Galaxies In Low Density Regions Research Articles

The evolutionary path of void galaxies in TNG300 simulation

ABSTRACT The properties of galaxies in low-density regions of the universe suggest an interplay between galaxy formation and environment. However, the specific reason why this particular large-scale environment influences the evolution of galaxies remains unclear. This paper examines the properties and evolutionary paths of galaxies within cosmic voids using the Illustris TNG300 simulation. The population of void galaxies at z = 0 has a higher star formation rate, a smaller stellar-to-halo-mass ratio, higher gas metallicity, and lower stellar metallicity in comparison with non-void galaxies at fixed stellar mass. Our analysis shows that these differences are mainly due to the characteristics of galaxies classified as satellites, for which the largest differences between void and non-void samples are found. Although the mean number of mergers is similar between void and non-void samples at a fixed stellar mass, void galaxies tend to experience mergers at later times, resulting in a more recent accumulation of accreted stellar mass. While the mean net accreted mass is comparable for high-mass galaxies, low-mass void galaxies tend to exhibit higher fractions of accreted stars than non-void galaxies. This finding challenges the common notion that void galaxies predominantly experience growth with infrequent mergers or interactions. With this paper, we also publicly release our void catalogue as part of the IllustrisTNG project.

Dissect two-halo galactic conformity effect for central galaxies: the dependence of star formation activities on the large-scale environment

ABSTRACT We investigate the two-halo galactic conformity effect for central galaxies, which is the spatial correlation of the star formation activities for central galaxies to several Mpcs, by studying the dependence of the star formation activities of central galaxies on their large-scale structure in our local Universe using the SDSS data. Here we adopt a novel environment metric using only central galaxies quantified by the distance to the nth nearest central galaxy. This metric measures the environment within an aperture from ∼1 to ≳ 10 Mpc, with a median value of ∼4 Mpc. We found that two kinds of conformity effects in our local Universe. The first one is that low-mass central galaxies are more quenched in high-density regions, and we found that this effect mainly comes from low-mass centrals that are close to a more massive halo. A similar trend is also found in the IllustrisTNG simulation, which can be entirely explained by backsplash galaxies. The second conformity effect is that massive central galaxies in low-density regions are more star-forming. This population of galaxies also possesses a higher fraction of spiral morphology and lower central stellar velocity dispersion, suggesting that their low quiescent fraction is due to less-frequent major merger events experienced in the low-density regions and, as a consequence, less-massive bulges and central black holes.

Living with Neighbors. III. The Origin of the Spin–Orbit Alignment of Galaxy Pairs: A Neighbor versus the Large-scale Structure

Abstract Recent observations revealed a coherence between the spin vector of a galaxy and the orbital motion of its neighbors. We refer to the phenomenon as “the spin–orbit alignment (SOA)” and explore its physical origin via the IllustrisTNG simulation. This is the first study to utilize a cosmological hydrodynamic simulation to investigate the SOA of galaxy pairs. In particular, we identify paired galaxies at z = 0 having the nearest neighbor with mass ratios from 1/10 to 10 and calculate the spin–orbit angle for each pair. Our results are as follows. (a) There exists a clear preference for prograde orientations (i.e., SOA) for galaxy pairs, qualitatively consistent with observations. (b) The SOA is significant for both baryonic and dark matter spins, being the strongest for gas and the weakest for dark matter. (c) The SOA is stronger for less massive targets and for targets having closer neighbors. (d) The SOA strengthens for galaxies in low-density regions, and the signal is dominated by central–satellite pairs in low-mass halos. (e) There is an explicit dependence of the SOA on the duration of interaction with its current neighbor. Taken together, we propose that the SOA witnessed at z = 0 has been developed mainly by interactions with a neighbor for an extended period of time, rather than tidal torque from the ambient large-scale structure.

Discrimination of heavy elements originating from Pop III stars in z = 3 intergalactic medium

ABSTRACT We investigate the distribution of metals in the cosmological volume at $z$ ∼ 3, in particular, provided by massive Population III (Pop III) stars using a cosmological N-body simulation in which a model of Pop III star formation is implemented. Owing to the simulation, we can choose minihaloes where Pop III star formation occurs at $z$ > 10 and obtain the spatial distribution of the metals at lower redshifts. To evaluate the amount of heavy elements provided by Pop III stars, we consider metal yield of pair-instability or core-collapse supernovae (SNe) explosions of massive stars. By comparing our results to the Illustris-1 simulation, we find that heavy elements provided by Pop III stars often dominate those from galaxies in low-density regions. The median value of the volume averaged metallicity is $Z\sim 10^{-4.5 - -2} \, \mathrm{Z}_{\odot }$ at the regions. Spectroscopic observations with the next generation telescopes are expected to detect the metals imprinted on quasar spectra.

The oxygen abundance gradients in the gas discs of galaxies in the EAGLE simulation

We use the EAGLE simulations to study the oxygen abundance gradients of gas discs in galaxies within the stellar mass range [10^9.5, 10^10.8]Mo at z=0. The estimated median oxygen gradient is -0.011 (0.002) dex kpc^-1, which is shallower than observed. No clear trend between simulated disc oxygen gradient and galaxy stellar mass is found when all galaxies are considered. However, the oxygen gradient shows a clear correlation with gas disc size so that shallower abundance slopes are found for increasing gas disc sizes. Positive oxygen gradients are detected for ~40 per cent of the analysed gas discs, with a slight higher frequency in low mass galaxies. Galaxies that have quiet merger histories show a positive correlation between oxygen gradient and stellar mass, so that more massive galaxies tend to have shallower metallicity gradients. At high stellar mass, there is a larger fraction of rotational-dominated galaxies in low density regions. At low stellar mass, non-merger galaxies show a large variety of oxygen gradients and morphologies. The normalization of the disc oxygen gradients in non-merger galaxies by the effective radius removes the trend with stellar mass. Conversely, galaxies that experienced mergers show a weak relation between oxygen gradient and stellar mass. Additionally, the analysed EAGLE discs show no clear dependence of the oxygen gradients on local environment, in agreement with current observational findings.

The Influence of Large-scale Environments on Galaxy Properties

The star formation properties of galaxies and their dependence on environments play an important role for understanding the formation and evolution of galaxies. Using the galaxy sample of the Sloan Digital Sky Survey (SDSS), different research groups have studied the physical properties of galaxies and their large-scale environments. Here, using the filament catalog from Tempel et al. and the galaxy catalog of large-scale structure classification from Wang et al., and taking the influence of the galaxy morphology, high/low local density environment, and central (satellite) galaxy into consideration, we have found that the properties of galaxies are correlated with their residential large-scale environments: the SSFR (specific star formation rate) and SFR (star formation rate) strongly depend on the large-scale environment for spiral galaxies and satellite galaxies, but this dependence is very weak for elliptical galaxies and central galaxies, and the influence of large-scale environments on galaxies in low density region is more sensitive than that in high density region. The above conclusions remain valid even for the galaxies with the same mass. In addition, the SSFR distributions derived from the catalogs of Tempel et al. and Wang et al. are not entirely consistent.

A DEEP SEARCH FOR FAINT GALAXIES ASSOCIATED WITH VERY LOW REDSHIFT C iv ABSORBERS. III. THE MASS- AND ENVIRONMENT-DEPENDENT CIRCUMGALACTIC MEDIUM

ABSTRACT Using Hubble Space Telescope Cosmic Origins Spectrograph observations of 89 QSO sightlines through the Sloan Digital Sky Survey footprint, we study the relationships between C iv absorption systems and the properties of nearby galaxies, as well as the large-scale environment. To maintain sensitivity to very faint galaxies, we restrict our sample to , which defines a complete galaxy survey to or stellar mass . We report two principal findings. First, for galaxies with impact parameter , C iv detection strongly depends on the luminosity/stellar mass of the nearby galaxy. C iv is preferentially associated with galaxies with lower-mass galaxies rarely exhibit significant C iv absorption (covering fraction for 11 galaxies with ). Second, C iv detection within the population depends on environment. Using a fixed-aperture environmental density metric for galaxies with ρ < 160 kpc at , we find that (8/14) of galaxies in low-density regions (regions with fewer than seven galaxies within 1.5 Mpc) have affiliated C iv absorption; however, none (0/7) of the galaxies in denser regions show C iv. Similarly, the C iv detection rate is lower for galaxies residing in groups with dark matter halo masses of . In contrast to C iv, H i is pervasive in the circumgalactic medium without regard to mass or environment. These results indicate that C iv absorbers with trace the halos of galaxies but also reflect larger-scale environmental conditions.

ELLIPTICITIES OF ELLIPTICAL GALAXIES IN DIFFERENT ENVIRONMENTS

ABSTRACT We studied the ellipticity distributions of elliptical galaxies in different environments. From the ninth data release of the Sloan Digital Sky Survey, we selected galaxies with absolute -band magnitudes between −21 and −22. We used the volume number densities of galaxies as the criterion for selecting the environments of the galaxies. Our samples were divided into three groups with different volume number densities. The ellipticity distributions of the elliptical galaxies differed considerably in these three groups of different density regions. We deprojected the observed 2D ellipticity distributions into intrinsic 3D shape distributions, and the result showed that the shapes of the elliptical galaxies were relatively spherically symmetric in the high density region (HDR) and that relatively more flat galaxies were present in the low density region (LDR). This suggests that the ellipticals in the HDRs and LDRs have different origins or that different mechanisms might be involved. The elliptical galaxies in the LDR are likely to have evolved from mergers in relatively anisotropic structures, such as filaments and webs, and might contain information on the anisotropic spatial distribution of their parent mergers. By contrast, elliptical galaxies in the HDR might be formed in more isotropic structures, such as galaxy clusters, or they might encounter more torqueing effects compared with galaxies in LDRs, thereby becoming rounder.

DEMOGRAPHICS OF ISOLATED GALAXIES ALONG THE HUBBLE SEQUENCE

Isolated galaxies in low-density regions are significant in the sense that they are least affected by the hierarchical pattern of galaxy growth, and interactions with perturbers, at least for the last few Gyr. To form a comprehensive picture of the star formation history of isolated galaxies, we constructed a catalog of isolated galaxies and their comparison sample in relatively denser environments. The galaxies are drawn from the SDSS DR7 in the redshift range of $0.025<z<0.044$. We performed a visual inspection and classified their morphology following the Hubble classification scheme. For the spectroscopic study, we make use of the OSSY catalog. We confirm most of the earlier understanding on isolated galaxies. The most remarkable additional results are as follows. Isolated galaxies are dominantly late type with the morphology distribution (E: S0: S: Irr) = (9.9: 11.3: 77.6: 1.2)\%. The frequency of elliptical galaxies among isolated galaxies is only a third of that of the comparison sample. Most of the photometric and spectroscopic properties are surprisingly similar between isolated and comparison samples. However, early-type isolated galaxies are less massive by 50\% and younger (by H$\beta$) by 20\% than their counterparts in the comparison sample. This can be explained as a result of different merger and star formation histories for differing environments in the hierarchical merger paradigm. We provide an on-line catalog for the list and properties of our sample galaxies.

Environment, morphology, and stellar populations of bulgeless low surface-brightness galaxies

Based on the Sloan Digital Sky Survey DR 7, we investigate the environment, morphology and stellar population of bulgeless low surface brightness (LSB) galaxies in a volume-limited sample with redshift ranging from 0.024 to 0.04 and $M_r$ $\leq$ $-18.8$. The local density parameter $\Sigma_5$ is used to trace their environments. We find that, for bulgeless galaxies, the surface brightness does not depend on the environment. The stellar populations are compared for bulgeless LSB galaxies in different environments and for bulgeless LSB galaxies with different morphologies. The stellar populations of LSB galaxies in low density regions are similar to those of LSB galaxies in high density regions. Irregular LSB galaxies have more young stars and are more metal-poor than regular LSB galaxies. These results suggest that the evolution of LSB galaxies may be driven by their dynamics including mergers rather than by their large scale environment.

A Galaxy in Transition: Structure, Globular Clusters, and Distance of the Star‐Forming S0 Galaxy NGC 1533 in Dorado

We use two-band imaging data from the Advanced Camera for Surveys onboard the Hubble Space Telescope for a detailed study of NGC 1533, an SB0 galaxy in the Dorado group surrounded by a ring of H I. NGC 1533 appears to be completing a transition from late to early type: it is red, but not quite dead. Faint spiral structure becomes visible following galaxy subtraction, and luminous blue stars can be seen in isolated areas of the disk. Dust is visible in the color map in the region around the bar, and there is a linear color gradient throughout the disk. We determine an accurate distance from the surface brightness fluctuation (SBF) method, finding (m − M) = 31.44 ± 0.12 mag, or d = 19.4 ± 1.1 Mpc. We then study the globular cluster (GC) colors, sizes, and luminosity function (GCLF). Estimates of the distance from the median of the GC half-light radii and from the peak of the GCLF both agree well with the SBF distance. The GC specific frequency is SN = 1.3 ± 0.2, typical for an early-type disk galaxy. The color distribution is bimodal, as commonly observed for bright galaxies. There is a suggestion of the redder GCs having smaller sizes, but the trend is not significant. The sizes do increase significantly with galactocentric radius, in a manner more similar to the Milky Way GC system than to those in Virgo. This difference may be an effect of the steeper density gradients in loose groups as compared to galaxy clusters. Additional studies of early-type galaxies in low-density regions can help determine if this is indeed a general environmental trend.

Central stellar populations of early-type galaxies in low-density environments

We have investigated the properties of a volume and magnitude limited sample of nearby early type galaxies that were carefully selected from the AAO two degree field galaxy redshift survey. We used images from the DSS to confirm the E/S0 morphologies, and augmented this sample with field galaxies from Colbert et al. 2001. We present spectroscopic observations of 22 galaxies from the combined sample, from which central velocity dispersions and the Lick stellar population indices were measured. After carefully correcting the spectra for nebular emission we derived luminosity-weighted ages, metallicities, and alpha-element abundance ratios. We compare these isolated galaxies with samples of early-type galaxies in the Virgo and Coma clusters, and also with the sample of galaxies in low-density regions of Kuntschner et al. (2002). We find that galaxies in low-density environments are younger and have a greater spread of ages compared to cluster galaxies. They also show a wider range of metallicities at a given velocity dispersion than cluster galaxies, which display only super-solar metallicities. On average cluster, as well as, isolated galaxies show non-solar abundance ratios in alpha-elements, suggesting that, independent of galactic environment, star formation occurred on short time-scales. We reason that early-type galaxies in low-density environments experienced merging-induced star-formation episodes over a longer and more recent period of time compared to a cluster environment, and speculate that a considerable fraction of their stars formed out of low-metallicity halo gaseous material during the slow growth of a stellar disk between merging events.

Early-type galaxies in low-density environments

We describe the construction and study of an objectively-defined sample of early-type galaxies in low-density environments. The sample galaxies are selected from a recently-completed redshift survey using uniform and readily-quantified isolation criteria, and are drawn from a sky area of ~700 deg^2, to a depth of 7000 k/ms and an apparent magnitude limit of b_J < 16.1. Their early-type (E/S0) morphologies are confirmed by subsequent CCD imaging. Five out of the nine sample galaxies show signs of morphological peculiarity such as tidal debris or blue circumnuclear rings. We confirm that E/S0 galaxies are rare in low-density regions, accounting for only ~8% of the total galaxy population in such environments. We present spectroscopic observations of nine galaxies in the sample, which are used, in conjunction with updated stellar population models, to investigate star-formation histories. Environmental effects on early-type galaxy evolution are investigated by comparison with a sample of Fornax cluster E/S0s. Results from both samples are compared with predictions from semi-analytic galaxy formation models. The Mg-sig relation of E/S0s in low-density regions is shown to be indistinguishable from that of the Fornax sample. Luminosity-weighted stellar ages and metallicities are determined by considering various combinations of line-indices. At a given luminosity, the E/S0 galaxies in low-density regions are younger than the E/S0s in clusters (by ~2-3 Gyr), and also more metal-rich (by ~0.2 dex). We infer that an anti-correlation of age and metallicity effects is responsible for maintaining the zero-point of the Mg-sig relation. The youngest galaxies in our sample show clear morphological signs of interaction. (Abridged)

Understanding the results of galaxy-galaxy lensing using galaxy-mass correlation in numerical simulations

McKay et al. (2002) have recently used measurements of weak galaxy-galaxy lensing in the Sloan Digital Sky Survey to estimate the cross correlation between galaxies and the projected dark matter density field. They derive a relation between aperture mass within a radius of $260\kpch$, $M_{260}$, and lens galaxy luminosity, that does not depend on galaxy luminosity, type or environment. In this paper, we study the cross-correlation between galaxies and dark matter using galaxy catalogs constructed from a high-resolution N-body simulation of a $\Lambda$CDM Universe. We show that our simulations reproduce the McKay et al. results reasonably well. In the simulation, $M_{260}$ is approximately equal to the halo virial mass for $L_*$ galaxies. $M_{260}$ overestimates the virial mass for fainter galaxies and underestimates it for brighter galaxies. We use the simulations to show that under certain circumstances the halo virial mass may be recovered by fitting an NFW profile to the projected galaxy-mass correlation function. If we apply our method to the observations, we find that $L_*$ galaxies typically reside in halos of $\sim 2\times 10^{12} h^{-1} \msun$, consistent with halo masses estimated from the observed Tully-Fisher relation. In the simulations, the halo virial mass scales with galaxy luminosity as $L^{1.5}$ for central galaxies in halos and for galaxies in low-density regions. For satellite galaxies, and for galaxies in high-density regions, there is no simple relation between galaxy luminosity and halo mass and care must be exercised when interpreting the lensing results.

Squelched Galaxies and Dark Halos

There is accumulating evidence that the faint end of the galaxy luminosity function might be very different in different locations. The luminosity function might be rising in rich clusters and flat in regions of low density. If galaxies form according to the model of hierarchical clustering, then there should be many small halos compared to the number of big halos. If this theory is valid, then there must be a mechanism that eliminates at least the visible component of galaxies in low-density regions. A plausible mechanism is photoionization of the intergalactic medium at a time before the epoch that most dwarf galaxies form in low-density regions but after the epoch of formation for similar systems that ultimately end up in rich clusters. The dynamical timescales are found to accommodate this hypothesis in a flat universe with Ωm 0.4. If small halos exist but simply cannot be located because they have never become the sites of significant star formation, they still might have dynamical manifestations. These manifestations are hard to identify in normal groups of galaxies because small halos do not make a significant contribution to the global mass budget. It could be entertained, however, that there are clusters of halos where there are only small systems, clusters that are at the low-mass end of the hierarchical tree. There may be places where only a few small galaxies managed to form, enough for us to identify and use as test probes of the potential. It turns out that such environments might be common. Four probable groups of dwarfs are identified within 5 Mpc, and the assumption that they are gravitationally bound suggests M/LB ~ 300-1200 M☉/L☉, a factor of 6 ± 2 times higher than typical values for groups with luminous galaxies.

An Imaging and Spectroscopic Survey of Galaxies within Prominent Nearby Voids. I. The Sample and Luminosity Distribution

We study the optical properties of a large sample of galaxies in low-density regions of the nearby universe. We make a 5 h-1 Mpc smoothed map of the galaxy density throughout the Center for Astrophysics Redshift Survey (CfA2) to identify galaxies within three prominent nearby voids with diameter 30 h-1 Mpc. We augment the CfA2 galaxy sample with fainter galaxies found in the same regions from the more recent and deeper Century and Redshift surveys . We obtain B and R CCD images and high signal-to-noise long-slit spectra for the resulting sample of 149 galaxies, as well as for an additional 131 galaxies on the periphery of these voids. Here we describe the photometry for the sample, including B isophotal magnitudes and B-R colors. For the 149 galaxies that lie in regions below the mean survey density, the luminosity functions in B and R are well fit by Schechter functions with respective parameters (αB = -0.5 ± 0.3, B* = -18.9 ± 0.2) and (αR = -0.9 ± 0.3, R* = -20.4 ± 0.3). The B luminosity function (LF) is consistent with typical survey LFs (e.g., the Southern Sky Redshift Survey), and the R LF is consistent with the Century Survey. The B and R LFs of 131 galaxies in the void periphery—regions between the mean density and twice the mean—have similar Schechter parameters. The CfA2 LF is inconsistent with both samples at the 3.5 σ level. When we narrow our analysis to the 46 galaxies in regions below half the mean density, the LF is significantly steeper: α ~ -1.4 ± 0.5. The typical survey LFs are inconsistent with this subsample at the ~2 σ level. The B-R color distribution of galaxies in the lowest density regions is also shifted significantly (~3 σ) blueward of the higher density samples. The most luminous red galaxies (R -21) are absent from the lowest density regions at the 2.5 σ level.

Surface photometry of galaxies in low density regions

We perform detailed surface photometry, based on B- and/or R-band CCD images of 92 faint galaxies. They are a subsample of those galaxies which were studied by Hopp et al. (1994) in the direction of three nearby voids. We derive integral photometric parameters and radial surface brightness proles and compare them to those of several faint galaxy samples, located in dierent environments, and of a bright eld galaxy sample. According to the obtained photometric characteristics, our sample is a mixture of intrinsically bright and faint galaxies, about 60% of them belonging to the bright subsample (MB h 19:0). These bright ones show mean characteristics of typical Freeman's disks and are mainly located in the background of the observed voids. The faint subsample (MB 19:0) has parameters typical for the low-surface-brightness (LSB) galaxies. There are a few well isolated galaxies both among the bright and faint subsamples. About 40% of studied galaxies reveal ellipticity and position angle variations along of the radius. The (B R) colour indices of the observed galaxies span over a wide range of 0.5 1.8 mag with a median value at 1.11 mag, rather blue. About 30% of the observed galaxies reveal radial colour gradients at a 5 signicance level. These galaxies show typically red centers and are getting bluer towards the periphery. The most isolated galaxies in our sample show relatively blue colours (BT RT ' 0.8). About 40% of the studied galaxies have surface brightness proles which can be described by the model of single exponential disk. 38% of our galaxies have more complicated proles and either can be approximated by two exponentials or show the presence of a small bulge. 20% of the observed galaxies have central light depression and outer irregularities { typical for some dwarf galaxies. These prole type frequencies are similar to those of faint eld galaxy sample. The evolutionary history of both the isolated and clustered galaxies obviously results in similar photometric and structural characteristics. Thus the photometric methods alone are inadequate for selecting galaxies with dierent isolation properties.

Pairs of Galaxies in Low Density Regions of a Combined Redshift Catalog

AbstractThe distributions of projected separations and radial velocity differences of pairs of galaxies in the CfA and SSRS redshift catalogs are examined. We focus on pairs that fall in low density environments rather than in clusters or large groups. The projected separation distribution is nearly flat, while uncorrelated galaxies would have given one linearly rising with rp. There is no break in this curve even below 50 kpc, the minimum halo size consistent with measured galaxy rotation curves. The significant number of pairs at small separations is inconsistent with the N-body result that galaxies with overlapping halos will rapidly merge, unless there are significant amounts of matter distributed out to a few hundred kpc of the galaxies. This dark matter may either be in distinct halos or more loosely distributed. Large halos would allow pairs at initially large separations to head toward merger, replenishing the distribution at small separations. In the context of this model, we estimate that roughly 10 — 25% of these low density galaxies are the product of a merger, compared with the elliptical / S0 fraction of 18%, observed in low density regions of the sample.

N-body experiments with gas in a cosmological model

view Abstract Citations (24) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS N-Body Experiments with Gas in a Cosmological Model Carlberg, R. G. Abstract The formation of galaxies and their clustering is studied with an N-body experiment altered to include gas along with collisionless dark matter. The gas is isothermal, the simplest equation of state that dissipates energy under compression. The gas resists only shallow potential wells, that is, it has a "minimal" physical biasing for galaxy formation. The background cosmology has a local {OMEGA} > 1 and the particles are perturbed with a spectrum of waves having P(k) proportional to k^-1^. About one-fourth of the particles form into galaxies, originating in "1 σ" peaks, where σ is the amplitude of density fluctuations on the mass scale of galaxies, although there is a large dispersion of overdensities. The expected correlation between initial overdensity and the clustering environment of the galaxy clearly emerges, the highest peaks tending to become galaxies in the core of the cluster. Galaxies completely avoid the initially underdense regions. The autocorrelation function of the galaxies is generally a factor of 2-3 times stronger than that of the dark matter. The most massive galaxies form from 2 and more σ peaks, making their autocorrelation 2-4 times higher than that of all galaxies. In this {OMEGA} > 1 model all the galaxies eventually come together to form a stable group of 21 galaxies, corresponding closely to a poor cD cluster. The galaxies orbit in a dark matter background having 1000 times the mass of a typical galaxy, where the residual dynamical friction is defeated by continuing infall. The virial mass provides a reliable indication of the mass within the orbits of the galaxies. Global mass estimators are vexed by the absence of galaxies in low-density regions, underestimating the mass present by a factor of 2-10. The main conclusion is that this relatively conservative gas model provides a physical justification for significant "biasing" in the sites of galaxy formation. Publication: The Astrophysical Journal Pub Date: January 1988 DOI: 10.1086/165926 Bibcode: 1988ApJ...324..664C Keywords: Astronomical Models; Cosmology; Dark Matter; Galactic Clusters; Many Body Problem; Autocorrelation; Galactic Evolution; Galactic Structure; Star Formation; Astrophysics; COSMOLOGY; GALAXIES: CLUSTERING; GALAXIES: FORMATION full text sources ADS |

Properties of Galaxies in Low Density Regions

Isolated galaxies seem to have more nuclear activity and less disk activity than nonisolated objects. This is interpreted as an expression of the mass-profile in their halos.The discovery of voids in the three-dimensional distribution of galaxies has rapidly been followed by claims that in at least one such void some objects, like Markarian and emission-line galaxies, are present. Why are only active galaxies found in these voids? Should we also expect the opposite to be true, namely that active galaxies ought to be found more often in voids, or in regions of the Universe with lower-than-average density? There are indications that this seems to be the case, as Gisler (1978) found that emission-line galaxies, Markarians and possibly Seyferts avoid environments of dense clusters. Differences between “field” and “cluster” populations are probably due more to the enhanced local density of galaxies than to the nature of the overall environment.