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Register a new userCorrelations between age, kinematics, and chemistry as seen by the RAVE survey
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We explore the connections between stellar age, chemistry, and kinematics across a Galactocentric distance of $7.5 < R,(mathrm{kpc}) < 9.0$, using a sample of $sim 12,000$ intermediate-mass (FGK) turnoff stars observed with the RAdial Velocity Experiment (RAVE) survey. The kinematics of this sample are determined using radial velocity measurements from RAVE, and parallax and proper motion measurements from the Tycho-Gaia Astrometric Solution (TGAS). In addition, ages for RAVE stars are determined using a Bayesian method, taking TGAS parallaxes as a prior. We divide our sample into young ($0 < tau < 3$ Gyr) and old ($8 < tau < 13$ Gyr) populations, and then consider different metallicity bins for each of these age groups. We find significant differences in kinematic trends of young and old, metal-poor and metal-rich, stellar populations. In particular, we find a strong metallicity dependence in the mean Galactocentric radial velocity as a function of radius ($partial {V_{rm R}}/partial R$) for young stars, with metal-rich stars having a much steeper gradient than metal-poor stars. For $partial {V_{phi}}/partial R$, young, metal-rich stars significantly lag the LSR with a slightly positive gradient, while metal-poor stars show a negative gradient above the LSR. We interpret these findings as correlations between metallicity and the relative contributions of the non-axisymmetries in the Galactic gravitational potential (the spiral arms and the bar) to perturb stellar orbits.
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The nature of the Triangulum-Andromeda (TriAnd) system has been debated since the discovery of this distant, low-latitude Milky Way (MW) overdensity more than a decade ago. Explanations for its origin are either as a halo substructure from the disruption of a dwarf galaxy or a distant extension of the Galactic disk. We test these hypotheses using chemical abundances of a dozen TriAnd members from the Sloan Digital Sky Surveys 14th Data Release of Apache Point Observatory Galactic Evolution Experiment (APOGEE) data to compare to APOGEE abundances of stars with similar metallicity from both the Sagittarius (Sgr) dSph, and the outer MW disk. We find that TriAnd stars are chemically distinct from Sgr across a variety of elements, (C+N), Mg, K, Ca, Mn, and Ni, with a separation in [X/Fe] of about 0.1 to 0.4 dex depending on the element. Instead, the TriAnd stars, with a median metallicity of about -0.8, exhibit chemical abundance ratios similar to those of the lowest metallicity ([Fe/H] ~ -0.7) stars in the outer Galactic disk, and are consistent with expectations of extrapolated chemical gradients in the outer disk of the MW. These results suggest that TriAnd is associated with the MW disk, and, therefore, that the disk extends to this overdensity --- i.e., past a Galactocentric radius of 24 kpc --- albeit vertically perturbed about 7 kpc below the nominal disk midplane in this region of the Galaxy.
We apply the method of Burnett & Binney (2010) for the determination of stellar distances and parameters to the internal catalogue of the Radial Velocity Experiment (Steinmetz et al. 2006). Subsamples of stars that either have Hipparcos parallaxes or belong to well-studied clusters, inspire confidence in the formal errors. Distances to dwarfs cooler than ~6000 K appear to be unbiased, but those to hotter dwarfs tend to be too small by ~10% of the formal errors. Distances to giants tend to be too large by about the same amount. The median distance error in the whole sample of 216,000 stars is 28% and the error distribution is similar for both giants and dwarfs. Roughly half the stars in the RAVE survey are giants. The giant fraction is largest at low latitudes and in directions towards the Galactic Centre. Near the plane the metallicity distribution is remarkably narrow and centred on [M/H]-0.04 dex; with increasing |z| it broadens out and its median moves to [M/H] ~ -0.5. Mean age as a function of distance from the Galactic centre and distance |z| from the Galactic plane shows the anticipated increase in mean age with |z|.
Aims: We study the relations between stellar kinematics and chemical abundances of a large sample of RAVE giants in search for selection criteria needed for disentangling different Galactic stellar populations. Methods: We select a sample of 2167 giant stars with signal-to-noise per spectral measurements above 75 from the RAVE chemical catalogue and follow the analysis performed by Gratton and colleagues on 150 subdwarf stars spectroscopically observed at high-resolution. We then use a larger sample of 9131 giants (with signal-to-noise above 60) to investigate the chemo-kinematical characteristics of our stars by grouping them into nine subsamples with common eccentricity ($e$) and maximum distance achieved above the Galactic plane ($Z_max$). Results: The RAVE kinematical and chemical data proved to be reliable by reproducing the results by Gratton et al. obtained with high-resolution spectroscopic data. Our analysis, based on the $e$-$Z_max$ plane combined with additional orbital parameters and chemical information, provides an alternative way of identifying different populations of stars. In addition to extracting canonical thick- and thin-disc samples, we find a group of stars in the Galactic plane ($Z_max<1$ kpc and 0.4 $< e < $0.6), which show homogeneous kinematics but differ in their chemical properties. We interpret this as a clear sign that some of these stars have experienced the effects of heating and/or radial migration, which have modified their original orbits. The accretion origin of such stars cannot be excluded.
We use the second data releases of the ESA Gaia astrometric survey and the high-resolution GALAH spectroscopic survey to analyse the structure of our Galaxys disc components. With GALAH, we separate the alpha-rich and alpha-poor discs (with respect to Fe), which are superposed in both position and velocity space, and examine their distributions in action space. We study the distribution of stars in the zV_z phase plane, for both V_phi and V_R, and recover the remarkable phase spiral discovered by Gaia. We identify the anticipated quadrupole signature in zV_z of a tilted velocity ellipsoid for stars above and below the Galactic plane. By connecting our work with earlier studies, we show that the phase spiral is likely to extend well beyond the narrow solar neighbourhood cylinder in which it was found. The phase spiral is a signature of corrugated waves that propagate through the disc, and the associated non-equilibrium phase mixing. The radially asymmetric distribution of stars involved in the phase spiral reveals that the corrugation, which is mostly confined to the alpha-poor disc, grows in z-amplitude with increasing radius. We present new simulations of tidal disturbance of the Galactic disc by the Sagittarius (Sgr) dwarf. The effect on the zV_z phase plane lasts >2 Gyr but a subsequent disc crossing wipes out the coherent structure. We find that the phase spiral was excited < 0.5 Gyr ago by an object like Sgr with total mass 3 x 10^10 Msun (stripped down from 5 x 10^10 Msun when it first entered the halo) passing through the plane.
Hot, dust-obscured galaxies (Hot DOGs) are a population of hyper-luminous obscured quasars identified by WISE. We present ALMA observations of the [CII] fine-structure line and underlying dust continuum emission in a sample of seven of the most extremely luminous (EL; L$_{rm bol}$ $ge$ 10$^{14}$ L$_odot$) Hot DOGs, at redshifts z ~ 3.0-4.6. The [CII] line is robustly detected in four objects, tentatively in one, and likely red-shifted out of the spectral window in the remaining two based on additional data. On average, [CII] is red-shifted by ~ 780 km/s from rest-frame ultraviolet emission lines. EL Hot DOGs exhibit consistently very high ionized gas surface densities, with $Sigma_{rm [CII]}$ ~ 1-2 x 10$^{9}$ L$_odot$ kpc$^{-2}$; 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 [CII] 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 $pm$ 0.10, suggesting they could be linearly proportional. The [CII] 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 $gtrsim$ 500 km/s, which on average are larger than optically and IR-selected quasars at similar or higher redshifts. We argue that one possible hypothesis for 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, rather than a single event, the EL Hot DOG phase could be recurrent.
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