Past Merger Research Articles

The globular clusters-stellar haloes connection in early-type galaxies

This paper explores if, and to what an extent, the stellar populations of early type galaxies can be traced through the colour distribution of their globular cluster systems. The analysis, based on a galaxy sample from the Virgo ACS data, is an extension of a previous approach that has been successful in the cases of the giant ellipticals NGC 1399 and NGC 4486, and assumes that the two dominant GC populations form along diffuse stellar populations sharing the cluster chemical abundances and spatial distributions. The results show that a) Integrated galaxy colours can be matched to within the photometric uncertainties and are consistent with a narrow range of ages; b) The inferred mass to luminosity ratios and stellar masses are within the range of values available in the literature; c) Most globular cluster systems occupy a thick plane in the volume space defined by the cluster formation efficiency, total stellar mass and projected surface mass density. The formation efficiency parameter of the red clusters shows a dependency with projected stellar mass density that is absent for the blue globulars. In turn, the brightest galaxies appear clearly detached from that plane as a possible consequence of major past mergers; d) The stellar mass-metallicity relation is relatively shallow but shows a slope change at $M_*\approx 10^{10} M_\odot$. Galaxies with smaller stellar masses show predominantly unimodal globular cluster colour distributions. This result may indicate that less massive galaxies are not able to retain chemically enriched intestellar matter.

Spatial Distributions of Dust and Polycyclic Aromatic Hydrocarbons in the Nearby Elliptical Galaxy NGC4589 Observed with AKARI

Abstract We report on near- to far-infrared (IR) observations of the nearby elliptical galaxy NGC4589 by AKARI. NGC4589 has a complex stellar rotation field with a position angle difference of 45$^{\circ}$ between the kinematic and morphological axes, although its morphology shows a smooth optical profile that closely follows the de Vaucouleurs law. The galaxy has a minor-axis dust lane, which is twisted toward the main stellar rotation. These properties strongly support that the galaxy is a relatively old merger remnant. We have obtained the spatial distributions of dust from far-IR emission and near-IR absorption, separately. We have also estimated the distribution of polycyclic aromatic hydrocarbons (PAHs) from the difference between images in the mid-IR bands. The spatial distributions of the dust and PAHs resemble each other, both showing significant deviations from the smooth stellar distribution. They extend to the main stellar rotation and the direction of the twisted inner dust lane, thus following the relics of the merger. Therefore, a large fraction of the dust and PAHs in NGC4589 are likely to originate from a past merger event.

THE ASSEMBLY OF GALAXY CLUSTERS

We study the formation of fifty-three galaxy cluster-size dark matter halos (M = 10{sup 14.0-14.76} M{sub {circle_dot}}) formed within a pair of cosmological {Lambda}CDM N-body simulations, and track the accretion histories of cluster subhalos with masses large enough to host {approx} 0.1L{sub *} galaxies. By associating subhalos with cluster galaxies, we find the majority of galaxies in clusters experience no 'pre-processing' in the group environment prior to their accretion into the cluster. On average, {approx} 70% of cluster galaxies fall into the cluster potential directly from the field, with no luminous companions in their host halos at the time of accretion; and less than {approx} 12% are accreted as members of groups with five or more galaxies. Moreover, we find that cluster galaxies are significantly less likely to have experienced a merger in the recent past ({approx}< 6 Gyr) than a field halo of the same mass. These results suggest that local, cluster processes like ram-pressure stripping, galaxy harassment, or strangulation play the dominant role in explaining the difference between cluster and field populations at a fixed stellar mass; and that pre-evolution or past merging in the group environment is of secondary importance for setting cluster galaxy properties for mostmore » clusters. The accretion times for z = 0 cluster members are quite extended, with {approx} 20% incorporated into the cluster halo more than 7 Gyr ago and {approx} 20% within the last 2 Gyr. By comparing the observed morphological fractions in cluster and field populations, we estimate an approximate time-scale for late-type to early-type transformation within the cluster environment to be {approx} 6 Gyr.« less

Formation of the Small Magellanic Cloud: An Ancient Major Merger as a Solution to the Kinematical Differences between Old Stars and H i Gas

Recent observations of the Small Magellanic Cloud (SMC) have revealed that the H I gas shows a significant amount of rotation (Vc ~ 60 km s−1), while no or little rotation is evident for the old stellar populations. We suggest that this unique kinematical difference between these components in the SMC can be caused by a major merger event which occurred in the early stage of the SMC formation. Our simulations show that dissipative dwarf-dwarf merging can transform two gas-rich dwarf irregulars into a new dwarf, which consists of a spheroidal stellar component and a rotating extended H I disk. The remnant of this dwarf-dwarf merging shows significantly different kinematics between stars and gas, in the sense that a gas disk rotates rapidly while a stellar component shows little rotation. We thus suggest that the simulated dwarf having a dynamically hot spheroid and an extended gas disk finally evolves into the present SMC after efficient stripping of the outer gas via tidal fields of the Galaxy and the Large Magellanic Cloud. We also suggest that spatial distributions and kinematics of RGB and AGB stars with different ages in the possible spheroidal component of the SMC can provide valuable information on whether and when a past major merger event really occurred in the SMC.

The different physical mechanisms that drive the star formation histories of giant and dwarf galaxies

We present an analysis of star formation and nuclear activity in galaxies as a function of both luminosity and environment in the fourth data release of the Sloan Digital Sky Survey. Using a sample of 27 753 galaxies in the redshift range 0.005 < z < 0.037 that is ≥90 per cent complete to M r = -18.0, we find that the Ha equivalent width, EW(Hα), distribution is strongly bimodal, allowing galaxies to be robustly separated into passively evolving and star-forming populations about a value EW(Ha) = 2 A. In high-density regions ∼70 per cent of galaxies are passively evolving independent of luminosity. In the rarefied field, however, the fraction of passively evolving galaxies is a strong function of luminosity, dropping from 50 per cent for M r ≤ -21 to zero by M r ∼ -18. Indeed for the lowest luminosity range covered (-18 < M r < -16) none of the ∼600 galaxies in the lowest-density quartile is passively evolving. The few passively evolving dwarf galaxies in field regions appear as satellites to bright (≥ L*) galaxies. We find a systematic reduction of ∼30 per cent in the Ha emission from dwarf (-19 < M r < - 18) star-forming galaxies in high-density regions with respect to field values, implying that the bulk of star-forming dwarf galaxies in groups and clusters are currently in the process of being slowly transformed into passive galaxies. The fraction of galaxies with the optical signatures of an active galactic nucleus (AGN) decreases steadily from ∼50 per cent at M r ∼ -21 to ∼0 per cent by M r ∼ -18 closely mirroring the luminosity dependence of the passive galaxy fraction in low-density environments. This result reflects the increasing importance of AGN feedback with galaxy mass for their evolution, such that the star formation histories of massive galaxies are primarily determined by their past merger history. In contrast, the complete absence of passively evolving dwarf galaxies more than ∼2 virial radii from the nearest massive halo (i.e. cluster, group or massive galaxy) indicates that internal processes, such as merging, AGN feedback or gas consumption through star formation, are not responsible for terminating star formation in dwarf galaxies. Instead the evolution of dwarf galaxies is primarily driven by the mass of their host halo, probably through the combined effects of tidal forces and ram-pressure stripping.

Quest for Efficiency: Assessing the Impact of Mergers on Performance in the US Telecommunications Industry

We evaluate the impact of the various mergers of the local exchange companies that took place between 1988 and 2001 on several measures of performance of the firms that have undergone the mergers. Our analysis reveals that relative cash flows decrease after mergers, the pattern of accompanying sales growth is ambiguous and driven by increased market presence while the impact of mergers on the measures of efficiency and synergy are negative. If the efficiency motive is primary in influencing merger approval, then the past mergers approved have led to inefficiencies and welfare losses for the American consumer and the mergers of communication common carriers have not been in the public interest. On the other hand, given the inefficiency outcomes views that the quiet life, hubris and a quest for possible market power have motivated the mergers cannot be discarded.

Evidence for Tidal Interactions and Mergers as the Origin of Galaxy Morphology Evolution in Compact Groups

We present the results of a morphological study based on NIR images of 25 galaxies, with different levels of nuclear activity (star formation or AGN), in eight compact groups (CGs) of galaxies. We independently perform two different analyses: a study of the deviations of the isophotal levels from pure ellipses and a study of morphological asymmetries. The results yielded by the two analyses are highly consistent. For the first time, it is possible to show that deviations from pure ellipses are produced by inhomogeneous stellar mass distributions related to galaxy interactions and mergers. We find evidence of mass asymmetries in 74% of the galaxies in our sample. In 59% of these cases, the asymmetries come in pairs and are consistent with tidal effects produced by the proximity of companion galaxies. The symmetric galaxies are generally small in size or mass and inactive, and have an early-type morphology. They may have already lost their gas and least-attached envelope of stars to their more massive companions. In 20% of the galaxies we find evidence for cannibalism: a big galaxy swallowing a smaller companion. In 36% of the early-type galaxies the color gradient is positive (blue nucleus) or flat. Summing up these results, as much as 52% of the galaxies in our sample could show evidence of an ongoing or past merger. Our observations also suggest that galaxies in CGs merge more frequently under dry conditions (that is, once they have lost most of their gas). The high frequency of interacting and merging galaxies observed in our study is consistent with the bias of our sample toward CGs of type B, which represent the most active phase in the evolution of the groups. In these groups we also find a strong correlation between asymmetries and nuclear activity in early-type galaxies. This correlation allows us to identify tidal interactions and mergers as the cause of galaxy morphology transformation in CGs.

Large-scale HI in nearby radio galaxies: segregation in neutral gas content with radio source size

We present results of a study of neutral hydrogen (HI) in a complete sample of nearby non-cluster radio galaxies. We find that radio galaxies with large amounts of extended HI (M_HI >= 10^9 M_solar) all have a compact radio source. The host galaxies of the more extended radio sources, all of Fanaroff & Riley type-I, do not contain these amounts of HI. We discuss several possible explanations for this segregation. The large-scale HI is mainly distributed in disk- and ring-like structures with sizes up to 190 kpc and masses up to 2 x 10^10 M_solar. The formation of these structures could be related to past merger events, although in some cases it may also be consistent with a cold-accretion scenario.

The Effects of Past Entry, Market Consolidation, and Expansion by Incumbents on the Probability of Entry

The threat of entry is an important factor in the evaluation of the potential competitive effects of proposed mergers and acquisitions. In the evaluation of proposed bank mergers, a high probability of entry, or strong potential competition, is often found to mitigate the potential anticompetitive effect of a proposed horizontal merger. Because the probability of entry is not directly observed for each local market, variables such as per capita income, population growth and past entry are typically used to predict the probability of future entry. This study extends previous research on the determinants of entry into local banking markets. In addition to variables considered by past research, such as market demographic characteristics, branching deregulation and past merger activity, this study considers the effects on future entry of past entry and strategic barriers to entry, which are proxied by changes in incumbent branching, the presence of small incumbent firms and market concentr

A cluster in a crowded environment: XMM-Newton and Chandra observations of A3558

Combining XMM-Newton and Chandra data, we have performed a detailed study of Abell 3558. Our analysis shows that its dynamical history is more complicated than previously thought. We have found some traits typical of cool core clusters (surface brightness peaked at the center, peaked metal abundance profile) and others that are more common in merging clusters, like deviations from spherical symmetry in the thermodynamic quantities of the ICM. This last result has been achieved with a new technique for deriving temperature maps from images. We have also detected a cold front and, with the combined use of XMM-Newton and Chandra, we have characterized its properties, such as the speed and the metal abundance profile across the edge. This cold front is probably due to the sloshing of the core, induced by the perturbation of the gravitational potential associated with a past merger. The hydrodynamic processes related to this perturbation have presumably produced a tail of lower entropy, higher pressure and metal rich ICM, which extends behind the cold front for 500 kpc. The unique characteristics of A3558 are probably due to the very peculiar environment in which it is located: the core of the Shapley supercluster.

Non-Uniform Temperature Distribution in the Galaxy Clusters 2A 0335+096 and Abell 496 Observed by XMM-Newton

Abstract We present the XMM-Newton results of the galaxy clusters 2A 0335$+$096 and A496. From an investigation of 2-dimensional ICM profiles, we obtained new features related to dynamical activities in the ICM. In 2A 0335$+$096, we found a spiral cool tail around the central cold core, which is trailing from the opposite side of the cold front observed by Chandra. The deprojected temperature of the tail is 1.7 keV. The spiral feature apparently shows a turnaround motion of the core. We suggest that the cool tail feature represents evidence for turbulent viscous stripping of the remaining subcluster core. For A496, we found another cold front $4^{\prime}$ south of the X-ray emission peak. From the pressure profile across the cold front, we derived an upper limit to the Mach number of a gas body of 0.2. Regarding the temperature distribution in the outer region, we found non-uniformity on the cluster scale for both clusters, which suggests that both clusters experienced a merger in the past. We argue that the gas dynamical motions seen in the central regions of both clusters can be attributed to the past merger.

Halo Wide Binaries and Moving Clusters as Probes of the Dynamical and Merger History of our Galaxy

AbstractWide or fragile pairs are sensitive probes of the galactic potential, and they have been used to provide information about the galactic tidal field, the density of GMC and the masses of dark matter perturbers present in both the disk and the halo. Halo wide binaries and moving clusters, since they are likely to be the remains of past mergers or of dissolved clusters, can provide information on the dynamical and merger history of our Galaxy. Such remnants should continue to show similar motions over times of the order of their ages. We have looked for phase space groupings among the low-metallicity stars of Schuster et al (2006) and have identified a number of candidate moving clusters. In several of the moving clusters we found a wide CPM binary already identified in our catalogue of wide binaries among high-velocity and metal-poor stars (Allen et al 2000a). Spectroscopic follow-up studies of these stars would confirm the physical reality of the groups, as well as allow us to distinguish whether their progenitors are dissolved clusters or accreted extragalactic systems.

Asymmetrical structure of ionization and kinematics in the Seyfert galaxy NGC 5033

We present integral field spectroscopy of NGC 5033, a low luminosity Seyfert galaxy. The observations were made with INTEGRAL, a fiber based system operating at the WHT. The intensity map of the H emission line represents a spiral or ring-like pattern of HII regions. On the contrary, the [OIII] intensity map morphology is markedly anisotropic. The strong morphological differences imply that the [ OIII] emitters represent highly ionized gas illuminated by the central source. The [ OIII] map morphology is compatible with a biconical structure of ionization induced by strong extinction in the galaxy disc that also obscures half of the spheroidal stellar bulge. We identify the spectrum corresponding to the Seyfert 1 nucleus from the presence of H broad emission lines. This spectrum is located in a region where strong extinction is expected but exhibits the bluest spectral energy distribution. The Seyfert 1 nucleus seems to be off center with respect to the stellar rotation center. This result has been also found in other Seyfert galaxies and interpreted in terms of a past merger. The off centering could indicate the presence of nonsymmetric departures in the gravitational potential which could be fueling the active nucleus. The kinematics of the [ OIII] emitters show important deviations at a kpc scale with respect to the stellar velocity field and show features related to the asymmetrical morphology of the high ionization region.

Galaxy morphology – halo gas connections

We studied a sample of 38 intermediate redshift MgII absorption-selected galaxies using (1) Keck/HIRES and VLT/UVES quasar spectra to measure the halo gas kinematics from MgII absorption profiles and (2) HST/WFPC-2 images to study the absorbing galaxy morphologies. We have searched for correlations between quantified gas absorption properties, and host galaxy impact parameters, inclinations, position angles, and quantified morphological parameters. We report a 3.2-sigma correlation between asymmetric perturbations in the host galaxy morphology and the MgII absorption equivalent width. We suggest that this correlation may indicate a connection between past merging and/or interaction events in MgII absorption-selected galaxies and the velocity dispersion and quantity of gas surrounding these galaxies.

Integral Field Spectroscopy of the Central Regions of 3C 120: Evidence of a Past Merging Event

Optical integral field spectroscopy (IFS), combined with Hubble Space Telescope (HST) WFPC imaging, was used to characterize the central regions of the Seyfert 1 radio galaxy 3C 120. We carried out the analysis of the data, deriving intensity maps of different emission lines and the continua at different wavelengths from the observed spectra. Applying a two-dimensional modeling to the HST images, we decoupled the nucleus and the host galaxy and analyzed the host morphology. The host is a highly distorted bulge-dominated galaxy, rich in substructures. We developed a new technique to model the IFS data extending the two-dimensional modeling (hereafter three-dimensional modeling). Using this technique, we separated the Seyfert nucleus and the host galaxy spectra and derived a residual data cube with spectral and spatial information of the different structures in 3C 120. Three continuum-dominated structures (named A, B, and C) and three other extended emission-line regions (EELRs, named E1, E2, and E3) are found in 3C 120, which does not follow the general behavior of a bulge-dominated galaxy. We also found shells in the central kiloparsec that may be remnants of a past merging event in this galaxy. The origin of E1 is most probably due to the interaction of the radio jet of 3C 120 with the intergalactic medium (Axon et al. 1989; Sanchez et al. 2004a). Structures A, B, and the shell at the southeast of the nucleus seem to correspond to a larger morphological clumpy structure that may be a tidal tail, a consequence of the past merging event. We found a bright EELR (E2) in the innermost part of this tidal tail, nearby the nucleus, which shows a high ionization level. The kinematics of the E2 region and its connection to the tidal tail suggest that the tail has channeled gas from the outer regions to the center.

The Complex Cooling Core of A2029: Radio and X‐Ray Interactions

We present an analysis of Chandra observations of the central regions of the cooling flow cluster A2029. We find a number of X-ray filaments in the central 40 kpc, some of which appear to be associated with the currently active central radio galaxy. The outer southern lobe of the steep-spectrum radio source appears to be surrounded by a region of cool gas and is at least partially surrounded by a bright X-ray rim similar to that seen around radio sources in the cores of other cooling flow clusters. Spectroscopic fits show that the overall cluster emission is best fitted by either a two-temperature gas (kThigh = 7.47 keV and kTlow = 0.11 keV) or a cooling flow model with gas cooling over the same temperature range. This large range of temperatures (over a factor of 50) is relatively unique to A2029 and may suggest that this system is a very young cooling flow in which the gas has only recently started cooling to low temperatures. The cooling flow model gives a mass deposition rate of = 56 M☉ yr-1. In general, the cluster emission is elongated along a position angle of 22° with an ellipticity of 0.26. The distribution of the X-ray emission in the central region of the cluster is asymmetric, however, with excess emission to the northeast and southeast compared with that to the southwest and northwest, respectively. Fitting and subtracting a smooth elliptical model from the X-ray data reveals a dipolar spiral excess extending in a clockwise direction from the cluster core to radii of ~150 kpc. We estimate a total mass of Mspr ~ 6 × 1012 M☉ in the spiral excess. The most likely origin of the excess is either stripping of gas from a galaxy group or from bare dark matter potential that has fallen into the cluster or sloshing motions in the cluster core induced by a past merger.

More evidence of substructure in the motions of nearby halo stars

We explore the stellar halo of the Milky Way to search for fossil signatures of past mergers. We use the Beers et al. (2000) catalog of non-kinematically selected metal poor stars in the solar neighborhood to select subsets of halo stars within 1-2 kpc of the Sun. Motivated by the results of high resolution CDM simulations, we look for substructure in the kinematics of the fastest stars. When a two-point velocity correlation function is applied to these subsets, statistical evidence of substructure is found. This appears to be due to a small moving group with dynamical properties similar to the stellar “stream” previously discovered by Helmi et al. (1999).To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The kinematic properties of isolated elliptical galaxies

Ellipticals in very low density environments are extremely rare but hold important clues about galaxy formation and evolution. We present results of an investigation of their internal dynamics. We observe a dichotomy in kinematic properties similar to that in normal ellipticals. We compare the positions of isolated ellipticals with respect to the Fundamental Plane of normal ellipticals and find that they fall onto it, with the exception of NGC 2865. This shows that isolated ellipticals are structurally similar to normal ellipticals, and do not have very young ages, although some have signs of past mergers or interactions.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Will the Proposed Application of Basel II in the United States Encourage Increased Bank Merger Activity? Evidence from Past Merger Activity

This paper presents two tests of the hypothesis that adoption of the internal ratings-based approach to determining minimum capital requirements, as proposed in applying the Basel II capital accord in the United States, will cause adopting banking organizations to increase acquisition activity. The first test estimates the relationship between excess regulatory capital and subsequent merger activity, including organization and time fixed effects, while the second test employs a "difference in difference" analysis of the change in merger activity that occurred the last time regulatory capital standards were changed. Estimated coefficients and observed differences have signs consistent with the hypothesis, but results are either statistically insignificant or imply differences that are small in magnitude.

Dynamics of the Globular Cluster System Associated with M49 (NGC 4472): Cluster Orbital Properties and the Distribution of Dark Matter

Using the Low Resolution Imaging Spectrometer on the Keck I and II telescopes, we have measured radial velocities for 196 globular clusters (GCs) around M49 (NGC 4472), the brightest member of the Virgo Cluster. Combined with published data, they bring the total number of GCs with measured radial velocities in this galaxy to 263. In terms of sample size, spatial coverage, velocity precision, and the availability of metallicity estimates from Washington photometry, this radial velocity database resembles that presented recently for M87 (NGC 4486), Virgo's cD galaxy and its second-ranked member. We extract the projected kinematics of the full sample of GCs and of separate subsamples of 158 metal-poor and 105 metal-rich GCs. In agreement with previous results for the global GC kinematics based on smaller data sets, we find that the GC system as a whole exhibits a slow overall rotation that is due almost entirely to a net rotation of the metal-poor GC subsystem alone. In a spatial average, the metal-rich GCs show essentially no rotation. As a function of galactocentric position, the metal-poor GCs rotate roughly about the photometric minor axis of M49 and at an approximately constant level of ΩR ~ 100-150 km s-1 out to R 2Reff. The metal-rich GC system shows some evidence (at roughly 1 σ significance) for weak rotation (ΩR ~ 50 km s-1) beyond R 0.5Reff, also about the galaxy's minor axis, but in the opposite direction from the metal-poor GCs. Outside of R ~ Reff, the line-of-sight velocity dispersion of the metal-poor GCs exceeds that of their metal-rich counterparts by ~50%. We also note the presence of a well-defined grouping of 10 metal-rich GCs that are located at opposite poles along the galaxy's major axis and that appear to be rotating at nearly 300 km s-1 about the minor axis. This grouping may be the relic of a past merger or accretion event. The dynamics of the GC system is modeled by using published catalogs and number counts to define three-dimensional GC density distributions as input to a Jeans equation analysis. We show that the GC radial velocities alone point unequivocally, and independently of X-ray observations, to the need for a massive dark halo associated with M49 and the Virgo B subcluster around it. We then use a mass model for M49/Virgo B, constructed without reference to any GC data and described in detail in a forthcoming paper, to infer the orbital properties of the M49 globulars. The GC system as a whole is shown to be consistent with an almost perfectly isotropic velocity ellipsoid. It is more difficult to draw any firm conclusions on the orbital (an)isotropy of the two metallicity subsamples, as a result of the large uncertainties in their individual spatial density profiles and the poorly observationally defined kinematics of the metal-rich GCs in particular. After M87, M49 is the second elliptical galaxy for which we have been able to demonstrate velocity isotropy in the GC system overall, when no division based on GC color or metallicity is attempted. Thus, the data for these two galaxies lend support to the general assumption of isotropy when using GC kinematics to study the dark matter distribution in early-type galaxies. We also compare the kinematic properties of the GC system of M49 to those of M87, M31, and the Milky Way, the other galaxies for which samples of 100 or more GC velocities have been accumulated. We argue that, contrary to the traditional view of GCs as nonrotating, or slowly rotating, systems, rotation may in fact be a common by-product of the formation of GC systems. However, the quantitative details of the rotation are still not clear, particularly with regard to the question of possible differences between metal-poor and metal-rich globulars.