We study the properties of black holes and their host galaxies across cosmic time in the Illustris simulation. Illustris is a large-scale cosmological hydrodynamical simulation which resolves a ...(106.5 Mpc)3 volume with more than 12 billion resolution elements and includes state-of-the-art physical models relevant for galaxy formation. We find that the black hole mass density for redshifts z = 0–5 and the black hole mass function at z = 0 predicted by Illustris are in very good agreement with the most recent observational constraints. We show that the bolometric and hard X-ray luminosity functions of active galactic nuclei (AGN) at z = 0 and 1 reproduce observational data very well over the full dynamic range probed. Unless the bolometric corrections are largely underestimated, this requires radiative efficiencies to be on average low, ϵr ≲ 0.1, noting however that in our model radiative efficiencies are degenerate with black hole feedback efficiencies. Cosmic downsizing of the AGN population is in broad agreement with the findings from X-ray surveys, but we predict a larger number density of faint AGN at high redshifts than currently inferred. We also study black hole–host galaxy scaling relations as a function of galaxy morphology, colour and specific star formation rate. We find that black holes and galaxies co-evolve at the massive end, but for low mass, blue and star-forming galaxies there is no tight relation with either their central black hole masses or the nuclear AGN activity.
Abstract
Mergers and the spin of the dark matter halo are factors traditionally believed to determine the morphology of galaxies within a Λ cold dark matter (ΛCDM) cosmology. We study this hypothesis ...by considering approximately 18 000 central galaxies at z = 0 with stellar masses M
* = 109–1012 M⊙ selected from the Illustris cosmological hydrodynamic simulation. The fraction of accreted stars – which measures the importance of massive, recent and dry mergers – increases steeply with galaxy stellar mass, from less than 5 per cent in dwarfs to 80 per cent in the most massive objects, and the impact of mergers on galaxy morphology increases accordingly. For galaxies with M
* ≳ 1011 M⊙, mergers have the expected effect: If gas-poor, they promote the formation of spheroidal galaxies, whereas gas-rich mergers favour the formation and survivability of massive discs. This trend, however, breaks at lower masses. For objects with M
* ≲ 1011 M⊙, mergers do not seem to play any significant role in determining the morphology, with accreted stellar fractions and mean merger gas fractions that are indistinguishable between spheroidal and disc-dominated galaxies. On the other hand, halo spin correlates with morphology primarily in the least massive objects in the sample (M
* ≲ 1010 M⊙), but only weakly for galaxies above that mass. Our results support a scenario where (1) mergers play a dominant role in shaping the morphology of massive galaxies, (2) halo spin is important for the morphology of dwarfs, and (3) the morphology of medium-sized galaxies – including the Milky Way – shows little dependence on galaxy assembly history or halo spin, at least when these two factors are considered individually.
Galaxy mergers are expected to have a significant role in the mass assembly of galaxies in the early universe, but there are very few observational constraints on the merger history of galaxies at z ...> 2. We present the first study of galaxy major mergers (mass ratios <1:4) in mass-selected samples out to z 6. Using all five fields of the Hubble Space Telescope/CANDELS survey and a probabilistic pair-count methodology that incorporates the full photometric redshift posteriors and corrections for stellar mass completeness, we measure galaxy pair-counts for projected separations between 5 and 30 kpc in stellar mass selected samples at 9.7 < log10(M /M ) < 10.3 and log10(M /M ) > 10.3. We find that the major merger pair fraction rises with redshift to z 6 proportional to (1 + z)m, with m = 0.8 0.2 (m = 1.8 0.2) for log10(M /M ) > 10.3 (9.7 < log10(M /M ) < 10.3). Investigating the pair fraction as a function of mass ratio between 1:20 and 1:1, we find no evidence for a strong evolution in the relative numbers of minor to major mergers out to z < 3. Using evolving merger timescales, we find that the merger rate per galaxy ( ) rises rapidly from 0.07 0.01 Gyr−1 at z < 1 to 7.6 2.7 Gyr−1 at z = 6 for galaxies at log10(M /M ) > 10.3. The corresponding comoving major merger rate density remains roughly constant during this time, with rates of Γ 10−4 Gyr−1 Mpc−3. Based on the observed merger rates per galaxy, we infer specific mass accretion rates from major mergers that are comparable to the specific star formation rates for the same mass galaxies at z > 3 - observational evidence that mergers are as important a mechanism for building up mass at high redshift as in situ star formation.
ABSTRACT We study the stellar angular momentum of thousands of galaxies in the Illustris cosmological simulation, which captures gravitational and gas dynamics within galaxies, as well as feedback ...from stars and black holes. We find that the angular momentum of the simulated galaxies matches observations well, and in particular two distinct relations are found for late-type versus early-type galaxies. The relation for late-type galaxies corresponds to the value expected from full conservation of the specific angular momentum generated by cosmological tidal torques. The relation for early-type galaxies corresponds to retention of only ∼30% of that, but we find that those early-type galaxies with low angular momentum at z = 0 nevertheless reside at high redshift on the late-type relation. Some of them abruptly lose angular momentum during major mergers. To gain further insight, we explore the scaling relations in simulations where the galaxy formation physics is modified with respect to the fiducial model. We find that galactic winds with high mass-loading factors are essential for obtaining the high angular momentum relation typical for late-type galaxies, while active galactic nucleus feedback largely operates in the opposite direction. Hence, feedback controls the stellar angular momentum of galaxies, and appears to be instrumental for establishing the Hubble sequence.
ABSTRACT
Using the TNG50 cosmological simulation and observations from the Kilo-Degree Survey (KiDS), we investigate the connection between galaxy mergers and optical morphology in the local Universe ...over a wide range of galaxy stellar masses (8.5 ≤ log (M*/M⊙) ≤ 11).
To this end, we have generated over 16 000 synthetic images of TNG50 galaxies designed to match KiDS observations, including the effects of dust attenuation and scattering, and used the statmorph code to measure various image-based morphological diagnostics in the r-band for both data sets. Such measurements include the Gini–M20 and concentration–asymmetry–smoothness statistics. Overall, we find good agreement between the optical morphologies of TNG50 and KiDS galaxies, although the former are slightly more concentrated and asymmetric than their observational counterparts. Afterwards, we trained a random forest classifier to identify merging galaxies in the simulation (including major and minor mergers) using the morphological diagnostics as the model features, along with merger statistics from the merger trees as the ground truth. We find that the asymmetry statistic exhibits the highest feature importance of all the morphological parameters considered. Thus, the performance of our algorithm is comparable to that of the more traditional method of selecting highly asymmetric galaxies. Finally, using our trained model, we estimate the galaxy merger fraction in both our synthetic and observational galaxy samples, finding in both cases that the galaxy merger fraction increases steadily as a function of stellar mass.
ABSTRACT We present a detailed, multi-wavelength study of star formation (SF) and active galactic nucleus (AGN) activity in 11 near-infrared (IR) selected, spectroscopically confirmed massive ( 1014 ...M ) galaxy clusters at 1 < z < 1.75. Using new deep Herschel/PACS imaging, we characterize the optical to far-IR spectral energy distributions (SEDs) for IR-luminous cluster galaxies, finding that they can, on average, be well described by field galaxy templates. Identification and decomposition of AGNs through SED fittings allows us to include the contribution to cluster SF from AGN host galaxies. We quantify the star-forming fraction, dust-obscured SF rates (SFRs) and specific SFRs for cluster galaxies as a function of cluster-centric radius and redshift. In good agreement with previous studies, we find that SF in cluster galaxies at z 1.4 is largely consistent with field galaxies at similar epochs, indicating an era before significant quenching in the cluster cores (r < 0.5 Mpc). This is followed by a transition to lower SF activity as environmental quenching dominates by z ∼ 1. Enhanced SFRs are found in lower mass ( ) cluster galaxies. We find significant variation in SF from cluster to cluster within our uniformly selected sample, indicating that caution should be taken when evaluating individual clusters. We examine AGNs in clusters from z = 0.5-2, finding an excess AGN fraction at z 1, suggesting environmental triggering of AGNs during this epoch. We argue that our results-a transition from field-like to quenched SF, enhanced SF in lower mass galaxies in the cluster cores, and excess AGNs-are consistent with a co-evolution between SF and AGNs in clusters and an increased merger rate in massive halos at high redshift.
Abstract
We compare mass-selected close pairs at z > 1 with the intrinsic galaxy merger rate in the Illustris Simulations. To do so, we construct three 140 arcmin2 lightcone catalogues and measure ...pair fractions, finding that they change little or decrease with increasing redshift at z > 1. Consistent with current surveys, this trend requires a decrease in the merger-pair observability time, roughly as τ∝(1 + z)−2, in order to measure the merger rates of the same galaxies. This implies that major mergers are more common at high redshift than implied by the simplest arguments assuming a constant observability time. Several effects contribute to this trend: (1) The fraction of massive, major (4:1) pairs that merge by today increases weakly from ∼0.5 at z = 1 to ∼0.8 at z = 3. (2) The median time elapsed between an observed pair and final remnant decreases by a factor of 2 from z ∼ 1 to 3. (3) An increasing specific star formation rate decreases the time during which common stellar-mass-based pair selection criteria could identify the mergers. The average orbit of the pairs at observation time varies only weakly, suggesting that the dynamical time is not varying enough to account by itself for the pair fraction trends. Merging pairs reside in dense regions, having overdensity δ ∼ 10 to ∼100 times greater than the average massive galaxy. We forward model the pairs to reconstruct the merger remnant production rate, showing that it is consistent with a rapid increase in galaxy merger rates at z > 1.
ABSTRACT
We present image-based evolution of galaxy mergers from the Illustris cosmological simulation at 12 time-steps over 0.5 < z < 5. To do so, we created approximately one million synthetic deep ...Hubble Space Telescope and James Webb Space Telescope images and measured common morphological indicators. Using the merger tree, we assess methods to observationally select mergers with stellar mass ratios as low as 10:1 completing within ±250 Myr of the mock observation. We confirm that common one- or two-dimensional statistics select mergers so defined with low purity and completeness, leading to high statistical errors. As an alternative, we train redshift-dependent random forests (RFs) based on 5–10 inputs. Cross-validation shows the RFs yield superior, yet still imperfect, measurements of the late-stage merger fraction, and they select more mergers in bulge-dominated galaxies. When applied to CANDELS morphology catalogues, the RFs estimate a merger rate increasing to at least z = 3, albeit two times higher than expected by theory. This suggests possible mismatches in the feedback-determined morphologies, but affirms the basic understanding of galaxy merger evolution. The RFs achieve completeness of roughly $70{{\ \rm per\ cent}}$ at 0.5 < z < 3, and purity increasing from $10{{\ \rm per\ cent}}$ at z = 0.5–60 per cent at z = 3. At earlier times, the training sets are insufficient, motivating larger simulations and smaller time sampling. By blending large surveys and large simulations, such machine learning techniques offer a promising opportunity to teach us the strengths and weaknesses of inferences about galaxy evolution.
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