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Register a new userInsights into the Formation and Evolution History of the Galactic Disk System
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We present a kinematic analysis of a sample of 23,908 G- and K-type dwarfs in the Galactic disk. Based on the $alpha$-abundance ratio, [$alpha$/Fe], we separated our sample into low-$alpha$ thin-disk and high-$alpha$ thick-disk stars. We find a $V_{rm phi}$ gradient of $-$28.2 km s$^{-1}$ dex$^{-1}$ over [Fe/H] for the thin disk, and an almost flat trend of the velocity dispersions of $V_{rm R}$, $V_{rm phi}$, and $V_{rm Z}$ components with [Fe/H]. The metal-poor (MP; [Fe/H] $<$ $-$0.3) thin-disk stars with low-$V_{rm phi}$ velocities have high eccentricities ($e$) and small perigalacticon distances ($r_{rm p}$), while the high-$V_{rm phi}$ MP thin-disk stars possess low $e$ and large $r_{rm p}$. Interestingly, half of the super metal-rich ([Fe/H] $>$ $+$0.1) stars in the thin disk exhibit low-$e$, solar-like orbits. Accounting for the inhomogeneous metallicity distribution of the thin-disk stars with various kinematics requires radial migration by churning $-$ it apparently strongly influences the current structure of the thin disk; we cannot rule out the importance of blurring for the high-$e$ stars. We derive a rotation velocity gradient of $+$36.9 km s$^{-1}$ dex$^{-1}$ for the thick disk, and decreasing trends of velocity dispersions with increasing [Fe/H]. The thick-disk population also has a broad distribution of eccentricity, and the number of high-$e$ stars increases with decreasing [Fe/H]. These kinematic behaviors could be the result of a violent mechanism, such as a gas-rich merger or the presence of giant turbulent clumps, early in the history of its formation. Dynamical heating by minor mergers and radial migration may also play roles in forming the current thick-disk structure.
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Satellites constitute an important fraction of the overall galaxy population and are believed to form in dark matter subhalos. Here we use the cosmological hydrodynamic simulation TNG100 to investigate how the formation histories of subhalos affect the properties and evolution of their host galaxies. We use a scaled formation time ($a_{rm nf}$) to characterize the mass assembly histories of the subhalos before they are accreted by massive host halos. We find that satellite galaxies in young subhalos (low $a_{rm nf}$) are less massive and more gas rich, and have stronger star formation and a higher fraction of ex situ stellar mass than satellites in old subhalos (high $a_{rm nf}$). Furthermore, these low $a_{rm nf}$ satellites require longer timescales to be quenched as a population than the high $a_{rm nf}$ counterparts. We find very different merger histories between satellites in fast accretion (FA, $a_{rm nf}<1.3$) and slow accretion (SA, $a_{rm nf}>1.3$) subhalos. For FA satellites, the galaxy merger frequency dramatically increases just after accretion, which enhances the star formation at accretion. While, for SA satellites, the mergers occur smoothly and continuously across the accretion time. Moreover, mergers with FA satellites happen mainly after accretion, while a contrary trend is found for SA satellites. Our results provide insight into the evolution and star formation quenching of the satellite population.
There is a well known disparity between the evolution the star formation rate density, {psi}*, and the abundance of neutral hydrogen (HI), the raw material for star formation. Recently, however, we have shown that {psi}* may be correlated with the fraction of cool atomic gas, as traced through the 21-cm absorption of HI. This is expected since star formation requires cold (T ~ 10 K) gas and so this could address the issue of why the star formation rate density does not trace the bulk atomic gas. The data are, however, limited to redshifts of z < 2, where both {psi}* and the cold gas fraction exhibit a similar steep climb from the present day (z = 0), and so it is unknown whether the cold gas fraction follows the same decline as {psi}* at higher redshift. In order to address this, we have used unpublished archival observations of 21-cm absorption in high redshift damped Lyman-{alpha} absorption systems to increase the sample at z > 2. The data suggest that the cold gas fraction does exhibit a decrease, although this is significantly steeper than {psi}* at z ~ 3. This is, however, degenerate with the extents of the absorbing galaxy and the background continuum emission and upon removing these, via canonical evolution models, we find the mean spin temperature of the gas to be <T> ~ 3000 K, compared to the ~2000 K expected from the fit at z < 2. These temperatures are consistent with the observed high neutral hydrogen column densities, which require T < 4000 K in order for the gas not to be highly ionised.
We have assembled a sample of 5 X-ray-absorbed and submm-luminous type 1 QSOs at $z sim 2$ which are simultaneously growing their central black holes through accretion and forming stars copiously. We present here the analysis of their rest-frame UV to submm Spectral Energy Distributions (SEDs), including new Herschel data. Both AGN (direct and reprocessed) and Star Formation (SF) emission are needed to model their SEDs. From the SEDs and their UV-optical spectra we have estimated the masses of their black holes $M_{BH}sim 10^{9}-10^{10},M_{odot}$, their intrinsic AGN bolometric luminosities $L_{BOL}sim(0.8 - 20)times 10^{13} L_{odot}$, Eddington ratios $L_{BOL}/L_{Edd}sim 0.1 - 1.1$ and bolometric corrections $L_{BOL}/L_{X,2-10}sim 30 - 500$. These values are common among optically and X-ray-selected type 1 QSOs (except for RX~J1249), except for the bolometric corrections, which are higher. These objects show very high far-infrared luminosities $L_{FIR}sim$ (2 - 8)$times10^{12},M_{odot}$ and Star Formation Rates SFR$sim 1000 M_{odot}/$y. From their $L_{FIR}$ and the shape of their FIR-submm emission we have estimated star-forming dust masses of $M_{DUST}sim 10^9,M_odot$. We have found evidence of a tentative correlation between the gas column densities of the ionized absorbers detected in X-ray (N$_{H_{ion}}$) and $SFR$. Our computed black hole masses are amongst the most massive known.
We present a detailed determination and analysis of 3D stellar mass distribution of the Galactic disk for mono-age populations using a sample of 0.93 million main-sequence turn-off and subgiant stars from the LAMOST Galactic Surveys. Our results show (1) all stellar populations younger than 10,Gyr exhibit strong disk flaring, which is accompanied with a dumpy vertical density profile that is best described by a $sech^n$ function with index depending on both radius and age; (2) Asymmetries and wave-like oscillations are presented in both the radial and vertical direction, with strength varying with stellar populations; (3) As a contribution by the Local spiral arm, the mid-plane stellar mass density at solar radius but 400--800,pc (3--6$^circ$) away from the Sun in the azimuthal direction has a value of $0.0594pm0.0008$,$M_odot$/pc$^3$, which is 0.0164,$M_odot$/pc$^3$ higher than previous estimates at the solar neighborhood. The result causes doubts on the current estimate of local dark matter density; (4) The radial distribution of surface mass density yields a disk scale length evolving from $sim$4,kpc for the young to $sim$2,kpc for the old populations. The overall population exhibits a disk scale length of $2.48pm0.05$,kpc, and a total stellar mass of $3.6(pm0.1)times10^{10}$,$M_odot$ assuming $R_{odot}=8.0$,kpc, and the value becomes $4.1(pm0.1)times10^{10}$,$M_odot$ if $R_{odot}=8.3$,kpc; (5) The disk has a peak star formation rate ({rm SFR}) changing from 6--8,Gyr at the inner to 4--6,Gyr ago at the outer part, indicating an inside-out assemblage history. The 0--1,Gyr population yields a recent disk total {rm SFR} of $1.96pm0.12$,$M_odot$/yr.
In this paper we give a brief review of the astrophysics of active galactic nuclei (AGN). After a general introduction motivating the study of AGNs, we discuss our present understanding of the inner workings of the central engines, most likely accreting black holes with masses between a million and ten billion solar masses. We highlight recent results concerning the jets (collimated outflows) of AGNs derived from X-ray observations (Chandra) of kpc-scale jets and gamma-ray observations of AGNs (Fermi, Cherenkov telescopes) with jets closely aligned with the lines of sight (blazars), and discuss the interpretation of these observations. Subsequently, we summarize our knowledge about the cosmic history of AGN formation and evolution. We conclude with a description of upcoming observational opportunities.
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