The Inside-out Growth of the Galactic Disk

© 2019. The American Astronomical Society. All rights reserved.. We quantify the inside-out growth of the Milky Way's low-α stellar disk, modeling the ages, metallicities, and Galactocentric radii of APOGEE red clump stars with 6 kpc < R < 13 kpc. The current stellar distribution differs significantly from that expected from the star formation history due to the redistribution of stars through radial orbit mixing. We propose and fit a global model for the Milky Way disk, specified by an inside-out star formation history, radial orbit mixing, and an empirical, parametric model for its chemical evolution. We account for the spatially complex survey selection function, and find that the model fits all data well. We find distinct inside-out growth of the Milky Way disk; the best-fit model implies that the half-mass radius of the Milky Way disk has grown by 43% over the last 7 Gyr. Yet, such inside-out growth still results in a present-day age gradient weaker than 0.1 Gyr kpc-1. Our model predicts the half-mass and half-light sizes of the Galactic disk at earlier epochs, which can be compared to the observed redshift-size relations of disk galaxies. We show that radial orbit migration can reconcile the distinct disk-size evolution with redshift, also expected from cosmological simulations, with the modest present-day age gradients seen in the Milky Way and other galaxies

The fundamental plane of cluster elliptical galaxies at z=1.25

Using deep Hubble Space Telescope Advanced Camera for Surveys imaging and Very Large Telescope FOcal Reducer/low dispersion Spectrograph 2 spectra, we determined the velocity dispersions, effective radii, and surface brightnesses for four early-type galaxies in the z = 1.237 cluster RDCS 1252.9 - 2927. All four galaxies are massive, greater than 10(11) M-.. These four galaxies, combined with three from RDCS 0848 + 4453 at z = 1.276, establish the fundamental plane of massive early-type cluster galaxies at (z) over bar = 1.25. The offset of the fundamental plane shows that the luminosity evolution in rest-frame B is Deltaln(M/L-B) = (-0.98 +/- 0.06)z for galaxies with M > 10(11.5) M-.. To reproduce the observed mass-to-light ratio (M/L) evolution, we determine the characteristic age of the stars in these M > 10(11.5) M-. galaxies to be 3.0(-0.3)(+0.3) Gyr; i.e., z* = 3.4(-0.4)(+0.5). Including selection effects caused by morphological bias (the "progenitor bias"), we estimate an age of 2.1(-0.2)(+0.2) Gyr, or z* = 2.3(-0.2)(+0.2) for the elliptical galaxy population. Massive cluster early-type galaxies appear to have a large fraction of stars that formed early in the history of the universe. However, there is a large scatter in the derived M/L values, which is confirmed by the spread in the galaxies' colors. Two lower mass galaxies in our sample have much lower values, implying significant star formation close to the epoch of observation. Thus, even in the centers of massive clusters, there appears to have been significant star formation in some massive, M similar or equal to 10(11) M-., galaxies at z similar or equal to 1.

Spectroscopic confirmation of a substantial population of luminous red galaxies at redshifts z ≳ 2

We confirm spectroscopically the existence of a population of galaxies at z greater than or similar to 2 with rest-frame optical colors similar to normal nearby galaxies. The galaxies were identified by their red near-infrared colors in deep images obtained with the Infrared Spectrometer and Array Camera on the Very Large Telescope of the field around the foreground cluster MS 1054-03. Redshifts of six galaxies with J(s)-K-s > 2.3 were measured from optical spectra obtained with the W. M. Keck telescope. Five out of six are in the range, demonstrating that the 2.43 = z = 3.52 J(s)-K-s color selection is quite efficient. The rest-frame ultraviolet spectra of confirmed z > 2 galaxies display a range of properties, with two galaxies showing emission lines characteristic of active galactic nuclei, two having Lyalpha in emission, and one showing interstellar absorption lines only. Their full spectral energy distributions are well described by constant star formation models with ages 1.4-2.6 Gyr, except for one galaxy whose colors indicate a dusty starburst. The confirmed z > 2 galaxies are very luminous: their K-s magnitudes are in the range 19.2-19.9, corresponding to rest-frame absolute V magnitudes from -24.8 to -23.2. Assuming that our bright spectroscopic sample is representative for the general population of J(s)-K-s selected objects, we find that the surface density of red z greater than or similar to 2 galaxies is approximate to0.9 arcmin(-2) to K-s = 21. The surface density is comparable to that of Lyman break-selected galaxies with K-s < 21, when corrections are made for the different redshift distributions of the two samples. Although there will be some overlap between the two populations, most "optical-break" galaxies are too faint in the rest-frame ultraviolet to be selected as Lyman break galaxies. The most straightforward interpretation is that star formation in typical optical-break galaxies started earlier than in typical Lyman break galaxies. Optical-break galaxies may be the oldest and most massive galaxies yet identified at, and they z 1 2 could evolve into early-type galaxies and bulges

Simulating and interpreting deep observations in the Hubble Ultra Deep Field with the JWST/NIRSpec low-resolution 'prism'

The James Webb Space Telescope (JWST) will enable the detection of optical emission lines in galaxies spanning a broad range of luminosities out to redshifts z 10. Measurements of key galaxy properties, such as star formation rate and metallicity, through these observations will provide unique insight into, e.g. the role of feedback from stars and active galactic nuclei (AGNs) in regulating galaxy evolution, the co-evolution of AGNs and host galaxies, the physical origin of the 'main sequence' of star-forming galaxies, and the contribution by star-forming galaxies to cosmic reionization. We present an original framework to simulate and analyse observations performed with the near-infrared spectrograph (NIRSpec) on board JWST. We use the BEAGLE tool (Bayesian Analysis of GaLaxy sEds) to build a semi-empirical catalogue of galaxy spectra based on photometric spectral energy distributions of dropout galaxies in the Hubble Ultra Deep Field (HUDF).We demonstrate that the resulting catalogue of galaxy spectra satisfies different types of observational constraints on high-redshift galaxies, and use it as an input to simulate NIRSpec/prism (R∼100) observations.We show that a single 'deep' (∼100 ks) NIRSpec/prism pointing in the HUDF will enable S/N > 3 detections of multiple optical emission lines in∼30 (∼60) galaxies at z6 (z ∼ 4 - 6) down tomF160W 30 AB mag. Such observations will allowmeasurements of galaxy star formation rates, ionization parameters, and gas-phasemetallicitieswithin factors of 1.5,mass-to-light ratioswithin a factor of 2, galaxy ages within a factor of 3, and V-band attenuation optical depths with a precision of 0.3

Origin of chemically distinct discs in the Auriga cosmological simulations

The stellar disc of the Milky Way shows complex spatial and abundance structure that is central to understanding the key physical mechanisms responsible for shaping our Galaxy. In this study, we use six very high resolution cosmological zoom-in simulations of Milky Way-sized haloes to study the prevalence and formation of chemically distinct disc components. We find that our simulations develop a clearly bimodal distribution in the [α/Fe]-[Fe/H] plane. We find two main pathways to creating this dichotomy, which operate in different regions of the galaxies: (a) an early (z > 1) and intense high-[α/Fe] star formation phase in the inner region (R ≲ 5 kpc) induced by gas-rich mergers, followed by more quiescent low-[α/Fe] star formation; and (b) an early phase of high-[α/Fe] star formation in the outer disc followed by a shrinking of the gas disc owing to a temporarily lowered gas accretion rate, after which disc growth resumes. In process (b), a double-peaked star formation history around the time and radius of disc shrinking accentuates the dichotomy. If the early star formation phase is prolonged (rather than short and intense), chemical evolution proceeds as per process (a) in the inner region, but the dichotomy is less clear. In the outer region, the dichotomy is only evident if the first intense phase of star formation covers a large enough radial range before disc shrinking occurs; otherwise, the outer disc consists of only low-[α/Fe] sequence stars. We discuss the implication that both processes occurred in the Milky Way

Confirming chemical clocks: asteroseismic age dissection of the Milky Way disk(s)

Investigations of the origin and evolution of the Milky Way disk have long relied on chemical and kinematic identification of its components to reconstruct our Galactic past. Difficulties in determining precise stellar ages have restricted most studies to small samples, normally confined to the solar neighbourhood. Here we break this impasse with the help of asteroseismic inference and perform a chronology of the evolution of the disk throughout the age of the Galaxy. We chemically dissect the Milky Way disk population using a sample of red giant stars spanning out to 2~kpc in the solar annulus observed by the {\it Kepler} satellite, with the added dimension of asteroseismic ages. Our results reveal a clear difference in age between the low- and high-$\alpha$ populations, which also show distinct velocity dispersions in the $V$ and $W$ components. We find no tight correlation between age and metallicity nor [$\alpha$/Fe] for the high-$\alpha$ disk stars. Our results indicate that this component formed over a period of more than 2~Gyr with a wide range of [M/H] and [$\alpha$/Fe] independent of time. Our findings show that the kinematic properties of young $\alpha$-rich stars are consistent with the rest of the high-$\alpha$ population and different from the low-$\alpha$ stars of similar age, rendering support to their origin being old stars that went through a mass transfer or stellar merger event, making them appear younger, instead of migration of truly young stars formed close to the Galactic bar

Confirming chemical clocks: asteroseismic age dissection of the Milky Way disc(s)

Investigations of the origin and evolution of the Milky Way disc have long relied on chemical and kinematic identifications of its components to reconstruct our Galactic past. Difficulties in determining precise stellar ages have restricted most studies to small samples, normally confined to the solar neighbourhood. Here, we break this impasse with the help of asteroseismic inference and perform a chronology of the evolution of the disc throughout the age of the Galaxy. We chemically dissect the Milky Way disc population using a sample of red giant stars spanning out to 2 kpc in the solar annulus observed by the Kepler satellite, with the added dimension of asteroseismic ages. Our results reveal a clear difference in age between the low- and high-α populations, which also show distinct velocity dispersions in the V and W components. We find no tight correlation between age and metallicity nor [α/Fe] for the high-α disc stars. Our results indicate that this component formed over a period of more than 2 Gyr with a wide range of [M/H] and [α/Fe] independent of time. Our findings show that the kinematic properties of young α-rich stars are consistent with the rest of the high-α population and different from the low-α stars of similar age, rendering support to their origin being old stars that went through a mass transfer or stellar merger event, making them appear younger, instead of migration of truly young stars formed close to the Galactic bar

The RAVE-on Catalog of Stellar Atmospheric Parameters and Chemical Abundances for Chemo-dynamic Studies in the Gaia Era

The orbits, atmospheric parameters, chemical abundances, and ages of individual stars in the Milky Way provide the most comprehensive illustration of galaxy formation available. The Tycho-Gaia Astrometric Solution (TGAS) will deliver astrometric parameters for the largest ever sample of Milky Way stars, though its full potential cannot be realized without the addition of complementary spectroscopy. Among existing spectroscopic surveys, the RAdial Velocity Experiment (RAVE) has the largest overlap with TGAS ($\gtrsim$200,000 stars). We present a data-driven re-analysis of 520,781 RAVE spectra using The Cannon. For red giants, we build our model using high-fidelity APOGEE stellar parameters and abundances for stars that overlap with RAVE. For main-sequence and sub-giant stars, our model uses stellar parameters from the K2/EPIC. We derive and validate effective temperature $T_{\rm eff}$, surface gravity $\log{g}$, and chemical abundances of up to seven elements (O, Mg, Al, Si, Ca, Fe, Ni). We report a total of 1,685,851 elemental abundances with a typical precision of 0.07 dex, a substantial improvement over previous RAVE data releases. The synthesis of RAVE-on and TGAS is the most powerful data set for chemo-dynamic analyses of the Milky Way ever produced

The Gaia-ESO Survey: Hydrogen lines in red giants directly trace stellar mass

Red giant stars are perhaps the most important type of stars for Galactic and extra-galactic archaeology: they are luminous, occur in all stellar populations, and their surface temperatures allow precise abundance determinations for many different chemical elements. Yet, the full star formation and enrichment history of a galaxy can be traced directly only if two key observables can be determined for large stellar samples - age and chemical composition. While spectroscopy is a powerful method to analyse the detailed abundances of stars, stellar ages are the "missing link in the chain", since they are not a direct observable. However, spectroscopy should be able to estimate stellar masses, which for red giants directly infer ages provided their chemical composition is known. Here we establish a new empirical relation between the shape of the hydrogen line in the observed spectra of red giants and stellar mass determined from asteroseismology. The relation allows to determine stellar masses and ages with the accuracy of 10-15%. The method can be used with confidence for stars in the following range of stellar parameters: 4000 < Teff < 5000 K, 0.5 < log g < 3.5, -2.0 < [Fe/H] < 0.3, and luminosities log L/LSun < 2.5. Our analysis provides observational evidence that the Halpha spectral characteristics of red giant stars are tightly correlated with their mass and therefore their age. We also show that the method samples well all stellar populations with ages above 1 Gyr. Targeting bright giants, the method allows to obtain simultaneous age and chemical abundance information far deeper than would be possible with asteroseismology, extending the possible survey volume to remote regions of the Milky Way and even to neighbouring galaxies like Andromeda or the Magellanic Clouds already with present instrumentation, like VLT and Keck facilities

The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope II. Multi-object spectroscopy (MOS)

We provide an overview of the capabilities and performance of the Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST) when used in its multi-object spectroscopy (MOS) mode employing a novel Micro Shutter Array (MSA) slit device. The MSA consists of four separate 98 arcsec $\times$ 91 arcsec quadrants each containing $365\times171$ individually addressable shutters whose open areas on the sky measure 0.20 arcsec $\times$ 0.46 arcsec on a 0.27 arcsec $\times$ 0.53 arcsec pitch. This is the first time that a configurable multi-object spectrograph has been available on a space mission. The levels of multiplexing achievable with NIRSpec MOS mode are quantified and we show that NIRSpec will be able to observe typically fifty to two hundred objects simultaneously with the pattern of close to a quarter of a million shutters provided by the MSA. This pattern is fixed and regular, and we identify the specific constraints that it yields for NIRSpec observation planning. We also present the data processing and calibration steps planned for the NIRSpec MOS data. The significant variation in size of the mostly diffraction-limited instrument point spread function over the large wavelength range of 0.6-5.3 $\mu$m covered by the instrument, combined with the fact that most targets observed with the MSA cannot be expected to be perfectly centred within their respective slits, makes the spectrophotometric and wavelength calibration of the obtained spectra particularly complex. These challenges notwithstanding, the sensitivity and multiplexing capabilities anticipated of NIRSpec in MOS mode are unprecedented, and should enable significant progress to be made in addressing a wide range of outstanding astrophysical problems