Merger Fraction Research Articles

The Importance of Spheroidal and Mixed Mergers for Early-Type Galaxy Formation

We use semianalytical modeling techniques to investigate the progenitor morphologies of present-day elliptical galaxies. We find that, independent of the environment, the fraction of mergers of bulge-dominated galaxies (early types) increases with time. The last major merger of bright present-day elliptical galaxies with MB -21 was preferentially between bulge-dominated galaxies, while those with MB ~ -20 have mainly experienced last major mergers between a bulge-dominated and a disk-dominated galaxy. Independent of specific model assumptions, more than 50% of present-day elliptical galaxies in clusters with MB -18 had major mergers that were not of spiral galaxies, as usually expected within the standard merger scenario.

Evidence for a Major Merger Origin of High-Redshift Submillimeter Galaxies

Submillimeter-detected galaxies located at redshifts z > 1 host a major fraction of the bolometric luminosity at high redshifts due to thermal emission from heated dust grains, yet the nature of these objects remains a mystery. The major problem in understanding their origin is whether the dust-heating mechanism is predominantly caused by star formation or active galactic nuclei and what triggered this activity. We address this issue by examining the structures of 11 submillimeter galaxies imaged with STIS on the Hubble Space Telescope. We argue that ~61% ± 21% of these submillimeter sources are undergoing an active major merger using the CAS (concentration, asymmetry, clumpiness) quantitative morphological system. We rule out at ~5 σ confidence that these submillimeter galaxies are normal Hubble types at high redshift. This merger fraction appears to be higher than for Lyman break galaxies undergoing mergers at similar redshifts. Using reasonable constraints on the stellar masses of Lyman break galaxies and these submillimeter sources, we further argue that at redshifts z ~ 2-3, systems with high stellar masses are more likely than lower mass galaxies to be involved in major mergers.

A Direct Measurement of Major Galaxy Mergers atz3

This paper presents direct evidence for hierarchical galaxy assembly out to redshifts z ~ 3. We identify major mergers using the model-independent CAS (concentration, asymmetry, clumpiness) physical morphological system on galaxies detected, and photometrically selected, in the WFPC2 and NICMOS Hubble Deep Field North. We specifically use the asymmetric distributions of rest-frame optical light measured through the asymmetry parameter (A) to determine the fraction of galaxies undergoing major mergers as a function of redshift (z), stellar mass (M*), and absolute magnitude (MB). We find that the fraction of galaxies consistent with undergoing a major merger increases with redshift for all galaxies, but most significantly, at 5?10 ? confidence, for the most luminous and massive systems. The highest merger fractions we find are 40%?50% for galaxies with MB 1010 M? at z > 2.5, e.g., objects identified as Lyman-break galaxies. Using these results, we model the merger fraction evolution in the form fm(A, M*, MB, z) = f0 ? (1 + z). We find mA values ~4?6 for the most luminous and massive galaxies, while lower mass and less luminous galaxies have smaller mA values. We use these merger fractions, combined with merger timescales calculated from N-body simulations, to derive galaxy merger rates to z ~ 3. We also use stellar masses of HDF-N galaxies to determine the mass accretion rate of field galaxies involved in major mergers. We find an average stellar mass accretion rate of G ~ 4 ? 108 M? Gyr-1 per galaxy at z ~ 1 for galaxies with stellar masses M* > 109 M?. This accretion rate changes with redshift as G = 1.6 ? 108(1 + z)0.99?0.32 M? Gyr-1 per galaxy. We also find that the fraction of stellar mass density in galaxies involved in major mergers increases with redshift, with a peak mass fraction ~0.5 for the brightest, MB 1010 M?, systems near z ~ 2.5. By comparing merger fractions predicted in cold dark matter semianalytic models with our results we find a reasonably good agreement for the largest and brightest systems, although we find more low-mass galaxy mergers at lower redshifts than what these models predict.

Lensing Effects of Misaligned Disks in Dark Matter Halos

In this paper we study the observational signatures of the lensing signal produced by dark matter halos with embedded misaligned disks. This issue is of particular interest at present, since several of the observed multiple-lens systems have magnification ratios and image geometries that are not simultaneously well fitted by standard mass models. The presence of substructure exterior to the lens has been invoked by several authors in the context of cold dark matter (CDM) models in order to explain the anomalous magnification ratios. We emphasize that the anomalous magnification ratios may be an artifact of the simple mass models currently in use; the inclusion of a misaligned disk may be able to mimic the effect of substructure. These slight spatial offsets between the dark matter halo and the disk, which are likely to occur as a consequence of interactions or mergers, lead to complex image configurations and nonstandard magnification ratios. We investigate the effects of disk misalignment on two illustrative lenses: a spiral disk embedded within a dark matter halo, and a compact, disklike component within an early-type galaxy. The expected fraction of galaxies with a misaligned disk is estimated to be of the order of the merger fraction, since all mergers are expected to progress via a transiently misaligned phase. We find that there is a significant change in all observed lensing properties due to the added complexity of such mass profiles. In particular cases, the resulting lensing geometries are unusual, with high image multiplicities. The caustic structures—both radial and tangential—are drastically modified, and the magnification ratios differ fromexpectations from standard lens models. The additional parameters required to specify the relative alignment of multiple mass components in the primary lens introduce yet another source of uncertainty in the mass modeling of gravitational lens systems.

μJy radio sources in the z=0.83 cluster MS1054-03

An extremely deep 5-GHz radio observation is presented of the rich cluster MS1054−03 at redshift z=0.83. 34 radio sources are detected down to a 6σ level of 32 μJy, compared with about 25 expected from previous blank-field radio source count determinations; the sources giving rise to these excess counts lie predominantly within 2 arcmin (∼700 kpc) of the cluster centre. Existing imaging and spectroscopic observations have provided optical identifications for 21 of the radio sources and redshifts for 11, of which eight are confirmed cluster members. Four of these eight confirmed cluster sources are associated with close galaxy pairs (10–25 kpc projected offset) of similar magnitude, implying that the radio source may be triggered by an interaction. However, although MS1054−03 has a very high fraction (17 per cent) of ongoing mergers (separations ≪10 kpc), no radio emission is detected towards any of these merger events, setting a mean upper limit of 10 M⊙ yr−1 for any star-formation associated with these mergers. This supports the hypothesis that low-luminosity radio sources may be onset by initial weak interactions rather than direct mergers. The host galaxies of the other four confirmed cluster radio sources are all isolated, and show a range of morphologies from early type to Sc. A comparison between the emission-line and radio luminosities suggests that two of these four radio sources are low-luminosity active galactic nuclei (AGNs), whilst for at least one of the other two the radio emission is associated with ongoing star formation. All the radio sources associated with the galaxy pairs appear more likely to be AGN than starburst in origin. The overall proportion of radio sources associated with AGNs in this cluster (≳75 per cent) is higher than that detected at these flux-density levels in the field (40–50 per cent).

The Origin, Evolution, and Classification of Field and Cluster Galaxy Populations

In this thesis I address the formation and evolution of lowmass galaxies and investigate their importance in the general galaxy evolution/formation process. Currently favored models of galaxy formation, such as cold dark matter (CDM), suggest that the first galaxies to form in the universe have low masses and sizes, similar to modern dwarf galaxies, that later merge to form more massive systems. The most common galaxies in the nearby universe are dwarf ellipticals in clusters that could be descendants of these early galaxies. This thesis addresses two questions related to these objects: (1) Are the dwarf elliptical galaxies seen in clusters an old primordial population that survived since their initial formation, or are these dwarfs the end product of some evolutionary dynamical process? (2) Do galaxy mergers occur in the universe, and if so, how does this merging evolve through time? To determine whether dwarf elliptical galaxies are an old cluster population, kinematic, structural, and photometric information for these objects was obtained using the WisconsinIndiana-Yale-NOAO 3.5 m telescope. Radial velocities of dwarf elliptical galaxies in the Virgo Cluster suggest that these objects are not an old cluster population but were accreted sometime in the past (C. J. Conselice, J. S. Gallagher III, & R. F. G. Wyse 2001, ApJ, 559, 791). Kinematics also demonstrate that these objects could have originated from a more massive progenitor. Photometry of dwarf elliptical galaxies in the Perseus Cluster further suggests that the stellar populations of these objects are old, with little to no recent star formation or population gradients. The broadband UBR colors of these objects are also consistent with color differences originating from metallicity. Low-mass galaxies in the Perseus Cluster however do not follow the color-magnitude relationship found for the giant elliptical galaxies, with some too red and others too blue for their magnitudes. Through comparisons with LCDM models of galaxy formation, I show that the blue lowmass cluster objects in Perseus are consistent with being old CDM galaxies. Low-mass cluster galaxies that are redder than the color-magnitude relationship are shown to be composed of stars with high metallicities. These red low-mass cluster galaxies must have very high mass-to-light ratios near 250 in solar units or must be the remnants of higher mass galaxies whose stellar material was stripped during high-speed impulsive interactions with other cluster galaxies. To address the issue of galaxy mergers, I develop and use a morphological system to identify robustly, and unambiguously, objects undergoing major mergers (C. J. Conselice, M. A. Bershady, & A. Jangren 2000, ApJ, 529, 886; C. J. Conselice, M. A. Bershady, & J. S. Gallagher III 2000, A&A, 354, L21). This system uses an asymmetry index, such that galaxies undergoing mergers have asymmetry values , with the A 1 0.35 range of possible values being 0–2. When this is applied to the galaxies in the Hubble Deep Field–North, an increase with redshift of galaxies undergoing major mergers is found. Depending on the method of computation, the merger fraction evolution out to can be characterized as , where m m z ∼ 3.5 f p (1 z) ranges from 1 to 2.5. The best fit gives a variable m, such that . m p 2.96 0.32z I also investigate the properties of a nearby massive central cluster galaxy, NGC 1275 in the Perseus Cluster, whose unusual properties and structural aspects can be partially accounted for by recent and past mergers (C. J. Conselice, J. S. Gallagher III, & R. F. G. Wyse 2001, AJ, 122, 2281). Finally, after summarizing the results of this thesis, as well as previous work, I present a new galaxy classification system according to which all galaxies can be fitted based on the three most salient properties that drive galaxy evolution: dissipation, star formation, and galaxy interactions.

The Q-Theory of Mergers

The Q-theory of investment says that a firm's investment rate should rise with its Q. We argue here that this theory also explains why some firms buy other firms. We find that 1. A firm's merger and acquisition (M&A) investment responds to its Q more -- by a factor of 2.6 -- than its direct investment does, probably because M&A investment is a high fixed cost and a low marginal adjustment cost activity, 2. The typical firm wastes some cash on M&As, but not on internal investment, i.e., the 'Free-Cash Flow' story works, but explains a small fraction of mergers only, and 3. The merger waves of 1900 and the 1920's, `80s, and `90s were a response to profitable reallocation opportunities, but the `60s wave was probably caused by something else.

Redshift Evolution of the Merger Fraction of Galaxies in Cold Dark Matter Cosmologies

We use semianalytical modeling of galaxy formation to study the redshift evolution of the galaxy merger fractions and merger rates in Λ cold dark matter (ΛCDM) and quintessence (QCDM) cosmologies, their dependence on physical parameters such as the environment, the merger timescale, the way major mergers are defined, and the minimum mass of objects taken into account. We find that for a given final halo mass the redshift dependence of the merger fraction Fmg and the resulting merger rate can be fitted well by a power law for redshifts z 1. The normalization Fmg(0) and the slope m depend on the final halo mass. For a given merger timescale tmerg and an assumed maximum mass ratio Rmajor for major mergers, Fmg(0) and m depend exponentially on each other. The slope m depends logarithmically on the ratio of the final halo mass and the minimum halo mass taken into account. In addition, the local normalization Fmg(0) increases for larger Rmajor while m decreases. We compare the predicted merger fraction with recent observations and find that the model cannot reproduce both the merger index and the normalization at the same time. In general the model underestimates Fmg(0) and m by a factor of 2.

The evolution and merging history of cluster ellipticals from z = 0 to z = 0.83

The evolution of galaxies is likely to be complex, involving mergers, starbursts, and other dramatic changes in morphology and luminosity. The measurement of the evolution of the mass function of galaxies is therefore essential. This can be accomplished by measuring the evolution of the mass–to–light ratios of galaxies as a function of redshift. The Fundamental Plane relation is uniquely suited to measure the evolution of the mass–to–light ratio of early–type galaxies. We show that the evolution depends sensitively on cosmology and star–formation history. We present results on the evolution of the mass–to–light ratio from the Fundamental Plane out to z = 0.83. The early-type galaxies in clusters follow a well–defined relation out the highest redshift. The mass–to–light evolution is very slow, and implies a high mean stellar age in an open universe. One of the main uncertainties in the interpretation is morphological evolution. If the youngest early types at low redshift appear as other morphological types at high redshift, then the study of early–type galaxies at high redshifts will produce biased results. We discuss the effects of this ‘progenitor bias’. We show evidence for significant morphological evolution for all early types (elliptical and S0 galaxies). We find a high fraction of mergers in MS 1054–03, comparable with the fraction of ellipticals. Furthermore, the total fraction of early types in rich clusters decreases from z = 0 to z = 0.83. These results suggest that the set of early types is not a closed set, but evolving. The effects on the derived evolution of the mass–to–light ratio is relatively small, due to the small scatter. The next step will be to extend these studies to high redshift clusters, and to the field. This work can provide very strong constraints on the mass evolution of galaxies.

Evolution of Cluster Galaxies in Hierarchical Clustering Universes

Abstract Using cosmological N-body simulations of critical (SCDM) and open (Ω = 0.3, OCDM) cold dark matter models, we investigated the evolution of cluster galaxies. Based on our numerical simulation, we constructed merging history trees of the galaxies. By following their merging history, we could show that the major merger fractions of the galaxies in cluster-forming regions is roughly proportional to (1+z)4.5 at low redshifts (z < 2), and has a steep peak at z ≃ 2.5 and z ≃ 3 in SCDM and OCDM, respectively. We also show that the cluster galaxies are affected by the tidal interaction after the clusters are formed. Because the formation redshift of the cluster in SCDM, zform = 0.15, is much more recent than that of the cluster in OCDM, zform = 1.6, the cluster galaxies in SCDM show more rapid evolution by tidal interactions from z = 0.5 than those in OCDM.

Hubble Space Telescope imaging of the CFRS and LDSS redshift surveys--IV. Influence of mergers in the evolution of faint field galaxies from z 1

HST images of a sample of 285 galaxies with measured z from the CFRS and Autofib-LDSS redshift surveys are analysed to derive the evolution of the merger fraction out to z~1. We have performed visual and machine-based merger identifications, as well as counts of bright pairs of galaxies with magnitude differences less than 1.5 mag. We find that the pair fraction increases with z, with up to ~20% of the galaxies being in physical pairs at z~0.75-1. We derive a merger fraction varying with z as (1+z)^{3.2 +/- 0.6}, after correction for line-of-sight contamination, in excellent agreement with the merger fraction derived from the visual classification of mergers for which m = 3.4 +/- 0.6. After correcting for seeing effects on the ground-based selection of survey galaxies, we conclude that the pair fraction evolves as (1+z)^{2.7 +/- 0.6}. This implies that an average L* galaxy will have undergone 0.8 to 1.8 merger events from z=1 to 0, with 0.5 to 1.2 merger events occuring in a 2 Gyr time span at z~0.9. This result is consistent with predictions from semi-analytical models of galaxy formation. From the simple co-addition of the observed luminosities of the galaxies in pairs, physical mergers are computed to lead to a brightening of 0.5 mag for each pair on average, and a boost in star formation rate of a factor of 2, as derived from the average [O II] equivalent widths. Mergers of galaxies are therefore contributing significantly to the evolution of both the luminosity function and luminosity density of the Universe out to z~1.

Dust-obscured star formation and AGN fuelling in hierarchical models of galaxy evolution

A large fraction of the luminous distant submillimetre-wave galaxies recently detected using the Submillimetre Common-User Bolometer Array (SCUBA) camera on the James Clerk Maxwell Telescope appear to be associated with interacting optical counterparts. We investigate the nature of these systems using a simple hierarchical clustering model of galaxy evolution, in which the large luminosity of the SCUBA galaxies is assumed to be generated at the epoch of galaxy mergers in a burst of either star formation activity or the fuelling of an active galactic nucleus (AGN). The models are well constrained by the observed spectrum of the far-infrared/submillimetre-wave background radiation and the 60-µm counts of low-redshift IRAS galaxies. The ratio between the total amount of energy released during mergers and the mass of dark matter involved must increase sharply with redshift z at z < 1, and then decrease at greater redshifts. This result is independent of the fraction of the luminosity of mergers that is produced by starbursts and AGN. One additional parameter – the reciprocal of the product of the duration of the enhanced luminosity produced by the merger and the fraction of mergers that induce an enhanced luminosity, which we call the activity parameter – is introduced, to allow the relationship between merging dark matter haloes and the observed counts of distant dusty galaxies to be investigated. The observed counts can only be reproduced if the activity parameter is greater by a factor of about 5 and 100 at redshifts of 1 and 3 respectively, as compared with the present epoch. Hence, if merging galaxies account for the population of SCUBA galaxies, then the merger process must have been much more violent at high redshifts. We discuss the counts of galaxies and the intensity of background radiation in the optical/nearinfrared wavebands in the context of these hierarchical models, and thus investigate the relationship between the populations of submillimetre-selected and Lyman-break galaxies.

A High Merger Fraction in the Rich Cluster MS 1054−03 at [CLC][ITAL]z[/ITAL][/CLC] = 0.83: Direct Evidence for Hierarchical Formation of Massive Galaxies

We present a morphological study of the galaxy population of the luminous X-ray cluster MS 1054-03 at z = 0.83. The sample consists of 81 spectroscopically confirmed cluster members in a 3 × 2 h Mpc area imaged in F606W and F814W with WFPC2. We find 13 ongoing mergers in MS 1054-03, comprising 17% of the L L* cluster population. Most of these mergers will likely evolve into luminous (~2L*) elliptical galaxies, and some may evolve into S0 galaxies. Assuming the galaxy population in MS 1054-03 is typical for its redshift, it is estimated that ~50% of present-day cluster elliptical galaxies experienced a major merger at z < 1. The mergers are preferentially found in the outskirts of the cluster and probably occur in small infalling clumps. Morphologies, spectra, and colors of the mergers show that their progenitors were typically E/S0 or early-type spiral galaxies with mean stellar formation redshifts z* 1.7. The red colors of the merger remnants are consistent with the low scatter in the color-magnitude relation in rich clusters at lower redshift. The discovery of a high fraction of mergers in this young cluster is direct evidence against formation of elliptical galaxies in a single monolithic collapse at high redshift and is in qualitative agreement with predictions of hierarchical models for structure formation.

The difference between radio-loud and radio-quiet active galaxies

The recent development of unified theories of active galactic nuclei (AGN) has indicated that there are two physically distinct classes of these objects - radio-loud and radio-quiet. The primary observational distinctions between the two types are: (1) The radio-loud objects produce large scale radio jets and lobes, with the kinetic power of the jets being a significant fraction of the total bolometric luminosity. On the other hand, the weak radio ejecta of the radio-quiet objects are energetically insignificant. (2) The radio-loud objects are associated with elliptical galaxies which have undergone recent mergers, while the radio-quiets prefer spiral hosts. (3) The space density of the radio-louds at a given optical luminosity is $\approx$ 10 times lower than that of the radio-quiets. Despite these differences, the (probably) thermal emissions from the AGN (continua and lines from X-ray to infrared wavelengths) are quite similar in the two classes of object. We argue that this last result suggests that the black hole masses and mass accretion rates in the two classes are not greatly different, and that the difference between the classes is associated with the spin of the black hole. We assume that the normal process of accretion through a disk does not lead to rapidly spinning holes, and propose instead that galaxies (e.g. spirals) which have not suffered a recent major merger event contain non-rotating or only slowly rotating black holes. When two such galaxies merge, the two black holes are known to form a binary and we assume that they eventually coalesce. In the small fraction of mergers in which the two ``parent'' galaxies contain very massive holes of roughly equal mass, a rapidly spinning, very massive hole results. It is proposed that such mergers are the progenitors of powerful radio

The frequency of mergers in compact groups

The far-infrared colors, optical morphologies, and emission-line rotation curves of galaxies in compact groups are used to estimate the number of these galaxies which are currently merging. Each of these independent methods indicates that roughly 7% of the galaxies in compact groups are in the process of merging. Although each technique is somewhat uncertain, the agreement among the independent methods for the fraction of ongoing mergers and the generally good correlation between different measures of merging for individual systems lend added weight to the results

What is the evidence on antigalaxies?

Recent observations show collisions or mergers among galaxies to be far more frequent than formerly anticipated. IRAS data have shown the most luminous FIR sources to involve such mergers. The luminosity of some of those mergers could be fueled by annihilation of matter and antimatter, and it is possible that this mechanism provides a way in which the existence of antigalaxies in the universe could become evident. If galaxy-antigalaxy collisions were prevalent among IRAS mergers, the 100-MeV gamma-ray flux from the most luminous merging galaxies would be roughly twice as high as the observed IR flux. Current data on a few of the brightest sources disqualify these as colliding galaxy-antigalaxy pairs. A systematic absence of gamma radiation from all ultraluminous IRAS sources would imply that antigalaxies must be rare or else located at extreme redshifts. In any case, galaxy-antigalaxy collisions cannot constitute a substantial fraction of all IRAS mergers, because their combined gamma flux would exceed the observed gamma-ray background.