# Deep imaging with Milanković telescope: Linking merger history to kinematics
of elliptical galaxies

###### Abstract

Kinematical and morphological features observed in early-type galaxies provide
valuable insights into the evolution of their hosts. We studied the origin of
prolate rotation (i.e., rotation around the long axis) in Illustris large-
scale cosmological hydrodynamical simulations. We found that basically all the
simulated massive prolate rotators were created in relatively recent major
mergers of galaxies. Such mergers are expected to produce tidal features such
as tails, shells, asymmetric stellar halos.

We investigated deep optical images of prolate rotators, including newly
obtained Milanković data, revealing signs of galaxy interaction in all of
them. This correlation proves to be statistically very significant when
compared with a general sample of early-type galaxies from the MATLAS deep
imaging survey.

In an ongoing project, we use Milanković to assemble deep images of the
complete sample of all known nearby massive prolate rotators. Additionally, we
searched these data for asteroids to improve the accuracy of trajectories and
even discover one previously unknown main-belt asteroid.

The most frequent tidal features among the prolate rotators happen to be
shells. We developed methods to calculate the probable time of the merger from
optical images. This will allow us to compare the merger history of the sample
with predictions from Illustris. Our plan is to expand these methods to even
larger samples of shell galaxies supplied by upcoming large surveys like LSST
at Rubin Observatory. This will provide an unprecedented amount of
statistically significant data on the recent merger history of our Universe
and allow extensive investigation of the impact of mergers to a wide range of
other astrophysical phenomena.

IVANA EBROVÁ 1,∗, MICHAL BÍLEK 1,2,3, ANA LALOVIĆ 4, MUSTAFA K. YıLDıZ 5,6,

PIERRE-ALAIN DUC 7, MARTIN MAŠEK 1, and MICHAEL PROUZA 1

1FZU -- Institute of Physics of the Czech Academy of Sciences, Na Slovance
1999/2, 182 00 Prague, Czechia

∗E-mail<EMAIL_ADDRESS>
2LERMA, Observatoire de Paris, CNRS, PSL Univ., Sorbonne Univ., 75014 Paris,
France

3Collège de France, 11 place Marcelin Berthelot, 75005 Paris, France

4Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia

5Astronomy and Space Sciences Department, Science Faculty, Erciyes University,
Kayseri, 38039 Türkiye

6Erciyes University, Astronomy and Space Sciences Observatory Applied and
Research Center (UZAYBİMER), 38039, Kayseri, Türkiye

7Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg
(ObAS), UMR 7550, 67000 Strasbourg, France

##  1  INTRODUCTION

The closer we look at a galaxy, the more remarkable characteristics we see.
Integral-field spectroscopy of inner parts reveals kinematical peculiarities,
while deep imaging can uncover various tidal features in the outskirts. Even
many elliptical galaxies that previously seemed featureless have become highly
attractive objects to study. Our work explores the connections between the
kinematical and morphological attributes in early-type galaxies (ETGs). To
unveil these connections, we combine and utilize data and findings from large-
scale cosmological simulations on the theoretical side, and integral-field
spectroscopy as well as ultra deep imaging on the observational side.

Figure 1: Surface density (top row) and kinematics (bottom row) of stellar
particles of two galaxies from the Illustris simulation. Left column: galaxy
with a normal disky rotation and with stellar shells visible in the top panel.
Right column: galaxy with prolate rotation and a kinematically distinct core.

##  2  RESULTS

###  2.1  Simulations

In Ebrová et al. (2021b) and Ebrová and Łokas (2017), we studied galaxies with
kinematical peculiarities in the Illustris project – hydrodynamic cosmological
simulations (Vogelsberger et al., 2014; Nelson et al., 2015). We examined the
formation and merger histories of selected galaxies drawn from a global sample
of 7697 Illustris galaxies with more than $10^{4}$ stellar particles (i.e.,
LMC-like and heavier) in the last snapshot of the Illustris-1 run. By visually
inspecting kinematic maps, we identified 134 galaxies with kinematically
distinct cores (KDCs) and automatically selected 59 galaxies with prolate
rotation.

Fig.1 shows two galaxies from Illustris. On the left, an ETG that maintained
normal disky rotation, even though the galaxy suffered a relatively recent
merger, as can be inferred from the presence of stellar shells in the surface
density map. On the right, an ETG with a kinematically distinct core (KDC) and
prolate rotation (aalso known as “minor-axis rotation”). Prolate rotators
exhibit a substantial misalignment between the photometric and kinematic axes;
in other words, the galaxy appears to be rotating predominantly around its
major morphological axis. The galaxy on the right had both kinematic features
created in a major 1:1 merger 6.1 Gyr before the end of the simulation.

Figure 2: Correlation of the birth of prolate rotation (left) and KDCs (right)
with the time of the merger experienced by the host galaxy in the Illustris
simulation. For prolate rotators, the merger time represents the last
significant merger, while for the KDC hosts, it indicates the time of the
merger closest to the KDC birth. Circle areas are proportional to the host
stellar mass at the end of the simulation.

While both, prolate rotation and KDCs, can emerge from mergers, Illustris data
indicate systematic differences between those two. Prolate rotation is more
common among massive ETGs, consistent with observations. In contrast, KDCs
display no clear dependence on the host mass and environment. We specifically
examined the role of galaxy mergers in creating both kinematic features, see
Fig.2. KDCs more often have other origins, and if their origins are associated
with mergers, the mergers can be minor or ancient. Other KDCs are induced by
galaxy fly-bys or without an apparent cause. Moreover, KDCs can be long-
lasting features and survive subsequent significant mergers.

On the other hand, basically all massive prolate rotators were created in
major mergers (at least 1:5) during the last 6 Gyr of the Illustris
simulation. Such mergers are expected to produce tidal features that should
be, in the majority of cases, visible in current deep imaging surveys. Based
on the Illustris data, we predicted that the frequency of tidal features in
host galaxies should be higher for prolate rotation than for KDCs.

Figure 3: Images from the Milanković telescope: recent deep images of three
prolate rotators and, in the bottom right panel, the newly discovered asteroid
2022 TO6.

###  2.2  Observations

In Ebrová et al. (2021a) we examined 19 observed prolate rotators with
available deep optical images and found morphological signs of galaxy
interaction in all of them, which proves to be a statistically very
significant correlation when compared with a general sample of ETGs in MATLAS
– a deep imaging survey (Bílek et al., 2020).

In our current project, we use the Serbian 1.4m Milanković telescope at the
Astronomical Station Vidojevica to assemble deep optical images of the
complete sample of all known nearby massive prolate rotators. Between Feb 2021
and Oct 2023 we observed 5 out of 8 additional prolate rotators, each at least
5.5 h integrated on-source exposure time in the $L$-band. Fig.3 shows
preliminary processed deep images of three of these prolate rotators. All show
signs of galaxy interactions.

###  2.3  Small Solar System bodies

We search and measure small Solar System bodies as a by-product of deep
imaging of galaxies with the Milanković telescope. We use Tycho-Tracker to
explore the field of view of the images of prolate rotators and other
galaxies. In some cases, we performed follow-up and dedicated observations. So
far, we have significantly improved trajectory measurements of more than 50
objects of 17 – 22 mag and even one asteroid as faint as 23.2 mag in stacked
images.

However, the most remarkable is the discovery of a previously unknown main-
belt asteroid that received a temporary designation 2022 TO6, see the bottom
right panel of Fig.3 – the stacked image of 4$\times$300 s exposures, each
centered on the asteroid before stacking. As far as we know, this is the first
and currently the sole asteroid discovery at the Astronomical Station
Vidojevica.

###  2.4  Merger histories

The most frequent tidal features among the prolate rotators happen to be
stellar shells. Therefore, we can estimate the timing of mergers for a large
portion of the sample and compare it with the predictions of the merger
history of prolate rotators in the Illustris simulation.

We developed the ‘shell identification method’ in Bílek et al. (2013) and
Bílek et al. (2014). So far, we have applied it to several special cases of
shell galaxies to explore the host gravitational potential and derive the time
of the galaxy mergers undergone by the hosts (Bílek et al., 2014; Ebrová et
al., 2020; Bílek et al., 2022; see also Bílek et al., 2015).

There are hundreds of known shell galaxies, even more hidden in current data,
and much more will be observed in the next few years in upcoming large deep
surveys like the Large Survey of Space and Time (LSST) at the Vera C. Rubin
Observatory. We are developing tools to extract fairly accurate estimates of
the merger times for large samples. This will transform shell galaxies from a
position of curiosity to utility, allowing statistical applications using the
merger data on thousands of shell galaxies. This will provide an unprecedented
amount of data on the recent merger history of our Universe and allow
extensive investigation of the impact of mergers on a wide range of other
astrophysical phenomena such as star formation, stellar dynamics, active
galactic nuclei, transient events, and more.

##  3  CONCLUSIONS

We investigated the link between kinematical and morphological features in
early-type galaxies through simulations as well as observations. In Illustris,
we found that while the origin of kinematically distinct cores is partially
associated with mergers of galaxies, basically all massive prolate rotators
were created in relatively recent major mergers. Such mergers are expected to
leave tidal features that should be detectable today in sufficiently deep
images.

In our analysis of available observational data, we found an overabundance of
morphological signs of galaxy interactions in prolate rotators. With the help
of the Milanković telescope, we are assembling deep optical images of the
complete sample of all known nearby massive prolate rotators. Initial data
processing confirms the high statistical significance of previous findings,
showcasing Milanković as a valuable tool for studying low-surface-brightness
tidal features. Additionally, the same data can be used to search for small
Solar System bodies. We significantly improved the accuracy of trajectories
for more than 50 such objects and even discovered a previously unknown main-
belt asteroid 2022 TO6.

The most frequent tidal features among the prolate rotators are stellar shells
that can be used to constrain the time since the merger. That will enable us
to compare merger histories of prolate rotators with the Illustris
predictions. Our plan is to extend such analyses to hundreds, potentially
thousands, of shell galaxies. This will significantly deepen our understanding
of the impact of mergers on various astrophysical phenomena.

Acknowledgements

This project has received funding from the European Union’s Horizon Europe
Research and Innovation program under the Marie Skłodowska-Curie grant
agreement No. 101067618. We acknowledge support by the Astronomical Station
Vidojevica and funding from the Ministry of science, technological development
and innovation of the Republic of Serbia, contract No.
451-03-47/2023-01/200002 and by the EC through project BELISSIMA (call
FP7-REGPOT-2010-5, No. 256772). IE acknowledges the support from the Polish
National Science Centre under the grant 2017/26/D/ST9/00449.

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