Abstract
Collisions and interactions between gas-rich galaxies are thought to be pivotal stages in their formation and evolution, causing the rapid production of new stars, and possibly serving as a ...mechanism for fueling supermassive black holes (BHs). Harnessing the exquisite spatial resolution (∼0${^{\prime\prime}_{.}}$5) afforded by the first ∼170 deg2 of the Hyper Suprime-Cam (HSC) survey, we present our new constraints on the importance of galaxy–galaxy major mergers (1 : 4) in growing BHs throughout the last ∼8 Gyr. Utilizing mid-infrared observations in the WISE all-sky survey, we robustly select active galactic nuclei (AGN) and mass-matched control galaxy samples, totaling ∼140000 spectroscopically confirmed systems at i < 22 mag. We identify galaxy interaction signatures using a novel machine-learning random forest decision tree technique allowing us to select statistically significant samples of major mergers, minor mergers / irregular systems, and non-interacting galaxies. We use these samples to show that galaxies undergoing mergers are a factor of ∼2–7 more likely to contain luminous obscured AGN than non-interacting galaxies, and this is independent of both stellar mass and redshift to z < 0.9. Furthermore, based on our comparison of AGN fractions in mass-matched samples, we determine that the most luminous AGN population (LAGN ≳ 1045 erg s−1) systematically reside in merging systems over non-interacting galaxies. Our findings show that galaxy–galaxy interactions do, on average, trigger luminous AGN activity substantially more often than in secularly evolving non-interacting galaxies, and we further suggest that the BH growth rate may be closely tied to the dynamical time of the merger system.
Abstract
The recent Chandra-JWST discovery of a quasar in the
z
≈ 10.1 galaxy UHZ1 reveals that accreting supermassive black holes were already in place 470 million years after the Big Bang. The ...Chandra X-ray source detected in UHZ1 is a Compton-thick quasar with a bolometric luminosity of
L
bol
∼ 5 × 10
45
erg s
−1
, which corresponds to an estimated black hole (BH) mass of ∼4 × 10
7
M
⊙
, assuming accretion at the Eddington rate. JWST NIRCAM and NIRSpec data yield a stellar mass estimate for UHZ1 comparable to its BH mass. These characteristics are in excellent agreement with prior theoretical predictions for a unique class of transient, high-redshift objects, overmassive black hole galaxies (OBGs) by Natarajan et al., that harbor a heavy initial black hole seed that likely formed from the direct collapse of the gas. Given the excellent agreement between the observed multiwavelength properties of UHZ1 and theoretical model template predictions, we suggest that UHZ1 is the first detected OBG candidate. Our assertion rests on multiple lines of concordant evidence between model predictions and the following observed properties of UHZ1: its X-ray detection and the estimated ratio of the X-ray flux to the IR flux, which is consistent with theoretical expectations for a heavy initial BH seed; its high measured redshift of
z
≈ 10.1, as predicted for the transient OBG stage (9 <
z
< 12); the amplitude and shape of the detected JWST spectral energy distribution (SED) between 1 and 5
μ
m, which is in very good agreement with simulated template SEDs for OBGs; and the extended JWST morphology of UHZ1, which is suggestive of a recent merge and is also expected for the formation of transient OBGs. As the first OBG candidate, UHZ1 provides compelling evidence for the formation of heavy initial seeds from direct collapse in the early Universe.
ABSTRACT
Classifying catalogue objects as stars, galaxies, or active galactic nuclei (AGNs) is a crucial part of any statistical study of galaxies. We describe our pipeline for binary (star/galaxy) ...and multiclass (star/galaxy/Type I AGN/Type II AGN) classification developed for the very deep CLAUDS + HSC-SSP u*grizy data set. Our method uses the XGBoost implementation of gradient boosted trees (GBTs) to train ensembles of models that take photometry, colours, maximum surface brightnesses, and effective radii from all available bands as input, and output the probability that an object belongs to each of the classes under consideration. At iAB < 25 our binary star/galaxy model has AUC = 0.9974 and at the threshold that maximizes our sample’s weighted F1 score, selects a sample of galaxies with 99.7 per cent purity and 99.8 per cent completeness. We test the model’s ability to generalize to objects fainter than those seen during training and find that extrapolation of ∼1−2 mag is reasonable for most applications provided that the galaxies in the training sample are representative of the range of redshifts and colours of the galaxies in the target sample. We also perform an exploratory analysis of the method’s ability to identify AGNs using a small X-ray-selected sample and find that it holds promise for classifying Type I AGN, although it performs less well for Type II AGN. Our results demonstrate that GBTs provide a flexible, robust, and efficient method for performing classification of catalogue objects in large astronomical imaging surveys.
We investigate the effect of active galactic nucleus (AGN) variability on the observed connection between star formation and black hole accretion in extragalactic surveys. Recent studies have ...reported relatively weak correlations between observed AGN luminosities and the properties of AGN hosts, which has been interpreted to imply that there is no direct connection between AGN activity and star formation. However, AGNs may be expected to vary significantly on a wide range of timescales (from hours to Myr) that are far shorter than the typical timescale for star formation (gap 100 Myr). This variability can have important consequences for observed correlations. We present a simple model in which all star-forming galaxies host an AGN when averaged over ~100 Myr timescales, with long-term average AGN accretion rates that are perfectly correlated with the star formation rate (SFR). We show that reasonable prescriptions for AGN variability reproduce the observed weak correlations between SFR and L sub(AGN) in typical AGN host galaxies, as well as the general trends in the observed AGN luminosity functions, merger fractions, and measurements of the average AGN luminosity as a function of SFR. These results imply that there may be a tight connection between AGN activity and SFR over galaxy evolution timescales, and that the apparent similarities in rest-frame colors, merger rates, and clustering of AGNs compared to "inactive" galaxies may be due primarily to AGN variability. The results provide motivation for future deep, wide extragalactic surveys that can measure the distribution of AGN accretion rates as a function of SFR.
ABSTRACT Studies of the physical properties of local elliptical galaxies are shedding new light on galaxy formation. Here we present the hot-gas properties of 33 early-type systems within the MASSIVE ...galaxy survey that have archival Chandra X-ray observations, and we use these data to derive X-ray luminosities ( ) and plasma temperatures ( ) for the diffuse gas components. We combine this with the survey to investigate the X-ray-optical properties of a statistically significant sample of early-type galaxies across a wide range of environments. When X-ray measurements are performed consistently in apertures set by the galaxy stellar content, we deduce that all early types (independent of galaxy mass, environment, and rotational support) follow a universal scaling law such that . We further demonstrate that the scatter in around both K-band luminosity (LK) and the galaxy stellar velocity dispersion ( ) is primarily driven by , with no clear trends with halo mass, radio power, or angular momentum of the stars. It is not trivial to tie the gas origin directly to either stellar mass or galaxy potential. Indeed, our data require a steeper relation between , and than predicted by standard mass-loss models. Finally, we find that is set by the galaxy potential inside the optical effective radius. We conclude that within the innermost 10-30 kpc region, early types maintain pressure-supported hot gas, with a minimum set by the virial temperature, but the majority show evidence for additional heating.
Though smooth, extended spheroidal stellar outskirts have long been observed around nearby dwarf galaxies, it is unclear whether dwarfs generically host an extended stellar halo. We use imaging from ...the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) to measure the shapes of dwarf galaxies out to four effective radii for a sample of 6758 dwarfs at 0.005 < z < 0.2 and 107.0 < M /M < 109.6. We find that dwarfs are slightly triaxial, with (where the ellipsoid is characterized by three principal semiaxes constrained by C ≤ B ≤ A). At M > 108.5 M , the galaxies grow from thick disk-like at one effective radius toward the spheroidal extreme at four effective radii. We also see that although blue dwarfs are, on average, characterized by thinner disks than red dwarfs, both blue and red dwarfs grow more spheroidal as a function of radius. This relation also holds true for a comparison between field and satellite dwarfs. This uniform trend toward relatively spheroidal shapes as a function of radius is consistent with an in situ formation mechanism for stellar outskirts around low-mass galaxies, in agreement with proposed models where star formation feedback produces round stellar outskirts around dwarfs.
Abstract
Quantifying the connection between galaxies and their host dark matter halos has been key for testing cosmological models on various scales. Below
M
⋆
∼ 10
9
M
⊙
, such studies have ...primarily relied on the satellite galaxy population orbiting the Milky Way (MW). Here we present new constraints on the connection between satellite galaxies and their host dark matter subhalos using the largest sample of satellite galaxies in the Local Volume (
D
≲ 12 Mpc) to date. We use 250 confirmed and 71 candidate dwarf satellites around 27 MW-like hosts from the Exploration of Local VolumE Satellites (ELVES) Survey and use the semianalytical
SatGen
model for predicting the population of dark matter subhalos expected in the same volume. Through a Bayesian model comparison of the observed and the forward-modeled satellite stellar mass functions (SSMFs), we infer the satellite stellar-to-halo mass relation. We find that the observed SSMF is best reproduced when subhalos at the low-mass end are populated by a relation of the form
M
⋆
∝
M
peak
α
, with a moderate slope of
α
const
=
2.10
±
0.01
and a low scatter, constant as a function of the peak halo mass, of
σ
const
=
0.06
−
0.05
+
0.07
. A model with a steeper slope (
α
grow
= 2.39 ± 0.06) and a scatter that grows with decreasing
M
peak
is also consistent with the observed SSMF but is not required. Our new model for the satellite–subhalo connection, based on hundreds of Local Volume satellite galaxies, is in line with what was previously derived using only MW satellites.
We present optical long-slit spectroscopy and far-ultraviolet to mid-infrared spectral energy distribution fitting of two diffuse dwarf galaxies, LSBG-285 and LSBG-750, which were recently discovered ...by the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We measure redshifts using H line emission and find that these galaxies are at comoving distances of 25 and 41 Mpc, respectively, after correcting for the local velocity field. They have effective radii of reff = 1.2 and 1.8 kpc and stellar masses of M (2-3) × 107 M . There are no massive galaxies ( ) within a comoving separation of at least 1.5 Mpc from LSBG-285 and 2 Mpc from LSBG-750. These sources are similar in size and surface brightness to ultradiffuse galaxies, except they are isolated, star-forming objects that were optically selected in an environmentally blind survey. Both galaxies likely have low stellar metallicities Z /Z < −1.0 and are consistent with the stellar mass-metallicity relation for dwarf galaxies. We set an upper limit on LSBG-750's rotational velocity of ∼50 km s−1, which is comparable to dwarf galaxies of similar stellar mass with estimated halo masses <1011 M . We find tentative evidence that the gas-phase metallicities in both of these diffuse systems are high for their stellar mass, though a statistically complete, optically selected galaxy sample at very low surface brightness will be necessary to place these results into context with the higher surface brightness galaxy population.
Abstract Drawing from the Chandra archive and using a carefully selected set of nearby dwarf galaxies, we present a calibrated high-mass X-ray binary (HMXB) luminosity function in the low-mass galaxy ...regime and search for an already hinted at dependence on metallicity. Our study introduces a new sample of local dwarf galaxies ( D < 12.5 Mpc and M * < 5 × 10 9 M ⊙ ), expanding the specific star formation rates (sSFR) and gas-phase metallicities probed in previous investigations. Our analysis of the observed X-ray luminosity function indicates a shallower power-law slope for the dwarf galaxy HMXB population. In our study, we focus on dwarf galaxies that are more representative in terms of sSFR compared to prior work. In this regime, the HMXB luminosity function exhibits significant stochastic sampling at high luminosities. This likely accounts for the pronounced scatter observed in the galaxy-integrated HMXB population’s L X /SFR versus metallicity for our galaxy sample. Our calibration is necessary to understand the active galactic nuclei content of low-mass galaxies identified in current and future X-ray survey fields and has implications for binary population synthesis models, as well as X-ray-driven cosmic heating in the early Universe.
Supermassive black hole (SMBH) binaries with masses of ∼108-109M are expected to dominate the contribution to the as-yet undetected gravitational wave background (GWB) signal at the nanohertz ...frequencies accessible to pulsar timing arrays. We currently lack firm empirical constraints on the amplitude of the GWB due to the dearth of confirmed SMBH binaries in the required mass range. Using Hubble Space Telescope/Wide Field Camera 3 images, we have discovered a z ∼ 0.2 quasar hosted in a merger remnant with two closely separated (0 13 or ∼430 pc) continuum cores at the heart of the galaxy SDSS J1010+1413. The two cores are spatially coincident with two powerful O iii-emitting point sources with quasar-like luminosities (LAGN ∼ 5 × 1046 erg s−1), suggesting the presence of a bound SMBH system, each with MBH > 4 × 108M . We place an upper limit on the merging timescale of the SMBH pair of 2.5 billion years, roughly the universe lookback time at z ∼ 0.2. There is likely a population of quasar binaries similar to SDSS J1010+1413 that contribute to a stochastic GWB that should be detected in the next several years. If the GWB is not detected this could indicate that SMBHs merge only over extremely long timescales, remaining as close separation binaries for many Hubble times, the so-called "final-parsec problem."
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