Fraction Of Spirals Research Articles

The Host Galaxies of Narrow-Line Seyfert 1 Galaxies: Nuclear Dust Morphology and Starburst Rings

We present a study of the nuclear morphology of a sample of narrow- and broad-line Seyfert 1 galaxies (NLS1s and BLS1s, respectively) based on broadband images in the Hubble Space Telescope archives. In our previous study we found that large-scale stellar bars at >1 kpc from the nucleus are more common in NLS1s than BLS1s. In this paper we find that NLS1s preferentially have grand-design dust spirals within ~1 kpc of their centers. We also find that NLS1s have a higher fraction of nuclear star-forming rings than BLS1s. We find that many of the morphological differences are due to the presence or absence of a large-scale stellar bar within the spiral host galaxy. In general, barred Seyfert 1 galaxies tend to have grand-design dust spirals at their centers, confirming the results of other researchers. The high fraction of grand-design nuclear dust spirals and stellar nuclear rings observed in NLS1s' host galaxies suggests a means for efficient fueling of their nuclei to support their high Eddington ratios.

GALEX Observations of "Passive Spirals" in the Cluster Cl 0024+17: Clues to the Formation of S0 Galaxies

We present new results from deep GALEX UV imaging of the cluster Cl 0024+17 at z ~ 0.4. Rest-frame far-UV emission is detected from a large fraction of so-called passive spiral galaxies—a significant population that exhibits spiral morphology with little or no spectroscopic evidence of ongoing star formation. This population is thought to represent infalling galaxies whose star formation has been somehow truncated by environmental processes, possibly in morphological transition to S0 galaxies. Compared to normal cluster spirals, we find that passive spirals are redder in FUV-optical color, while exhibiting much stronger UV emission than cluster E/S0 galaxies—as expected for recently truncated star formation. By modeling the different temporal sensitivities of UV and spectroscopic data to recent activity, we show that star formation in passive spirals decayed on timescales of less than 1 Gyr, consistent with starvation—a process where the cluster environment prevents cold gas from accreting onto the spiral disk. Intriguingly, the fraction of spirals currently observed in the passive phase is consistent with the longer period expected for the morphological transformation and the subsequent buildup of cluster S0 galaxies observed since z 0.4.

Galaxy halo masses and satellite fractions from galaxy-galaxy lensing in the Sloan Digital Sky Survey: stellar mass, luminosity, morphology and environment dependencies

The relationship between galaxies and dark matter (DM) can be characterized by the halo mass of the central galaxy and the fraction of galaxies that are satellites. Here, we present observational constraints from the Sloan Digital Sky Survey on these quantities as a function of r-band luminosity and stellar mass using galaxy-galaxy weak lensing, with a total of 351 507 lenses. We use stellar masses derived from spectroscopy and virial halo masses derived from weak gravitational lensing to determine the efficiency with which baryons in the halo of the central galaxy have been converted into stars. We find that an L* galaxy with a stellar mass of 6 x 1010 M ⊙ is hosted by a halo with mass of 1.4 x 10 12 h -1 M ⊙ , independent of morphology, yielding baryon conversion efficiencies of 17 +10 -5 per cent (early-types) and 16 +15 -6 per cent (late-types) at the 95 per cent confidence level (statistical, not including systematic uncertainty due to assumption of a universal initial mass function). We find that for a given stellar mass, the halo mass is independent of morphology below M stellar = 10 11 M ⊙ , in contrast to typically a factor of 2 difference in halo mass between ellipticals and spirals at a fixed luminosity. This suggests that stellar mass is a good proxy for halo mass in this range and should be used preferentially whenever a halo mass selected sample is needed. For higher stellar masses, the conversion efficiency is a declining function of stellar mass, and the differences in halo mass between early- and late-types become larger, reflecting the fact that most group and cluster haloes with masses above 10 13 M ⊙ host ellipticals at the centre, while even the brightest central spirals are hosted by haloes of mass below 10 13 M ⊙ We find that the fraction of spirals that are satellites is roughly 10-15 per cent independent of stellar mass or luminosity, while for ellipticals this fraction decreases with stellar mass from 50 per cent at 10 10 M ⊙ to 10 per cent at 3 x 10 11 M ⊙ or 20 per cent at the maximum luminosity considered. We split the elliptical sample by local density, and find that at a given luminosity there is no difference in the signal on scales below 100 h -1 kpc between high- and low-density regions, suggesting that tidal stripping inside large haloes does not remove most of the DM from the early-type satellites. This result is dominated by haloes in the mass range 10 13 -10 14 h -1 M ⊙ , and is an average over all separations from the group or cluster centre.

Galics- VI. Modelling hierarchical galaxy formation in clusters

High-resolution N-body re-simulations of 15 massive (10 14 10 15 M⊙) dark matter haloes have been combined with the hybrid galaxy formation model GalICS (Hatton et al. 2003), to study the formation and evolution of galaxies in clusters, within the framework of the hierarchical merging scenario. This paper describes the high-resolution resimulation technique used to build the dark matter halo sample, and discusses its reliability. New features incorporated in GalICS include a better description of galaxy positioning after dark matter halo merger events, a more reliable computation of the temperature of the inter-galactic medium as a function of redshift, that also takes into account the re ionisation history of the Universe, and a semi-analytic description of the ram pressure strippi ng of cold gas from galactic discs, suffered by galaxies during their motion through the diffuse hot intra-cluster medium. Within the multitude of available model results, we choose to focus here on the luminosity functions, morphological fractions and colour distributions of galax ies in clusters and in cluster outskirts, at z = 0. No systematic dependency on cluster richness is found either for the galaxy luminosity functions, morphological mixes, or colour distributions. Moving from higher density (cluster cores), to lower density environments (clust er outskirts), we detect a progressive flattening of the luminosity functions, an increase of the fr action of spirals and a decrease of that of ellipticals and S0s, and the progressive emergence of a bluer tail in the distributions of galaxy colours, especially for spirals. As compared to cluster spirals, early-type galaxies show a flatter luminosity function, and more homogeneous and redder colours. An overall good agreement is found between our results and the observations, particularly in terms of the cluster luminosity functions and morphological mixes. However, some discrepancies are also apparent, with too faint magnitudes of the brightest cl uster members, especially in the B band, and galaxy colours tendentially too red (or not blue enough) in the model, with respect to the observations. Finally, ram pressure stripping appea rs to affect very little our results.

M/L, Hα Rotation Curves, and HiGas Measurements for 329 Nearby Cluster and Field Spirals. II. Evidence for Galaxy Infall

We have conducted a study of optical and H I properties of spiral galaxies (size, luminosity, Hflux distribu- tion, circular velocity, H I gas mass) to explore the role of gas stripping as a driver of morphological evolution in clusters. We find a strong correlation between the spiral a nd S0 fractions within clusters, and the spiral fraction scales tightly with cluster X-ray gas luminosity. We explore young star formation and identify spirals that are (1) asymmetric, with truncated Hemission and H I gas reservoirs on the leading edge of the disk, on a first pass through the dense intracluster medium in the core s of rich clusters; (2) strongly H I deficient and stripped, with star formation confined to the inner 5 h -1 kpc and 3 disk scale lengths; (3) reddened, extremely H I deficient and quenched, where star formation has been halted across the entire disk. We propose that these spirals are in successive stages of morphological transfor mation, between infalling field spirals and cluster S0s, and that the process which acts to remove the H I gas reservoir suppresses new star formation on a similarly fast timescale. These data suggest that gas stripping plays a significant role in morphological transformation and rapid truncation of star formation across the disk. Subject headings:galaxies: clusters — galaxies: evolution — galaxies: kinematics and dynamics

The GEMS project: X-ray analysis and statistical properties of the group sample

The Group Evolution Multiwavelength Study (GEMS) involves a multiwavelength study of a sample of 60 galaxy groups, chosen to span a wide range of group properties. Substantial ROSAT Position Sensitive Proportional Counter (PSPC) observations, available for all of these groups, are used to characterize the state of the intergalactic medium in each. We present the results of a uniform analysis of these ROSAT data and a statistical investigation of the relationship between X-ray and optical properties across the sample. Our analysis improves in several respects on previous work: (i) we distinguish between systems in which the hot gas is a group-scale medium and those in which it appears to be just a hot halo associated with a central galaxy; (ii) we extrapolate X-ray luminosities to a fixed overdensity radius (r 500 ) using fitted surface brightness models, in order to avoid biases arising from the fact that cooler systems are detectable to smaller radii, and (iii) optical properties have been rederived in a uniform manner from the NASA Extragalactic Database, rather than relying on the data in the disparate collection of group catalogues from which our systems are drawn. The steepening of the L X -T X relation in the group regime reported previously is not seen in our sample, which fits well on to the cluster trend, albeit with large non-statistical scatter. A number of biases affect the fitting of regression lines under these circumstances, and until the impact of these has been thoroughly investigated it seems best to regard the slope of the group L X -T X relation as being poorly determined. A significant problem in comparing the properties of groups and clusters is the derivation of system radii, to allow different systems to be compared within regions having the same overdensity. We find evidence that group velocity dispersion (σ v ) provides a very unreliable measure of system mass (and hence radius), with a number of groups having remarkably low values of σ v , given that they appear from their X-ray properties to be collapsed systems. We confirm that the surface brightness profiles of groups are significantly flatter than those of clusters - the maximum value of the β fit parameter for our sample is 0.58, lower than the typical value of 0.67 seen in clusters - however, we find no significant tendency within our sample for cooler groups to show flatter profiles. This result is inconsistent with simple universal pre-heating models. The morphology of the galaxies in the GEMS groups is correlated to their X-ray properties in a number of ways: we confirm the very strong relationship between X-ray emission and a dominant early-type central galaxy, which has been noted since the early X-ray studies of groups, and also find that spiral fraction is correlated with the temperature of the hot gas and hence the depth of the gravitational potential. A class of spiral-rich groups with little or no X-ray emission probably corresponds to groups that have not yet fully collapsed.

The morphology—density relation in X-ray-bright galaxy groups

We have examined the morphological make-up of X-ray bright groups. The brighter galaxies in these groups exhibit clear morphology-density and morphology-radius relations. The group morphology-density relation is offset from the cluster relation in the sense that at a given surface density, X-ray bright groups have a lower spiral fraction. After correcting for projection effects the morphology-3D density relation is still shifted towards fewer spirals for a given 3D density, in comparison with clusters. A simple model which corrects the group data for the effects of projection and for the expected higher merging rate in groups, brings the morphology-density relation into good agreement with that of clusters, suggesting that the relation may be driven by two-body interactions. The fraction of S0 galaxies in these X-ray bright groups is at least as high as that observed in nearby clusters. Given the low velocity dispersion of groups, this indicates that ram pressure stripping is not the dominant mechanism for S0 formation.

Evolutionary synthesis models for the formation of S0 galaxies in clusters

Rich galaxy clusters in the local Universe show a large population of SO galaxies (∼40% of all luminous galaxies). With increasing redshift the fraction of this SO galaxy population is observed to strongly decrease (e.g. by a factor ∼2-3 to z = 0.5) in favor of the spiral galaxy fraction while the number of bright ellipticals does not seem to change. The infalling field galaxy population that successively builds up the cluster also is spiral rich and SO poor. It has hence been suspected that galaxy transformation processes, either due to galaxy - galaxy or to galaxy - ICM interactions, are responsible for this change. Complementing dynamical and morphological studies, we use evolutionary synthesis models describing various possible effects of those interactions on the star formation rates of the infalling spirals. We study the effects of starbursts of various strengths as well as of the truncation of star formation on the color and luminosity eyolution of model galaxies of various spectral types. Comparison with observed properties of the local SO galaxy population is used to constrain possible SO formation mechanisms. We find that star formation truncation in spiral galaxies earlier than Sd-type, if occurring not too long ago, as well as starbursts more than 3 Gyr ago and followed by complete star formation extinction in spirals - again earlier than Sd- may well account for the observed average SO luminosities and colors. Late-type galaxies (Sd), even after a strong burst, remain either too blue or too faint. Our results are in agreement with studies of spectral features of cluster SOs but allow for stronger constraints.

AGNs in Shakhbazian Compact Groups

The results of CCD spectroscopic observations of Shakhbazian compact groups of galaxies (SHCGs) with the 1.54-m (La Silla, Chile), 2.2-m (Calar Alto, Spain) and 2.6-m (Byurakan) telescopes are presented. According to these preliminary data, about 10% of member galaxies in SHCGs are emission-line galaxies (ELGs) including the broad-line AGNs (of classical Seyfert 1 type) and the narrow-emission-line galaxies.A research program has been developed in the University of Potsdam, Potsdam Astrophysikalisches Institut in cooperation with other observatories (particularly with Byurakan Astrophysical Observatory) to perform photometric and spectroscopic investigations of galaxies in the SHCGs. Within the framework of this program the redshift (radial velocity) measurements have been carried out for more than 200 galaxies in 36 SHCGs. The MIDAS software package was used for processing and interpreting of the galaxy spectra. Most of these redshifts were measured for the first time. 180 member galaxies (90%) in these groups have absorption spectra typical of E and SO galaxies. Twenty galaxies (10%) turn out to be ELGs. They are in the range 0.02 ≤ z ≤ 0.17, i.e., the SHCGs lie in approximately the same redshift space as Abell clusters. These compact groups contain predominantly elliptical and lenticular galaxies (del Olmo 1988; Amirkhanian 1989) like the cores of rich, regular, centrally condensed clusters of galaxies. The fraction of spirals falls in the densest matter concentrations. On the other hand, it is a well-established fact that in the local universe the active objects tend to avoid the cores of dense clusters of galaxies (e.g. Green and Yee 1984). That is why the discovery of an emission-line population with broad-line AGNs in SHCGs (Tiersch et al. 1999) was unexpected. As shown by Dressier, Thompson and Shectman (1985) in their sample of 1268 galaxies in the feilds of 14 rich clusters the ELGs comprise 31% of the field galaxies but only 7% of the cluster galaxies. Similarly, according to their statistics AGNs make up 5% of the field sample, but only 1% of the cluster sample. They note that the difference in the distribution of morphological types can only partially explain this effect. Obviously, some sort of environmental influence is present.

On the Association of Hickson Compact Groups with Loose Groups

ABSTRACTCounts of galaxies are made in the vicinity of Hickson compact groups (HCGs). Loose group galaxies are detected in ∼70% of nearby groups with z<0.025. It is shown that members of the loose group associated with a certain HCG are located in a relatively narrow strip oriented along the elongation of the HCG. Such a peculiar distribution of the loose group galaxies and the found correlation of the radial velocity dispersion of the loose group galaxies with the elongation of the group allows us to conclude that the loose group galaxies are rotating around a common gravitational center. This is a clue for the solution of the known puzzles related to HCGs: the lack of strong radio and luminous far‐infrared sources, a surprisingly large spiral fraction, the lack of blue elliptical galaxies, and so on. Only those of such elongated formations in which the bright members happen to be oriented close to each other on the sky are detected as compact groups.

Optical Luminosities and Mass‐to‐Light Ratios of Nearby Galaxy Clusters

We analyze a sample of 105 clusters having virial mass homogeneously estimated and for which galaxy magnitudes are available with a well-defined high degree of completeness. In particular, we consider a subsample of 89 clusters with Bj-band galaxy magnitudes taken from the COSMOS/UKST Southern Sky Object Catalog. After suitable magnitude corrections and uniform conversions to Bj band, we compute cluster luminosities L within several clustercentric distances, 0.5, 1.0, 1.5 h-1 Mpc and within the virialization radius Rvir. In particular, we use the luminosity function and background counts estimated by Lumsden et al. on the Edinburgh/Durham Southern Galaxy Catalog, which is the well-calibrated part of the COSMOS catalog. We analyze the effect of several uncertainties connected to photometric data, fore/background removal, and extrapolation below the completeness limit of the photometry, in order to assess the robustness of our cluster luminosity estimates. We draw our results on the relations between luminosity and dynamical quantities from the COSMOS sample by considering mass and luminosities determined within the virialization radius. We find a very good correlation between cluster luminosity, L, and galaxy velocity dispersion, σv, with L ∝ σ. Our estimate of the typical value for the mass-to-light ratio is M/L ~ 250 h M☉/L☉. We do not find any correlation of M/L with cluster morphologies, i.e., Rood-Sastry and Bautz-Morgan types, and only a weak significant correlation with cluster richness. We find that mass has a slight, but significant, tendency to increase faster than the luminosity does, M ∝ L. We verify the robustness of this relation against a number of possible systematics. We verify that this increasing trend of M/L with cluster mass cannot be entirely due to a higher spiral fraction in poorer clusters, thus suggesting that a similar result would also be found by using R-band galaxy magnitudes.

Tidally induced star formation in Abell 1367

Our principal aim is to compare global star formation rates between cluster galaxies and field galaxies in order to clarify environmental influence on star formation. We use an objective prism technique to survey over 200 Zwicky catalogue (CGCG) galaxies within ∼ 2°.5 of Abell 1367 for Hα emission. After a brief discussion of the survey characteristics, we consider first the dependence of Hα detection on Hubble type, galaxy disturbance and the presence of a bar. As expected, we rarely detect early-type galaxies and consequently restrict further discussion to spirals (type S0/a and later), of which we detect ∼ 35 per cent in Hα. We find that an extremely valuable distinction to make is between galaxies with diffuse Hα and galaxies with compact Hα. There is a very significant tendency for galaxies with compact Hα emission to be disturbed, and there may be a weak tendency for them to be barred. Neither of these tendencies is found for galaxies with diffuse Hα emission. We infer that compact emission results from tidally induced star formation, while diffuse emission results from more normal disc star formation. After considering field contamination, we adopt as a ‘predominantly cluster’ sample the spiral population inside 0.5 rA; a ‘predominantly field’ sample outside 0.5 rA; and a ‘pure field’ sample outside 1.5 rA. We consistently find a much larger fraction of spirals detected with compact Hα in the cluster sample compared to the field samples (e.g. 38 versus 0 per cent detected in cluster and ‘pure field’ samples, χ2 significance 3.6 σ). This increased fraction detected in the cluster is found for early-, mid- and late-type spirals separately. No such cluster/field differences are found for galaxies with diffuse Hα emission. We conclude that tidal perturbations are more common in the cluster than in the field, leading to a higher incidence of compact tidally triggered star formation. By combining information on galaxy disturbance, galaxy companions, and the location of galaxies within the cluster, we have tried to identify the origin of the tidal perturbations. We find strong evidence that near-neighbour interaction plays a significant role in triggering star formation. However, we also find candidate objects near the cluster core which may be perturbed by the overall cluster tidal field, and candidate objects which may be influenced by a higher speed ‘harassment’ interaction between galaxies.

Morphological Studies of the Galaxy Populations in Distant “Butcher‐Oemler” Clusters with theHubble Space Telescope. II. AC 103, AC 118, and AC 114 atz= 0.31

We present new results of a program to study the detailed morphologies of galaxies in intermediate-redshift clusters and hence understand the physical origin of the enhanced star formation seen in these environments at earlier epochs. Deep, high-resolution imagery has been obtained of three rich clusters, AC 103, AC 118, and AC 114 at z = 0.31, through the R (F702W) filter of the Wide Field Planetary Camera (WFPC2) of the Hubble Space Telescope (HST). For AC 103 and AC 118, single three-orbit pointings covering a central ~0.5 × 0.5 h-1 Mpc area have been obtained. For AC 114, a mosaic of four six-orbit pointings has provided coverage of a central ~1.2 × 0.7 h-1 Mpc area, for the first time allowing a study extending to the outer regions of a more distant cluster. Morphological classification has been conducted visually using these images, with Hubble types plus evidence of dynamical interactions and/or structural abnormalities recorded for all galaxies down to R702 = 22.25 in AC 103 and AC 118 and R702 = 23.00 in AC 114. New spectroscopy limited at K' ≤ 18 has also been acquired, providing within our WFPC2 images a total sample of 129 confirmed cluster members, of which 117 have secure star formation classifications (e.g., starburst, poststarburst, Hδ strong). Our study reveals that the mix of Hubble types within the distant clusters is, overall, significantly different from that seen in the same high-density environments nearby: there are proportionally more spiral galaxies and fewer E/S0 galaxies at these earlier epochs. Within the core (r 200 h-1 kpc) regions of the three clusters, spirals covering the full Sa-Sdm/Irr range are present in numbers up to a factor of ~4 higher than that seen in present-day clusters, the highest fraction being observed in the least massive cluster within our sample, AC 103. Only in the virialized core of our most massive, regular cluster, AC 114, do we see morphological fractions approaching those of the present epoch. However, outside the core of this cluster the spiral fraction rises to ~60%: 3 times above the present-day level. Dynamical interactions are also widespread throughout the populations of all three clusters, with ~20% of the members, on average, showing morphological evidence of this phenomenon. The various subsets of cluster members show emergent morphological trends. The majority of blue galaxies either undergoing a starburst or seen shortly (<0.5 Gyr) thereafter are systems involved in major mergers. These galaxies, however, are generally of modest luminosity (L ~ L* + 1 mag) even in this brightened phase; in their faded state they appear destined to become dwarfs, too faint to be included in magnitude-limited spectroscopic samples such as ours. Cluster members with ongoing star formation typical of present-day spirals are mostly normal Sb-Sdm/Irr Hubble types. Galaxies identified as having completed their last major episode of star formation 1-2 Gyr prior to the epoch of observation are conspicuous by their commonality in morphology, all being early-type (S0-Sb) disk systems. The red Hδ-strong objects, interpreted from their colors and spectra as being the remnants of secondary star formation in old dormant systems, have morphologies consistent with this picture, being a mixture of normal E and S0 galaxies. In combination, these results point to cluster evolution being driven by not one but several different physical processes. The only conspicuous one is galaxy-galaxy interactions and merging. However, our study reveals many galaxies where this process could not have been responsible for altering the course of their star formation activity. Furthermore, the normal disk morphology of the majority of these previously active galaxies indicates the operation of process(es) that halt star formation but leave the basic disk structure intact and largely unperturbed. This tends to favor mechanisms that affect the gas supply (e.g., ram-pressure stripping, infall) rather than processes like galaxy harassment, where we simply do not see the population of severely distorted spirals expected if its operation is widespread.

9.8. Bars and black holes

Massive black holes are now thought to be present at the centre of a fair fraction of nearby galaxies. The origin of these central dark masses is still unknown, although tentative explanations have been proposed in an attempt to reconcile non-active galaxies with AGNs and quasars. The activity of a galaxy may then mostly depend on the efficiency of gas accretion onto the central dark object. It is important to note that many of the galaxies which are today candidates for the presence of a massive black hole are spirals. In this context, bars should play an important role in the evolution (and formation?) of a central mass, since (1) they are present in a significant fraction of spirals, (2) they may be efficient drivers of gas accretion. If indeed most of present day galaxies hosts a central dark mass, then bars and black holes should coexist in a significant fraction of them. We examine here the cases of 3 edge-on galaxies which are candidates for the presence of a central black holes: NGC 4570, NGC 3115 and M 104.

Evolution sincez= 0.5 of the Morphology‐Density Relation for Clusters of Galaxies

Using traditional morphological classifications of galaxies in 10 intermediate-redshift (z~0.5) clusters observed with WFPC-2 on the Hubble Space Telescope, we derive relations between morphology and local galaxy density similar to that found by Dressler for low-redshift clusters. Taken collectively, the `morphology-density' relationship, M-D, for these more distant, presumably younger clusters is qualitatively similar to that found for the local sample, but a detailed comparison shows two substantial differences: (1) For the clusters in our sample, the M-D relation is strong in centrally concentrated ``regular'' clusters, those with a strong correlation of radius and surface density, but nearly absent for clusters that are less concentrated and irregular, in contrast to the situation for low redshift clusters where a strong relation has been found for both. (2) In every cluster the fraction of elliptical galaxies is as large or larger than in low-redshift clusters, but the S0 fraction is 2-3 times smaller, with a proportional increase of the spiral fraction. Straightforward, though probably not unique, interpretations of these observations are (1) morphological segregation proceeds hierarchically, affecting richer, denser groups of galaxies earlier, and (2) the formation of elliptical galaxies predates the formation of rich clusters, and occurs instead in the loose-group phase or even earlier, but S0's are generated in large numbers only after cluster virialization.

The luminosity function of dwarf galaxies in four spiral-rich clusters

We measure luminosity functions in the cores of four spiral-rich, poor clusters of galaxies at median redshift $z = 0.016$. In the red magnitude range -14 < M_R < -10, our data imply that the luminosity functions phi(L) \propto L^{alpha} are steep, -1.8 < alpha < -1.6, in the central 200-300 kpc of Abell 262 and of the NGC 507 Group. Abell 194 also shows signs of a steep luminosity function, alpha < -1.6, in this magnitude range. In Pegasus, the dwarf galaxy density is too low to let us constrain alpha. The NGC 507 Group and Abell 194 have been interpreted as clusters that are forming today, based on morphology and velocity structure. The high spiral galaxy fraction in Abell 262 relative to clusters like Virgo and Coma also suggests that it is young. We therefore suggest that steep luminosity functions in the range -14 < M_R < -10 may be a universal feature of young clusters and possibly of the field. If this is true, then the observed paucity of gas-rich galaxies in such environments suggests that we are finding galaxies similar to the low-surface-brightness, dark-matter-dominated dwarf spheroidal galaxies seen locally and in Virgo. This interpration is also consistent with the distribution of colors and sizes of the faint galaxies in Abell 262. If we are indeed detecting dwarf spheroidal galaxies and if they are as numerous relative to bright galaxies in the field as they are in the young clusters observed here, then the contribution of their halos to the cosmological mass density is Omega_{dSph halo} \approx 0.01. This is much smaller than values of Omega derived from dynamical measurements.

Deep Optical Observations of Compact Groups of Galaxies

Compact groups of galaxies appear to be extremely dense, making them likely sites of intense galaxy interaction, while their small populations make them relatively simple to analyze. In order to search for optical interaction tracers such as diffuse light and galaxy tidal features in Hickson compact groups (HCGs), we carried out deep photometry in three filters on a sample of HCGs with ROSAT observations. Using a modeling procedure to subtract the light of bright early-type galaxies, we found shell systems and extended envelopes around many, but not all, of those galaxies. Only one group in our sample, HCG 94, has diffuse light in the group potential (with a luminosity of 7 L^*^); the other groups do not contain more than 113 L^*^ in diffuse light. With the exception of HCG 94 (which is the most x-ray-luminous HCG), we found no correlation between the presence of shells or other tidal features and the x-ray luminosity of a group. Better predictors of detectable group x-ray emission are a low spiral fraction and belonging to a larger galaxy condensation-neither of which are correlated with optical disturbances in the group galaxies. No elliptical galaxies that are extremely optically luminous but x-ray faint are found to have shells and very complex color structures. This is likely due to recent infall of gas-rich material into the galaxies, which would produce both the disruption of stellar orbits and a significant amount of star formation.

ROSAT observations of compact groups of galaxies

We have systematically analyzed a sample of 13 new and archival ROSAT PSPC observations of compact groups of galaxies: 12 Hickson Compact Groups plus the NCG 2300 group. We find that approximately two-thirds of the groups have extended X-ray emission and, in four of these, the emission is resolved into diffuse emission from gas at a temperature of $k$T $\sim 1$ keV in the group potential. All but one of the groups with extended emission have a spiral fraction of less than 50\%. The baryon fraction of groups with diffuse emission is 5--19\%, similar to the values in clusters of galaxies. However, with a single exception (HCG 62), the gas-to-stellar-mass ratio in our groups has a median value near 5\%, somewhat greater than the values for individual early-type galaxies and two orders of magnitude lower than in clusters of galaxies. The X-ray luminosities of individual group galaxies are comparable to those of similar field galaxies, although the L$_X$-L$_B$ relation for early-type galaxies may be flatter in compact groups than in the field.

What determines the morphological fractions in clusters of galaxies?

view Abstract Citations (309) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS What Determines the Morphological Fractions in Clusters of Galaxies? Whitmore, Bradley C. ; Gilmore, Diane M. ; Jones, Christine Abstract A reexamination of Dressler's sample of nearly 6000 galaxies in 55 clusters shows that the morphology-clustercentric radius relation is more fundamental than the morphology-local density relation. This conclusion is supported by improved correlations when the clustercentric radius is used as the independent parameter, and by a comparison of galaxies with the same normalized clustercentric radii but different values of the local density. The morphology-radius relation, when normalized by a characteristic cluster radius, R_c_^opt^, does not vary as a function of the number density within 0.5 Mpc, N_0.5_, the X-ray luminosity, L_x_, or the velocity dispersion of the cluster, V_disp_. This surprising result means that only one parameter is needed to determine the morphological fractions in clusters, namely R/R_c_^opt^. The elliptical fraction in the outer regions of clusters is relatively constant for all types of clusters, with a slight rise from about 10% in the outermost region to about 16% at R/R_c_^opt^ = 1. For radii smaller than R/R_c_^opt^= 1 (generally about 0.5 Mpc for most clusters) the elliptical fraction rises rapidly, reaching values of 60%-70% near the center of the cluster. The S0 fraction rises moderately as the center is approached, and then falls sharply within about 0.2 Mpc of the center. The spiral fraction falls moderately as the clustercentric radius decreases and then falls rapidly near the center. The spiral fraction is essentially zero at the cluster center, even though spirals dominate everywhere else in the universe. These results indicate that some property of the cluster center plays the key role in determining the morphological fractions in clusters, and suggests the possibility that a destructive mechanism is controlling the morphological fractions rather than a formation mechanism. Using this basic idea as a starting point we developed the following simple model. The three basic assumptions are (1) the intrinsic morphological mix of galaxies is E/(S0 + S + I) = 10%/90%, (2) elliptical galaxies form first, followed by the cluster collapse, S0 galaxies, and finally spiral and irregular galaxies, and (3) during the cluster collapse, galaxies which are still protogalactic clouds of gas are destroyed, and the gas from these failed galaxies is added to the intracluster medium. The destruction of the late-forming spiral and S0 protogalaxies near the cluster centers results in the increase in the elliptical fraction. This simple model can explain a wide range of observations. Besides explaining the morphological fractions, it also suggests that roughly half of the intracluster gas resulted from "failed" galaxies. The model is also consistent with the tidal radii that would be imposed by the mean tidal field of the clusters. Publication: The Astrophysical Journal Pub Date: April 1993 DOI: 10.1086/172531 Bibcode: 1993ApJ...407..489W Keywords: Elliptical Galaxies; Galactic Clusters; Spiral Galaxies; Astronomical Models; Galactic Evolution; Galactic Structure; X Ray Astronomy; Astrophysics; GALAXIES: CLUSTERING; GALAXIES: FUNDAMENTAL PARAMETERS full text sources ADS | data products SIMBAD (51) NED (51)

The nature of compact groups of galaxies

Monte Carlo numerical simulation is used to calculate the probability for the chance occurrence of four galaxies projected on the sky satisfying the Hickson isolation criterion within a loose group of eight members. For the models which match most closely the size and galaxy multiplicity function of observed groups, this chance occurrence is found to be smaller by a factor of about 100 than the value obtained previously by Mamom from numerical simulations of dynamical models of groups. This and other direct independent observational results from the literature constitute strong evidence that nearly all of the Hickson compact groups are real physical systems. It is concluded that the tendency for the spiral fraction of a compact group to be larger than the value inferred from the galaxy morphology-group density relation of rich clusters and loose groups is a real physical effect indicating that galaxy morphology depends strongly on a second parameter which, it is suggested, is the velocity dispersion of a system. 21 references.