Dry Mergers Research Articles

On the survival of metallicity gradients to major dry-mergers

Using a large suite of galaxies with a variety of concentrations and metallicity gradients, we study the evolution of non-dissipative (“dry”) equal mass mergers. Our purpose in generating these simulations is to explore how the metallicity gradient in dry mergers depends on the structure and metallicity gradients of the galaxies involved in the merger. Specifically, we would like to answer: Could dry mergers lead to metallicity gradients as observed in elliptical galaxies in the local Universe? Do dry mergers always lead to a flattening of the initial (i.e., pre-merger) gradient? From this modeling, we conclude that: The ratio of the remnant and the initial galaxy slopes span a wide range of values, up to values greater than 1 (with values greater than one resulting only when companions have gradients twice the progenitor). For a merger between two ellipticals having identical initial metallicity slopes (i.e., equal companion and galaxy slopes), the metallicity profile of the remnant flattens, with a final gradient about 0.6 times the initial one. Ellipticals can maintain their original pre-merger metallicity gradient if the companion slope is sufficiently steep. The amount of flattening neither depends on the characteristics of the orbit of the progenitors or on their initial concentration. Given the diversity in outcomes of the mergers, we conclude that dry mergers do not violate any observational constraints on the systematic characteristics of metallicity gradients in local ellipticals. In fact, dry mergers may be important within the context of the results of our simulations and may explain the large scatter in gradients for massive ellipticals and the relative paucity of massive ellipticals with no or shallow metallicity gradients.

The VIMOS VLT Deep Survey

<i>Context. <i/>The rate at which galaxies grow via successive mergers is a key element in understanding the main phases of galaxy evolution.<i>Aims. <i/>We measure the evolution of the fraction of galaxies in pairs and the merging rate since redshift <i>z<i/> ~ 1 assuming a (<i>H<i/><sub>0<sub/> = 70 km s<sup>-1<sup/> Mpc<sup>-1<sup/>, = 0.3 and = 0.7) cosmology.<i>Methods. <i/>From the VIMOS VLT Deep Survey we use a sample of 6464 galaxies with <i>I<i/><sub>AB<sub/><i>≤<i/> 24 to identify 314 pairs of galaxies, each member with a secure spectroscopic redshift, which are close in both projected separation and in velocity.<i>Results. <i/>We estimate that at <i>z<i/> ~ 0.9, 10.9 <i>±<i/> 3.2% of galaxies with <i>≤<i/> -18-Qz (<i>Q<i/> = 1.11) are in pairs with separations <i>≤<i/> 20 h<sup>-1<sup/> kpc, <i>≤<i/> 500 km s<sup>-1<sup/>, and with <i>≤<i/> 1.5, significantly larger than 3.8 <i>±<i/> 1.7% at <i>z<i/> ~ 0.5; thus, the pair fraction evolves as (1 + <i>z<i/>)<sup><i>m<i/><sup/> with <i>m<i/> = 4.73 <i>±<i/> 2.01. For bright galaxies with <i>≤<i/> -18.77, the pair fraction is higher and its evolution with redshift is flatter with <i>m<i/> = 1.50 <i>±<i/> 0.76, a property also observed for galaxies with increasing stellar masses. Early-type pairs (dry mergers) increase their relative fraction from 3% at <i>z<i/> ~ 0.9 to 12% at <i>z<i/> ~ 0.5. The star formation rate traced by the rest-frame [OII] <i>EW<i/> increases by 26 <i>±<i/> 4% for pairs with the smallest separation <i>r<i/><sub>p<sub/><i>≤<i/> 20 h<sup>-1<sup/> kpc. Following published prescriptions to derive merger timescales, we find that the merger rate of -18-Qz galaxies evolves as <i>N<i/><sub>mg<sub/> = (4.96 <i>±<i/> 2.07)10<sup>-4<sup/> mergers Mpc<sup>-3<sup/> Gyr<sup>-1<sup/>.<i>Conclusions. <i/>The merger rate of galaxies with <i>≤<i/> -18-Qz has significantly evolved since <i>z<i/> ~ 1 and is strongly dependent on the luminosity or stellar mass of galaxies. The major merger rate increases more rapidly with redshift for galaxies with fainter luminosities or stellar mass, while the evolution of the merger rate for bright or massive galaxies is slower, indicating that the slow evolution reported for the brightest galaxies is not universal. The merger rate is also strongly dependent on the spectral type of galaxies involved. Late-type mergers were more frequent in the past, while early-type mergers are more frequent today, contributing to the rise in the local density of early-type galaxies. About 20% of the stellar mass in present day galaxies with <i>≥<i/> 9.5 has been accreted through major merging events since <i>z<i/> = 1. This indicates that major mergers have contributed significantly to the growth in stellar mass density of bright galaxies over the last half of the life of the Universe.

Shape and kinematics of elliptical galaxies: evolution due to merging at z ${&lt;}$ 1.5

Aims:In this study we investigate the evolution of shape and kinematics of elliptical galaxies in a cosmological framework. Methods: We use a set of hydrodynamic, self-consistent simulations operating in the context of a concordance cosmological model where relaxed elliptical-like objects (ELOs) were identified at redshifts z=0, z=0.5, z=1 and z=1.5. Results: The population of elliptical systems analysed here present a systematic change through time, i.e. evolution, by becoming rounder in general at z=0 and, at the same time more velocity dispersion supported. This is found to be primarily due to major dry mergers where only a modest amount of angular momentum is involved into the merger event. Despite the general trend, in a significant amount of cases the merger event involves a higher specific angular momentum, which in general causes the system to acquire a higher rotational support and/or a more oblate shape. These evolutionary patterns are still present when we study our systems in projection, mimicking real observations, and thus they should become apparent in future observations.

UPPER LIMITS ON THE MASSES OF 105 SUPERMASSIVE BLACK HOLES FROMHUBBLE SPACE TELESCOPE/SPACE TELESCOPE IMAGING SPECTROGRAPH ARCHIVAL DATA

Based on the modeling of the central emission-line width measured over sub-arcsecond apertures with the Hubble Space Telescope, we present stringent upper bounds on the mass of the central supermassive black hole, MBH, for a sample of 105 nearby galaxies (D<100Mpc) spanning a wide range of Hubble types (E-Sc) and values of the central stellar velocity dispersion, sigma (58-419km/s). For the vast majority of the objects the derived MBH upper limits run parallel and above the well-known MBH-sigma relation independently of the galaxy distance, suggesting that our nebular line-width measurements trace rather well the nuclear gravitational potential. For values of sigma between 90 and 220km/s the 68% of our upper limits falls immediately above the MBH-sigma relation without exceeding the expected MBH values by more than a factor 4.1. No systematic trends or offsets are observed in this sigma range as a function of the galaxy Hubble type or with respect to the presence of a bar. For 6 of our 12 MBH upper limits with sigma<90km/s our line-width measurements are more sensitive to the stellar contribution to the gravitational potential, either due to the presence of a nuclear stellar cluster or because of a greater distance compared to the other galaxies at the low-sigma end of the MBH-sigma relation. Conversely, our MBH upper bounds appear to lie closer to the expected MBH in the most massive elliptical galaxies with values of sigma above 220km/s. Such a flattening of the MBH-sigma relation at its high-sigma end would appear consistent with a coevolution of supermassive black holes and galaxies driven by dry mergers, although better and more consistent measurements for sigma and K-band luminosity are needed for these kind of objects before systematic effects can be ruled out.

Evolution in the structural properties of early-type brightest cluster galaxies at small lookback time and dependence on the environment

At the present time, early-type brightest cluster galaxies (BCGs) in the Sloan Digital Sky Survey (SDSS) MaxBCG and C4 catalogues have larger sizes than early-type galaxies of similar luminosity, whether these other objects are in the field or are satellites in clusters. BCG sizes are also stronger functions of luminosity R e α L than are the sizes of the bulk of the population; this remains true if one restricts attention to narrow bins in velocity dispersion. At fixed stellar mass and formation time, objects at lower redshift are larger and have smaller velocity dispersions - i.e. the sizes increase and velocity dispersions decrease with age. In addition, at any given redshift, younger BCGs have slightly larger sizes than older BCGs of the same stellar mass; however, they have similar velocity dispersions. As a result, at redshifts ∼0.25, corresponding to lookback times of order 3 Gyr, BCGs are smaller than their lower redshift counterparts by as much as ∼70 per cent for the brightest BCGs: the sizes evolve as (1 + z) 0.85(Mr+21) . Qualitatively, similar but weaker evolution in the sizes is also seen in the bulk of the early-type population: at M r -22 the evolution is approximately (1 + z) ―0.7 , independent of M r . The velocity dispersion-luminosity correlation also evolves: (1 + z) ―0.2(Mr+21) at M r < ―22 (as for the BCGs) and (1 + z) 0.2 for fainter galaxies. The size- and velocity dispersion-stellar mass correlations yield consistent results, although, in this case, accounting for selection effects is less straightforward. These trends, in particular the fact that the velocity dispersions at fixed stellar mass decrease with age, are most easily understood if early-type BCGs grew from many dry minor mergers rather than a few major mergers. Only in such a scenario can BCGs be the descendents of the super-dense galaxies seen at z ∼ 2; major dry mergers, which increase the size in proportion to the mass, cannot bring these galaxies on to the BCG R e ― M * relation at z ∼ 0. We also compared the ages and sizes of our early-type BCGs with other cluster galaxies (satellites). BCGs are larger than satellites of similar luminosity or stellar mass at the same redshift. Although both satellites and BCGs trace the same weak age-L or age―M * relation, this can be understood by noting that BCGs are typically about 1 Gyr older than the satellites in their group, and they are about 0.5 mag more luminous. Finally, we find that the mean satellite luminosity is approximately independent of BCG luminosity, in agreement with recent predictions based on the luminosity dependence of clustering.

DISSIPATION AND EXTRA LIGHT IN GALACTIC NUCLEI. IV. EVOLUTION IN THE SCALING RELATIONS OF SPHEROIDS

We develop a model for the origins and redshift evolution of spheroid scaling relations. We consider spheroid sizes, velocity dispersions, masses, profile shapes (Sersic indices), and black hole (BH) masses, and their related scalings. Our approach combines advantages of observational constraints in halo occupation models and hydrodynamic merger simulations. This allows us to separate the relative roles of dissipation, dry mergers, formation time, and progenitor evolution, and identify their effects on scalings at each redshift. Dissipation is the most important factor determining spheroid sizes and fundamental plane (FP) scalings, and can account for the FP tilt and differences between disk and spheroid scalings. Because disks at high-z have higher gas fractions, mergers are more gas-rich, yielding more compact spheroids. This predicts mass-dependent evolution in spheroid sizes, in agreement with observations. This relates to subtle evolution in the FP, important to studies that assume a fixed intrinsic FP. This also predicts mild evolution in BH-host correlations, towards larger BHs at higher z. Dry mergers are significant, but only for massive systems which form early: they form compact, but undergo dry mergers (consistent with observations) such that their sizes at later times are similar to spheroids of similar mass formed more recently. We model descendants of observed compact high-z spheroids: most will become cores of BCGs, with sizes, velocity dispersions, and BH masses consistent with observations, but we identify a fraction that might survive to z=0 intact.

CORRELATIONS BETWEEN SUPERMASSIVE BLACK HOLES, VELOCITY DISPERSIONS, AND MASS DEFICITS IN ELLIPTICAL GALAXIES WITH CORES

High-dynamic-range surface photometry in a companion paper makes possible accurate measurement of the stellar light deficits L_def and mass deficits M_def associated with the cores of elliptical galaxies. We show that L_def correlates with the velocity dispersion sigma of the host galaxy bulge averaged outside the central region that may be affected by a supermassive black hole (BH). We confirm that L_def correlates with BH mass MBH. Also, the fractional light deficit L_def/L correlates with MBH/M, the ratio of BH mass to the galaxy stellar mass. All three correlations have scatter similar to or smaller than the scatter in the well known correlation between MBH and sigma. The new correlations are remarkable in view of the dichotomy between ellipticals with cores and those with central extra light. Core light deficit correlates closely with MBH and sigma, but extra light does not. This supports the suggestion that extra light Es are made in wet mergers with starbursts whereas core Es are made in dry mergers. After dry mergers, cores are believed to be scoured by BH binaries that fling stars away as their orbits decay or by BHs that sink back to the center after recoiling from anisotropic gravitational radiation emitted when they merge. Direct evidence has been elusive. We interpret the new correlations as the "smoking gun" that connects cores with BHs. Together, the MBH - sigma and MBH - L_def correlations give us two independent ways to estimate BH masses in core ellipticals.

THE MASS-METALLICITY GRADIENT RELATION OF EARLY-TYPE GALAXIES

We present a newly observed relation between galaxy mass and radial metallicity gradients of early-type galaxies. Our sample of 51 early-type galaxies encompasses a comprehensive mass range from dwarf to brightest cluster galaxies. The metallicity gradients are measured out to one effective radius by comparing nearly all of the Lick absorption-line indices to recent models of single stellar populations. The relation shows very different behaviour at low and high masses, with a sharp transition being seen at a mass of ~ 3.5 x 10^10 M_sun (velocity dispersion of ~140 km/s, M_B ~ -19). Low-mass galaxies form a tight relation with mass, such that metallicity gradients become shallower with decreasing mass and positive at the very low-mass end. Above the mass transition point several massive galaxies have steeper gradients, but a clear downturn is visible marked by a broad scatter. The results are interpreted in comparison with competing model predictions. We find that an early star-forming collapse could have acted as the main mechanism for the formation of low-mass galaxies, with star formation efficiency increasing with galactic mass. The high-mass downturn could be a consequence of merging and the observed larger scatter a natural result of different merger properties. These results suggest that galaxies above the mass threshold of ~ 3.5 x 10^10 M_sun might have formed initially by mergers of gas-rich disc galaxies and then subsequently evolved via dry merger events. The varying efficiency of the dissipative merger-induced starburst and feedback processes have shaped the radial metallicity gradients in these high-mass systems.

THE SPATIAL DISTRIBUTIONS OF RED AND BLUE GLOBULAR CLUSTERS IN MAJOR DRY MERGER REMNANTS

Using high-resolution N-body simulations, we examine whether a major dry merger mitigates the difference in the radial density distributions between red and blue globular clusters (GCs). To this end, we study the relation between the density slope of the GCs in merger progenitors and that in a merger remnant, when the density distribution is described by n GC r –α. We also study how our results depend on the merger orbit and the size of the core radius of the initial GC density distribution. We find that a major dry merger makes the GC profile flatter, and the steeper initial GC profile leads to more significant flattening, especially if the initial slope is steeper than α ~ 3.5. Our result suggests that if there is a major dry merger of elliptical galaxies whose red GCs have a steeper radial profile than the blue GCs, as currently observed, and their slopes are steeper than α ~ 3.5, the difference in the slopes between the two populations becomes smaller after dry mergers. Therefore, the observed slopes of red and blue GCs can be a diagnostic of the importance of dry merger. The current observational data show that the red and blue GCs have more comparable and shallower slopes in some luminous galaxies, which may indicate that they have experienced dry mergers.

EQUAL- AND UNEQUAL-MASS MERGERS OF DISK AND ELLIPTICAL GALAXIES WITH BLACK HOLES

We present binary galaxy merger simulations with varying mass ratios and different progenitor morphologies. The simulations include mergers of gas-rich disks (Sp-Sp), of early-type galaxies and disks (E-Sp, mixed mergers), and mergers of early-type galaxies (E-E, dry mergers). We follow the dynamics of gas, stars, and dark matter, and include radiative cooling, star formation and black hole (BH) accretion, and the associated feedback processes as in Springel et al. We study the mass assembly of the BHs and discuss technical issues of the implemented model in detail. For Sp-Sp mergers, the peak star formation rate and BH accretion rate decrease and the growth timescales of the central BHs and newly formed stars increase with higher progenitor mass ratios. The peak BH accretion rate typically occurs shortly after the time of BH merging for low progenitor mass ratios (e.g., 3:1 and lower), whereas for higher progenitor mass ratios there is no clear correlation between the peak BH accretion rate and the BH merging time. The termination of star formation by BH feedback in disk mergers is significantly less important for higher progenitor mass ratios (e.g., 3:1 and higher). In addition, the inclusion of BH feedback efficiently suppresses star formation in dry E-E mergers and mixed E-Sp mergers. All merger remnants, independent of their progenitors, follow the observed relations between the central BH mass and the stellar velocity dispersion (MBH − σ), the bulge mass (MBH − M*), and the bulge binding energy (MBH − M*σ2), with the dominant source of scatter arising from variations in the initial gas mass fraction. The normalizations for all relations and the simulated slope of the MBH − σ and MBH − M*σ2 relations are in good agreement with the observations, whereas the simulated slope of the MBH − M* relation is slightly steeper compared to the observations. This indicates that the simple BH feedback model self-regulates the mass growth of the BHs in accordance with the observed relations for a very wide range of merger progenitors.

Termination of star formation by BH feedback in equal‐ and unequal‐mass mergers of disk and elliptical galaxies

AbstractWe present binary galaxy merger simulations of gas‐rich disks (Sp‐Sp), of early‐type galaxies and disks (E‐Sp, mixed mergers), and mergers of early‐type galaxies (E‐E, dry mergers) with varying mass ratios and different progenitor morphologies. The simulations include radiative cooling, star formation and black hole (BH) accretion and the associated feedback processes. We find for Sp‐Sp mergers, that the peak star formation rate and BH accretion rate decrease and the growth timescales of the central black holes and newly formed stars increase with higher progenitor mass ratios. The termination of star formation by BH feedback in disk mergers is significantly less important for higher progenitor mass ratios (e.g. 3:1 and higher). In addition, the inclusion of BH feedback suppresses efficiently star formation in dry E‐E mergers and mixed E‐Sp mergers. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A MORPHOLOGICAL RE-EVALUATION OF GALAXIES IN COMMON FROM THE CATALOG OF ISOLATED GALAXIES AND THE SLOAN DIGITAL SKY SURVEY (DR6)

We present a re-evaluation of the optical morphology for 549 galaxies from the Catalog of Isolated Galaxies in the Northern Hemisphere (CIG) that are available in the Sloan Digital Sky Survey (SDSS; DR6). Both the high resolution and high dynamic range of the SDSS images and our semiautomatic image processing scheme allow for a major quality and uniform morphological analysis. The processing scheme includes (1) sky-subtracted, cleaned, and logarithmic scaled g-band images, (2) filtered-enhanced versions of the images in (1), and (3) the corresponding red-green-blue (RGB) composed images available in the SDSS database. We propose an empirical method to distinguishing between E, S0, and Sa candidates through an additional analysis of (4) the surface brightness, position angle, ellipticity and A 4 B 4 coefficients of the Fourier series expansion profiles. An atlas of mosaics containing (1), (2), and (3) images for Sab-Sm/Irr types and (1), (2), (3), (4) images for E/S0/Sa types was produced and is available on the Web. The median type in the sample corresponds to Sbc, with 65% of the sample being of this type or later. A scarce population of early-type E (3.5%) and S0 (5%) galaxies amounting to 8.5% and a non-negligible 14% fraction of early-type (SaSab) spirals are identified. We compare our results with a previous reclassification of the CIG based on the digitized Palomar Observatory Sky Survey II images. We calculate also the gri absolute magnitudes corrected by Galactic and internal extinctions and present the g – i color distribution and the color-magnitude diagram. Among the spirals, we find tentative fractions of strong and suspected bars of 65.8% and of 33.3% of rings. A detailed image analysis of the E galaxies (18) allows us to find a richness of distinct substructure in their isophotal shape and also of morphological distortions. At least 78% of the E galaxies show some kind of morphological distortion (shells, dust lanes, diffuse halos, etc.), suggesting that these galaxies suffered late dry mergers. The isophotes of 42% (37%) of the E galaxies are boxy (disky). Among four blue Es, three are disky. Finally, we calculate the CAS (concentration, asymmetry and clumpiness) structural parameters in the ugriz bands for the entire sample. We analyze the loci of these galaxies in different projections of the CAS volume diagram and discuss some trends of the CAS parameters with the color band, as well as with the morphological type and the galaxy color.

Evidence for powerful AGN winds at high redshift: dynamics of galactic outflows in radio galaxies during the “Quasar Era”

AGN feedback now appears as an attractive mechanism to resolve some of the outstanding problems with the standard cosmological models, in particular those related to massive galaxies. To directly constrain how this may influence the formation of massive galaxies near the peak in the redshift distribution of powerful quasars, z~2, we present an analysis of the emission-line kinematics of 3 powerful radio galaxies at z~2-3 (HzRGs) based on rest-frame optical integral-field spectroscopy obtained with SINFONI on the VLT. HzRGs are among the most massive galaxies, so AGN feedback may have a particularly clear signature. We find evidence for bipolar outflows in all HzRGs, with kinetic energies that are equivalent to 0.2% of the rest-mass of the supermassive black hole. Velocity offsets in the outflows are ~800-1000 km s^-1 between the blueshifted and redshifted line emission, FWHMs ~1000 km s^-1 suggest strong turbulence. Ionized gas masses estimated from the Ha luminosity are of order 10^10 M_s, similar to the molecular gas content of HzRGs, underlining that these outflows may indicate a significant phase in the evolution of the host galaxy. The total energy release of ~10^60 erg during a dynamical time of ~10^7 yrs corresponds to about the binding energy of a massive galaxy. Geometry, timescales and energy injection rates of order 10% of the kinetic energy flux of the jet suggest that the outflows are most likely driven by the radio source. The global energy density release of ~10^57 erg s^-1 Mpc^-3 may also influence the subsequent evolution of the HzRG by enhancing the entropy and pressure in the surrounding halo and facilitating ram-pressure stripping of gas in satellite galaxies that may contribute to the subsequent mass assembly of the HzRG through low-dissipation dry mergers.

Downsizing by shutdown in red galaxies

We address the origin of the 'downsizing' of elliptical galaxies, according to which the stars in more massive galaxies formed earlier and over a shorter period than those in less massive galaxies. We show that this could be the natural result of a shutdown of star formation in dark matter haloes above a critical mass of ∼10 12 M ⊙ . This is demonstrated using a semi-analytic simulation of galaxy formation within the standard hierarchical scenario of structure formation. The assumed threshold mass is motivated by the prediction of stable shock heating above this mass and the finding that such a shutdown reproduces the observed distribution of galaxies in luminosity and colour. The shutdown at a critical halo mass introduces a characteristic stellar mass for the transition of galaxies into the 'red sequence' of the galaxy colour-magnitude diagram. Central galaxies of haloes that are more massive today have reached this mass earlier and can therefore grow further along the red sequence by dry mergers, ending up more massive and containing older stars. Small galaxies formed in haloes below the critical mass can shutdown late, when they fall into haloes above the critical mass and become satellites. While our semi-analytic simulation that incorporates an explicit shutdown reproduces downsizing as inferred from the stellar ages of ellipticals, we explain why it is much harder to detect downsizing using the mass functions of different galaxy types.

Morphologies and Color Gradients of Luminous Evolved Galaxies atz∼ 1.5

We have examined in detail the morphologies of seven -->z ~ 1.5 passively evolving luminous red galaxies using high-resolution HST NICMOS and ACS imaging data. Almost all of these galaxies appear to be relaxed systems, with smooth morphologies at both rest-frame UV and visible wavelengths. Previous results from spectral synthesis modeling favor a single burst of star formation more than 1 Gyr before the observed epoch. The prevalence of old stellar populations, however, does not correlate exclusively with early-type morphologies as it does in the local universe; the light profiles for some of these galaxies appear to be dominated by massive exponential disks. This evidence for massive old disks, along with the apparent uniformity of stellar age across the disk, suggests formation by a mechanism better described as a form of monolithic collapse than as a hierarchical merger. These galaxies could not have undergone a single major merging event since the bulk of their stars were formed, more than 1 Gyr earlier. There is at least one case, however, that appears to be undergoing a dry merger, which may be an example of the process that converts these unusual galaxies into the familiar spheroids that dominate galaxies comprising old stellar populations at the present epoch.

The Redshift Evolution of Wet, Dry, and Mixed Galaxy Mergers from Close Galaxy Pairs in the DEEP2 Galaxy Redshift Survey

We study the redshift evolution of galaxy pair fractions and merger rates for different types of galaxies using kinematic pairs selected from the DEEP2 Redshift Survey, combined with other surveys at lower redshifts. By parameterizing the evolution of the pair fraction as -->(1 + z)m, we find that the companion rate increases mildly with redshift with -->m = 0.41 ± 0.20 for all galaxies with – -->21 m = 1.27 ± 0.35, while the red companion rate of red galaxies is better fitted with the negative slope -->m = − 0.92 ± 0.59. For the chosen luminosity range, we find that at low redshift the pair fraction within the red sequence exceeds that of the blue cloud, indicating a higher merger probability among red galaxies compared to that among the blue galaxies. With further assumptions on the merger timescale and the fraction of pairs that will merge, the galaxy major merger rates for -->0.1 10−3 h3 Mpc −3 Gyr −1 with a factor of 2 uncertainty. At -->z ~ 1.1, 68% of mergers are wet, 8% of mergers are dry, and 24% of mergers are mixed, compared to 31% wet mergers, 25% dry mergers, and 44% mixed mergers at -->z ~ 0.1. Wet mergers dominate merging events at -->z = 0.2–1.2, but the relative importance of dry and mixed mergers increases over time. About 22%-54% of present-day -->L* galaxies have experienced major mergers since -->z ~ 1.2, depending on the definition of major mergers. Moreover, 24% of the red galaxies at the present epoch have had dry mergers with luminosity ratios between $1t{:}t 4$ --> and $4t{:}t 1$ --> since -->z ~ 1. Our results also suggest that the wet mergers and/or mixed mergers may be partially responsible for producing red galaxies with intermediate masses, while a significant portion of massive red galaxies are assembled through dry mergers at later times.

Inclination‐Independent Galaxy Classification

We present a new method to classify galaxies from large surveys like the Sloan Digital Sky Survey using inclination-corrected concentration, inclination-corrected location on the color-magnitude diagram, and apparent axis ratio. Explicitly accounting for inclination tightens the distribution of each of these parameters and enables simple boundaries to be drawn that delineate three different galaxy populations: Early-type galaxies, which are red, highly concentrated, and round; Late-type galaxies, which are blue, have low concentrations, and are disk dominated; and Intermediate-type galaxies, which are red, have intermediate concentrations, and have disks. We have validated our method by comparing to visual classifications of high-quality imaging data from the Millennium Galaxy Catalogue. The inclination correction is crucial to unveiling the previously unrecognized Intermediate class. Intermediate-type galaxies, roughly corresponding to lenticulars and early spirals, lie on the red sequence. The red sequence is therefore composed of two distinct morphological types, suggesting that there are two distinct mechanisms for transiting to the red sequence. We propose that Intermediate-type galaxies are those that have lost their cold gas via strangulation, while Early-type galaxies are those that have experienced a major merger that either consumed their cold gas, or whose merger progenitors were already devoid of cold gas (the ``dry merger'' scenario).

Are dry mergers of ellipticals the way to reconcile model predictions with downsizing?

Aims. We show that the bulk of the star formation and the galaxy assembly should occur simultaneously in order to reproduce at the same time the downsizing and the chemical properties of present-day massive spheroids within one effective radius. Methods. By means of chemical evolution models, we create galactic building blocks of several masses and different chemical properties. We then construct a sample of possible merger histories going from a multiple minor merger scenario to a single major merger event aimed at reproducing a single massive elliptical galaxy. We compare our results against the mass-[Mg/Fe] and the massmetallicity relations. Results. We found that a series of multiple dry mergers (no star formation in connection with the merger) involving building-blocks that have been created ad hoc to satisfy the [Mg/Fe]-mass relation cannot fit the mass-metallicity relation and viceversa. A major dry merger, instead, does not make the agreement with observations worse if it happens between galaxies that already obey both the mass(σ)-[Mg/Fe] and the mass(σ)-metallicity relations. However, this process alone cannot explain the physical reasons for these trends. Conclusions. Dry mergers alone are not be the way to reconcile the need of a more efficient star formation in the most massive galaxies with the late-time assembly suggested in the hierarchical paradigm to recover the galaxy downsizing.

The Growth of Luminous Red Galaxies by Merging

We study the role of major and minor mergers in the mass growth of luminous red galaxies. We present small-scale (0.01 h−1 Mpc < r < 8 h−1 Mpc ) projected cross-correlation functions of 23,043 luminous early-type galaxies from the Sloan Digital Sky Survey (SDSS) luminous red galaxy (LRG) sample (0.16 < z < 0.30, M0.3 i − 5log h ≈ − 22.75 mag) with all their companions in the SDSS imaging sample, split into color and luminosity subsamples with M0.3 i − 5log h < − 18 mag. We deproject the two-dimensional functions to obtain three-dimensional real-space LRG-galaxy cross-correlation functions for each companion subsample. We find that the cross-correlation functions are not pure power laws and that there is a clear one-halo to two-halo transition near 1 h−1 Mpc. We convert these results into close pair statistics and estimate the LRG accretion rate from each companion galaxy subsample using timescales from dynamical friction arguments for each subsample of the companions. We find that the accretion onto LRGs is dominated by dry mergers of galaxies that are more luminous than L*. We integrate the luminosity accretion rate from mergers over all companion galaxy subsamples and find that LRGs are growing by 1.7 ± 0.1 h percent per Gyr, on average, from merger activity at redshift z ~ 0.25. This rate is almost certainly an overestimate, because we have assumed that all close pairs are merging as quickly as dynamical friction allows; nonetheless, it is on the low side of the panoply of measurements in the literature, and it is lower than any rate predicted from theory.

Deficiency of Broad-Line AGNs in Compact Groups of Galaxies

Based on a new survey of AGN activity in compact groups of galaxies, we report a remarkable deficiency of broad-line AGNs as compared to narrow-line AGNs. The cause of such deficiency could be related to the average low luminosity of AGNs in CGs: 1039 ergs s−1. This result may imply lower accretion rates in CG AGNs, making broad-line regions (BLRs) undetectable, or may indicate a genuine absence of BLRs. Both phenomena are consistent with gas stripping through tidal interaction and dry mergers.