arXiv subjects

K. C. Freeman

Publications and source records attributed to K. C. Freeman.

At least 19 recordsLinked to original sources

arXiv2025

An Integral Field Spectroscopy Study Of Thin And Thick Discs In IC 2531

Thin and thick discs are prominent components in the Milky Way and other spiral galaxies, but their formation histories are not yet understood, which is particularly true for thick discs. This is partially due to the fact that we lack sufficient understanding of thin and thick discs in other galaxies to determine how common features of the Milky Way's discs are. Here, we conduct an integral field spectroscopy study of the thin and thick discs in the edge-on Milky Way analogue IC 2531, using observational data from the WiFeS IFU spectrograph on the ANU 2.3m telescope. We provide spectral analysis of IC 2531's kinematics and stellar populations in the thin and thick disc regions by conducting pPXF fitting with Vazdekis models. We found that IC 2531's disc above the dust plane generally has chemical properties between the Milky Way's chemical thin and thick disc. IC 2531's thick disc is somewhat similar to the Milky Way's in terms of stellar populations: on average, it is older, more metal-poor than its corresponding thin disc, but similarly alpha-enhanced in general. But we do find a clearly alpha-rich thick disc bin though one of our "thick disc" bins may be dominated by a warped thin disc. These results may help to constrain formation theories of thick discs.

astro-ph.GA↗

arXiv2024

The GECKOS Survey: Identifying kinematic sub-structures in edge-on galaxies

The vertical evolution of galactic discs is governed by the sub-structures within them. We examine the diversity of kinematic sub-structure present in the first 12 galaxies observed from the GECKOS survey, a VLT/MUSE large programme providing a systematic study of 36 edge-on, Milky Way-mass disc galaxies. Employing the nGIST analysis pipeline, we derive the mean line-of-sight stellar velocity ($V_{\star}$), velocity dispersion ($\sigma_{\star}$), skew ($h_{3}$), and kurtosis ($h_{4}$) for the sample, and examine 2D maps and 1D line profiles. Visually, the majority of this sample (8/12) are found to possess boxy-peanut bulges and host the corresponding kinematic structure predicted for stellar bars viewed in projection. Four galaxies exhibit strong evidence for the presence of nuclear discs, including central $h_{3}$-$V_{\star}$ sign mismatch, `croissant'-shaped central depressions in $\sigma_{\star}$ maps, strong gradients in $h_{3}$, and positive $h_{4}$ plateaus over the expected nuclear disc extent. The strength of the $h_{3}$ feature corresponds to the size of the nuclear disc, measured from the $h_{3}$ turnover radius. We can explain the features within the kinematic maps of all sample galaxies via disc structure(s) alone. We do not find any need to invoke the existence of dispersion-dominated bulges. Obtaining the specialised data products for this paper and the broader GECKOS survey required significant development of existing integral field spectroscopic (IFS) analysis tools. Therefore, we also present the nGIST pipeline: a modern, sophisticated, and easy-to-use pipeline for the analysis of galaxy IFS data. We conclude that the variety of kinematic sub-structures seen in GECKOS galaxies requires a contemporary view of galaxy morphology, expanding on the traditional view of galaxy structure, and uniting the kinematic complexity observed in the Milky Way with the extragalactic.

astro-ph.GA↗

arXiv2024

The GALAH Survey: Data Release 4

The stars of the Milky Way carry the chemical history of our Galaxy in their atmospheres as they journey through its vast expanse. Like barcodes, we can extract the chemical fingerprints of stars from high-resolution spectroscopy. The fourth data release (DR4) of the Galactic Archaeology with HERMES (GALAH) Survey, based on a decade of observations, provides the chemical abundances of up to 32 elements for 917 588 stars that also have exquisite astrometric data from the $Gaia$ satellite. For the first time, these elements include life-essential nitrogen to complement carbon, and oxygen as well as more measurements of rare-earth elements critical to modern-life electronics, offering unparalleled insights into the chemical composition of the Milky Way. For this release, we use neural networks to simultaneously fit stellar parameters and abundances across the whole wavelength range, leveraging synthetic grids computed with Spectroscopy Made Easy. These grids account for atomic line formation in non-local thermodynamic equilibrium for 14 elements. In a two-iteration process, we first fit stellar labels to all 1 085 520 spectra, then co-add repeated observations and refine these labels using astrometric data from $Gaia$ and 2MASS photometry, improving the accuracy and precision of stellar parameters and abundances. Our validation thoroughly assesses the reliability of spectroscopic measurements and highlights key caveats. GALAH DR4 represents yet another milestone in Galactic archaeology, combining detailed chemical compositions from multiple nucleosynthetic channels with kinematic information and age estimates. The resulting dataset, covering nearly a million stars, opens new avenues for understanding not only the chemical and dynamical history of the Milky Way, but also the broader questions of the origin of elements and the evolution of planets, stars, and galaxies.

astro-ph.GA↗

arXiv2023

Planetary nebulae populations in the haloes of nearby massive early-type galaxies

Planetary nebulae (PNe) are excellent tracers of the metal-poor haloes of nearby early-type galaxies. They are commonly used to trace spatial distribution and kinematics of the halo and intracluster light at distances of up to 100 Mpcs. The results on the early-type galaxy M105 in the Leo I group represent a benchmark for the quantitative analysis of halo and intragroup light. Since the Leo I group lies at just a 10 Mpc distance, it is at the ideal location to compare results from resolved stellar populations with the homogeneous constraints over a much larger field of view from the PN populations. In M105, we have -- for the first time -- established a direct link between the presence of a metal-poor halo as traced by resolved red-giant branch stars and a PN population with a high specific frequency ($\alpha$-parameter). This confirms our inferences that the high $\alpha$-parameter PN population in the outer halo of M49 in the Virgo Cluster traces the metal-poor halo and intra-group light.

astro-ph.GA↗

arXiv2022

The halo of M105 and its group environment as traced by planetary nebula populations: II. Using kinematics of single stars to unveil the presence of intragroup light around the Leo I galaxies NGC 3384 and M105

M105 is an early-type galaxy in the nearby Leo I group, the closest galaxy group to contain all galaxy types and therefore an excellent environment to explore the low-mass end of intra-group light (IGL) assembly. We present a new extended kinematic survey of planetary nebulae (PNe) in M105 and the surrounding 30'x30' in the Leo I group with the Planetary Nebula Spectrograph. We use PNe as kinematic tracers of the diffuse stellar light in the halo and IGL and employ Gaussian mixture models to separate contributions from the companion galaxy NGC 3384 and associate PNe with halo and IGL components around M105. We present a catalogue of 314 PNe and firmly associate 93 with NGC 3384 and 169 with M105. The PNe in M105 are further associated with its halo and the surrounding exponential envelope. We construct smooth velocity and velocity dispersion fields and calculate projected rotation, velocity dispersion, and $λ_R$ profiles for each component. Halo PNe exhibit declining velocity dispersion and rotation profiles, while the velocity dispersion and rotation of the exponential envelope increase notably at large radii. We identify three regimes with distinct kinematics that are linked to distinct stellar population properties: (i) the rotating core (within $1~R_\mathrm{eff}$) formed in situ and dominated by metal-rich ([M/H]~0) stars likely formed in situ, (ii) the halo from 1 to $7.5~R_\mathrm{eff}$ consisting of intermediate-metallicity stars ([M/H]>-1), either formed in situ or brought in through major mergers, and (iii) the exponential envelope reaching beyond our farthest data point at 16 $R_\mathrm{eff}$, predominately composed of metal-poor ([M/H]<-1) stars. The high velocity dispersion and moderate rotation of the latter are consistent with that measured for dwarf satellite galaxies in the Leo I group, indicating that the exponential envelope traces the transition to the IGL.

astro-ph.GA↗

arXiv2021

The GALAH survey: effective temperature calibration from the InfraRed Flux Method in the Gaia system

In order to accurately determine stellar properties, knowledge of the effective temperature of stars is vital. We implement Gaia and 2MASS photometry in the InfraRed Flux Method and apply it to over 360,000 stars across different evolutionary stages in the GALAH DR3 survey. We derive colour-effective temperature relations that take into account the effect of metallicity and surface gravity over the range 4000 to 8000 kelvin, from very metal-poor stars to super solar metallicities. The internal uncertainty of these calibrations is of order 40-80 kelvin depending on the colour combination used. Comparison against solar-twins, Gaia benchmark stars and the latest interferometric measurements validates the precision and accuracy of these calibrations from F to early M spectral types. We assess the impact of various sources of uncertainties, including the assumed extinction law, and provide guidelines to use our relations. Robust solar colours are also derived.

astro-ph.SR↗

arXiv2021

A2A: 21,000 bulge stars from the ARGOS survey with stellar parameters on the APOGEE scale

We use the data-driven method, The Cannon, to bring 21,000 stars from the ARGOS bulge survey, including 10,000 red clump stars, onto the parameter and abundance scales of the cross-Galactic survey, APOGEE, obtaining rms precisions of 0.10 dex, 0.07 dex, 74 K, and 0.18 dex for [Fe/H], [Mg/Fe], Teff, and log(g), respectively. The re-calibrated ARGOS survey - which we refer to as the A2A survey - is combined with the APOGEE survey to investigate the abundance structure of the Galactic bulge. We find X-shaped [Fe/H] and [Mg/Fe] distributions in the bulge that are more pinched than the bulge density, a signature of its disk origin. The mean abundance along the major axis of the bar varies such that the stars are more [Fe/H]-poor and [Mg/Fe]-rich near the Galactic center than in the long bar/outer bulge region. The vertical [Fe/H] and [Mg/Fe] gradients vary between the inner bulge and long bar with the inner bulge showing a flattening near the plane that is absent in the long bar. The [Fe/H]-[Mg/Fe] distribution shows two main maxima, an ``[Fe/H]-poor [Mg/Fe]- rich'' maximum and an ``[Fe/H]-rich [Mg/Fe]-poor'' maximum, that vary in strength with position in the bulge. In particular, the outer long bar close to the Galactic plane is dominated by super-solar [Fe/H], [Mg/Fe]-normal stars. Stars composing the [Fe/H]-rich maximum show little kinematic dependence on [Fe/H], but for lower [Fe/H] the rotation and dispersion of the bulge increase slowly. Stars with [Fe/H]<-1 dex have a very different kinematic structure than stars with higher [Fe/H]. Comparing with recent models for the Galactic boxy-peanut bulge, the abundance gradients and distribution, and the relation between [Fe/H] and kinematics suggest that the stars comprising each maximum have separate disk origins with the ``[Fe/H]-poor [Mg/Fe]-rich'' stars originating from a thicker disk than the ``[Fe/H]-rich [Mg/Fe]-poor'' stars.

astro-ph.GA↗

arXiv2021

The GALAH Survey and Symbiotic Stars. I. Discovery and follow-up of 33 candidate accreting-only systems

We have identified a first group of 33 new candidates for symbiotic stars (SySt) of the accreting-only variety among the 600,255 stars so far observed by the GALAH high-resolution spectroscopic survey of the Southern Hemisphere, more than doubling the number of those previously known. GALAH aims to high latitudes and this offers the possibility to sound the Galaxy for new SySt away from the usual Plane and Bulge hunting regions. In this paper we focus on SySt of the M spectral type, showing an Halpha emission with a peak in excess of 0.5 above the adjacent continuum level, and not affected by coherent radial pulsations. These constraints will be relaxed in future studies. The 33 new candidate SySt were subjected to a vast array of follow-up confirmatory observations (X-ray/UV observations with the Swift satellite, search for optical flickering, presence of a near-UV upturn in ground-based photometric and spectroscopic data, radial velocity changes suggestive of orbital motion, variability of the emission line profiles). According to Gaia eDR3 parallaxes, the new SySt are located at the tip of the Giant Branch, sharing the same distribution in M(Ks) of the well established SySt. The accretion luminosities of the new SySt are in the range 1-10 Lsun, corresponding to mass-accretion rates of 0.1-1x10(-9) Msun/yr for WDs of 1 Msun. The M giant of one of the new SySt presents a large Lithium over-abundance.

astro-ph.SR↗

arXiv2020

Resolving the Disc-Halo Degeneracy II: NGC 6946

The mass-to-light ratio (M/L) is a key parameter in decomposing galactic rotation curves into contributions from the baryonic components and the dark halo of a galaxy. One direct observational method to determine the disc M/L is by calculating the surface mass density of the disc from the stellar vertical velocity dispersion and the scale height of the disc. Usually, the scale height is obtained from near-IR studies of edge-on galaxies and pertains to the older, kinematically hotter stars in the disc, while the vertical velocity dispersion of stars is measured in the optical band and refers to stars of all ages (up to ~10 Gyr) and velocity dispersions. This mismatch between the scale height and the velocity dispersion can lead to underestimates of the disc surface density and a misleading conclusion of the sub-maximality of galaxy discs. In this paper we present the study of the stellar velocity dispersion of the disc galaxy NGC 6946 using integrated star light and individual planetary nebulae as dynamical tracers. We demonstrate the presence of two kinematically distinct populations of tracers which contribute to the total stellar velocity dispersion. Thus, we are able to use the dispersion and the scale height of the same dynamical population to derive the surface mass density of the disc over a radial extent. We find the disc of NGC 6946 to be closer to maximal with the baryonic component contributing most of the radial gravitational field in the inner parts of the galaxy (Vmax(bar) = 0.76($\pm$0.14)Vmax).

astro-ph.GA↗

arXiv2020

The halo of M105 and its group environment as traced by planetary nebula populations: I. Wide-field photometric survey of planetary nebulae in the Leo I group

M105 (NGC 3379) is an early-type galaxy in the Leo I group. This group is the nearest group that contains all main galaxy types and can thus be used as a benchmark to study the properties of the intra-group light (IGL) in low-mass groups. We use PNe as discrete stellar tracers of the diffuse light around M105. PNe were identified on the basis of their bright [OIII]5007 AA emission and the absence of a broad-band continuum. We compare the PN number density profile with the galaxy surface-brightness profile decomposed into metallicity components using published HST photometry in two halo fields. We identify 226 PNe candidates within a limiting magnitude of mlim = 28.1 from our Subaru-SuprimeCam imaging, covering 67.6 kpc along the major axis of M105 and the halos of NGC 3384 and NGC 3398. We find an excess of PNe at large radii compared to the stellar surface brightness profile from broad-band surveys. This excess is related to a variation in the luminosity-specific PN number $α$ with radius. The $α$-parameter value of the extended halo is more than 7 times higher than that of the inner halo. We also measure an increase in the slope of the PN luminosity function at fainter magnitudes with radius. We infer that the radial variation of the PN population properties is due to a diffuse population of metal-poor stars ([M/H] < -1.0) following an exponential profile, in addition to the M105 halo. The spatial coincidence between the number density profile of these metal-poor stars and the increase in the $α$-parameter value with radius establishes the missing link between metallicity and the post-AGB phases of stellar evolution. We estimate that the total bolometric luminosity associated with the exponential IGL population is 2.04x10^9 Lsun as a lower limit, corresponding to an IGL fraction of 3.8%. This work sets the stage for kinematic studies of the IGL in low-mass groups.

astro-ph.GA↗

arXiv2019

HERBS II: Detailed chemical compositions of Galactic bulge stars

This work explores the detailed chemistry of the Milky Way bulge using the HERMES spectrograph on the Anglo-Australian Telescope. Here we present the abundance ratios of 13 elements for 832 red giant branch and clump stars along the minor bulge axis at latitudes $b=-10^{\circ}, -7.5$ and $-5^{\circ}$. Our results show that none of the abundance ratios vary significantly with latitude. We also observe {\color{red}disk-like} [Na/Fe] abundance ratios, which indicates the bulge does not contain helium-enhanced populations as observed in some globular clusters. Helium enhancement is therefore not the likely explanation for the double red-clump observed in the bulge. We confirm that bulge stars mostly follow abundance trends observed in the disk. However, this similarity is not confirmed across for all elements and metallicity regimes. The more metal-poor bulge population at [Fe/H] $\lesssim -0.8$ is enhanced in the elements associated with core collapse supernovae (SNeII). In addition, the [La/Eu] abundance ratio suggests higher $r$-process contribution, and likely higher star formation in the bulge compared to the disk. This highlights the complex evolution in the bulge, which should be investigated further, both in terms of modelling; and with additional observations of the inner Galaxy.

astro-ph.GA↗

arXiv2019

HERBS I: Metallicity and alpha enhancement along the Galactic bulge minor axis

To better understand the origin and evolution of the Milky Way bulge, we have conducted a survey of bulge red giant branch and clump stars using the HERMES spectrograph on the Anglo-Australian Telescope. We targeted ARGOS survey stars with pre-determined bulge memberships, covering the full metallicity distribution function. The spectra have signal-to-noise ratios comparable to, and were analysed using the same methods as the GALAH survey. In this work, we present the survey design, stellar parameters, distribution of metallicity and alpha-element abundances along the minor bulge axis at latitudes $b$ = $-10^{\circ}, -7.5^{\circ}$ and $-5^{\circ}$. Our analysis of ARGOS stars indicates that the centroids of ARGOS metallicity components should be located $\approx$0.09 dex closer together. The vertical distribution of $α$-element abundances is consistent with the varying contributions of the different metallicity components. Closer to the plane, alpha abundance ratios are lower as the metal-rich population dominates. At higher latitudes, the alpha abundance ratios increase as the number of metal-poor stars increases. However, we find that the trend of alpha-enrichment with respect to metallicity is independent of latitude. Comparison of our results with those of GALAH DR2 revealed that for [Fe/H] $\approx -0.8$, the bulge shares the same abundance trend as the high-$α$ disk population. However, the metal-poor bulge population ([Fe/H] $\lesssim -0.8$) show enhanced alpha abundance ratios compared to the disk/halo. These observations point to fairly rapid chemical evolution in the bulge, and that the metal-poor bulge population does not share the same similarity with the disk as the more metal-rich populations.

astro-ph.GA↗

arXiv2018

The GALAH survey: An abundance, age, and kinematic inventory of the solar neighbourhood made with TGAS

The overlap between the spectroscopic Galactic Archaeology with HERMES (GALAH) survey & $Gaia$ provides a high-dimensional chemodynamical space of unprecedented size. We present a first analysis of a subset of this overlap, of 7066 dwarf, turn-off, & sub-giant stars. [...] We investigate correlations between chemical compositions, ages, & kinematics for this sample. Stellar parameters & elemental abundances are derived from the GALAH spectra with the spectral synthesis code SME. [...] We report Li, C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, as well as Ba & we note that we employ non-LTE calculations for Li, O, Al, & Fe. We show that the use of astrometric & photometric data improves the accuracy of the derived spectroscopic parameters, especially $\log g$. [...] we recover the result that stars of the high-$α$ sequence are typically older than stars in the low-$α$ sequence, the latter spanning $-0.7<$[Fe/H]$<+0.5$. While these two sequences become indistinguishable in [$α$/Fe] vs. [Fe/H] at the metal-rich regime, we find that age can be used to separate stars from the extended high-$α$ & the low-$α$ sequence even in this regime. [...] we find that the old stars ($>8$ Gyr have lower angular momenta $L_z$ than the Sun, which implies that they are on eccentric orbits & originate from the inner disk. Contrary to some previous smaller scale studies we find a continuous evolution in the high-$α$-sequence up to super-solar [Fe/H] rather than a gap, which has been interpreted as a separate "high-$α$ metal-rich" population. Stars in our sample that are younger than 10 Gyr, are mainly found on the low $α$-sequence & show a gradient in $L_z$ from low [Fe/H] ($L_z>L_{z,\odot}$) towards higher [Fe/H] ($L_z<L_{z,\odot}$), which implies that the stars at the ends of this sequence are likely not originating from the close solar vicinity.

astro-ph.SR↗

arXiv2018

The extended Planetary Nebula Spectrograph (ePN.S) early-type galaxy survey. The kinematic diversity of stellar halos and the relation between halo transition scale and stellar mass

In the hierarchical two-phase formation scenario, the extended halos of early type galaxies (ETGs) are expected to have different physical properties from those of the galaxies' central regions. This work aims at characterizing the kinematic properties of ETG halos using planetary nebulae (PNe) as tracers, which allow us to overcome the limitations of absorption line spectroscopy of continuum at low surface brightness. We present two-dimensional velocity and velocity dispersion fields for 33 ETGs, including both fast (FRs) and slow rotators (SRs), making this the largest kinematic survey to-date of extragalactic PNe. The velocity fields extend out to a median 5.6 effective radii (Re), with a range [3Re-13Re] for the PN.S ETGs. We complemented the PN kinematics with absorption line data from the literature. We find that ETGs typically show a kinematic transition between inner regions and halo. Estimated transition radii in units of Re anticorrelate with stellar mass. SRs have increased but still modest rotational support at large radii, while most of the FRs show a decrease in rotation, due to the fading of the stellar disk in the outer, more slowly rotating spheroid. 30% of the FRs are dominated by rotation also at large radii. Most ETGs have flat or slightly falling halo velocity dispersion profiles, but 15% of the sample have steeply falling profiles. 40% of the FRs show kinematic twists, misalignments, or rotation along two axes, indicating that they turn from oblate at the center to triaxial in the halo, consistently with the distribution of isophote twists from extended photometry. ETGs have more diverse kinematic properties in their halos than in the central regions. The observed kinematic transition to the halo and its dependence on stellar mass is consistent with LambdaCDM simulations and supports a two-phase formation scenario. [abridged]

astro-ph.GA↗

arXiv2018

Near-identical star formation rate densities from H$α$ and FUV at redshift zero

For the first time both H$α$ and far-ultraviolet (FUV) observations from an HI-selected sample are used to determine the dust-corrected star formation rate density (SFRD: $\dotρ$) in the local Universe. Applying the two star formation rate indicators on 294 local galaxies we determine log($\dotρ$$ _{Hα}) = -1.68~^{+0.13}_{-0.05}$ [M$_{\odot} $ yr$^{-1} $ Mpc$^{-3}]$ and log($\dotρ_{FUV}$) $ = -1.71~^{+0.12}_{-0.13}$ [M$_\odot $ yr$^{-1} $ Mpc$^{-3}]$. These values are derived from scaling H$α$ and FUV observations to the HI mass function. Galaxies were selected to uniformly sample the full HI mass (M$_{HI}$) range of the HI Parkes All-Sky Survey (M$_{HI} \sim10^{7}$ to $\sim10^{10.7}$ M$_{\odot}$). The approach leads to relatively larger sampling of dwarf galaxies compared to optically-selected surveys. The low HI mass, low luminosity and low surface brightness galaxy populations have, on average, lower H$α$/FUV flux ratios than the remaining galaxy populations, consistent with the earlier results of Meurer. The near-identical H$α$- and FUV-derived SFRD values arise with the low H$α$/FUV flux ratios of some galaxies being offset by enhanced H$α$ from the brightest and high mass galaxy populations. Our findings confirm the necessity to fully sample the HI mass range for a complete census of local star formation to include lower stellar mass galaxies which dominate the local Universe.

astro-ph.GA↗

arXiv2018

Three dynamically distinct stellar populations in the halo of M49

M49 (NGC 4472) is the dominant galaxy in subcluster B of the Virgo Cluster, and a benchmark for studying the build-up of the extended halos of brightest group galaxies in the outskirts of galaxy clusters. We investigate the kinematics in the outer halo of M49, look for substructures, and describe the transition to the surrounding intra-group light. As kinematic tracers we use planetary nebulae (PNe), combining kinematics from the extended Planetary Nebula Spectrograph (PN.S) early-type galaxy survey with our recent deep photometric sample. We study the position-velocity-plane for bright and faint PN populations out to 95 kpc radius, and employ a multi-Gaussian model for the velocity distribution to identify stellar populations with distinct kinematics and histories. We report the detection of stellar-kinematic substructure associated with the interaction of M49 with the dwarf irregular galaxy VCC 1249. We find two kinematically distinct PN populations associated with the main M49 halo and the extended intra-group light (IGL). These have velocity dispersions $σ_{halo}\simeq 170$ km/s and $σ_{IGL} \simeq400$ km/s at 10-80 kpc radii. The overall luminosity profile and velocity dispersion at $\sim80$ kpc are consistent with a flat circular velocity curve extrapolated from X-ray observations. The dispersion of the PNe associated with the IGL joins onto that of the satellite galaxies in subcluster B at $\sim100$ kpc radius. This is the first time that the transition from halo to IGL is observed based on the velocities of individual stars. Therefore the halo of M49, consisting of at least three distinct components, has undergone an extended accretion history within its parent group potential. The blue colours of the IGL component are consistent with a population of stars formed in low-mass galaxies at redshift $\sim0.5$ that has since evolved passively, as suggested by other data.

astro-ph.GA↗

arXiv2018

The GALAH Survey: Second Data Release

The Galactic Archaeology with HERMES (GALAH) survey is a large-scale stellar spectroscopic survey of the Milky Way and designed to deliver chemical information complementary to a large number of stars covered by the $Gaia$ mission. We present the GALAH second public data release (GALAH DR2) containing 342,682 stars. For these stars, the GALAH collaboration provides stellar parameters and abundances for up to 23 elements to the community. Here we present the target selection, observation, data reduction and detailed explanation of how the spectra were analysed to estimate stellar parameters and element abundances. For the stellar analysis, we have used a multi-step approach. We use the physics-driven spectrum synthesis of Spectroscopy Made Easy (SME) to derive stellar labels ($T_\mathrm{eff}$, $\log g$, $\mathrm{[Fe/H]}$, $\mathrm{[X/Fe]}$, $v_\mathrm{mic}$, $v \sin i$, $A_{K_S}$) for a representative training set of stars. This information is then propagated to the whole survey with the data-driven method of $The~Cannon$. Special care has been exercised in the spectral synthesis to only consider spectral lines that have reliable atomic input data and are little affected by blending lines. Departures from local thermodynamic equilibrium (LTE) are considered for several key elements, including Li, O, Na, Mg, Al, Si, and Fe, using 1D MARCS stellar atmosphere models. Validation tests including repeat observations, Gaia benchmark stars, open and globular clusters, and K2 asteroseismic targets lend confidence in our methods and results. Combining the GALAH DR2 catalogue with the kinematic information from $Gaia$ will enable a wide range of Galactic Archaeology studies, with unprecedented detail, dimensionality, and scope.

astro-ph.SR↗

arXiv2018

The GALAH survey: properties of the Galactic disk(s) in the solar neighbourhood

Using data from the GALAH pilot survey, we determine properties of the Galactic thin and thick disks near the solar neighbourhood. The data cover a small range of Galactocentric radius ($7.9 \leq R_\mathrm{GC} \leq 9.5$ kpc), but extend up to 4 kpc in height from the Galactic plane, and several kpc in the direction of Galactic anti-rotation (at longitude $260 ^\circ \leq \ell \leq 280^\circ$). This allows us to reliably measure the vertical density and abundance profiles of the chemically and kinematically defined `thick' and `thin' disks of the Galaxy. The thin disk (low-$α$ population) exhibits a steep negative vertical metallicity gradient, at d[M/H]/d$z=-0.18 \pm 0.01$ dex kpc$^{-1}$, which is broadly consistent with previous studies. In contrast, its vertical $α$-abundance profile is almost flat, with a gradient of d[$α$/M]/d$z$ = $0.008 \pm 0.002$ dex kpc$^{-1}$. The steep vertical metallicity gradient of the low-$α$ population is in agreement with models where radial migration has a major role in the evolution of the thin disk. The thick disk (high-$α$ population) has a weaker vertical metallicity gradient d[M/H]/d$z = -0.058 \pm 0.003$ dex kpc$^{-1}$. The $α$-abundance of the thick disk is nearly constant with height, d[$α$/M]/d$z$ = $0.007 \pm 0.002$ dex kpc$^{-1}$. The negative gradient in metallicity and the small gradient in [$α$/M] indicate that the high-$α$ population experienced a settling phase, but also formed prior to the onset of major SNIa enrichment. We explore the implications of the distinct $α$-enrichments and narrow [$α$/M] range of the sub-populations in the context of thick disk formation.

astro-ph.GA↗