Loose Groups Of Galaxies Research Articles

Tidal Remnants and Intergalactic H II Regions

We report the discovery of two small intergalactic H II regions in the loose group of galaxies around the field elliptical NGC 1490. The H II regions are located at least 100 kpc from any optical galaxy but are associated with a number of large H I clouds that are lying along an arc 500 kpc in length and that have no optical counterpart on the Digital Sky Survey. The sum of the H I masses of the clouds is almost 1010M⊙ and the largest H I cloud is about 100 kpc in size. Deep optical imaging reveals a very low surface brightness counterpart to this largest H I cloud, making this one of the H I richest optical galaxies known (MHI/LV ~ 200). Spectroscopy of the H II regions indicates that the abundance in these H II regions is only slightly sub-solar, excluding a primordial origin of the H I clouds. The H I clouds are perhaps remnants resulting from the tidal disruption of a reasonably sized galaxy, probably quite some time ago, by the loose group to which NGC 1490 belongs. Alternatively, they are remnants of the merger that created the field elliptical NGC 1490. The isolated H II regions show that star formation on a very small scale can occur in intergalactic space in gas drawn from galaxies by tidal interactions. Many such intergalactic small star formation regions may exist near tidally interacting galaxies.

An H I Census of Loose Groups of Galaxies

We present results from our Parkes Multibeam H I survey of 3 loose groups of galaxies that are analogous to the Local Group. This is a survey of groups containing only spiral galaxies with mean separations of a few hundred kpc, and total areas of approximately 1 Mpc2; groups similar to our own Local Group. We present a census of the H I-rich objects in these groups down to a 1σ MHI sensitivity ~7×105M⊙, as well as the detailed properties of these detections from follow-up Compact Array observations. We found 7 new H I-rich members in the 3 groups, all of which have stellar counterparts and are, therefore, typical dwarf galaxies. The ratio of low-mass to high-mass gas-rich galaxies in these groups is less than in the Local Group meaning that the “missing satellite” problem is not unique. No high-velocity cloud analogs were found in any of the groups. If HVCs in these groups are the same as in the Local Group, this implies that HVCs must be located within ~300-400 kpc of the Milky Way.

Galaxies in group and field environments: A comparison of optical–NIR luminosities and colors

We compare properties of galaxies in loose groups with those in field environment by analyzing the Nearby Optical Galaxy (NOG) catalog of galaxy systems. We consider as group galaxies objects belonging to systems with at least five members identified by means of the of friends method, and, as field galaxies, all galaxies with no companions. We analyze both a magnitude-limited sample of 959 and 2035 galaxies (groups vs. field galaxies, respectively, B < 14 mag, and 2000 < cz < 6000 km s - 1 ) and a volume-limited sample (M B < -19.01 + 5 log 1 0 h h mag, 2000 < cz < 4000 km s - 1 369 group and 548 field galaxies). For all these galaxies, blue corrected magnitudes and morphological types are available. The cross-correlation of NOG with the 2MASS second release allow us to assign K magnitudes and obtain B - K colors for about half of the galaxies in our samples. We analyze luminosity and color segregation-effects in relation with the morphological segregation. For both B and K bands, we find that group galaxies are, on average, more luminous than field galaxies and this effect is not entirely a consequence of the morphological segregation. After taking into account the morphological segregation, the luminosity difference between group and field galaxies is about 10%. When considering only very early-type galaxies (T < -2) the difference is larger than 30%. We also find that group galaxies are redder than field galaxies, A(B - K) ∼ 0.4 mag. However, after taking into account the morphological segregation, we find a smaller B - K difference, poorly significant (only at the c.l. of ∼80%). We discuss our results considering that the analyzed groups define a very low density environment (projected mean density ∼5-6 h 2 Mpc - 2 galaxies).

Morphology and luminosity segregation of galaxies in nearby loose groups

We study morphology and luminosity segregation of galaxies in loose groups. We analyze the two catalogs of groups identified in the Nearby Optical Galaxy (NOG) sample, by means of hierarchical and percolation “friends-of-friends” methods (HG and PG catalogs, respectively). In the first part of our analysis we consider 387 and 436 groups of HG and PG and compare morphology- (luminosity-) weighted to unweighted group properties: velocity dispersion, mean pairwise distance, and mean groupcentric distance of member galaxies. The second part of our analysis is based on two ensemble systems, one for each catalog, built by suitably combining together galaxies of all groups (1584 and 1882 galaxies for HG and PG groups). We find that earlier-type (brighter) galaxies are more clustered and lie closer to the group centers, both in position and in velocity, than later-type (fainter) galaxies. Spatial segregations are stronger than kinematical segregations. These effects are generally detected at the $\mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{~}}}\hbox{$>$}}}$3-sigma level. Luminosity segregation is shown to be independent of morphology segregation. Our main conclusions are strengthened by the detection of segregation in both hierarchical and percolation catalogs. Our results agree with a continuum of segregation properties of galaxies in systems, from low-mass groups to massive clusters.

The mass function of the Las Campanas loose groups of galaxies

We have determined the mass function of loose groups of galaxies in the Las Campanas Redshift Survey. Loose groups of galaxies in the LCRS range in mass from $M \sim 10^{12}~M_{\odot}$ to $10^{15}~{M}_{\odot}$. We find that the sample is almost complete for masses in the interval $5\times 10^{13}{-}8\times 10^{14}~{M}_{\odot}$. Comparison of the observed mass function with theoretical mass functions obtained from N -body simulations shows good agreement with a CDM model with the parameters $\Omega_{\rm m} = 0.3$, $\Omega_{\Lambda} = 0.7$ and the amplitude of perturbations about $\sigma_8=0.78{-}0.87$. For smaller masses the mass function of LCRS loose groups flattens out, differing considerably from the group mass function found by Girardi & Giuricin (2000) and from mass functions obtained by numerical simulations.

HI observations of loose galaxy groups

At Nancay, 21-cm H i line observations were made of 15 spiral-dominated loose groups of galaxies, divided into two samples: an sample containing at least one pair of interacting galaxies, and a sample having no optical evidence of interactions or morphological disturbances among the group members. The interacting sample consists of 62 galaxies representing 9 different groups, and the control sample contains 40 galaxies representing 6 groups. Of the 91 galaxy and galaxy pairs observed, 74 were detected, while upper limits were placed on the remaining 17 objects. These homogeneous H i data, which will be used in future analyses, provide comparative information on the H i content of groups and serve as a probe of the vicinity of the target spirals for H i clouds or very low surface brightness gas-rich galaxies.

Relationship between the Morphological Type of the Brightest Galaxy and the Morphological Content of the Remaining Members in a Loose Group of Galaxies

An investigation of groups of galaxies showed that there is a correlation between the morphological type of the brightest galaxy and the morphological types of the remaining population of the group. The distributions by morphological type of the brightest galaxies and of the remaining group members do not differ from each other. This means that the observed morphological types of both the brightest and the remaining group members cannot be explained by their interaction with their surroundings. They may be due to the initial conditions at the epoch of galaxy formation.

The Dependence of the Galaxy Luminosity Function on Environment

We present luminosity functions for galaxies in loose groups in the Las Campanas Redshift Survey, differentiated by their environment (defined by the line-of-sight velocity dispersion sigma of the host groups) and also by their spectral type (emission or non-emission, defined by the equivalent width of the 3727-Angstrom [OII] line). We find systematic variations in the Schechter parameters alpha and M* for non-emission line galaxies over a range of 0 < sigma < 800 km/s. Alpha varies from 0.20 to -0.91, indicating an increase in the steepness of the faint end slope with increasing sigma. The accompanying variation in M* appears to be accounted for by the intrinsic correlation with alpha and does not indicate a significant physical variation in the bright end of the luminosity function. For emission line galaxies, we find no significant systematic variation of the luminosity function with the environment. Our results show that emission and non-emission galaxies generally occupy two distinct regions in the alpha-M* parameter space. From our luminosity functions, we derive the number ratios of emission to non-emission galaxies as a function of environment and absolute magnitude, showing that the relative abundance of non-emission line galaxies generally increases for all magnitudes -23 < M_R < -17.5 towards high-sigma environments, from ~80% to >90% at M_R = -22 and from ~10% to >50% at M_R = -18 (H_0 = 100 km s^{-1} Mpc^{-1} and q_0 = 0.5).

On the Nature of Andromeda IV

Lying at a projected distance of 40' or 9 kpc from the center of M31, Andromeda IV is an enigmatic object first discovered during van den Bergh's search for dwarf spheroidal companions to M31. Since it is bluer, more compact, and has a higher surface brightness than other known dwarf spheroidals, it has been suggested that And IV is either a relatively old star cloud in the outer disk of M31 or a background dwarf galaxy. We present deep Hubble Space Telescope WFPC2 observations of And IV and the surrounding field, which, along with ground-based long-slit spectroscopy and H? imagery, are used to decipher the true nature of this puzzling object. We find compelling evidence that And IV is a background galaxy seen through the disk of M31. The moderate surface brightness ( ~ 24), very blue color (V-I 0.6), low current star formation rate (~0.001 M? yr-1), and low metallicity (~10% Z?) reported here are consistent with And IV being a small dwarf irregular galaxy, perhaps similar to Local Group dwarfs such as IC 1613 and Sextans A. Although the distance to And IV is not tightly constrained with the current data set, various arguments suggest it lies in the range 5 D 8 Mpc, placing it well outside the confines of the Local Group. It may be associated with a loose group of galaxies, containing major members UGC 64, IC 1727, and NGC 784. We report an updated position and radial velocity for And IV.

The intragroup medium in loose groups of galaxies

We have used the ROSAT PSPC to study the properties of a sample of 24 X-ray-bright galaxy groups, representing the largest sample examined in detail to date. Hot plasma models are fitted to the spectral data to derive temperatures, and modified King models are used to characterize the surface brightness profiles. In agreement with previous work, we find evidence for the presence of two components in the surface brightness profiles. The extended component is generally found to be much flatter than that observed in galaxy clusters, and there is evidence that the profiles follow a trend with system mass. We derive relationships between X-ray luminosity, temperature and optical velocity dispersion. The relation between X-ray luminosity and temperature is found to be LX∝T4.9, which is significantly steeper than the same relation in galaxy clusters. These results are in good agreement with pre-heating models, in which galaxy winds raise the internal energy of the gas, inhibiting its collapse into the shallow potential wells of poor systems.

The Nature of Compact Groups

AbstractWe show that compact groups (CGs) are real physical entities, that almost all HCGs are dynamically associated with generally elongated loose groups of galaxies, that HCGs are compact cores of loose groups, and that they are longer living formations.

Warm Gas and Ionizing Photons in the Local Group

Several lines of argument suggest that a large fraction of the baryons in the universe may be in the form of warm (T ~ 105-107 K) gas. In particular, loose groups of galaxies may contain substantial reservoirs of such gas. Observations of the cosmic microwave background by COBE place only weak constraints on such an intragroup medium within the Local Group. The idea of a Local Group corona dates back at least 40 years (Kahn & Woltjer). Here we show that gas at T ~ (2-3) × 106 K (the approximate virial temperature of the Local Group)—extremely difficult to observe directly—can in principle radiate a large enough flux of ionizing photons to produce detectable Hα emission from embedded neutral clouds. However, additional constraints on the corona—the most stringent being pulsar dispersion measures toward the Magellanic Clouds, and the timing mass—rule out an intragroup medium whose ionizing flux dominates over the cosmic background or the major Local Group galaxies. A cosmologically significant coronal gas mass could remain invisible to Hα observations. More massive galaxy groups could contain extensive coronae which are important for the baryon mass and produce a strong, local ionizing flux.

Loose groups of galaxies in the Perseus–Pisces survey

We present a large catalog 1 of loose groups of galaxies in the Southern Galactic Hemisphere, selected from the Perseus{Pisces redshift Survey (PPS). Particular care is taken in order to obtain group samples as ho- mogeneous as possible to previously published catalogs. All our catalogs contain about 200 groups, signicantly more than in most previous studies where group samples were obtained from galaxy data sets of comparable qual- ity to (but smaller extent than) PPS. Groups are iden- tied with the adaptive Friends{Of{Friends (FOF) algo- rithm of Huchra & Geller (1982), with suitable normal- izations D0 =0 :231 h 1 Mpc and V0 = 350 km s 1 at cz0 = 1000 km s 1 . The luminosity function (LF) normal- ization =0 :02 h 3 Mpc 3 appropriate for PPS yields a number density threshold n=n 180 for the adopted D0, instead of n=n 80 used in previous studies of other samples. However, the customary choice of D0 obtained (through the LF) from a xed mass overdensity= = 80, well motivated in theory, suers from important observa- tional uncertainties and sample{to{sample variations of the LF normalization, and from major uncertainties in the relation between galaxy densityn and mass density .W e discuss how to self{consistently match FOF parameters among dierent galaxy samples. We then separately vary several FOF and sample parameters, and discuss their ef- fect on group properties. Loose groups in PPS nicely trace the large scale structure (LSS) in the parent galaxy sam- ple. The group properties vary little with dierent redshift corrections, redshift cut{o, and galaxy LF, but are rather sensitive to the adopted links D0 and V0. More precisely, the typical group size (velocity dispersion) is linearly re-

Study of a Slice at +9° to +15° of Declination. I. The Neutral Hydrogen Content of Galaxies in Loose Groups

We examine the H1 content of spiral galaxies in groups by using a catalog of loose groups of galaxies identified in a magnitude limited sample m < 15.7 spanning the range 8 h to 18 h in right ascension and +9 to +15 in declination. The redshift completeness of the galaxy sample is ~95%. No significant effect of H1 depletion is found, although there may be a hint that the earliest type spirals are slightly deficient.

The Discovery of a Small Group of Galaxies in Ursa Major by the Hubble Space Telescope

view Abstract Citations References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Discovery of a Small Group of Galaxies in Ursa Major by the Hubble Space Telescope Malumuth, Eliot M. ; Sandoval, Jennifer L. Abstract We present HST/WFPC2 images of a previously unknown loose group of galaxies in Ursa Major. These images were obtained as part of the GTO parallel program in which WFPC2 images are taken using the F606W (wide V) and F814W (I band) filters in parallel with GTO observations using other HST instruments. These WFPC2 images, obtained in parallel with FGS observations of BD +67552^deg^ on 1994 April 22 contain a number of galaxies between 4" and 11" in diameter as well as a multitude of other, smaller galaxies. The group is dominated by eight spiral galaxies and one elliptical galaxy in an area smaller than one Wide Field Camera chip (80"X 80"). We present V and I band photometry, V-I colors, and V-I color gradients of each galaxy in the group. The luminosity function provides evidence that the smaller, fainter galaxies are background to the group. We estimate the redshift of the cluster to be z ~ 0.2(H_0_ = 75 and q_0_ = 0). Publication: The Astronomical Journal Pub Date: September 1996 DOI: 10.1086/118061 Bibcode: 1996AJ....112..887M Keywords: GALAXIES: CLUSTERING; GALAXIES: DISTANCES AND REDSHIFTS full text sources ADS | data products SIMBAD (12) NED (9) MAST (1) ESA (1)

A 2572 and HCG 94 - galaxy clusters but not as we know them: an X-ray case study of optical misclassifications

We present the results of a spectral-imaging analysis of X-ray data obtained with the Position Sensitive Proportional Counter aboard the ROSAT Observatory in a 32 ks pointed observation of Hickson's Compact Group (HCG) #94. Besides HCG94, A2572, a richness class 0 Abell cluster, is also contained in the central region of the field of view. Both systems are at a redshift of z~0.04 and are falling toward each other at a velocity of about 1000 km/s. Their three-dimensional spatial separation is probably of the order of an Abell radius; however, as yet, no clear signs of dynamical interaction are discernible in the X-ray. We find HCG94's gas temperature and unabsorbed X-ray luminosity to be far too high for a galaxy group thereby confirming the claim of Ebeling, Voges & B"ohringer (1994) that HCG94 should be classified as a galaxy cluster. The opposite is true for the Abell cluster A2572, the optical richness of which has been overestimated due to the inclusion of HCG94. In the X-ray, A2572 appears at first sight like the prototypical binary cluster with two equally massive and X-ray bright subclusters in the process of merging. However, the available X-ray, optical, and radio data strongly suggest that A2572 proper is in fact merely a loose group of galaxies, while the second component is a much richer and more distant cluster seen in superposition. A deprojection analysis shows HCG94 to host a moderate cooling flow; this picture is supported by a radial increase in the column density of absorbing material and a decrease in the gas temperature toward the cluster centre. HCG94's total gravitating mass is much higher than what could be anticipated from its appearance in the optical. Our findings hence underline the need for

The redshift-space neighborhoods of 36 loose groups of galaxies. 1: The data

We have selected 36 loose groups of galaxies (RGH89) with at least five members, and with mean redshift average value of CZ is greater than 3200 km/s. These groups all lie within the first two slices of the CfA redshift survey 8(sup h) less than or equal to alpha less than or equal to 17(sup h) and 26.5 deg less than or equal to delta less than or equal to 38.5 deg). For each of these groups, we define the redshift-space neighborhood as a region centered on the group coordinates and delimited by a circle of projected radius R(sub cir) = 1.5/h Mpc on the sky, and by a velocity interval delta (sub cz) = 3000 km/s. Here we give the redshifts of 334 galaxies in these redshift-space neighborhoods. For completeness, we also give the redshifts of the 232 original members. These data include 199 new redshifts. We demonstrate that these samples of fainter galaxies significantly increase the number of members.

Hydrogen and metal absorption lines in PKS 0405-123 from the halos of low redshift galaxies

HST ultraviolet spectra and ground-based optical spectra of the bright quasar PKS 0405-123 are used to identify two absorption-line systems probably produced by the halos of foreground galaxies. The quasar lies in a rich galaxy field. Loose groups of galaxies are present in the field at mean z1 = 0.1669 and mean z2 = 0.3519. Ly-alpha, C IV 1549 A, and probably Mg II 2798 A resonance lines are detected in an absorption system at z = 0.1670. An absorption system consisting of only Ly-alpha is observed at z = 0.3516. We cannot be completely certain which foreground galaxy is responsible for the metal line absorption system at z1. A likely candidate is a large, luminous, early-type spiral 40 arcsec to the east of the quasar. However, the origin of the z1 system could possibly be a small undetected galaxy more precisely aligned along our line of sight to PKS 0405-123, or a very faint galaxy whose redshift has not yet been measured.

The neighbourhoods of the nearest hickson groups

AbstractThe comparison of the spatial distribution of galaxies in the surroundings of the nearest compact and loose groups of galaxies confirms that compact groups are very likely to be part of looser groups and shows that loose groups associated to compact groups tend to have higher spatial density than normal ones.

On the properties of galaxies in groups

view Abstract Citations (29) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Properties of Galaxies in Groups Maia, M. A. G. ; da Costa, L. Nicolaci Abstract We make use of the recently compiled catalog of groups of galaxies in the southern hemisphere to examine properties of galaxies in loose groups. Several statistical tests are applied to the data to evaluate the possible role of physical processes, which could influence the evolution of the individual members and explain the present-day structure of the galaxies. Among the properties examined are the relations morphology-density and morphology-velocity dispersion, the level of Hα emission, and the size of first-ranked galaxies. When selection effects in the observed sample are properly taken into account, we find no strong evidence that would indicate the presence of evolutionary processes expected to occur in virial equilibrium systems. Publication: The Astrophysical Journal Pub Date: April 1990 DOI: 10.1086/168551 Bibcode: 1990ApJ...352..457M Keywords: Astronomical Catalogs; Emission Spectra; Galactic Clusters; Galactic Evolution; Galactic Structure; H Alpha Line; Southern Hemisphere; Statistical Analysis; Astrophysics; GALAXIES: CLUSTERING; GALAXIES: EVOLUTION; GALAXIES: STRUCTURE full text sources ADS | data products NED (87)