# The Extended Local Supercluster

P. J. E. Peebles,1
1Joseph Henry Laboratories, Princeton University, Princeton, NJ 08544, USA
E-mail<EMAIL_ADDRESS>
###### Abstract

It has long been established but seldom noticed that we are in a region at
least 170 Mpc across in which different types of galaxies show different
degrees of alignment with the plane of the de Vaucouleurs Local Supercluster.
While clusters of galaxies and radio galaxies at redshifts $z<0.02$ are
concentrated at low supergalactic latitudes, the most luminous galaxies in the
infrared, LIRGs, show little correlation with this plane. The most luminous
early-type galaxies are concentrated at low supergalactic latitudes, but
similarly luminous spirals are not noticeably so. The cross-correlations of
the positions of what might be considered galaxies selected for their stellar
mass with positions of clusters and LIRGs offer a measure of the situation.
The mean density at distance $\sim 0.5$ Mpc from a LIRG is comparable to the
mean density at that distance from a cluster of galaxies, but the mean density
5 Mpc from a LIRG is well below the mean density at that distance from a
cluster and not much greater than the cosmic mean density. Discussion of
issues arising is brief.

###### keywords:

catalogues — galaxies: formation — galaxies: starburst — quasars: supermassive
black holes — X-rays: galaxies: clusters — large-scale structure of Universe

††pubyear: 2021††pagerange: The Extended Local Supercluster–References

##  1 Introduction

Einasto, Corwin, Huchra et al. (1983, Fig. 1) and Tully (1986, 1987) pointed
out that clusters of galaxies tend to be close to the plane of the de
Vaucouleurs Local Supercluster in a region around us that is several hundred
megaparsecs across. Shaver and Pierre (1989) and Shaver (1990, 1991) took note
of this phenomenon and added the evidence that radio galaxies at comparably
large distances also tend to be at low supergalactic latitudes.

Shaver (1991, Fig. 5) made the point that motivates the present paper. At
redshifts $z<0.02$, where most clusters and radio galaxies are at low
supergalactic latitudes SGB, close to the plane of the Local Supercluster,
ordinary $L\sim L_{\ast}$ galaxies are not so noticeably concentrated to this
plane. Shaver’s effect has not been discussed much in the literature. Among
the 25 citations to Shaver (1991) listed in NASA’s Astrophysics Data System I
find explicit mention of Shaver’s effect in only three papers, all review
articles I wrote. I continue to think the phenomenon merits closer attention;
maybe it has something to teach us about the origin of cosmic structure. In
this paper I add to the scanty literature on Shaver’s effect by an exploration
of the phenomenon in the all-sky catalogs now available, with a few thoughts
about what it might mean.

We should pause to recall that Gérard de Vaucouleurs (1953) drew attention to
the tendency of the galaxies nearer than about 30 Mpc to be close to the plane
of what he first termed the Local Supergalaxy and later the Local Supercluster
(or first the local super-cluster, in de Vaucouleurs 1958). More recent terms
are the Virgo Supercluster, and a lobe of the Laniakea Supercluster (Tully,
Courtois, Hoffman, and Pomarède 2014). Coordinates aligned with the plane of
this system still are termed supergalactic, and their worth is demonstrated by
inclusion in the angular coordinate conversions provided by the NASA/IPAC
Extragalactic Database.

Community acceptance that de Vaucouleurs’ phenomenon is likely to be more than
a chance alignment was aided by Abell’s (1961) review of Carpenter’s (1961)
numerical evidence of Vaucouleurs’ effect. More recently Baleisis, Lahav,
Loan, and Wall (1998) found reasonably convincing evidence of the tendency of
radio galaxies to be near the plane of the Local Supercluster; Lahav,
Santiago, Webster, Strauss, Davis, Dressler, and Huchra (2000) found measures
of the distributions of galaxies selected in optical and infrared surveys
relative to the supergalactic plane; and Böhringer, Chon, & Trümper (2021a)
added the evidence that X-ray detected clusters of galaxies also tend to be
close to the plane of the Local Supercluster out to distances comparable to
that of the Einasto et al. (1983) sample. McCall (2014) pointed out that all
the galaxies, large and small, within about 6 Mpc distance from us are
remarkably close to what McCall termed the Local Sheet (a term used earlier by
Tully, Shaya, Karachentsev, et al. 2008 to describe the coherent motion of the
nearby galaxies away from the Local Void). McCall’s sheet is slightly tilted
from the plane of de Vaucouleurs’ Local Supercluster. This Local Sheet
includes the radio galaxy Centaurus A, at distance 3.6 Mpc, which is at
supergalactic latitude SGB$=-5^{\circ}$. But almost all galaxies at this
distance are at low SGB.

For the history of how radio astronomy became a part of physical cosmology I
refer to Sullivan (2009) and chapters 35 and 36 in Goss, Hooker, and Ekers
(2022). A natural part of this early research was to look for radio radiation
from the concentration of galaxies around the Virgo Cluster and in the plane
of de Vaucouleurs’ Local Supercluster. The earliest reports of detection I
have found are by Kraus and Ko (1953) at the Ohio State University and
Hanbury-Brown and Hazard (1953) at the Jodrell Bank Experimental Station.
Hanbury-Brown (1962) reported the detection of luminous large angular size
radio sources afforded by the better angular resolution at the Mullard Radio
Astronomy Observatory, and showed that they are largely at low supergalactic
latitude SGB. Hanbury-Brown concluded that “Although this evidence [of the
concentration to low SGB] is far from conclusive, it is at least consistent
with the model [of the Supergalaxy] we have assumed." This was an oblate
concentration centered in the direction of Virgo. Hanbury-Brown’s distribution
of radio sources in SGB in his Fig. 7 resembles the distribution in Panel
radio12 in Figure 3 below. The observable effect in surveys of radio sources
remained subtle, however. Thus Pauliny-Toth, Witzel, Preuss, Kühr, Kellermann,
Fomalont, and Davis (1978) presented evidence that extragalactic radio sources
are more abundant at lower SGB and toward the Virgo cluster, and that “The
counts indicate a possible anisotropy related to the supergalactic system.”
Shaver and Pierre (1989) and Shaver (1990; 1991) had better data to make the
relation of radio sources to the Local Supercluster more definite. It allowed
Shaver (1991) to see that at distances out to redshift $z=0.02$, or about 85
Mpc on the present distance scale, radio galaxies tend to be at low
supergalactic latitudes while ordinary $L\sim L_{\ast}$ galaxies are more
nearly isotropically distributed. The results presented in Section 3 below
illustrate the present state of the evidence of this effect: different kinds
of objects at the same distance from us have different degrees of
concentration to low SGB.

Section 2 presents conventions adopted in aid of comparisons of the
distributions of supergalactic latitudes of a variety of samples of objects
that have been mapped in reasonable approximations to all-sky surveys.
Understanding these results may be aided by the cross-correlations of
positions of some of the samples that are presented in Section 4. I offer in
Section 5 summary comments on the results and steps that could be taken that
might improve our understanding of Shaver’s effect, along with brief thoughts
about the possible physical significance of what is now known.

##  2 Conventions

The distribution of supergalactic latitudes of clusters of galaxies is to be
compared to the distributions of galaxies selected at several different
wavelength bands. The measure used in Section 3 is the distribution of
$\sin{\rm SGB}$, where SGB is the supergalactic latitude of an object.
Baleisis, Lahav, Loan, and Wall (1998) used the same measure for the same
purpose. Since equal intervals of $\sin{\rm SGB}$ mark equal elements of solid
angle, the expected counts of an isotropic random process in equal intervals
of $\sin{\rm SGB}$ are independent of SGB.

The obscuration of objects at low galactic latitudes requires a correction to
the expected distribution of $\sin{\rm SGB}$. In this paper all objects at
galactic latitudes $|b|<10^{\circ}$ are rejected, and the effect on the
distribution of $\sin{\rm SGB}$ is computed from the distribution of points
placed uniformly at random in the allowed bounds of galactic latitude. If
incompleteness is significant at somewhat higher galactic latitudes it tends
to suppresses the counts of galaxies at high SGB. The observed distributions
in sin SGB suggest this effect is not a serious problem.

Apart from the nearest sample to be considered, in which galaxy distances are
measured, the samples are placed in separate intervals of redshift. It is to
be borne in mind that different galaxies at the same physical distance may
appear in different redshift bins, in a greater degree among galaxies with
larger peculiar velocities, perhaps those near clusters of galaxies. Measured
redshifts are translated to physical distances by using Hubble’s constant

$H_{\rm o}=70~{}\hbox{km}~{}\hbox{s}^{-1}~{}\hbox{Mpc}^{-1}.$ (1)

The individual measurements of distances and redshifts of relatively nearby
galaxies are reasonably consistent with this value of $H_{\rm o}$ (Tully,
Courtois, and Sorce 2016).

Table 1: Sample Sizes Redshift range | 0.01 - 0.02 | 0.02 - 0.04 | 0.04 - 0.08
---|---|---|---
Brightest Cluster Galaxies | 12 | 58 | 359
Cluster X-ray Sources | 30 | 114 | 302
Radio Galaxies | 32 | 141 | 253
IRAS PSCz galaxies | 61 | 523 | –
2MASS 2MRS galaxies | 6515a | 7908b | –
aluminous enough to be detected at $z=0.02$.
bluminous enough to be detected at $z=0.04$.

The issue to be considered is the differences of the degree of alignment to
the plane of the Local Supercluster by clusters of galaxies and by galaxies
selected by luminosities at different wavelengths. Catalogs of galaxies
selected in different wavelength bands, if counted to large enough number
densities, must contain many galaxies in common. Since it would be difficult
if not impossible to sort the objects into separate interesting categories, I
instead choose for a reference base the large numbers of 2MRS galaxies in the
redshift catalog based on the 2MASS galaxy survey selected by apparent
magnitudes $K_{s}$ at $2.2\mu$ (Skrutskie, Cutri, Stiening, et al. 2006;
Huchra, Macri, Masters, et al. 2012; the data used here are drawn from the
VizieR On-line Data Catalog). I consider the rare most luminous of the objects
in other catalogs, with spatial number densities that are comparable to a
typical density of clusters of galaxies. The hope is that this may lead us to
other comparably rare and perhaps comparably interesting objects.

The translation to the mean volume number density $n$ from the number $N$ of
objects in the redshift bin $0.01<z<0.02$, with the bound $|b|>10^{\circ}$ on
the galactic latitude, and the Hubble constant in equation (1), is

$n=FN,\quad F=5.24\times 10^{-7}\hbox{ Mpc}^{-3}.$ (2)

Doubling the redshift bounds multiplies $F$ by a factor of eight. The redshift
bin $0.01<z<0.02$ includes 12 Brightest Cluster Galaxies (Lauer, Postman,
Strauss, Graves, and Chisari 2014), and 6515 2MRS galaxies that are luminous
enough to be detected at $z=0.02$. They translate to number densities

$\begin{split}n_{\rm BCG}&=6.3\times 10^{-6}\hbox{ Mpc}^{-3},\\\ n_{\rm
2MRS}&=3.4\times 10^{-3}\hbox{ Mpc}^{-3}.\end{split}$ (3)

The 2MRS density is comparable to that of optically selected $L\sim L_{\ast}$
galaxies. The space distribution of the $L\sim L_{\ast}$ galaxies is a useful
approximation to the mass distribution that fits the $\Lambda$CDM theory to
the measured anisotropy spectrum of the Cosmic Microwave Background (Planck
Collaboration et al. 2021 and references therein). Thus I take it that the
reference 2MRS sample detectable out to redshift $z=0.02$ likely is a useful
approximation to the large-scale mass distribution.

##  3 Distributions in Supergalactic Latitude

This section deals with distributions of supergalactic latitudes as measures
of concentrations to the plane of the Local Supercluster. Several varieties of
extragalactic objects are considered.

Figure 1: Distributions of supergalactic latitudes of the galaxies at
distances of 2 to 6 Mpc, plotted in black for the 25 most luminous galaxies,
plotted in blue and shifted slightly to the right for the 231 less luminous
galaxies, and in red for the expected close to flat distribution of a fair
sample of an isotropic random process truncated at galactic latitude
$|b|=10^{\circ}$. The counts are scaled to a common area under each histogram.

###  3.1 Nearby Galaxies

The nearby galaxies are known to be concentrated to the plane of the Local
Supercluster. The data for the illustration in Figure 1 were drawn from the
Catalog and Atlas of the LV galaxies111Available at
https://www.sao.ru/lv/lvgdb/tables.php. Angular positions and distances are in
separate tables that do not include all the same galaxies. I have checked for
matching galaxy names. expanded from Karachentsev, Karachentseva, Huchtmeier,
and Makarov (2004). I use the galaxies for which there are tabulated nonzero
distance modulus uncertainties eDM$<0.434$, for fractional distance
uncertainties less than 20%. The histograms in Figure 1 show the counts of LV
galaxies in the equal bins of $\sin{\rm SGB}$ indicated by the horizontal line
segments, and at distances $2<r<6$ Mpc. The lower bound removes Local Group
galaxies that are close to the plane of the Local Supercluster but might be at
high supergalactic latitude. The upper bound is about at the edge of McCall’s
(2014) Local Sheet.

The histogram plotted as the black lines in Figure 1 is the distribution of
the 25 galaxies that are most luminous at absolute B magnitude. The vertical
axis is scaled to get the same area under this histogram as the distribution
plotted in blue, and displaced slightly to the right, for the 231 less
luminous galaxies. The vertical axis is the actual counts of these less
luminous galaxies. The close to flat red histogram is the distribution
expected of an isotropic random process cut off at $|b|=10^{\circ}$, again
with the same area under the curve. It was computed from counts of positions
uniformly placed at randomly at galactic latitude $|b|>10^{\circ}$, and again
normalized to the same area under the curve.

The prominent peaks in the distributions in Figure 1 illustrate the known
tendency of nearby galaxies to be close to the plane of the Local
Supercluster. The peaks would be even tighter if coordinates had been referred
to the slightly tilted Local Sheet (McCall 2014). Within the modest precision
afforded by just 25 galaxies in the more luminous sample the histograms of
distributions of luminous and faint galaxies look to be essentially the same.
This effect is discussed further in Section 5.3. But the point of particular
relevance for the discussion to follow is the close concentration of the
nearby galaxies to the plane of the Local Supercluster.

Figure 2: Distributions of supergalactic latitudes of clusters of galaxies
identified by the Lauer et al (2014) compilation of redshifts of Brightest
Cluster Galaxies, BCGs, and by the NASA HEASARC compilation of X-ray luminous
clusters. The redshift bounds are indicated by the subscripts on the panel
names: 0.01 to 0.02, 0.02 to 0.04, and 0.04 to 0.08.

###  3.2 Clusters of Galaxies

The distributions of galaxies and clusters of galaxies at redshifts greater
than $z=0.01$, distances $D\lower
2.15277pt\hbox{$\;\buildrel>\over{\sim}\;$}45$ Mpc, are beyond a conventional
size of the Local Supercluster, but they are in the range of distances at
which Einasto et al. (1983), Shaver and Pierre (1989), and Shaver (1990, 1991)
saw evidence of alignment to the plane of the Local Supercluster. The angular
distributions of clusters of galaxies at these greater distances are
illustrated by the two samples shown in Figure 2. The data on the left are
from the Lauer, Postman, Strauss, Graves, and Chisari (2014) catalog of
redshifts of the Brightest Cluster Galaxies, BCG, in the catalogs of clusters
compiled by Abell (1958) and Abell, Corwin, and Olowin (1989). These data were
downloaded from the SIMBAD database. The right-hand column shows distributions
of X-ray sources identified as intracluster plasma. (These data were
downloaded from the NASA HEASARC compilation.) The subscripts on the panel
labels are reminders of the three ranges of redshift sampled: $0.01<z<0.02$,
$0.02<z<0.04$, and $0.04<z<0.08$. As in Figure 1, Figure 2 shows histograms in
equal bins of $\sin\hbox{SGB}$, and the near flat red curves are the mean
distributions of an isotropic process truncated at $|\ell|=10^{\circ}$ and
normalized to the same area under the curve.

I use the full BCG catalog without bounds on the galaxy luminosity or the
cluster richness. I remove from the sample of X-ray sources the faint tail of
clusters with compiled measures of the X-ray luminosity less than $10^{42}$
erg s-1. This removes three clusters at $0.01<z<0.02$, leaving 30 X-ray
clusters with 12 BCGs, for volume number densities $1.6\times 10^{-5}$ Mpc-3
and $6\times 10^{-6}$ Mpc-3 (from Eq. [3]). It removes 11 clusters at
$0.02<z<0.04$, leaving 114 X-ray clusters with 58 BCGs. It leaves all 302
X-ray clusters at $0.04<z<0.08$ with 359 BCGs.

The distributions of sin SGB in the two samples in the lowest redshift bin are
quite similar but differ in number and detail, meaning Abell’s visual
selections and the X-ray detections differently sample the varieties of groups
and clusters of galaxies. As Einasto, Corwin, Huchra et al. (1983) saw, and
Böhringer, Chon, & Trümper (2021a) checked in their compilation of clusters
detected by the ROSAT All Sky Survey, clusters of galaxies at redshifts up to
$z=0.02$, within distance $\sim 85$ Mpc, are closely concentrated to low SGB,
aligned with the plane of the Local Supercluster.

The peaks in the distributions of sin SGB at greater distances, in the bottom
two rows of panels in Figure 2, are not as prominent. It seems likely to be
significant, however, that in three of the four panels the peaks are at low
SGB. This is discussed further in Section 5.

Figure 3: Distributions of supergalactic latitudes of radio-luminous galaxies,
in the data compiled by van Velzen et al. (2012).

###  3.3 Radio Galaxies

Shaver and Pierre (1989) showed evidence that radio galaxies at redshifts
$z<0.02$ tend to be close to the plane of the Local Supercluster. Figure 3
illustrates the present situation in the data drawn from the all-sky catalog
of radio galaxies compiled by van Velzen, Falcke, Schellart, Nierstenhöfer,
and Kampert (2012; the data were downloaded from the VizieR Online Data
Catalog). As in Figure 2, the subscript on the panel name represents the
redshift range. The samples used here are bounded by the Van Velzen et al.
compilation of radio luminosities interpolated to 1.1 GHz, $\nu
L_{\nu}>3\times 10^{39}~{}\hbox{ergs s}^{-1}$. This cutoff was chosen to make
the counts of radio galaxies in Table 1 for the distributions in the three
redshift bins shown in Figure 3 comparable to the counts of clusters
represented by BGGs or intracluster plasma X-ray sources.

Consistent with what Shaver and Pierre (1989, Fig. 5) saw, the distinct peak
in the distribution of sin SGB in the redshift range $0.01<z<0.02$, in the top
panel in Figure 3, shows that radio galaxies at distances out to 85 Mpc are
concentrated near the plane of the Local Supercluster, along with clusters of
galaxies. Some of these radio galaxies are in clusters, and some surely are in
rich groups; Centaurus A is a nearby example not in the redshift range of the
figure. The differences in details of the histograms in Panels BCG12, X-ray12,
and radio12 show that observations of radio galaxies are in effect yet another
way to sample the varieties of groups and clusters.

The distribution of supergalactic latitudes of radio galaxies at redshifts
$0.02<z<0.04$ is reasonably consistent with statistical isotropy. This can be
taken to suggest that the position of the peak at low SGB in the more distant
sample, $0.04<z<0.08$, likely is accidental. But arguing against this is the
coincidence that the position of this peak agrees with the peaks in three of
the four distributions of clusters at redshifts greater than $z=0.02$ in
Figure 2.

Figure 4: Angular distributions of radio galaxies and clusters of galaxies at
$0.01<z<0.02$ in supergalactic latitude and quadrants of supergalactic
longitude.

###  3.4 Distributions in Supergalactic Longitude

The four rows of histograms in Figure 4 show distributions of radio galaxies
and clusters of galaxies in sin SGB and in separate quadrants of supergalactic
longitude SGL. This is a check that at $0.01<z<0.02$ the tendencies to low
supergalactic latitudes of these objects are best described as clumpy
distributions across the two dimensions projected on the plane of the Local
Supercluster.

If considered separately each of the distributions in Figure 4 would not be
likely to attract attention, but instead would bring to mind the general
clumpy distributions of extragalactic objects. As already seen in the
distributions summed over supergalactic latitude in Panel X-ray12 in Figure 2
and Panel radio12 in Figure 3, most of these objects are at low SGB. The point
added in Figure 4 is that there are radio galaxies in comparable numbers in
all four quadrants, and that while only one cluster is detected as an X-ray
source in the first quadrant, and it is at high SGB, there are larger numbers
of cluster X-ray sources in each of the other three histograms, and most are
at low SGB.

I conclude from Figure 4 that in a region around us that is 170 Mpc across the
radio galaxies and clusters of galaxies are scattered around the plane of the
Local Supercluster, and significantly closer to the plane than the size of
this region.

Figure 5: Distributions of elliptical, S0, and spiral 2MRS galaxies from the
2MASS survey at redshifts $0.01<z<0.02$ in three bins in absolute magnitude:
most luminous on the left, intermediate luminosity on the right, and also on
the right the least luminous of each morphological type shifted slightly to
the right.

###  3.5 Distributions of Morphological Types

It has long been known that galaxies that are exceptionally luminous at radio
wavelengths tend to be in large elliptical or irregular galaxies in clusters
and rich groups of galaxies. For example, in a pioneering paper on synchrotron
radiation as the source of radio galaxy emission, Burbidge (1958a) commented
on the radio galaxies in the Virgo and Perseus clusters and possibly in the
Coma cluster; Einasto, Joeveer, and Saar (1980) presented maps showing that
radio galaxies tend to be in or in the neighborhood of rich clusters of
galaxies that are in superclusters; and in the presentation of the radio
galaxy catalog used here Van Velzen et al. (2012) reported that “the majority
of the radio galaxies in our sample have massive, early-type hosts.”

To follow up on this I show in Figure 5 distributions in sin SGB of the early-
and late-type galaxies at redshifts $0.01<z<0.02$ drawn from the 2MRS redshift
catalog of 2MASS galaxies (Skrutskie et al. 2006; Huchra et al. 2012). At the
absolute magnitude limits in the figure the apparent magnitude cutoff at
redshift $z=0.02$ is $K_{s}=9.5$ in the histograms for the most luminous
subsets in the left-hand panels. The intermediate and lowest luminosity limits
in the distributions in the right-hand panels correspond to apparent magnitude
bounds $9.5<K_{s}<10.9$ and $10.9<K_{s}<11.75$ at redshift $z=0.02$. The
histograms for the least luminous galaxies in the right-hand panels are
shifted to larger sin SGB by 0.02 to separate them from the distributions of
the intermediate luminosity galaxies. The red near flat curves in the right-
hand panels are normalized to the counts of the intermediate luminosities of
each type.

The morphological types are sorted by the Huchra et al. (2012) tabulation of
the morphological parameter T: elliptical galaxies with T$\leq-5$ are in
Panels 2ME; lenticular and S0 galaxies, here collectively termed S0, with
$-5<$T$\leq 0$, are in Panels 2MS0; and spiral galaxies with $1\leq{\rm T}\leq
9$ are in Panels 2MSp. Galaxies with $T>9$ include various kinds of irregulars
along with those that are difficult to classify or have not been visually
examined. They are not entered in these distributions. In the most luminous,
less luminous, and least luminous of the samples there are respectively 53,
319, and 325 ellipticals; 73, 1038, and 900 S0s; and 54, 1440, and 1782
spirals at $0.01<z<0.02$.

Consistent with the observed concentration of clusters of galaxies to low SGB,
and the tendency of clusters to contain early-type galaxies, the distributions
of the most luminous of the early-type galaxies in Panels 2MEbright and
2MS0bright in the left-hand side of Figure 5 have prominent peaks at low SGB.
In this redshift bin there are 126 luminous early type 2MRS galaxies,
ellipticals plus S0s, considerably more than the 12 optically selected BCGs or
the 30 clusters detected as X-ray sources. That is, most of these luminous
ellipticals and S0s are in groups rather than clusters. Perhaps the fractions
of the most luminous ellipticals and S0s at high SGB are somewhat larger than
the fractions of BCGs and radio galaxies, but the samples are too small to
allow a clear case.

At the adopted luminosity cut, about two-thirds of the most luminous of the
classified 2MRS galaxies are early types, with the rest spirals. The angular
distribution of these luminous spirals in Panel 2MSpbright has a modest peak
at sin SGB $\sim-0.3$, but the number of spirals is too small to judge its
likely significance. We can only conclude that, if a tendency to larger counts
at lower SGB is present, then the excess of luminous spirals at low SGB is
more modest than for ellipticals and S0s.

The two disjoint lower luminosity bins contain totals of 2582 early types,
ellipticals plus S0s, and 3222 spirals. The lower fraction of early types in
the lower luminosity bins agrees with the known increasing ratio of early to
late types with increasing luminosity.

In all six histograms in the right-hand panels in Figure 5 we see a reasonably
clear indication of excess numbers over isotropy at low SGB. The anisotropy is
least in the two disjoint spiral samples in Panel 2MSpfainter. Considering the
small number of the most luminous spirals in the distribution in Panel
2MSpbright, there could be a fractional departure from isotropy consistent
with what is seen in the lower luminosity late-type 2MRS galaxies. That is,
the fractional anisotropies in sin SGB favouring low supergalactic latitudes
might be present in all three spiral samples, but the effect is smaller than
in the six early type samples, and much smaller than the concentrations of
radio galaxies and clusters of galaxies to low SGB. This refines what Shaver
(1991) saw in the samples available then.

Figure 6: Comparisons of distributions of 2MRS (Huchra et al. 2012) and PSCz
(Saunders et al. 2000) galaxies. The samples in panels 2MRS12b and 2MRS24 are
complete to the same limiting absolute magnitude. The sample 2MRS12f is
complete to $z=0.02$ but does not include the more luminous 2MRS12b galaxies.
Panels PSCz12α,β show angular distributions of the 29 galaxies at $z=0.01$ to
$0.02$ that are most luminous at $60\mu$ and the next 32 most luminous. The
histograms in panels PSCz24α,β are at the same two luminosity bounds as
PSCz12α,β respectively.

###  3.6 Luminous PSCz Galaxies

Figure 6 compares the distributions of supergalactic latitudes of the 2MRS
galaxies selected at the 2MASS $2.2\mu$ wavelength band and the PSCz galaxies
selected for this figure at $60\mu$. The former includes all morphologies in
the Huchra et al. (2012) catalog, and all the objects without an estimate of
morphology but with measured redshifts and apparent magnitudes in the wanted
ranges, because these data argue for detections of galaxies of some sort. It
does not include galaxies with listed redshifts but without entries for the
uncertainty in the redshift, because the redshifts might be seriously
uncertain. The PSCz samples in the right-hand side of Figure 6 are from the
galaxy redshift catalog that Saunders, Sutherland, Maddox, et al. (2000) drew
from the IRAS (infrared astronomical satellite) sky survey at wavelengths from
12 to $100\mu$. This valuable compilation is sampled in a close to uniform way
across a large part of the sky. (The data used here were downloaded from the
NASA HEASARC IRASPSCZ catalog, class GALAXY.)

Luminous Infrared Galaxies, LIRGs (Magnelli, Elbaz, Chary, et al. 2011 and
references therein), are a subset of the PSCz galaxies. I have not attempted
to separate LIRGs from the more luminous ULIRGs and the less luminous IRAS
galaxies that are not considered LIRGs; the name is used here as a convenient
abbreviation for the most luminous of the IRAS PSCz objects to be considered
here, with space number density comparable to that of clusters of galaxies.

Panel 2MRS12f in Figure 6 shows the distribution in SGB of the galaxies that
are luminous enough to be detected at $z=0.02$, at limiting apparent magnitude
$K_{s}=11.75$, but not luminous enough to be detected at $z=0.04$. Panel
2MRS12b shows the distribution of the disjoint sample of PSCz galaxies in the
lower redshift bin that are luminous enough to be detected at $z=0.04$. This
can be compared to the distribution in Panel 2MRS24 at $z=0.02$ to 0.04 and
the same luminosity bound as for 2MRS12b.

The 5393 2MRS galaxies in the lower luminosity sample in Panel 2MRS12f amount
to space number density 0.003 Mpc-3. This is comparable to the density of
ordinary $L\sim L_{\ast}$ galaxies, meaning the space distribution of these
galaxies likely is a useful approximation to the spatial distribution of mass
(as argued in Sec. 2). The angular distribution in this sample has a distinct
peak at low SGB, but a considerable fraction of these galaxies is at higher
supergalactic latitudes. The distribution in the disjoint sample of 1123 more
luminous PSCz galaxies in Panel 2MRS12b is quite similar. As expected, it also
is reasonably similar to the distributions of 2MRS galaxies separated by
morphological types, in the right-hand panels in Figure 5. But the
distributions in Panels 2MRS12b,f differ from the tighter concentrations of
the most luminous of the early types in the top two left-hand panels in Figure
5.

The 2MRS galaxies in panels 2MRS12b and 2MRS24 are luminous enough to be
detected at redshift $z=0.04$ at the $2.2\mu$ limiting magnitude $K_{s}\leq
11.75$. The common absolute magnitude limit allows comparison of distributions
at about the same number density in the two redshift bins (at counts 1123 and
7908, the number density is about $6\times 10^{-4}$ Mpc-3). The difference is
pronounced: the counts at $0.02<z<0.04$ look like an isotropic random process.
The distribution at $0.04<z<0.08$ is not shown; it too looks like an isotropic
random process.

For the present purpose the point of particular interest in Fgure 6 is that it
completes the illustrations of Shaver’s effect. The angular distribution of
the 29 most luminous PSCz galaxies in Panel PSCz12α and, in the check of
reproducibility in Panel PSCz12β, the distribution of the 32 next most
luminous PSCz galaxies, show no indication of a correlation or anticorrelation
with angular position relative to the plane of the Local Supercluster. The
most luminous spirals in Panel 2MSpbright in Figure 5 also are not more or
less numerous than average near the plane of the Local Supercluster. These
examples are to be compared to comparable numbers of radio galaxies in Panel
radio12 in Figure 3 and luminous ellipticals in Panel 2MEbright in Figure 5,
which are tightly concentrated to low SGB, along with clusters of galaxies.

We should pause to take note of the low counts of 2MRS galaxies near the north
supergalactic pole in Panels 2MRS12b and 2MRS12f, and in most of the panels in
Figure 5. This departure from isotropy may or may not be related to the Local
Supercluster. It should be noted also that the distribution of PSCz galaxies
at a significantly lower luminosity cutoff, not shown, is peaked toward low
SGB. This must be expected because at lower infrared luminosities the sample
must include 2MRS galaxies and optically selected galaxies that tend to be at
low SGB.

###  3.7 A Summary Sky Map

Figure 7: Map of extragalactic objects at $z<0.02$. The 2MRS galaxy positions
are plotted as small black circles; radio galaxies are shown as larger red
triangles; and clusters of galaxies as still larger blue circles.

The equal area map in Figure 7 illustrates the distributions of galaxies and
clusters of galaxies at redshift $z<0.02$, including those at low galactic
latitudes. The plane of the Local Supercluster is close to the central
horizontal line. The plane of the Milky Way runs close to the supergalactic
pole ${\rm SGB}=-90^{\circ}$ in the left-hand side of the figure, then through
SGB = 0 at supergalactic longitude near $180^{\circ}$, then up to the pole
${\rm SGB}=+90^{\circ}$ in the right-hand side. Obscuration by interstellar
dust causes the empty regions at low galactic latitudes. This is taken into
account in the previous figures by the truncation at galactic latitude
$|b|=10^{\circ}$ of the samples and the red nearly flat curves showing the
expected distribution of an isotropic random process.

The small black circles in the figure mark the positions of 2MRS galaxies that
are luminous enough to be detected at $z=0.02$. This sample from the 2MASS
survey is taken to be a good approximation to the distribution of $L\sim
L_{\ast}$ galaxies, and a useful approximation to the mass distribution. The
large blue circles show positions of the BGC and X-ray markers of clusters.
Duplicates appear as overprinted blue circles. The red triangles show the
positions of radio galaxies. A radio galaxy in a cluster appears as a triangle
on a large circle. There are many examples of this in the figure, there are
many clusters without a tabulated radio galaxy, and there are many radio
galaxies not in tabulated clusters.

Panels 2MRS12b and 2MRS12f in Figure 6 show counts of the galaxies represented
as small black circles in Figure 7 summed over supergalactic longitude. The
counts in excess of isotropy at low supergalactic latitudes are seen in the
two disjoint samples, and the anisotropy is in the direction of the excess
counts of clusters and radio galaxies at low SGB. It is difficult to make all
this out in the map in Figure 7. We see that the alignment to the plane of the
Local Supercluster is not sharply defined; it is a statistical effect in the
clumpy distribution of galaxies, but convincingly established by the
reproducibility of the effect in the distributions in SGB reviewed in this
section.

Figure 8: Cross-correlations of the positions of 2MRS galaxies with LIRGs in
the lower red line, and with clusters of galaxies in the upper two blue lines.
The middle black line is the autocorrelation function of the 2MRS galaxies.
Figure 9: The same as in Figure 8, but plotted in linear coordinates.

##  4 Position Correlation Functions

Some indication of the meaning of the differences in the distributions of
different types of extragalactic objects relative to the Local Supercuster
might be found in the cross-correlations of positions of these objects.
Considered here are extreme examples that illustrate Shaver’s effect: clusters
of galaxies and luminous PSCz galaxies along with the numerous 2MRS galaxies
that I take to approximate the mass distribution.

The two-point correlation functions are estimated in redshift space, where
distance is proportional to redshift. Peculiar motions tend to smooth the two-
point functions relative to real space, perhaps a larger effect for 2MRS
galaxies closer to BCGs. Estimates of the two-point correlation as a function
of orientation in redshift space, along the lines introduced in Davis and
Peebles (1983) and Kaiser (1987), would enable separation of the effect of
peculiar motions from the position space correlation, but the cluster and PSCz
samples are too small for this.

We have only modest constraints on the cross-correlation of cluster and LIRG
positions at $0.02<z<0.04$. There is one BCG–LIRG pair separated in redshift
space by less than 5 Mpc. A large number of points placed uniformly at random
in the bounds of galactic latitude and redshift indicates that 1.0 pairs would
be expected if positions were independent and random. At separation $5<r<10$
Mpc there are 18 pairs in the data, and 6.8 are expected if positions are
independent and random. And there are 69 BCG–LIRG pairs at $10<r<20$ Mpc
compared to 50 expected. Since the positions of these objects are not random
we can only conclude from these numbers that there is not a strong tendency
for LIRGs to be near clusters of galaxies, or to avoid them.

The larger number of 2MRS galaxies from the 2MASS survey offers better
estimates of the cross-correlations of positions of the 2MRS sample with LIRGs
and clusters. To begin, let us review the sample sizes. At $0.01<z<0.02$ there
are 12 BCGs, 30 X-ray clusters, the sum of 62 LIRGs in Panels PSCz12α and
PSCz12β in Figure 6, and 6515 2MRS galaxies that are luminous enough to be
detected at $z<0.02$. At $0.02<z<0.04$ there are 58 BCGs, 114 X-ray clusters,
the sum of 523 LIRGs in Panels PSCz12α,β, and 7908 2MRS galaxies that are
luminous enough to be detected at $z=0.04$. We must expect serious shot noise
from the small numbers of clusters and LIRGs, but the results are interesting.

The cross-correlations are estimated as follows. In a sample of $N_{\rm S}$
objects let the count of objects in distance bin $n$ in redshift space from
any of the 2MRS galaxies be Count2M,S,n. Place $N_{\rm R}$ points uniformly at
random in the bounds of redshift and galactic latitude, and let Count2M,R,n be
the number of randomly placed points in the distance bin n from any 2MRS
galaxy. If $N_{\rm R}$ is large enough the probability that a point placed
uniformly at random in the bounds of the region falls in the distance bin $n$
from any of the 2MRS galaxies in the region is Count${}_{{\rm 2M,R},n}/N_{\rm
R}$. This multiplied by the number $N_{\rm S}$ of sample objects is the
expected count of neighbors at distance bin $n$ from any 2MRS galaxy, if the
sample objects are uniformly distributed in the region. This divided into the
observed count gives an estimate of the cross-correlation between positions of
2MRS galaxies and the $N_{\rm S}$ objects in distance bin $n$:

$\xi_{\rm 2M,S}(n)=\frac{{\rm Count}_{{\rm 2M,S},n}}{{\rm Count}_{{\rm
2M,R},n}}\frac{N_{\rm R}}{N_{\rm S}}-1.$ (4)

The random counts are computed separately for the two redshift bins.222One
could instead measure the counts of random points in the distance bin $n$ from
any of the sample points. Perhaps this or some other approach is a better way.
I have not checked.

Figures 8 and 9 show the results of applying equation (4) to the samples in
two redshift bins, plotted in logarithmic and linear coordinates. The
horizontal line segments in the histograms mark the intervals in separation
$r$ within which equation (4) is applied. The interval in $r$ for the smaller
redshift bin is $\Delta r=0.5$ Mpc. The larger numbers of clusters and LIRGs
in the larger redshift bin allow a smaller interval, $\Delta r=0.25$ Mpc.

The cross-correlations of positions of 2MRS galaxies with the two cluster
samples, BCGs and X-ray detections, are plotted as the uppermost two blue
lines in both figures. The line for X-ray clusters is shifted to the right by
0.2 Mpc from the line for BCGs in the upper panel, and by 0.1 Mpc in the lower
panel. In the redshift bin $z=0.01$ to 0.02 the two cluster-2MRS functions are
systematically different, perhaps a simple result of the small sample sizes,
12 BCGs and 30 X-ray clusters. The samples are larger in the $0.02<z<0.04$
redshift bin, 58 and 114, and here the functions for the two cluster samples
are tolerably consistent.

The 2MRS position autocorrelation function plotted as the black centermost
line in the two figures is estimated by application of equation (4) with the
number $N_{\rm 2MRS}$ of 2MRS galaxies treated as the sample:

$\xi_{2M,2M}(n)=\frac{2\times{\rm Count}_{\rm 2M,2M,n}}{{\rm Count}_{\rm
2M,R,n}}\frac{N_{\rm R}}{N_{\rm 2MRS}}-1.$ (5)

The factor of two is required if each distinct 2MRS pair is counted once.

In both redshift bins the two 2MRS–cluster cross-correlation functions are
larger than the 2MRS galaxy autocorrelation function. That is, the broad-
spread concentration of mass around a typical cluster is greater than the mean
concentration of mass around a typical galaxy. This was already seen in the
angular cross-correlation of Abell clusters with Lick galaxy counts (Seldner
and Peebles 1977).

The lowest line in both figures, plotted in red, is the 2MRS-LIRG position
cross-correlation function. At separations $\lower
2.15277pt\hbox{$\;\buildrel<\over{\sim}\;$}0.5$ Mpc this function is much
larger than the 2MRS autocorrelation function, and perhaps comparable to the
2MRS–cluster functions, though this depends on the correction for galaxy
velocity dispersions. On this relatively small scale 2MRS galaxies are
concentrated around LIRGs far more than 2MRS galaxies are concentrated around
2MRS galaxies. At separation $r\lower
2.15277pt\hbox{$\;\buildrel>\over{\sim}\;$}2$ Mpc, where the correction for
peculiar velocities is less problematic, the mean 2MRS density around a LIRG
is less than the density around a 2MRS galaxy, and well below the density
around a cluster. This is consistent with other ways to establish that LIRGs
tend to appear in dense but less extended concentrations of galaxies, which is
to say in groups of galaxies (Tekola, Väisänen, and Berlind 2012). Perhaps
less expected is that at separations $5\lower
2.15277pt\hbox{$\;\buildrel<\over{\sim}\;$}r\lower
2.15277pt\hbox{$\;\buildrel<\over{\sim}\;$}10$ Mpc the LIRG-2MRS cross-
correlation function is smaller than the 2MRS-2MRS function, and the LIRG-2MRS
function is positive but close to zero. On these scales the mean density
around a LIRG is smaller than the mean density around a typical 2MRS galaxy,
and not much greater than the cosmic mean density.

##  5 Discussion

I offer summary thoughts about observational issues that might be relevant for
comparisons to numerical and analytic analyses of the formation of
configurations similar to the extended Local Supercluster, and conclude with
brief speculations.

###  5.1 Observations

It is well and clearly established that we are in a region at least 170 Mpc
across in which most of the clusters of galaxies and radio galaxies, along
with the most luminous early-type galaxies, are close to a common plane, the
Local Supercluster. This is seen in the histograms in the top panels in
Figures 2 and 3, and in the top two left-hand panels in Figure 5. Since these
objects all prefer dense regions the samples are related, but the differences
among the histograms are to be expected because they are different ways to
look at the nature of large-scale structure.

In contrast to this situation, the spirals at $z<0.02$ that are as luminous at
2.2 $\mu$ as the most luminous ellipticals are not noticeably concentrated to
this plane, and the same is observed of the galaxies that are most luminous at
$60~{}\mu$ (as seen in Panel 2MSpbright in Fig.5 and Panels PSCz12α,β in Fig.
6). These results generalize Shaver’s (1991) effect: different kinds of
extragalactic objects have different degrees of alignment to the plane of the
Local Supercluster.

The Local Universe catalog333Available at http://edd.ifa.hawaii.edu as “Local
Universe (LU)” in the Extragalactic Distance Database maintained by Brent
Tully at the University of Hawaii and colleagues. lists 31 galaxies at
distances less than 10 Mpc with K-band luminosities greater than $10^{10}$.
The large angular sizes of these galaxies require special treatment in the
2MASS survey, but it seems quite likely that similar galaxies at greater
distances are common 2MRS galaxies. Images of the large ones in the Local
Universe catalog can be found on the web; most are the usual beautiful
spirals, some are ellipticals, and a few look odd to my untrained eye. The
local number density of these 31 galaxies is 0.007 Mpc-3. This is similar to
the density of the 6515 2MRS galaxies detectable out to $z=0.02$, $n=0.003$
Mpc-3, and it it is not far from the mean number density of the $L\sim
L_{\ast}$ galaxies in an optically selected catalog. The space distribution of
optically selected galaxies is a useful approximation to the mass distribution
that fits the measured anisotropy of the thermal background radiation, and it
is reasonable to assume the 2MRS galaxies similarly approximate the mass. This
would mean the excess counts of 2MRS galaxies at low supergalactic latitudes
(as in Panel 2MSpfainter in Fig.5) signify a mass distribution at $z<0.02$
that at low SGB is systematically and exceptionally larger than the cosmic
mean. This would be consistent with the expected biased structure formation
that accounts for the tight concentration of clusters to low SGB.

There is evidence of the extension of the alignment phenomenon beyond redshift
$z=0.02$ from the coincidence of the well-established peaks in the
distribution of sin SGB at low SGB at $z<0.02$ with the peaks in the angular
distributions of clusters and radio galaxies in Panels BCG24, BCG48, X-ray48,
and radio48. We must bear in mind the absence of an interesting feature at low
SGB in the distributions in Panels X-ray24, radio24, and 2MRS24. But the
coincidences of the peaks in four of the seven distributions at higher
redshifts makes an arguable case that the alignment phenomenon extends to
redshift $z\sim 0.08$. This spans a region some 600 Mpc across. The length
scale is remarkable; the evidence merits attention.

Because we happen to be close to the plane of the Local Supercluster its
presence is manifest in the spread of galaxies across the sky. Surely there
are similar configurations that are not so readily seen. An example of
proposals of linear arrangements of galaxies on smaller on scales is the
evidence of a planar alignment of large and dwarf galaxies around M 101
extending to about 3 Mpc (Müller, Scalera, Binggeli, and Jerjen 2017). On the
larger scales considered in this paper, spreading over several hundred
megaparsecs, one might think first of the Pisces-Perseus supercluster
(Giovanelli, Haynes, and Chincarini 1986) and the Great Wall (Geller and
Huchra 1989). Many similar more distant configurations are seen for example in
the 6dF Galaxy Survey (Jones, Read, Saunders, et al. 2009, Figs. 8 and 9).
Böhringer, Chon, and Trümper (2021a,b) present a program of investigation of
these systems in X-ray detections.

The maps of the distributions of galaxies of different morphological types in
the Pisces-Perseus supercluster show that the distinct concentrations of early
types along the ridge of this nearly linear configuration are less distinct
among later types (Giovanelli, Haynes, and Chincarini 1986). This example of
the morphology-density relation (Dressler 1997 and earlier references therein)
also is seen in Figure 5: earlier-type galaxies are more strongly concentrated
to the plane of the Local Supercluster. Open issues are whether radio galaxies
and clusters of galaxies in the neighborhoods of these more distant
configurations are largely confined to the ridge or wall, in the manner of the
Local Supercluster, and whether galaxies that are exceptionally luminous in
the infrared are not so particularly concentrated.

###  5.2 Simulations

These observations invite comparisons with what is found in numerical
simulations of cosmic structure formation. White and Silk (1979) presented an
early study of the formation of the Local Supercluster modeled as a collapsing
homogeneous spheroid. Neuzil, Mansfield, and Kravtsov (2020) had far more
powerful computational tools to reach their conclusion that “Comparison with
simulations shows that the number density contrasts of bright and faint
galaxies within 8 Mpc alone make the Local Volume a $\simeq 2.5\sigma$
outlier” in the $\Lambda$CDM theory. Constrained simulations are a good start
to the exploration of the formation of structures on the larger scales of the
extended Local Supercluster (Yepes, Gottlöber, and Hoffman 2014). These
simulations can place mass concentrations in a near planar distribution in a
satisfactory approximation to the arrangement of the local clusters of
galaxies. Not yet investigated is whether these constrained simulations also
reproduce the observed scarcity of clusters at redshifts $z<0.02$ and well
away from the plane.

A useful next step in simulations might be pure dark matter simulations with
resolution sufficient for identification of mass concentrations that would be
expected to be clusters of galaxies if supplied with baryons, and with
simulation sizes large enough to discover the rate of occurrence of
arrangements of clusters similar to what is seen out to redshift $z=0.02$, and
perhaps even $z\sim 0.08$.

###  5.3 Challenges

The spiral galaxy NGC 6946 looks much like the Milky Way, and it seems likely
to be an equally good home for observers such as us. Since NGC 6946 is some 5
Mpc from the Local Sheet an observer in this galaxy would not so readily make
out the Local Sheet of galaxies that we observe in the angular distribution of
galaxies shown in Figure 1. Are large spirals typically in sheet-like
distributions, as we are, or might the sparser distribution of galaxies around
NGC 6946 be more typical?

Why are LIRGs, the galaxies that are exceptionally luminous at 12 to $100\mu$,
not noticeably more common than average, or less common, near the plane of the
Local Supercluster? The evidence is that these galaxies are passing through
phases of rapid formation of stars and the dust that converts starlight to
infrared radiation, maybe with sub-dominant AGN contributions (Tekola,
Väisänen, and Berlind 2012; Song, Linden, Evans, et al. 2021; Linden, Evans,
Larson, et al. 2021; Pérez-Torres, Mattila, Alonso-Herrero et al. 2021). LIRGs
tend to be in groups of galaxies, perhaps the best environment for present-day
major mergers that can enhance star formation in late-type galaxies. It agrees
with what is seen in the cross-correlation functions in Figures 8 and 9, which
indicate that on a scale of about $0.5$ Mpc LIRGs tend to be in concentrations
of 2MRS galaxies that are considerably denser than the typical concentration
of 2MRS galaxies around a 2MRS galaxy. But we also see that in the region
$\sim 10$ Mpc across around a typical LIRG the mean density of 2MRS galaxies
is less than the mean density around a 2MRS galaxy, and only about twice the
cosmic mean. We are challenged to understand why positions of LIRGs are not
significantly correlated with the enhanced mass density near the plane of the
Local Supercluster, and appear to be only weakly correlated with the mass
distribution well away from the plane.

A related challenge follows from the observed concentration of radio galaxies
to the plane of the Local Supercluster. The evidence is that the presence of a
powerful radio source in a galaxy depends on the presence of a massive central
black hole. Ordinary $L\sim L_{\ast}$ galaxies, spirals as well as
ellipticals, contain massive central black holes. So how do the black holes in
radio galaxies differ from the black holes in spiral galaxies? Maybe the mass
of the black hole in a radio galaxy is significantly greater than the one in
an $L\sim L_{\ast}$ spiral. But spiral galaxies with stellar masses as large
as in giant ellipticals, to judge by the luminosity at 2.2 $\mu$, might be
expected to have comparable massive central black holes, yet these spirals are
not noticeably concentrated to the plane of the Local Supercluster, while the
similarly luminous ellipticals are concentrated to this plane, along with the
radio galaxies.

Another way to look at the situation starts from the detection of the residual
effects of acoustic oscillations of the plasma-radiation fluid prior to
decoupling on the angular distribution of the thermal microwave background
radiation (e.g. Planck Collaboration et al. 2020) and the spatial distribution
of the galaxies (e. g. Beutler, Seo, Ross, et al. (2017). This is persuasive
evidence that cosmic structure grew by gravity. But the situation on the
smaller scales of galaxies looks more complicated. It would be natural to
expect that large $L\sim L_{\ast}$ galaxies are more strongly clustered than
$L\ll L_{\ast}$ dwarfs (Dekel and Silk 1986), but this is not observed. Visual
examples of the similar distributions of large and small galaxies include
Figure 1 in this paper, at distances $\la 6$ Mpc, and the much earlier
evidence in Figures 2a and 2d in Davis, Huchra, Latham, and Tonry (1982), at
distances $\la 40$ Mpc. Examples of more recent demonstrations include Einasto
(1991), Binggeli (1989), and Zehavi, Zheng, Weinberg, et al. (2011). Yet we
have the evidence that the most luminous early- and late-type galaxies, with
similar luminosities at $2.2\mu$, and presumably similar stellar masses, have
the quite different distributions relative to the plane of the Local
Supercluster shown in the left-hand panels in Figure 5.

The formation of the most massive galaxies seems to be bistable: early or late
with occasional mixed types and irregulars. The separatrix cannot be mass;
there are large and small early and late type galaxies. It cannot be angular
momentum, if angular momentum is transferred by gravity and as seems likely
this is not a bimodal process. We do have the hint from the evidence reviewed
here that the separatrix at the high mass end is correlated with the presence
of the plane of the Local Supercluster, in a manner that is difficult to
attribute to gravity.

###  5.4 Speculations

These considerations invite the speculation that cosmic structure grew out of
two kinds of seeds. One likely would be the usual Gaussian near scale-
invariant primordial departures from exact homogeneity, because this picture
fits many tests. The other, which at $z<0.02$ would be present largely near
the plane of the growing Local Supercluster, would seed the formation of the
kind of central black holes that host powerful radio sources. LIRGs would not
be correlated with this plane, as observed, if they had little or nothing to
do with the second kind of seed. In the $\Lambda$CDM theory clusters of
galaxies form out of primeval Gaussian mass fluctuations. We do not know yet
whether this Gaussian initial condition can account for the near complete
concentration of clusters to the plane observed at redshifts $z\lower
2.15277pt\hbox{$\;\buildrel<\over{\sim}\;$}0.02$. Perhaps the primeval
Gaussian mass density fluctuations tend to produce groups, and the burden for
formation of rich clusters of galaxies falls on the second kind of seed.

Carr and Silk (2018) reviewed the continuing interest in the idea that
primeval black holes, or the seeds for their formation, played a role in
cosmic structure formation. Might they promote the formation of the kind of
massive black holes that enable powerful radio sources in exceptionally
luminous elliptical galaxies, while leaving the formation of ordinary galaxies
with their kinds of central massive black holes to the primeval mass
fluctuations that cosmological inflation is expected to produce?

Why are radio galaxies and clusters of galaxies scattered about a plane at
redshift $z\lower 2.15277pt\hbox{$\;\buildrel<\over{\sim}\;$}0.02$? One might
think of the mass concentration that would be produced by a long fairly
straight but kinky cosmic string drawn across space. It is difficult to
imagine how the conventional picture of cosmic string formation could account
for such a configuration, but in the cyclic big bounce cosmology (Cook,
Glushchenko, Ijjas, Pretorius, and Steinhardt 2020 and references therein)
cosmic strings might survive the transition from contraction to expansion,
stretched out by the expansion after the bounce, maybe ending up moving at a
rate sufficient to pile up mass concentrations, or maybe piling up matter
where the string is kinky enough to be gravitating. This is speculation, but
maybe we need to think broadly.

There are in natural science many examples of phenomena that were known well
before community recognition of their significance; consider the large cosmic
abundance of helium. Burbidge (1958b) recognized the evidence and the
question: what produced the helium? Osterbrock and Rogerson (1961) added to
the evidence, and saw that it “can be understood without difficulty” in
Gamow’s (1948) relativistic hot big bang cosmology. But community recognition
came later, in 1965, prompted by the recognition that we are in a sea of
microwave radiation. Are there other curiosities in the phenomenology that,
when properly interpreted, will help us find a cosmology that is even better
than $\Lambda$CDM? It is wishful thinking, but surely worth bearing in mind.
Perhaps the extended Local Supercluster has something new and interesting to
teach us.

##  6 acknowledgements

I am grateful for discussions with Hans Böhringer, Bernard Carr, Jaan Einasto,
Merit Einasto, Ron Ekers, Yehuda Hoffman, Ken Kellerman, Andrey Kravtsov, Ofer
Lahav, Peter Shaver, Paul Steinhart, Brent Tully, and Jasper Wall, and helpful
comments from the referee. I acknowledge with thanks the moral support and
hospitality of Princeton University, but have received no monetary support for
this research.

##  7 Data Availability

This research made use of the data compiled by the University of Hawaii
Extragalactic Distance Database; the High Energy Astrophysics Science Archive
Research Center (HEASARC), which is a service of the Astrophysics Science
Division at NASA/GSFC; the SIMBAD and VizieR catalogue access tool, CDS,
Strasbourg, France; the Two Micron All Sky Survey, which is a joint project of
the University of Massachusetts and the Infrared Processing and Analysis
Center/California Institute of Technology, funded by the National Aeronautics
and Space Administration and the National Science Foundation; the NASA
Astrophysics Data System Bibliographic Services; and the NASA/IPAC
Extragalactic Database. I have not generated any new data.

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