

Before proceeding to analyse the properties of the stream, we revisit the
properties of the main body of F8D1 and update its measured structural
parameters. As the stars in the inner parts of the galaxy are poorly detected
due to aforementioned issues, a structural analysis using resolved stars is
not viable and we need to utilise diffuse light instead. However, as mentioned
in Sec. 2.3, our implementation of hscPipe does not preserve the LSB emission
of galaxies, leaving F8D1 with a large fraction of diffuse light missing in
the final stack. Instead, we investigate the structure of the main body of
F8D1 using the MegaCam LSB images.

[b]  Parameter Value R.A. (J2000.0) 09:44:45.95 Dec (J2000.0) +67:26:27.7
$M_{g_{0},{\rm direct}}$ (mag)a -13.42 $\pm$ 0.03 $M_{g_{0},{\rm
S\acute{e}rsic}}$ (mag)b -13.70 $\pm$ 0.04 $(g-r)_{0}$ c 0.764 $\pm$ 0.003
$(r-i)_{0}$ c 0.310 $\pm$ 0.004 $(m-M)_{0}$ $27.82\pm 0.03$ $D$ (Mpc) $3.67\pm
0.06$ $E(B-V)$ 0.11 $[$M/H$]$ (dex) $-1.14\pm 0.09$ $\sigma[$M/H$]$ (dex)
$0.37$ PA (deg)d $90.4\pm 5.1$ $\epsilon$d $0.12\pm 0.01$ $M_{*}$
($M_{\odot}$)e $\sim 6.8\times 10^{7}$

Table 1:  Properties of F8D1 measured in this paper.

  * a

The measured magnitude of the main body of F8D1 inside a radius of 3.6 arcmin.

  * b

The inferred total magnitude of the main body of F8D1 through extrapolation of
the Sérsic profile.

  * c

The mean colour inside a radius of 3.6 arcmin.

  * d

Average over the radial range 0.5$-$1.5 arcmin.

  * e

Main body stellar mass calculated using the inferred total magnitude.

We use the AutoProf software (Stone et al., 2021) to construct the surface
brightness profile of F8D1. AutoProf provides an end-to-end pipeline for non-
parametric profile extraction, including masking, sky determination,
centroiding and isophotal fitting. It can also work in a forced photometry
mode, where the parameters fit in one passband can be applied to extract
photometry from other passbands. AutoProf is based on the earlier isophotal-
fitting routine of Jedrzejewski (1987) and includes new regularization
techniques from machine learning which allow it to deliver more stable and
robust profiles. In our analysis, we use it in default mode to fit elliptical
isophotes at a series of radii and extract the surface brightness profile.

Based on visual inspection, the contaminating cirrus is less severe in the
$i$-band image (the galaxy gets brighter while the cirrus gets fainter with
respect to the higher sky brightness) and hence we decided to use that as our
primary image for fitting the isophotes and extracting surface photometry. The
ellipse parameters derived from this run of AutoProf were then applied to the
$g-$ and $r-$band images. The centre of F8D1 is chosen to be the point at
which the first FFT mode along a circular isophote333In fact ten circular
isophotes are used and averaged, these are chosen to be from $1-10$ times the
PSF FWHM so as to evaluate galaxy scale features and not individual peaks. is
minimized. This selects a location at which there is no global direction of
increasing brightness. Dwarf galaxies often have clumpy features that are
offset from the centre which could distract a conventional peak-finding
algorithm, thus the FFT based method is preferred. The updated central
position is provided in Table 1 and differs from the C98 position by $\sim 7$
arcsec in RA and $\sim 9$ arcsec in Declination.

Once the centre is selected, AutoProf then fits a series of isophotes which
extend until they reach below 2 times the pixel level SNR, which in our case
is a semimajor axis of $\sim 1.3^{\prime}$. When the limit of isophotal
fitting is reached, AutoProf can sample the profile further out by assuming
the average of the outer fit values of PA and ellipticity (defined as
$\epsilon=1-b/a$) (see Stone et al., 2021, for more details).

To assess the impact of contamination by foreground cirrus in the vicinity of
F8D1, the images were compared against WISE 12 $\mathrm{\mu m}$ observations
using the highest resolution maps from Meisner & Finkbeiner (2014). We
produced cirrus-corrected MegaCam images through an iterative scaling process
that flattens the background around F8D1 on the $g,r$ images. The $i$-band was
found to be unaffected and so does not require any correction. Comparing the
AutoProf photometry extracted from the $g,r$ images with and without this
cirrus correction shows no change to the profiles. However, it did reveal a
slight artificial boosting of the surface brightness due to cirrus, requiring
a correction of +0.22 mag to the $g$-band profile and +0.20 mag to the
$r$-band profile.

In Fig. 5 we show the three extracted surface brightness profiles (top) and
the colour profiles (bottom). These have been corrected for foreground
extinction using a single average value of E(B-V)=0.11. The uncertainties
returned by Autoprof are purely statistical in nature. Systematic effects,
such as those due to residual cirrus contamination and/or sky residuals, are
likely to be several times higher. We also overplot the surface brightness of
profile of C98 transformed to the $g-$band using the equations in Komiyama et
al. (2018) and adjusted for our assumed extinction. This shows very good
agreement with our own profile. The most significant departure is seen at
radii $\gtrsim 2$ arcmin, where the C98 profile shows a knee in the profile
that we do not see in our data. While there are no strong colour gradients
within the inner few arcmin of F8D1, there is a gradual trend to redder
colours with increasing radius. Table 1 reports the mean colours of F8D1
measured within 3.6 arcmin. The ellipticity and position angle derived by
Autoprof are essentially constant over the fit range, averaging to $0.12\pm
0.01$ and $90.4\pm 5.1$ degrees, respectively.

[b]  Parameter g r i $n$ 0.53 $\pm$ 0.01 0.56 $\pm$ 0.01 0.56 $\pm$ 0.01
$\mu(0)$ (mag arcsec-2) 25.69 $\pm$ 0.01 25.08 $\pm$ 0.01 24.68 $\pm$ 0.01
$R_{\rm h}$ (arcsec) 119 $\pm$ 0.6 119 $\pm$ 0.8 125 $\pm$ 1.1 $R_{\rm eff}$
(arcsec) 100 $\pm$ 0.8 105 $\pm$ 1.2 109 $\pm$ 1.6 $R_{\rm eff}$ (kpc) 1.70
$\pm$ 0.03 1.78 $\pm$ 0.04 1.85 $\pm$ 0.04 $M_{0,\rm S\acute{e}rsic}$ (mag)a
-13.70 $\pm$ 0.04 -14.32 $\pm$ 0.04 -14.82 $\pm$ 0.04

Table 2:  Sérsic fits and derived parameters for the main body of F8D1.

  * a

The inferred total magnitude through extrapolation of the Sérsic profile.

We fit a Sérsic profile ($I=I_{0}e^{{-(R/R_{h})^{\frac{1}{n}}}}$) to the
surface brightness profiles, leaving all parameters free (Sersic, 1968). These
fits, which are overlaid in Fig. 5, can be seen to provide a good description
of the light profile across the main body of F8D1, out to roughly 3.5 arcmin.
The values of the central surface brightness, $\mu(0)$, and the semimajor axis
scale radius, $R_{\rm h}$, are reported for each band in Table 2. The
effective radii, $R_{\rm eff}$, are also listed, which are obtained from the
fit parameters using $R_{\rm eff}=R_{\rm h}b^{n}$, where $b=2n-0.324$ (Ciotti,
1991). Adjusting for our derived distance, we find that the effective radii
slightly increase towards redder bands, going from 1.7 kpc in the $g-$band to
1.9 kpc in the $i-$band. These values lie in between the initial value of 2.45
kpc found by C98 and the smaller value of 1.2 kpc reported by Chiboucas et al.
(2009) from their $r$-band CFHT survey. When combined our central surface
brightnesses of $\mu(0)\sim 24.7-25.7$ mag arcsec-2, it is clear that F8D1 has
photometric and structural properties which firmly place it in the realm of
UDGs.

We also revisit the luminosity of F8D1 using two different methods. Firstly,
we sum the cumulative flux measured by AutoProf within the last measured
isophote at 3.6 arcmin ($\sim 2.2R_{\rm g,eff}$) and find $M_{g}=-13.42\pm
0.03$. Alternately, we can use the Sérsic fits to infer the total magnitude
using eqn. 12 of Graham & Driver (2005). This yields the slightly larger value
of $-13.70$ in the $g$-band (see Table 2), as expected given that it
extrapolates beyond the last directly measured datapoint. We adopt these two
measures as the lower and upper limits on the true luminosity of the main body
of the system. The latter value compares reasonably well to the total
magnitude reported by C98 which, when transformed from $V$ to $g_{\rm hsc}$
and adjusted for our distance and extinction, yields $M_{g}$ = $-13.9$ mag.
With the $i-$band magnitude and $g-i$ colour, we use eqn. 8 of Taylor et al.
(2011) to calculate that F8D1 contains $\sim 6.8\times 10^{7}M_{\odot}$ in
stellar mass.

Figure 5: (Top panel) The extinction-corrected, background-subtracted surface
brightness profiles of F8D1, derived from a diffuse light analysis of the
MegaCam LSB data. Sérsic fits are overlaid. For comparison, we also show the
V-band surface brightness profile of C98, which has been converted onto the
HSC photometric system and adjusted for our extinction (blue points). (Bottom
panel) The extinction-corrected $g-r$ and $g-i$ colour profiles.

###  3.3 Stream Profile

Visual inspection of Fig. 1 suggests that the tidal tail may have some
curvature along its length, as projected on the sky. To quantify this, we
consider the RGB star counts in $20\times 6$ arcmin wide bins aligned with the
direction of the tail. By fitting a Gaussian in the transverse direction, we
find the peak RGB star density location in each bin which we take to represent
the stream locus (Fig. 6). Bins lying 24 to 42 arcmin along the the stream
length are contaminated by stars from NGC 2976 and are omitted. We find that
the stream curves $\mathrm{\sim}$0.8 arcmin ($\sim 0.9$ kpc) West of the main
body at small radii (6–24 arcmin), and changes direction at larger radii,
curving $\mathrm{\sim}$1.1 arcmin ($\sim 1.2$ kpc) to the east at distances of
40–60 arcmin. This is consistent with the S-shaped behaviour that is commonly
seen in satellites that are being tidally stripped by a massive companion
(e.g., Johnston et al., 2002).

Figure 6: Peak RGB density across the width of the stream, in bins placed
sequentially along its length. The locations of the transverse peak densities
are measured relative to that of the first bin, which corresponds to $\sim
4R_{\rm g,eff}$ from F8D1. The region corresponding to $\sim$24-42 arcmin
along the length of the stream is omitted due to contamination from NGC 2976.
Clear curvature can be seen.

We next investigate the stellar density profile along the tail. For this, we
calculate the RGB surface density in $15\times 6$ rectangular bins aligned
along the direction of the tail. The $\sim 15$ kpc width of these bins matches
well the visible width of the stream. The bins begin at a distance of 6 arcmin
from the centre of F8D1 to avoid the regions where the crowding is severe and
completeness corrections are very large. The distance range 24 – 36 arcmin is
omitted due to the presence of NGC 2976. The stars counted are completeness
corrected. A correction is made for contaminants using the mean source density
inside the RGB selection box in nine $12\times 12$ reference fields positioned
far from the obvious overdensities in our HSC pointing. Fig. 7 shows the
resultant profile, with the stream clearly traced to $\approx 60$kpc. The
error bars reflect the combination of Poissonian uncertainties on the RGB and
contaminant source counts. The logarithmic stellar density profile can be
described reasonably well by a Sérsic model with $R_{h}\sim 28$ arcmin and
$n\sim 0.8$. Alternately, the outermost four points can be explained by a
power-law fall off, with index $\sim-3$.

Figure 7: The RGB stellar density profile along the stream length constructed
using 8 arcmin wide bins at intervals of 6 arcmin and corrected for
contaminants. The bins at $\sim 24-36$ arcmin were omitted due to the presence
of NGC 2976 at this location. The dashed line shows a Sérsic fit to the
profile.

We can calculate the luminosity of the stream by summing up the stellar light
along its length. We first consider two large rectangular bins that span 6–24
arcmin and 42–60 arcmin along the tail, each of width 15 arcmin. These bins
encompass the visible extent of the stream while avoiding the region
contaminated by NGC 2976. We convert the $g$-band magnitudes of the RGB stars
in each bin to flux and sum up the collective light, while also correcting for
completeness of each RGB star. We then correct for light from contaminants
using the mean contaminant density appropriately scaled to the size of our
rectangular bins. To account for stars with magnitudes fainter than our RGB
selection box, we simulate the luminosity function of a 10 Gyr stellar
population with [M/H] = $-1.1$ using the Padova isochrones (Bressan et al.,
2012). Once shifted to the F8D1 distance, the luminosity function shows that
our RGB selection box encompasses 43% of total luminosity of the population.
We therefore divide by this fraction to correct for unresolved stars. Using
the average of the luminosities in the two bins as a proxy for that in the
24–42 arcmin bin that is contaminated by NGC 2976, we calculate that the
luminosity contained in the visible stream is $M_{g}$ = $-12.03$ mag or
$7.17\times 10^{6}$ L⊙. The luminosity of the main body of F8D1 is estimated
to be $2.58-3.34\times 10^{7}$ L⊙, depending on whether the directly measured
flux inside of 3.6 arcmin or the extrapolated total magnitude is used. These
calculations make use of the absolute $g-$band magnitude of the Sun,
$M_{g\odot}$ = $5.11$, provided by Willmer (2018). Our HSC imagery currently
only captures one side of the F8D1 stream but we make the assumption that an
analogous tail exists to the South-West. Multiplying the stream luminosity by
a factor of two to account for this, we infer that 30-36% of F8D1’s light is
contained in the extratidal features. We also calculate that the average
surface brightness of the observed stream over the defined area is
$\mu_{g}\sim 32$ mag arcsec-2, which is significantly fainter than any
previous integrated light studies of this area have reached. We note that it
is not possible to directly join our star count profile of the stream (Fig. 7)
with our integrated light profile of the main body (Fig. 5) since there is no
radial range where the two profiles overlap.

Figure 8: The completeness-corrected and contaminant-subtracted MDF (blue) of
the main body of F8D1, constructed using RGB stars lying within $3R_{\rm eff}$
of the galaxy centre. The MDF for a similar-sized region away from F8D1 is
also shown to reflect the contribution of the contaminant population (orange).
The peak metallicity is derived from a Gaussian fit to the MDF after
subtracting the contribution of contaminants.

###  3.4 Metallicity of F8D1 and its stream

We investigate the metallicity distribution function (MDF) of resolved stars
in the main body and tidal stream of F8D1 using their photometric
metallicities ([M/H]). We select PARSEC isochrones of age 10 Gyr and spanning
a metallicity range of $-2.2$ to $-0.3$ dex in intervals of 0.05 dex, and
shift them to the distance of F8D1. Each star within our RGB selection box is
matched to the closest isochrone in colour (for its magnitude) and assigned
that metallicity if the colour difference is $\leq 0.05$ mag. The same
procedure is performed on a reference field, far removed from the stream, to
allow construction of the MDF of the contaminant population. We note that the
metallicities of the contaminant population are meaningless, since these
sources are either background galaxies or Milky Way foreground stars and hence
not at the distance of F8D1.

To calculate the main body MDF, we consider RGB stars lying within $\sim
3R_{\rm g,eff}$. Fig. 8 shows the completeness-corrected and contaminant-
subtracted MDF (blue) which exhibits a clear peak and significant dispersion.
The orange histogram represents the distribution of the contaminants in
reference field, scaled to the same area. To characterise the MDF, we fit a
Gaussian to estimate the mean metallicity and dispersion which we find to be
$\langle[$M/H$]\rangle$ of $-1.14\pm 0.09$ dex and $\sigma([$M/H$])=0.37$. The
errors come from sampling the selected stars from a Gaussian distribution
where the width was chosen to be the photometric error of each star centred on
the measured magnitude, repeated for N = 2000 times. Our mean metallicity and
dispersion compare reasonably well to the values of
$\langle[$Fe/H$]\rangle$=$-1.0\pm 0.15$ dex and $\sigma([$Fe/H$])=0.26\pm
0.03$ estimated by C98, although the metallicity scales are different between
these studies. It should also be kept in mind that our MDF for the main body
of F8D1, while based on a sample of several hundred stars, suffers from
significant incompleteness. As seen in Fig. 4, this is most likely to affect
stars with the reddest colours and hence highest metallicities for a fixed 10
Gyr age.

Figure 9: The completeness-corrected and contaminant-subtracted MDF in bins
along the length of the F8D1 stream, constructed in a similar way to that
shown in Fig.8. In the top left corner of each plot, the mean metallicity is
displayed. Below it is the dispersion of the Gaussian fit. On the top right
corners is the distance from the main body of F8D1 along the stream in both
arcmin and kpc. Figure 10: Luminosity-metallicity relation of Local Group
dwarf galaxies and F8D1. The black points are taken from Kirby et al. (2013),
including the best-fit and rms lines from eqn. 3 in the paper. The red and
blue stars represent F8D1 with $[\alpha/Fe]=$ 0.0 and 0.4 dex, respectively.
The lower luminosity points include the main body light only, the higher
luminosity represent the main body with added light from the two tidal tails.

Fig. 9 shows the completeness-corrected and contaminant-subtracted MDFs along
the stream, using the same rectangular bins as used to construct the stream
radial profile (Sec. 3). Again, we use Gaussian fits to characterise the mean
metallicity and dispersion in each bin. Beyond $\sim 20$ kpc, the stream MDFs
become increasingly noisy although a clear excess of stars above the expected
contaminant population is present in all cases. From inspection of the panels,
it is clear that there is no obvious metallicity gradient present. This
suggests that the main body of F8D1 was chemically well-mixed prior to
disruption, consistent with the lack of any significant colour gradient in the
main body.

Fig. 10 shows where F8D1 lies relative to the luminosity-metallicity relation
defined by Local Group dwarfs. We have taken the spectroscopic metallicities
and the fit relationship from Kirby et al. (2013). The luminosity of F8D1 is
transformed from the HSC $g-$band to V using the conversion factors of
(Komiyama et al., 2018). To convert our measured [M/H] to [Fe/H], we use eqn.
3 of Salaris et al. (1993) that requires knowledge of the alpha enhancement
$\mathrm{[\alpha/Fe]}$. While there are no $\mathrm{[\alpha/Fe]}$ measurements
for F8D1, Ruiz-Lara et al. (2018) find that the average $\mathrm{[\alpha/Fe]}$
for their UDGs is $\sim 0.4$. We place F8D1 on this diagram assuming both
$\mathrm{[\alpha/Fe]}\sim 0.0$ (red) and $\mathrm{[\alpha/Fe]}\sim 0.4$
(blue). While F8D1 lies more than $1\sigma$ off the relationship in both
cases, it falls on different sides of it depending on the assumed
$\mathrm{[\alpha/Fe]}$. This is true whether we consider just the present main
body luminosity or the luminosity if we add in the contribution of two tails.
Until a more precise spectroscopic measurement of [Fe/H] can be made for F8D1,
the extent to which it remains an outlier on the luminosity-metallicity
relation remains inconclusive.

##  4 Discussion

We have uncovered a previously-unknown giant tidal stream from the ultra-
diffuse galaxy F8D1. Considering its visible extent of $\sim 60$ kpc projected
on the sky (Fig. 1) and the fact that it contains $\sim$30–36% of the main
body light, it is clear that the galaxy is undergoing heavy tidal disruption.
The two most likely tidal disruptors of F8D1 are its closest projected
neighbour, the dwarf spiral NGC 2976, or the more massive central galaxy M81.
Inspection of HI maps of the M81 Group show a large filament pointing in the
direction of NGC 2976 and that the HI distribution of NGC 2976 is more
extended in the direction of M81 (de Blok et al., 2018; Sorgho et al., 2019).
This suggests a tidal interaction between these two systems but there are no
HI features around NGC 2976 that could be connected to a possible tidal
interaction with F8D1. HI has not been detected in F8D1 thus far. Sorgho et
al. (2019) notes that a cloud is visible in the South-East side of NGC 2976,
but it is unlikely to be tied to the disruption of F8D1 located in the South-
West.

Our estimated line-of-sight distances to F8D1 and NGC 2976 show that these
galaxies appear near each other only because of their projected positions on
the sky. Their actual 3D separation is 173 kpc, with F8D1 lying further away.
M81 lies at an angular distance of 1.91 degrees from F8D1. Taking this
together with a distance of 3.63 Mpc for this system (Jang et al., 2012), we
calculate that the 3D distance between M81 and F8D1 is 128 kpc. The total
masses of M81 and NGC 2976 are not well known but their stellar masses have
been reported to be $63.8\times 10^{9}$ M⊙ and $1.2\times 10^{9}$ M⊙ (Sorgho
et al., 2019), respectively. The stellar mass-halo mass relation of Behroozi
et al. (2013) implies that their halo masses differ by roughly one order of
magnitude, and hence that the tidal force exerted on F8D1 by M81 will be at
least 20 times stronger than that due to NGC 2976. The identification of M81
as the primary disruptor is also consistent with the fact that NGC 2976 shows
no obvious distortion in its outermost stellar isophotes. There is no radial
velocity measurement available for F8D1 but given its advanced state of
disruption it is natural to surmise that it has had at least one pericentric
passage to date. The fact that we see luminous AGB stars in F8D1 (see also
C98) indicates that star formation has taken place within the last few Gyr. We
speculate that this pericentric passage must have happened fairly recently and
that it has been responsible for the stripping the galaxy of its remaining
cold gas and quenching star formation.

There are several interesting and important implications of our findings.
Firstly, we can deduce that F8D1 has played a hitherto unrecognised and
potentially important role in the interaction history of the M81 Group. Most
famously known for its interacting triplet, all previous studies of the
dynamical evolution of the group have focused on the inner trio of M81, M82
and NGC 3077 alone. These three galaxies are strongly interacting at the
present day, with bridging structures of HI evident between them (Yun et al.,
1994; de Blok et al., 2018) as well as very extended and perturbed stellar
halos that are linked by a large stream of tidal debris (Barker et al., 2009;
Okamoto et al., 2015; Smercina et al., 2020).

Early modelling of the dynamical evolution of M81, M82 and NGC 3077 involved
studying the pairwise interactions between M81–M82 and M81–NGC 3077. This
suggested pericentric passages about 220 and 280 Myr ago and could match the
HI observations reasonably well. However, this was not full N-body modelling
so did not capture the effects of dynamical friction. More recently, Oehm et
al. (2017) have revisited the problem using N-body modelling. Based on the
current positions and velocities of the three inner galaxies, and imposing the
requirement that M82 and NGC 3077 experienced pericentric passages within 30
kpc of M81 in the last 500 Myr, they find it highly unlikely that the current
configuration of the system can be long-lived. Indeed, they argue that at
least one or both of M82 and NGC 3077 must have been initially unbound and
that we are observing the M81 Group at a special time, with a merger set to
take place within the next 1–2 Gyr. On the other hand, for the three galaxies
to be in an orbital configuration that is long-lived, they require that the
amount of dark matter in the galaxies must be significantly less than would be
expected on the basis of their stellar masses. It will be interesting to
explore if future modelling efforts that include F8D1 change these
conclusions.

The realisation that F8D1 is in an advanced state of tidal disruption provides
an explanation for the peculiar properties of this system, as first commented
on by C98. In particular, F8D1 has an unusually large size and metallicity for
its luminosity and central surface brightness, and C98 noted that it is offset
from the scaling relationships that are defined by dwarf galaxies in the Local
Group. We have shown that accounting for the visible stream, and assuming that
a similar feature lies on the side of the galaxy unprobed by our observations,
the luminosity of F8D1 is at least $\sim 43-56$% times larger (corresponding
to $M_{g}\sim-14$ mag) than would be inferred from measuring the main body
light. We note that this is likely to be a lower limit on its original
luminosity, as it may have experienced repeated earlier bouts of stripping as
it has orbited around M81. This moves the system closer to the expected
scaling relation behaviour, suggesting that initially, it may have been a
relatively normal galaxy.

The unveiling of F8D1 as a tidally-disrupted system also raises the question
of whether the properties of other UDGs can be explained in this way. In Table
3, we compare the properties of F8D1 that we have derived in this paper with
those of two very well-studied UDGs, the systems NGC1052-DF2 and NGC1052-DF4.
These systems have attracted much recent attention as examples of UDGs that
have little to no dark matter and unusually luminous globular cluster systems
(e.g., van Dokkum et al., 2018; van Dokkum et al., 2019). We use the
photometric and structural parameters from Román et al. (2021) for DF2 and
DF4, adjusted for their respective distances reported by Danieli et al. (2020)
and Shen et al. (2021). It can be seen that while F8D1 has a somewhat lower
central surface brightness and luminosity than these systems, their structural
properties compare surprisingly well. In particular, all systems have
effective radii $\gtrsim 1.6$ kpc, Sérsic indices smaller than unity and
rather circular isophotes in their central regions. Recently Keim et al.
(2022) have used new deep imaging data from the Dragonfly Telephoto array to
show that NGC1052-DF2 and DF4 exhibit position angle twists and increasingly
elongated isophotes when surface brightness levels of $\mu_{g}\sim 30$ mag
arcsec-2 are reached (although see Montes et al., 2021). While they see no
evidence for the strong signatures of ongoing disruption that we have
uncovered in F8D1, we note that the F8D1 tail has an even lower mean surface
brightness ($\mu_{g}\sim 32$ mag arcsec-2) than Dragonfly has reached. Indeed,
the F8D1 tail lies well below the surface brightness limit of all integrated
light studies of individual UDGs carried out to date444Mowla et al. (2017)
stack 231 UDGs in the Coma cluster and find no evidence of an S-shaped tidal
feature to $\gtrsim 30$ mag arcsec-2. However, this may not be unexpected
given that tidal features are likely to exhibit a variety of morphologies and
sizes around individual galaxies, which stacking will cancel out.. This leaves
open the possibility that many of these systems could have relatively regular
shapes and radial profiles in their inner regions, yet still be undergoing
significant tidal stripping.

Tidal stripping and heating have been discussed by several authors as a
mechanism for producing UDGs (e.g., Carleton et al., 2019; Jiang et al., 2019;
Sales et al., 2020; Benavides et al., 2022). These papers have mostly focused
on galaxies residing in dense environments but they have also discussed UDGs
in groups and in the field. They generally show that satellite galaxies born
in $\sim 10^{10}-10^{11}$ M⊙ haloes can suffer a dramatic reduction in surface
brightness and expansion in size due to tidal stripping and heating. However,
this is not the only way to form UDGs in the simulations, and many are also
born as intrinsically low surface brightness extended dwarfs. Jiang et al.
(2019) argue that roughly half of the population of group UDGs have started
off as normal dwarfs on highly eccentric orbits. For these systems, one
pericentric passage is sufficient for the system to puff up, and lose its gas
due to ram pressure stripping. The presence of a few Gyr old AGB stars in F8D1
is interesting in this respect, suggesting the pericentric passage around M81
occurred on this timescale. A precise measurement of the star formation
history of F8D1 through modelling its CMD will further constrain this, and
will also help distinguish between competing formation scenarios. For example,
Carleton et al. (2019) predict that the mean stellar age for UDGs with
effective radii between $1.5-3$ kpc is 4.8 Gyr, while Sales et al. (2020)
predict that tidal UDGs should be significantly older. Thus far, the available
age measurements for UDGs are consistent with ages $\gtrsim 7$ Gyr, albeit
with large uncertainties (e.g., Gu et al., 2018; Ferré-Mateu et al., 2018).
Furthermore, a spectroscopic measurement of F8D1’s metallicity will clarify if
it really is an outlier on the luminosity-metallicity relationship as would be
expected in the tidal-stripping scenario (e.g., Sales et al., 2020).

[b]  Parameter F8D1 NGC1052-DF2a NGC1052-DF4b $R_{\rm eff}$ (kpc) 1.70 $\pm$
0.03 2.28$\pm$ 0.03 1.63$\pm$ 0.04 $\mu_{g}(0)$ 25.69 $\pm$ 0.01 24.7 $\pm$
0.1 24.3$\pm$ 0.1 $(g-r)_{0}$ 0.764 $\pm$ 0.003 0.6 $\pm$ 0.01 0.63 $\pm$ 0.01
$n$ 0.52 $\pm$ 0.01 0.58$\pm$ 0.02 0.8 $\pm$ 0.04 $b/a$ 0.88$\pm 0.01$ 0.89
$\pm$ 0.01 0.86 $\pm$ 0.01 M${}_{g_{0}}$ (mag) -13.70 $\pm$ 0.04 -14.89$\pm$
0.04 -14.65$\pm$ 0.04

Table 3:  Comparison of F8D1 and NGC1052-DF2 and DF4.

  * a

Derived in this paper.

  * b

Taken from Román et al. (2021) and using the Danieli et al. (2020) distance.

  * c

Taken from Román et al. (2021) and using the Shen et al. (2021) distance.

##  5 Conclusions

We have used data from Subaru/HSC and CFHT/MegaCam to revisit the properties
of F8D1, a peculiar dwarf satellite companion of M81. Despite being the
closest known example of a UDG to the Milky Way, it has been very poorly
studied since its discovery more than 20 years ago. Indeed, the complex and
bright Galactic cirrus around F8D1 makes analysis of its faint structure very
challenging.

Using Subaru/HSC observations in the $g$ and $i$ filters, we have mapped the
distribution of individual RGB stars on the North-East side of F8D1. We have
found evidence for a giant tidal tail emanating from this system. This feature
can be traced for over 1 degree on the sky, corresponding to $\gtrsim 60$ kpc
at the distance of the galaxy. We have mapped the distribution of stars along
and across the stream, finding behaviour consistent with an S-shaped structure
that is characteristic of tidal stripping. Using the TRGB distance estimation
method, we confirm that F8D1 lies on the far side of the M81 Group at D = 3.67
Mpc $\pm$ 0.06 Mpc while the nearby dwarf spiral NGC 2976 is on the near side
at D = 3.50 Mpc $\pm$ 0.04 Mpc. We argue that the most likely cause of F8D1’s
tidal disruption is the central galaxy M81. We calculate that the luminosity
contained in the visible stream is $M_{g}$ = $-12.03$ mag or $7.17\times
10^{6}$ L⊙. We determine the photometric metallicity distribution of RGB stars
in the main body and stream of F8D1 and find that these are consistent with
each other, with no evidence for a significant gradient in either component.
The CMD of F8D1 shows no evidence of recent star formation in the form of
young MS stars, although it does reveal a significant population of AGB stars.
These stars indicate that some star formation took place within the last few
Gyr, and their presence may be used to time the last pericentric passage
around M81.

The deep CFHT/MegaCam data allow us to improve the photometric and structural
characterisation of the main body of F8D1 and place its tidal stream in
context. We find the effective radius to be in the range of 1.7-1.9 kpc, with
a trend towards larger values for redder filters. When combined with the
central surface brightnesses of $\mu(0)\sim 24.7-25.7$ mag arcsec-2, it is
clear that F8D1 is a bona fide example of an UDG. We update the main body
luminosity of F8D1 to be $2.58-3.34\times 10^{7}$ L⊙, depending on whether the
directly measured flux inside of 3.6 arcmin or the extrapolated total
magnitude is used. Assuming a comparable stream on the other side of F8D1
beyond the extent of our current imagery, we deduce that 30-36% of F8D1’s
light is contained in the extratidal features. Alternately, accounting for the
light in the tails increases the luminosity of F8D1 by 43-56%, or to
$M_{g}=-14$ mag.

Revealing F8D1 to be a system in an advanced state of tidal disruption has
implications for both the dynamical evolution of the M81 Group and for the
origin of galaxies that exhibit UDG properties. In the first instance, F8D1
has likely played a hitherto unrecognised role in the interaction history of
the group and it will be important to include its effects in future modelling
efforts. To this end, measuring an accurate radial velocity for this system
will be of great importance.

Furthermore, the severe tidal shredding of F8D1 is likely the origin of its
present-day extremely diffuse nature, and not any peculiar properties it might
have had at birth. It will be very important to determine the mass-to-light
ratio of F8D1 to constrain its dark matter content and compare it to other
UDGs. This could be achieved with integrated light spectroscopy of the main
body or with the use of dynamical tracers such as GCs, assuming a population
of such objects can be found.

Our results illustrate the critical role that surface brightness limiting
depth plays in detecting tidal features around UDGs. By virtue of F8D1’s
proximity, we have been able to apply the resolved star approach and thereby
detect surface brightness levels of $\mu_{g}\gtrsim 32$ mag arcsec-2. This is
significantly below what has been achieved in current integrated light studies
of other individual UDGs. As the only UDG that is close enough to allow
studies at extremely low surface brightness, and the first to be unambiguously
linked to tidal stripping, our results for F8D1 are of particular importance.
It leaves open the possibility that many other UDGs could be the result of
similar processes, with the most telling signatures of this lurking below
current detection limits.

Upcoming large-area sky surveys such as Euclid Wide Survey (Euclid
Collaboration et al., 2022), the Legacy Survey of Space and Time with the Vera
C. Rubin Observatory (Ivezić et al., 2019) and the High Latitude Survey of the
Nancy Grace Roman Space Telescope (Akeson et al., 2019) will enable giant
leaps in the number of systems that can have their large-scale structures and
stellar contents analysed via their resolved star populations. These surveys
are poised to be transformative for LSB Local Volume science and will yield
rich information on the statistical properties of galaxy outskirts within
$\sim 5$ Mpc (e.g., Pearson et al., 2019; Mutlu-Pakdil et al., 2021). However,
as we have demonstrated, the ability to conduct robust and quantitative
studies of LSB diffuse light, either in the high-crowding regions of local
galaxies or in galaxies that are too far away for the purely resolved star
approach, is also essential. This is a far more challenging requirement and
will greatly depend on the adopted strategies for pipeline processing and sky
subtraction (A. Watkins, private communication). If excellent performances can
be achieved, this will enable unprecedented statistical studies of UDG
structures and populations throughout the nearby universe and enable F8D1 to
be placed in context.

## Acknowledgements

R.Z. acknowledges a studentship received from UKRI STFC and supplementary
funding from the Institute for Astronomy at the University of Edinburgh
(2145313). We thank Aaron Watkins for useful discussions on sky subtraction
and helpful comments from an anonymous referee. S.O. acknowledges support from
JSPS Grant-in-Aid for Scientific Research (18H05875, 20K04031, 20H05855). N.A.
thanks the Brain Pool program for financial support, which is funded by the
Ministry of Science and ICT through the National Research Foundation of Korea
(2018H1D3A2000902).

The methods described in the paper have been implemented using Python code and
several Python libraries: Matplotlib for visualisation (Hunter, 2007), NumPy
and AstroPy for data handling (Harris et al., 2020; Astropy Collaboration et
al., 2013, 2018) and Scipy for model fitting (Virtanen et al., 2020). We use
Autoprof for the surface brightness analysis (Stone et al., 2021).

This research is based on observations collected at the Subaru Telescope,
which is operated by the National Astronomical Observatory of Japan, and on
observations obtained with MegaPrime/MegaCam, a joint project of CFHT and
CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by
the National Research Council (NRC) of Canada, the Institut National des
Science de l’Univers of the Centre National de la Recherche Scientifique
(CNRS) of France, and the University of Hawaii. We are honored and grateful
for the opportunity of observing the Universe from Maunakea, which has
cultural, historical, and natural significance in Hawaii.

This paper makes use of software developed for Vera C. Rubin Observatory. We
thank the Rubin Observatory for making their code available as free software
at http://pipelines.lsst.io/.

The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been
made possible through contributions by the Institute for Astronomy, the
University of Hawaii, the Pan-STARRS Project Office, the Max Planck Society
and its participating institutes, the Max Planck Institute for Astronomy,
Heidelberg, and the Max Planck Institute for Extraterrestrial Physics,
Garching, The Johns Hopkins University, Durham University, the University of
Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for
Astrophysics, the Las Cumbres Observatory Global Telescope Network
Incorporated, the National Central University of Taiwan, the Space Telescope
Science Institute, the National Aeronautics and Space Administration under
grant No. NNX08AR22G issued through the Planetary Science Division of the NASA
Science Mission Directorate, the National Science Foundation grant No.
AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the
Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation.

For the purpose of open access, the author has applied a Creative Commons
Attribution (CC BY) licence to any Author Accepted Manuscript version arising
from this submission.

## Data Availability

The data underlying this article will be shared on reasonable request to the
corresponding author.

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##  Appendix A Photometric Transformations between the HST and HSC Systems

In order to use HST observations to derive the completeness of our HSC
observations, we needed to convert photometry between the two filter systems.
Due to the lack of HST to HSC photometric transformations in the literature,
we derived our own equations. Following the procedure outlined in Komiyama et
al. (2018), we convolved the HSC and HST filter transmission curves of
interest with stellar spectra from the Gunn & Stryker (1983) and Pickles
(1998) spectral atlases, which together contain stars with a broad range of
colour. Only the HST F606W and F814W to HSC g and i band filters are needed
for our study. The integrated fluxes in the bandpasses were converted to
either AB magnitudes (HSC filters) or VEGA magnitudes (HST filters).

Figures 11 and 12 show the difference in derived magnitudes between the two
filter systems as a function the F606W$-$F814W colour. A quadratic polynomial
fit was found to be sufficient to describe the conversion between the two red
bands:

$i_{\rm HSC}-F814W=0.384+0.088(F606W-F814W)\\\ +0.011(F606W-F814W)^{2}$ (1)

Figure 11: Conversion from the HST F814W (Vega) to $i\mathrm{{}_{HSC}}$ (AB)
filter systems using a quadratic fit to the F606W$-$F814W colour. The top
panel shows the magnitude difference of the atlas stars as black points while
the bottom panel shows the residuals of the same stars from the quadratic fit.
Figure 12: Conversion from the HST F606W (Vega) to $g\mathrm{{}_{HSC}}$ (AB)
filter systems using a quadratic fit to the F606W$-$F814W. Divergent behaviour
is seen beyond F606W$-$F814W$=$1.0, thus requiring a separate quadratic fit in
this region. The top panel shows the magnitude difference of the atlas stars
as black points while the bottom panel shows the residuals of the same stars
from the quadratic fit.

The difference between the blue bands exhibits a break at
$\mathrm{F606W}-\mathrm{F814W}\sim 1$, requiring two separate quadratic
polynomial fits:

$g_{\rm HSC}-F606W=-0.124+0.892(F606W-F814W)\\\ +0.322(F606W-F814W)^{2}$ (2)

for $\mathrm{F606W}-\mathrm{F814W}<1.0$, and

$g_{\rm HSC}-F606W=0.507+0.742(F606W-F814W)\\\ -0.15(F606W-F814W)^{2}$ (3)

for $\mathrm{F606W}-\mathrm{F814W}>1.0$. In all cases, the HSC magnitudes are
on the AB system and the HST magnitudes are on the Vega system.

The i-band fit has a very low scatter – the residuals are typically less than
0.025 mag even at the reddest colours. While there is some higher-order
behaviour at colours bluer than F606W$-$F814W $=$ 0.75 which our polynomial
fit fails to capture, this level of precision is not required for our
purposes. On the other hand, the g-band fit exhibits higher residuals at all
colours, ranging from $\sim 0.075$ mag at colours bluer than F606W$-$F814W $=$
1.0 to $\sim 0.2$ mag beyond this limit. This is not unexpected given that
$g\mathrm{{}_{HSC}}$ is considerably bluer in transmission than F606W.
Nonetheless, this description is adequate for our study.