Hierarchical Galaxy Formation Models Research Articles

Is NGC 3108 transforming itself from an early- to late-type galaxy – an astronomical hermaphrodite?

A common feature of hierarchical galaxy formation models is the process of ‘inverse’ morphological transformation: a bulge dominated galaxy accretes a gas disc, dramatically reducing the system's bulge-to-disc mass ratio. During their formation, present-day galaxies may execute many such cycles across the Hubble diagram. A good candidate for such a ‘hermaphrodite’ galaxy is NGC 3108: a dust-lane early-type galaxy which has a large amount of H i gas distributed in a large-scale disc. We present narrow-band Hα and R-band imaging, and compare the results with the H i distribution from the literature. The emission is in two components: a nuclear bar and an extended disc component which coincides with the H i distribution. This suggests that a stellar disc is currently being formed out of the H i gas. The spatial distributions of the Hα and H i emission and the H ii regions are consistent with a barred spiral structure, extending some 20 kpc in radius. We measure an extinction-corrected star formation rate (SFR) of 0.42 M⊙ yr−1. The luminosity function of the H ii regions is similar to other spiral galaxies, with a power-law index of −2.1, suggesting that the star formation mechanism is similar to other spiral galaxies. We measured the current disc mass and find that it is too massive to have been formed by the current SFR over the last few Gyr. It is likely that the SFR in NGC 3108 was higher in the past. With the current SFR, the disc in NGC 3108 will grow to be ∼6.2 × 109M⊙ in stellar mass within the next 5.5 Gyr. While this is substantial, the disc will be insignificant compared with the large bulge mass: the final stellar mass disc-to-bulge ratio will be ∼0.02. NGC 3108 will fail to transform into anything resembling a spiral without a boost in the SFR and additional supply of gas.

The formation of galaxy discs in a hierarchical universe

The formation of galactic discs and the efficiency of star formation within them are issues central to our understanding of galaxy formation. We have developed a detailed and versatile model of disc formation which combines the strengths of previous studies of isolated discs with those of hierarchical galaxy formation models. Disc structure is inferred from the distribution of angular momentum in hot halo gas and the hierarchical build-up of dark matter, leading to theoretically generated systems where the evolution of surface density, rotation, velocity dispersion, stability and metallicity is predicted for annular regions of width 20-100 pc. The model will be used to establish whether the accepted theory of large-scale structure formation in the universe is consistent with observed trends in the properties of disc galaxies. This first paper explicitly examines the importance of embedding such calculations within a merging hierarchy of dark matter haloes, finding that this leads to dramatically different formation histories compared to models in which discs grow in isolation. Different models of star formation are explored, and are found to have only a secondary influence on the properties of the resulting galaxy discs, the main governing factor being the infalling gas supply from the hot halo.

On the onset of galactic winds in quiescent star forming galaxies

Context.The hierarchical model of galaxy formation, despite its many successes, still overpredicts the baryons fraction locked in galaxies as a condensed phase. The efficiency of supernovae feedback, proposed a long time ago as a possible solution for this so-called “overcooling” problem, is still under debate, mainly because modelling supernovae explosions within a turbulent interstellar medium, while capturing realistic large scale flows around the galaxy is a very demanding task.

Observational evidence for AGN feedback in early-type galaxies

A major amendment in recent models of hierarchical galaxy formation is the inclusion of so-called AGN feedback. The energy input from an active central massive black hole is invoked to suppress star formation in early-type galaxies at later epochs. A major problem is that this process is poorly understood, and compelling observational evidence for its mere existence is still missing. In search for signatures of AGN feedback, we have compiled a sample of 16,000 early-type galaxies in the redshift range 0.05<z<0.1 from the SDSS database. Key in our approach is the use of a purely morphological selection criterion through visual inspection which produces a sample that is not biased against recent star formation and nuclear activity. The objects with emission (~20 per cent) are offset from the red sequence and form a well-defined pattern in the colour-mass diagram. Star forming early-types inhabit the blue cloud, while early-types with AGN are located considerably closer to and almost on the red sequence. Star formation-AGN composites are found right between these two extremes. We further derive galaxy star formation histories using a novel method that combines multiwavelength photometry from near-UV to near-IR and stellar absorption indices. We find that in those objects deviating from the red sequence star formation occurred several 100 Myr in the past involving 1-10 per cent of the total stellar mass. We identify an evolutionary sequence from star formation via nuclear activity to quiescence. This transition process lasts about 1 Gyr, and the peak AGN phase occurs roughly half a Gyr after the starburst. The most likely interpretation is that star formation is suppressed by nuclear activity in these objects before they settle on the red sequence.

Dynamical Evolution of Globular Clusters in Hierarchical Cosmology

AbstractWe probe the evolution of globular clusters that could form in giant molecular clouds within high-redshift galaxies. Numerical simulations demonstrate that the large and dense enough gas clouds assemble naturally in current hierarchical models of galaxy formation. These clouds are enriched with heavy elements from earlier stars and could produce star clusters in a similar way to nearby molecular clouds. The masses and sizes of the model clusters are in excellent agreement with the observations of young massive clusters. Do these model clusters evolve into globular clusters that we see in our and external galaxies? In order to study their dynamical evolution, we calculate the orbits of model clusters using the outputs of the cosmological simulation of a Milky Way-sized galaxy. We find that at present the orbits are isotropic in the inner 50 kpc of the Galaxy and preferentially radial at larger distances. All clusters located outside 10 kpc from the center formed in the now-disrupted satellite galaxies. The spatial distribution of model clusters is spheroidal, with a power-law density profile consistent with observations. The combination of two-body scattering, tidal shocks, and stellar evolution results in the evolution of the cluster mass function from an initial power law to the observed log-normal distribution.

Cosmic menage a trois: the origin of satellite galaxies on extreme orbits

We examine the orbits of satellite galaxies identified in a suite of N-body/gasdynamical simulations of the formation of L* galaxies in a Lambda cold dark matter universe. The numerical resolution of the simulations allows us to track in detail the orbits of the ∼10 brightest satellites around each primary. Most satellites follow conventional orbits; after turning around, they accrete into their host halo and settle on orbits whose apocentric radii are steadily eroded by dynamical friction. As a result, satellites associated with the primary are typically found within its virial radius, r vir , and have velocities consistent with a Gaussian distribution with mild radial anisotropy. However, a number of outliers are also present. We find that a surprising number (about one-third) of satellites identified at z = 0 are on unorthodox orbits, with apocentres that exceed their turnaround radii. These include a number of objects with extreme velocities and apocentric radii at times exceeding ∼3.5 r vir (or, e.g. ≥1 Mpc when scaled to the Milky Way). This population of satellites on extreme orbits consists typically of the faint member of a satellite pair whose kinship is severed by the tidal field of the primary during first approach. Under the right circumstances, the heavier member of the pair remains bound to the primary, whilst the lighter companion is ejected on to a highly energetic orbit. Since the concurrent accretion of multiple satellite systems is a defining feature of hierarchical models of galaxy formation, a fairly robust prediction of this scenario is that at least some of these extreme objects should be present in the Local Group. We speculate that this three-body ejection mechanism may be the origin of (i) some of the newly discovered high-speed satellites around M31 (such as Andromeda XIV); (ii) some of the distant fast-receding Local Group members, such as Leo I and (iii) the oddly isolated dwarf spheroidals Cetus and Tucana in the outskirts of the Local Group. Our results suggest that care must be exercised when using the orbits of the most weakly bound satellites to place constraints on the total mass of the Local Group.

Evolution of Supermassive Black Hole Binaries and Acceleration of Jet Precession in Galactic Nuclei

Supermassive black hole binaries (SMBHBs) are expected with the hierarchical galaxy formation model. Currently, physics processes dominating the evolution of a SMBHB are unclear. An interesting question is whether we could observationally determine the evolution of SMBHBs and give constraints on the physical processes. Jet precession has been observed in many active galactic nuclei (AGNs) and is generally attributed to disk precession. In this paper we calculate the time variation of jet precession and conclude that jet precession is accelerated in SMBHB systems but decelerated in others. The acceleration of jet precession, -->dPpr/dt, is related to the jet precession timescale, -->Ppr, and the SMBHB evolution timescale, -->?a, as -->dPpr/dt ? ? (Ppr/?a) . Our calculations based on the models for jet precession and SMBHB evolution show that -->dPpr/dt can be as high as about ?1.0, with a typical value of ?0.2, and can be easily detected. We discuss the differential jet precession for NGC 1275 that has been observed in the literature. If its observed rapid acceleration of jet precession is true, the jet precession is due to the orbital motion of an unbound SMBHB with a mass ratio of -->q ? 0.76. When jets precess from ancient bubbles to the currently active jets, the separation of the SMBHB decreases from about 1.46 kpc to 0.80 kpc, with an averaged decreasing velocity of -->da/dt ? 1.54 ? 106 cm s?1 and an evolution timescale of -->?a ? 7.5 ? 107 yr. However, if we assume steady jet precession for many cycles, the observations imply a hard SMBHB with a mass ratio of a -->q ? 0.21 and a separation of -->a ? 0.29 pc.

Luminosity and stellar mass functions of discs and spheroids in the SDSS and the supermassive black hole mass function

Using the galactica (GALaxy Automated ComponenT Image Construction Algorithm) code of Benson et al., we obtain quantitative measurements of spheroid-to-disc ratios for a sample of 8839 galaxies observed in the Sloan Digital Sky Survey (SDSS). We carry out extensive tests of this code and of gim2d, finding that they perform similarly in all respects. From the spheroid and disc luminosities, we construct luminosity and stellar mass functions for each component and estimate the relative luminosity and stellar mass densities of discs and spheroids in the local Universe. Assuming a simple one-to-one mapping between spheroid mass and the mass of a central supermassive black hole, we provide the most accurate determination so far of the black hole mass function in the local universe. From this, we infer a cosmological mass density of black holes of ρ_• = (3.77 ± 0.97) × 10^5 h M_⊙ Mpc^(−3) . We compare our results to predictions from current hierarchical models of galaxy formation and these are found to fare well in predicting the qualitative trends observed. We find that stars in discs contribute 35–51 per cent of the local stellar mass density.

The reversal of the star formation-density relation in the distant universe

We study the relationship between the local environment of galaxies and their star formation rate (SFR) in the Great Observatories Origins Deep Survey, GOODS, at z~1, from ultradeep imaging at 24 microns with the MIPS camera onboard Spitzer. We show that the star formation-density relation observed locally was reversed at z~1: the average SFR of an individual galaxy increased with local galaxy density when the universe was less than half its present age. Hierarchical galaxy formation models (simulated lightcones from the Millennium model) predicted such a reversal to occur only at only at earlier epochs (z>2) and at a lower level. We present a remarkable structure at z~1.016, containing X-ray traced galaxy concentrations, which will eventually merge into a Virgo-like cluster. This structure illustrates how the individual SFR of galaxies increases with density at the ~1-2 Mpc scale. The SFR of z~1 galaxies is found to correlate with stellar mass suggesting that mass plays a role in the observed star formation-density trend. However the specific SFR (=SFR/M*) decreases with stellar mass while it increases with galaxy density, which implies that the environment does directly affect the star formation activity of galaxies. Major mergers do not appear to be the unique or even major cause for this effect since nearly half (46%) of the luminous infrared galaxies (LIRGs) at z~1 present the HST-ACS morphology of spirals, while only a third present a clear signature of major mergers. The remaining galaxies are divided into compact (9%) and irregular (14%) galaxies. Moreover, the specific SFR of major mergers is only marginally stronger than that of spirals. Reproducing the SFR-density relation at z ~ 1 is a new challenge for models, requiring a correct balance between mergers and in-situ star formation at early epochs.

The Star Formation Epoch of the Most Massive Early‐Type Galaxies

We present new spatially resolved Keck spectroscopy of early-type galaxies in three galaxy clusters at z ≈ 0.5. We focus on the fundamental plane (FP) relation and combine the kinematics with structural parameters determined from HST images. The galaxies obey clear FP relations, which are offset from the FP of the nearby Coma Cluster due to passive evolution of the stellar populations. The z ≈ 0.5 data are combined with published data for 11 additional clusters at 0.18 ≤ z ≤ 1.28, to determine the evolution of the mean M/L_B ratio of cluster galaxies with masses M ≳ 10^(11) M_☉, as implied by the FP. We find d log(M/L_B)/dz = -0.555 ± 0.042, stronger evolution than was previously inferred from smaller samples. The observed evolution depends on the luminosity-weighted mean age of the stars in the galaxies, the IMF, selection effects due to progenitor bias, and other parameters. Assuming a normal IMF but allowing for various other sources of uncertainty, we find z_* = 2.01^(+0.22)_(-0.17) for the luminosity-weighted mean star formation epoch. The main uncertainty is the slope of the IMF in the range 1-2 M_☉: we find z_* = 4.0 for a top-heavy IMF with slope x = 0. The M/L_B ratios of the cluster galaxies are compared to those of field early-type galaxies at 0.32 ≤ z ≤ 1.14. Assuming that progenitor bias and the IMF do not depend on environment, we find that the present-day age of stars in massive field galaxies is 4.1% ± 2.0% (≈0.4 Gyr) less than that of stars in massive cluster galaxies. This relatively small age difference is surprising in the context of expectations from standard hierarchical galaxy formation models and provides a constraint on the physical processes that are responsible for halting star formation in the progenitors of today's most massive galaxies.

Mass Deficits, Stalling Radii, and the Merger Histories of Elliptical Galaxies

A binary supermassive black hole leaves an imprint on a galactic nucleus in the form of a "mass deficit," a decrease in the mass of the nucleus due to ejection of stars by the binary. The magnitude of the mass deficit is in principle related to the galaxy's merger history, but the relation has never been quantified. Here, high-accuracy N-body simulations are used to calibrate this relation. Mass deficits are shown to be approximately 0.5M_{12}, with M_{12} the total mass of the binary; the coefficient in this relation depends only weakly on the binary mass ratio or on the galaxy's pre-existing density profile. Hence, after N mergers, the mass deficit is ~0.5 N M_h with M_h the final (current) black hole mass. When compared with observed mass deficits, this result implies between 1 and 3 mergers for most galaxies, in accord with hierarchical galaxy formation models. Implications for binary stalling radii, the origin of hyper-velocity stars, and the distribution of dark matter at the centers of galaxies are discussed.

The Influence of Mass and Environment on the Evolution of Early-Type Galaxies

We report on a uniform comparative analysis of the fundamental parameters of early-type galaxies at z ~ 1 down to MB ≤ -20, both in the field and in clusters. The changes in the /LB ratio from z ~ 1 to today are larger for lower mass galaxies in all environments, and are larger in the field than in clusters for galaxies with the same mass. By deriving ages from the /LB ratio, we estimate the formation redshift for early-type galaxies as a function of galaxy mass and environment. We find that (1) the age of early-type galaxies increases with galaxy mass (downsizing) in all environments, (2) cluster galaxies are older than field galaxies at any given galaxy mass, and (3) this age difference increases with galaxy mass. The first two results confirm similar ones obtained by other means, while the third one is new. The most recent incarnation of the hierarchical models of galaxy formation and evolution is capable of explaining the first two results, but predicts the opposite of our third result. We also find a total lack of massive early-type galaxies ( > 3 × 1011 ☉) with a formation redshift smaller than 2, which cannot be due to selection effects.

GEMS: The destiny of Blue Spheroidal Galaxies

One of the key predictions of hierarchical galaxy formation models is that a significant fraction of elliptical galaxies form in late merging events. One of the most important diagnostics of such an assembly is the existence of blue spheroidal galaxies, which have spheroid-dominated morphologies and blue colors indicating recent star formation, as an intermediate step in the evolution of elliptical galaxies.

Star Formation in Satellite Galaxies

The study of satellite galaxies can provide information on the merging and aggregation processes which, according to the hierarchical clustering models, form the larger spiral galaxies we observe. With the aim of testing hierarchical models of galaxy formation, we have conducted an observational program which comprises H$\alpha$ imaging for both the parent and the satellite galaxies, taken from the compilation by Zaritsky et al. (1997) that contains 115 galaxies orbiting 69 primary isolated spiral galaxies. We have observed a subsample of 37 spiral and irregular galaxies taken from the compilation mentioned above. The aim of this study is to determine star formation properties of the sample galaxies. In this work we present the preliminary results of this program that we have carried out with the 1.8-m Vatican Telescope (VATT).

Tidally Triggered Star Formation in Close Pairs of Galaxies: Major and Minor Interactions

We study star formation in a sample of 345 galaxies in 167 pairs and compact groups drawn from the original CfA2 Redshift Survey and from a follow-up search for companions. We construct our sample with attention to including pairs with luminosity contrast |\Delta m_R| >= 2. These 57 galaxies with |\Delta m_R| >= 2 provide a set of nearby representative cases of minor interactions, a central feature of the hierarchical galaxy formation model. Here we report the redshifts and positions of the 345 galaxies in our sample, and of 136 galaxies in apparent pairs that are superpositions. In the pairs sample as a whole, there are strong correlations between the equivalent width of the H\alpha emission line and the projected spatial and the line-of-sight velocity separation of the pair. For pairs of small luminosity contrast, |\Delta m_R| = 2, we detect no correlation between the equivalent width of H\alpha and the projected spatial separation. The relative luminosity of the companion galaxy is more important in a gravitational tidal interaction than the intrinsic luminosity of the galaxy. Central star formation across the entire pairs sample depends strongly on the luminosity ratio, |\Delta m_R|, a reasonable proxy for the mass ratio of the pair; pairs composed of similarly luminous galaxies produce the strongest bursts of star formation. Pairs with |\Delta m_R| >= 2 rarely have EW(H\alpha) >~ 70 Ang.

Type Ia Supernovae in a Hierarchical Galaxy Formation Model: The Milky Way

We investigate chemical evolution in Milky Way-like galaxies based on the cold dark matter model in which cosmic structures form via hierarchical merging. We introduce chemical enrichment due to Type Ia supernovae (SNe Ia) into the Mitaka semianalytic galaxy formation model developed by Nagashima & Yoshii. For the first time we derive distributions of stellar metallicities and their ratios in Milky Way-like galaxies treating chemical enrichment due to SNe Ia in a hierarchical galaxy formation model self-consistently. As a first attempt, we assume all SNe Ia to have the same lifetime and assume instantaneous recycling for Type II supernovae (SNe II). We find that our model reproduces well the metal abundance ratio [O/Fe] against [Fe/H] and the iron metallicity distribution function in the solar neighborhood. This means that the so-called G dwarf problem is resolved by the hierarchical formation of galaxies, and a gas infall term introduced in traditional monolithic collapse models to solve this problem is well explained by the mixture of some physical processes such as hierarchical merging of dark halos, gas cooling, energy feedback, and injection of gas and metals into hot gas due to supernovae. We also discuss how physical processes affect the metal abundance ratio by varying the lifetime of SNe Ia, the star formation timescale, and the strength of supernova feedback. We find that the supernova feedback plays a key role among them, and therefore there is no one-to-one correspondence between star formation histories and stellar metallicity ratio distributions.

Soft gamma-ray background and light dark matter annihilation

The bulk of the extragalactic background between 10 keV and 10 GeV is likely to be explained by the emission of Seyfert galaxies, type Ia supernovae, and blazars. However, as revealed by the INTEGRAL satellite, the bulge of our galaxy is an intense source of a 511 keV gamma-ray line, indicating the production of a large number of positrons that annihilate. The origin of the latter is debated, and they could be produced, in particular, by the ($S$- or $P$-wave) annihilation of light dark matter particles into ${e}^{+}{e}^{\ensuremath{-}}$. In any case, the cumulated effect of similar sources at all redshifts could lead to a new background of hard x-ray and soft gamma-ray photons. On the basis of the hierarchical model of galaxy formation, we compute analytically the SNIa contribution to the background, and add it to Seyfert and blazars emission models. Confronting these expected contributions to observation, we find that any extra contribution to this unresolved background around 511 keV should be lower than about $4\text{ }\text{ }\mathrm{keV}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}\text{ }{\mathrm{sr}}^{\ensuremath{-}1}$. We also estimate analytically the extragalactic background due to dark matter annihilation, increasing the accuracy of the earlier computations. Indeed, we take into account the large positron escape fraction from low mass dark matter halos, unable to confine a dense and magnetized interstellar medium. Our new background estimate turns out to be 1 order of magnitude lower so that the hypothesis of a light dark matter candidate remains compatible with the observed extragalactic background for a wider range of particle masses and cross sections.

The NOAO Fundamental Plane Survey - III. Variations in the stellar populations of red-sequence galaxies from the cluster core to the virial radius

We analyse absorption line-strength indices for ∼3000 red-sequence galaxies in 94 nearby clusters to investigate systematic variations of their stellar content with location in the host cluster. The data are drawn from the National Optical Astronomy Observatory (NOAO) Fundamental Plane Survey. Our adopted method is a generalization of that introduced by Nelan et al. to determine the global age–mass and metallicity–mass relations from the same survey. We find strong evidence for a change in galaxy properties, at fixed mass, over a range from the cluster centre to the virial radius, R200. For example, red-sequence galaxies further out in the clusters have weaker Mgb5177 (at ∼8σ significance) and stronger Hβ and Hγ absorption (∼3σ, ∼4σ) than galaxies of the same velocity dispersion in the cluster cores. The Fe5270 and Fe5335 indices show only very weak trends with radius. Using a total of 12 indices, the pattern of cluster-centric gradients is considered in light of their different dependences on stellar age and chemical composition. The measured gradients for all 12 indices can be reproduced by a model in which red-sequence galaxies at ∼1-R200 have on average younger ages (by 15 ± 4 per cent) and lower α-element abundance ratios (by 10 ± 2 per cent) than galaxies of the same velocity dispersion but located near the cluster centres. For the total metallicity, Z/H, no significant gradient is found (2 ± 3 per cent larger at R200 than in the cores). There are hints that the age trend may be stronger for galaxies of lower mass and/or for galaxies with more discy morphology. We show, however, that the trends cannot be driven primarily by changes in the morphological mix as a function of radius. The cluster-centric age and [α/Fe] gradients are in the sense expected if galaxies in the cluster core were accreted at an earlier epoch than those at larger radii, and if this earlier accretion contributed to an earlier cessation of star formation. The size of the observed age trend is comparable to predictions from semi-analytic models of hierarchical galaxy formation.

Evidence for Rapidly Spinning Black Holes in Quasars

It has long been believed that accretion onto supermassive black holes powers quasars, but there has been relatively few observational constraints on the spins of the black holes. We address this problem by estimating the average radiative efficiencies of a large sample of quasars selected from the Sloan Digital Sky Survey, by combining their luminosity function and their black hole mass function. Over the redshift interval $0.4<z<2.1$, we find that quasars have average radiative efficiencies of $\sim 30% - 35%$, strongly suggesting that their black holes are rotating very fast, with specific angular momentum $a \approx 1$, which stays roughly constant with redshift. The average radiative efficiency could be reduced by a factor of $\sim$2, depending on the adopted zeropoint for the black hole mass scale. The inferred large spins and their lack of significant evolution are in agreement with the predictions of recent semi-analytical models of hierarchical galaxy formation if black holes gain most of their mass through accretion.

Significant primordial star formation at redshifts z ≈ 3–4

Four recent observational results have challenged our understanding of high-redshift galaxies, as they require the presence of far more ultraviolet photons than should be emitted by normal stellar populations. First, there is significant ultraviolet emission from Lyman break galaxies (LBGs) at wavelengths shorter than 912 A. Second, there is strong Lyman alpha emission from extended 'blobs' with little or no associated apparent ionizing continuum. Third, there is a population of galaxies with unusually strong Lyman alpha emission lines. And fourth, there is a strong He II (1,640 A) emission line in a composite of LBGs. The proposed explanations for the first three observations are internally inconsistent, and the fourth puzzle has remained hitherto unexplained. Here we show that all four problems are resolved simultaneously if 10-30 per cent of the stars in many galaxies at z approximately 3-4 are mainly primordial--unenriched by elements heavier than helium ('metals'). Most models of hierarchical galaxy formation assume efficient intragalactic metal mixing, and therefore do not predict metal-free star formation at redshifts significantly below z approximately 5. Our results imply that micromixing of metals within galaxies is inefficient on an approximately gigayear timescale, a conclusion that can be verified with higher-resolution simulations, and future observations of the He ii emission line.