The Role of Mergers in Early-Type Galaxy Evolution and Black Hole Growth (original) (raw)
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Black Holes in Galaxy Mergers: The Formation of Red Elliptical Galaxies
The Astrophysical Journal, 2005
We use hydrodynamical simulations to study the color transformations induced by star formation and active galactic nuclei (AGN) during major mergers of spiral galaxies. Our modeling accounts for radiative cooling, star formation, and supernova feedback. Moreover, we include a treatment of accretion onto supermassive black holes embedded in the nuclei of the merging galaxies. We assume that a small fraction of the bolometric luminosity of an accreting black hole couples thermally to surrounding gas, providing a feedback mechanism that regulates its growth. The encounter and coalescence of the galaxies triggers nuclear gas inflow which fuels both a powerful starburst and strong black hole accretion. Comparing simulations with and without black holes, we show that AGN feedback can quench star formation and accretion on a short timescale, particularly in large galaxies where the black holes can drive powerful winds once they become sufficiently massive. The color evolution of the remnant differs markedly between mergers with and without central black holes. Without AGN, gas-rich mergers lead to ellipticals which remain blue owing to residual star formation, even after more than 7 Gyrs have elapsed. In contrast, mergers with black holes produce ellipticals that redden much faster, an effect that is more pronounced in massive remnants where a nearly complete termination of star formation occurs, allowing them to redden to u − r ≃ 2.3 in less than one Gyr. AGN feedback may thus be required to explain the population of extremely red massive early type-galaxies, and it appears to be an important driver in generating the observed bimodal color distribution of galaxies in the Local Universe.
Dry Mergers in GEMS: The Dynamical Evolution of Massive Early‐Type Galaxies
The Astrophysical Journal, 2006
We have used the 28 ′ × 28 ′ Hubble Space Telescope image mosaic from the GEMS (Galaxy Evolution from Morphology and SEDs) survey in conjunction with the COMBO-17 photometric redshift survey to constrain the incidence of major mergers between spheroid-dominated galaxies with little cold gas (dry mergers) since z = 0.7. A set of N-body merger simulations was used to explore the morphological signatures of such interactions: they are recognizable either as < 5 kpc separation close pairs or because of broad, low surface brightness tidal features and asymmetries. Data with the depth and resolution of GEMS are sensitive to dry mergers between galaxies with M V −20.5 for z 0.7; dry mergers at higher redshifts are not easily recovered in single-orbit HST imaging. Six dry mergers (12 galaxies) with luminosity ratios between 1:1 and 4:1 were found from a sample of 379 red early-type galaxies with M V < −20.5 and 0.1 < z < 0.7. The simulations suggest that the morphological signatures of dry merging are visible for ∼ 150 Myr and we use this timescale to convert the observed merger incidence into a rate. On this basis we find that present day spheroidal galaxies with M V < −20.5 on average have undergone between 0.5 and 2 major dry mergers since z ∼ 0.7. We have compared this result with the predictions of a Cold Dark Matter based semi-analytic galaxy formation model. The model reproduces the observed declining major merger fraction of bright galaxies and the space density of luminous early-type galaxies reasonably well. The predicted dry merger fraction is consistent with our observational result. Hence, hierarchical models predict and observations now show that major dry mergers are an important driver of the evolution of massive early-type galaxies in recent epochs.
The Role of Mergers in Galaxy Evolution
The Evolution of Galaxies, 2001
In the last decade the importance of mergers in the evolution of galaxies has become evident. In this paper we illustrate this importance by showing examples of merging galaxies, both local and at increasing redshift. However before getting carried away by the charms of the hierarchical model in which large galaxies have been built up by successive mergers of smaller objects, it is worth looking at what stellar population synthesis can tell us. Here I show that, using indices which allow us to separate the effects of metallicity and age on the spectra of the stellar populations of galaxies, we can show that the most massive galaxies have the oldest stellar populations, an effect which is enhanced within galaxy clusters and is maximized within the most massive clusters. These measurements imply that a model where mergers (even "dry" mergers) are the main driver for galaxy evolution cannot be giving us anything like a valid picture. The role of mergers must be considerably more subtle than one would infer from the standard semi-analytic models of galaxy evolution within a cosmological framework.
Monthly Notices of the Royal Astronomical Society, 2010
Following the study of hereafter D09a) we explore the environments, optical colours, stellar masses, star formation and AGN activity in a sample of 3003 pairs of merging galaxies drawn from the SDSS using visual classifications from the Galaxy Zoo project. While D09a found that the spiral-to-elliptical ratio in (major) mergers appeared higher than that of the global galaxy population, no significant differences are found between the environmental distributions of mergers and a randomly selected control sample. This makes the high occurrence of spirals in mergers unlikely to be an environmental effect and must, therefore, arise from differing time-scales of detectability for spirals and ellipticals. We find that merging galaxies have a wider spread in colour than the global galaxy population, with a significant blue tail resulting from intense star formation in spiral mergers. Galaxies classed as star-forming using their emission-line properties have average star-formation rates approximately doubled by the merger process though star formation is negligibly enhanced in merging elliptical galaxies. We conclude that the internal properties of galaxies significantly affect the time-scales over which merging systems can be detected (as suggested by recent theoretical studies) which leads to spirals being 'over-observed' in mergers. We also suggest that the transition mass 3 × 10 10 M ⊙ , noted by , below which ellipticals are rare could be linked to disc survival/destruction in mergers.
Inspiralling Supermassive Black Holes: A New Signpost for Galaxy Mergers
The Astrophysical Journal, 2009
We present a new technique for observationally identifying galaxy mergers spectroscopically rather than through host galaxy imaging. Our technique exploits the dynamics of supermassive black holes (SMBHs) powering active galactic nuclei (AGNs) in merger-remnant galaxies. Because structure in the universe is built up through galaxy mergers and nearly all galaxies host a central SMBH, some galaxies should possess two SMBHs near their centers as the result of a recent merger. These SMBHs spiral to the center of the resultant merger-remnant galaxy, and one or both of the SMBHs may power AGNs. Using the DEEP2 Galaxy Redshift Survey, we have examined 1881 red galaxies, of which 91 exhibit [O III] and Hβ emission lines indicative of Seyfert 2 activity. Of these, 32 AGNs have [O III] emission-line redshifts significantly different from the redshifts of the host galaxies' stars, corresponding to velocity offsets of ∼ 50 km s −1 to ∼ 300 km s −1. Two of these AGNs exhibit doublepeaked [O III] emission lines, while the remaining 30 AGNs each exhibit a single set of velocity-offset [O III] emission lines. After exploring a variety of physical models for these velocity offsets, we argue that the most likely explanation is inspiralling SMBHs in merger-remnant galaxies. Based on this interpretation, we find that roughly half of the red galaxies hosting AGNs are also merger remnants, which implies that mergers may trigger AGN activity in red galaxies. The AGN velocity offsets we find imply a merger fraction of ∼ 30% and a merger rate of ∼ 3 mergers Gyr −1 for red galaxies at redshifts 0.34 < z < 0.82.
Galaxy merger histories and the role of merging in driving star formation at z > 1
Monthly Notices of the Royal Astronomical Society, 2015
We use Horizon-AGN, a hydrodynamical cosmological simulation, to explore the role of mergers in the evolution of massive (M * > 10 10 M ⊙ ) galaxies around the epoch of peak cosmic star formation (1 < z < 4). The fraction of massive galaxies in major mergers (mass ratio R < 4 : 1) is around 3%, a factor of ∼2.5 lower than minor mergers (4 : 1 < R < 10 : 1) at these epochs, with no trend with redshift. At z ∼ 1, around a third of massive galaxies have undergone a major merger, while all such systems have undergone either a major or minor merger. While almost all major mergers at z > 3 are 'blue' (i.e. have significant associated star formation), the proportion of 'red' mergers increases rapidly at z < 2, with most merging systems at z ∼ 1.5 producing remnants that are red in rest-frame UV-optical colours. The star formation enhancement during major mergers is mild (∼20-40%) which, together with the low incidence of such events, implies that this process is not a significant driver of early stellar mass growth. Mergers (R < 10 : 1) host around a quarter of the total star formation budget in this redshift range, with major mergers hosting around two-thirds of this contribution. Notwithstanding their central importance to the standard ΛCDM paradigm, mergers are minority players in driving star formation at the epochs where the bulk of today's stellar mass was formed.
Context. Several studies have tried to ascertain whether or not the increase in abundance of the early-type galaxies (E-S0a's) with time is mainly due to major mergers, reaching opposite conclusions. Aims. We have tested it directly through semi-analytical modelling, by studying how the massive early-type galaxies with log(M * /M ⊙ ) > 11 at z ∼ 0 (mETGs) would have evolved backwards-in-time, under the hypothesis that each major merger gives place to an early-type galaxy. Methods. The study was carried out just considering the major mergers strictly reported by observations at each redshift, and assuming that gas-rich major mergers experience transitory phases as dust-reddened, star-forming galaxies (DSFs). Results. The model is able to reproduce the observed evolution of the galaxy LFs at z 1, simultaneously for different rest-frame bands (B, I, and K) and for different selection criteria on color and morphology. It also provides a framework in which apparentlycontradictory results on the recent evolution of the luminosity function (LF) of massive, red galaxies can be reconciled, just considering that observational samples of red galaxies can be significantly contaminated by DSFs. The model proves that it is feasible to build up ∼ 50-60% of the present-day number density of mETGs at z 1 through the coordinated action of wet, mixed, and dry major mergers, fulfilling global trends that are in general agreement with mass-downsizing. The bulk of this assembly takes place during ∼ 1 Gyr elapsed at 0.8 < z < 1, providing a straightforward explanation to the observational fact that redshift z ∼ 0.8 is a transition epoch in the formation of mETGs. The gas-rich progenitors of these recently-assembled mETGs reproduce naturally the observational excess by a factor of ∼ 4-5 of late-type galaxies at 0.8 < z < 1, as compared to pure luminosity evolution (PLE) models. Conclusions. The model suggests that major mergers have been the main driver for the observational migration of mass from the massive-end of the blue galaxy cloud to that of the red sequence in the last ∼ 8 Gyr.
Evolution of the mass, size, and star formation rate in high redshift merging galaxies
Astronomy & Astrophysics, 2014
Context. In Λ-CDM models, galaxies are thought to grow both through continuous cold gas accretion coming from the cosmic web and episodic merger events. The relative importance of these different mechanisms at different cosmic epochs is nevertheless not yet well understood. Aims. We aim at addressing the questions related to galaxy mass assembly through major and minor wet merging processes in the redshift range 1 < z < 2, an epoch corresponding to the peak of the cosmic star formation history. A significant fraction of Milky Way-like galaxies are thought to have undergone an unstable clumpy phase at this early stage. We focus on the behavior of the young clumpy disks when galaxies are undergoing gas-rich galaxy mergers. Methods. Using the adaptive mesh refinement code RAMSES, we build the Merging and Isolated high-Redshift Adaptive mesh refinement Galaxies (MIRAGE) sample. It is composed of 20 mergers and 3 isolated idealized disks simulations, which sample disk orientations and merger masses. Our simulations can reach a physical resolution of 7 parsecs, and include: star formation, metal line cooling, metallicity advection, and a recent physically-motivated implementation of stellar feedback which encompasses OB-type stars radiative pressure, photo-ionization heating, and supernovae. Results. The star formation history of isolated disks shows stochastic star formation rate, which proceeds from the complex behavior of the giant clumps. Our minor and major gas-rich merger simulations do not trigger starbursts, suggesting a saturation of the star formation due to the detailed accounting of stellar feedback processes in a turbulent and clumpy interstellar medium fed by substantial accretion from the circum-galactic medium. Our simulations are globally close to the normal regime of the disk-like star formation on a Schmidt-Kennicutt diagram. The mass-size relation and its rate of evolution in the redshift range 1 < z < 2 matches observations, suggesting that the inside-out growth mechanisms of the stellar disk do not necessarily require to be achieved through a cold accretion.
Merger History of Central Galaxies in Semi-analytic Models of Galaxy Formation
The Astrophysical Journal
We investigate the dynamical evolution of galaxies in groups with different formation epochs. Galaxy groups have been selected to be in different dynamical states, namely dynamically old and dynamically young, which reflect their early and late formation times, respectively, based on their halo mass assembly. Brightest galaxies in dynamically young groups have suffered their last major galaxy merger typically ∼ 2 Gyr more recently than their counterparts in dynamically old groups. Furthermore, we study the evolution of velocity dispersion in these two classes and compare them with the analytic models of isolated halos. The velocity dispersion of dwarf galaxies in high mass, dynamically young groups increases slowly in time, while the analogous dispersion in dynamically old high-mass groups is constant. In contrast, the velocity dispersion of giant galaxies in low mass groups decreases rapidly at late times. This increasing velocity bias is caused by dynamical friction, and starts much earlier in the dynamically old groups. The recent Radio-SAGE model of galaxy formation suggests that radio luminosities of central galaxies, considered to be tracers of AGN activity, are enhanced in halos that assembled more recently, independent of the time since the last major merger.
Hierarchical models predict that massive early-type galaxies (mETGs) derive from the most massive and violent merging sequences occurred in the Universe. However, the role of wet, mixed, and dry major mergers in the assembly of mETGs is questioned by some recent observations. We have developed a semi-analytical model to test the feasibility of the major-merger origin hypothesis for mETGs, just accounting for the effects on galaxy evolution of the major mergers strictly reported by observations. The model proves that it is feasible to reproduce the observed number density evolution of mETGs since z ∼ 1, just accounting for the coordinated effects of wet/mixed/dry major mergers. It can also reconcile the different assembly redshifts derived by hierarchical models and by mass downsizing data for mETGs, just considering that a mETG observed at a certain redshift is not necessarily in place since then. The model predicts that wet major mergers have controlled the mETGs buildup since z∼1, although dry and mixed mergers have also played an essential role in it. The bulk of this assembly took place at 0.7< z <1, being nearly frozen at z 0.7 due to the negligible number of major mergers occurred per existing mETG since then. The model suggests that major mergers have been the main driver for the observational migration of mass from the massive end of the blue galaxy cloud to that of the red sequence in the last ∼ 8 Gyr.