Galaxy formation and evolution

Abstract:

Hot, dust-obscured galaxies (Hot DOGs) are a population of hyper-luminous obscured quasars identified by WISE. We present ALMA observations of the [C ii] 158 μm fine-structure line and underlying dust continuum emission in a sample of seven of the most extremely luminous (EL; Lbol ≥ 1014 L·) Hot DOGs, at redshifts of z ∼ 3.0-4.6. The [C ii] line is robustly detected in four objects, tentatively in one, and likely to have been red-shifted out of the spectral window in the remaining two, based on additional data. On average, [C ii] is red-shifted by ∼780 km s-1 from rest-frame ultraviolet emission lines. EL Hot DOGs consistently exhibit very high [C ii] surface densities, with S[CII] ∼ 1-2 × 109 L kpc-2, which is as high as the most extreme cases seen in other high-redshift quasars. As a population, EL Hot DOG hosts seem to be roughly centered on the main sequence of star-forming galaxies, but the uncertainties are substantial and individual sources can fall above and below. The average, intrinsic [C ii] and dust continuum sizes (FWHMs) are ∼2.1 kpc and ∼1.6 kpc, respectively, with a very narrow range of line-to-continuum size ratios, 1.61 ± 0.10, suggesting they could be linearly proportional. The [C ii] velocity fields of EL Hot DOGs are diverse: from barely rotating structures, to resolved hosts with ordered, circular motions, to complex, disturbed systems that are likely the result of ongoing mergers. In contrast, all sources display large line-velocity dispersions, FWHM[CII] & 500 km s-1, which are, on average, larger than optically and IR-selected quasars at similar or higher redshifts. We argue that one possible hypothesis that explains the lack of a common velocity structure, the systematically large dispersion of the ionized gas, and the presence of nearby companion galaxies, may be that the EL Hot DOG phase could be recurrent, rather than a single event. The dynamical friction from the frequent in-fall of neighbor galaxies and gas clumps, along with the subsequent quasar feedback, would contribute to the high turbulence of the gas within the host in a process that could potentially trigger not only one continuous EL, obscured event but instead a number of recurrent, shorter-lived episodes as long as external accretion continues.

Abstract:

We investigate the dynamical evolution of an intermediate-mass black hole (IMBH) in a nuclear star cluster hosting a supermassive black hole (SMBH) and both a spherical and a flattened disk-like distribution of stellar-mass objects. We use a direct N-body (φGPU) and an orbit-averaged (N-ring) numerical integrator to simulate the orbital evolution of stars and the IMBH. We find that the IMBH's orbit gradually aligns with the stellar disk if their mutual initial inclination is less than 90°. If it is larger than 90°, i.e., counter-rotating, the IMBH does not align. Initially, the rate of orbital reorientation increases linearly with the ratio of the mass of the IMBH over the SMBH mass, and it is orders of magnitude faster than ordinary (i.e., Chandrasekhar) dynamical friction, particularly for high SMBH masses. The semimajor axes of the IMBH and the stars are approximately conserved. This suggests that the alignment is predominantly driven by orbit-averaged gravitational torques of the stars, a process that may be called resonant dynamical friction. The stellar disk is warped by the IMBH, and ultimately increases its thickness. This process may offer a test for the viability of IMBH candidates in the Galactic Center. Resonant dynamical friction is not limited to IMBHs; any object much more massive than disk particles may ultimately align with the disk. This may have implications for the formation and evolution of black hole disks in dense stellar systems and gravitational wave source populations for LIGO, VIRGO, KAGRA, and LISA.

Abstract:

We report the time-resolved spectral analysis of a bright near-infrared and moderate X-ray flare of Sgr A. We obtained light curves in the M, K, and H bands in the mid-and near-infrared and in the 2 -8 keV and 2 -70 keV bands in the X-ray. The observed spectral slope in the near-infrared band is νLν ? ν0.5 ± 0.2; the spectral slope observed in the X-ray band is νLν ? ν-0.7 ± 0.5. Using a fast numerical implementation of a synchrotron sphere with a constant radius, magnetic field, and electron density (i.e., a one-zone model), we tested various synchrotron and synchrotron self-Compton scenarios. The observed near-infrared brightness and X-ray faintness, together with the observed spectral slopes, pose challenges for all models explored. We rule out a scenario in which the near-infrared emission is synchrotron emission and the X-ray emission is synchrotron self-Compton. Two realizations of the one-zone model can explain the observed flare and its temporal correlation: one-zone model in which the near-infrared and X-ray luminosity are produced by synchrotron self-Compton and a model in which the luminosity stems from a cooled synchrotron spectrum. Both models can describe the mean spectral energy distribution (SED) and temporal evolution similarly well. In order to describe the mean SED, both models require specific values of the maximum Lorentz factor γmax, which differ by roughly two orders of magnitude. The synchrotron self-Compton model suggests that electrons are accelerated to γmax ∼ 500, while cooled synchrotron model requires acceleration up to γmax ∼ 5 × 104. The synchrotron self-Compton scenario requires electron densities of 1010 cm-3 that are much larger than typical ambient densities in the accretion flow. Furthermore, it requires a variation of the particle density that is inconsistent with the average mass-flow rate inferred from polarization measurements and can therefore only be realized in an extraordinary accretion event. In contrast, assuming a source size of 1 RS, the cooled synchrotron scenario can be realized with densities and magnetic fields comparable with the ambient accretion flow. For both models, the temporal evolution is regulated through the maximum acceleration factor γmax, implying that sustained particle acceleration is required to explain at least a part of the temporal evolution of the flare.