ABSTRACT
Since the advent of Gaia astrometry, it is possible to identify massive accreted systems within the Galaxy through their unique dynamical signatures. One such system, Gaia-Sausage-Enceladus ...(GSE), appears to be an early ‘building block’ given its virial mass $\gt 10^{10}\, \mathrm{M_\odot }$ at infall (z ∼ 1−3). In order to separate the progenitor population from the background stars, we investigate its chemical properties with up to 30 element abundances from the GALAH+ Survey Data Release 3 (DR3). To inform our choice of elements for purely chemically selecting accreted stars, we analyse 4164 stars with low-α abundances and halo kinematics. These are most different to the Milky Way stars for abundances of Mg, Si, Na, Al, Mn, Fe, Ni, and Cu. Based on the significance of abundance differences and detection rates, we apply Gaussian mixture models to various element abundance combinations. We find the most populated and least contaminated component, which we confirm to represent GSE, contains 1049 stars selected via Na/Fe versus Mg/Mn in GALAH+ DR3. We provide tables of our selections and report the chrono-chemodynamical properties (age, chemistry, and dynamics). Through a previously reported clean dynamical selection of GSE stars, including $30 \lt \sqrt{J_R / \, \mathrm{kpc\, km\, s^{-1}}} \lt 55$, we can characterize an unprecedented 24 abundances of this structure with GALAH+ DR3. With our chemical selection we characterize the dynamical properties of the GSE, for example mean $\sqrt{J_R / \, \mathrm{kpc\, km\, s^{-1}}} =$$26_{-14}^{+9}$. We find only $(29\pm 1){{\ \rm per\ cent}}$ of the GSE stars within the clean dynamical selection region. Our methodology will improve future studies of accreted structures and their importance for the formation of the Milky Way.
The structure of the Sagittarius stream in the southern Galactic hemisphere is analyzed with the Sloan Digital Sky Survey Data Release 8. Parallel to the Sagittarius tidal track, but ~10degrees away, ...there is another fainter and more metal-poor stream. We provide evidence that the two streams follow similar distance gradients but have distinct morphological properties and stellar populations. The brighter stream is broader, contains more metal-rich stars, and has a richer color-magnitude diagram with multiple turnoffs and a prominent red clump as compared to the fainter stream. Based on the structural properties and the stellar population mix, the stream configuration is similar to the Northern "bifurcation." In the region of the South Galactic Cap, there is overlapping tidal debris from the Cetus stream, which crosses the Sagittarius stream. Using both photometric and spectroscopic data, we show that the blue straggler population belongs mainly to Sagittarius and the blue horizontal branch stars belong mainly to the Cetus stream in this confused location in the halo.
Using data from the GALAH pilot survey, we determine properties of the Galactic thin and thick discs near the solar neighbourhood. The data cover a small range of Galactocentric radius (7.9 less than ...or similar to R-GC less than or similar to 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 degrees <= l <= 280 degrees). This allows us to reliably measure the vertical density and abundance profiles of the chemically and kinematically defined 'thick' and 'thin' discs of the Galaxy. The thin disc (low-alpha population) exhibits a steep negative vertical metallicity gradient, at dM/H/dz = -0.18 +/- 0.01 dex kpc(-1), which is broadly consistent with previous studies. In contrast, its vertical alpha-abundance profile is almost flat, with a gradient of dalpha/M/dz = 0.008 +/- 0.002 dex kpc(-1). The steep vertical metallicity gradient of the low-a population is in agreement with models where radial migration has a major role in the evolution of the thin disc. The thick disc (high-alpha population) has a weaker vertical metallicity gradient dM/H/dz = -0.058 +/- 0.003 dex kpc(-1). The aabundance of the thick disc is nearly constant with height, dalpha/M/dz = 0.007 +/- 0.002 dex kpc(-1). The negative gradient in metallicity and the small gradient in alpha/M indicate that the high-alpha population experienced a settling phase, but also formed prior to the onset of major Type I alpha supernova enrichment. We explore the implications of the distinct alpha-enrichments and narrow alpha/M range of the sub-populations in the context of thick disc formation.
We present the identification of potential members of nearby Galactic globular clusters using radial velocities from the RAdial Velocity Experiment Data Release 4 (RAVE-DR4) survey data base. Our ...identifications are based on three globular clusters – NGC 3201, NGC 5139 (ω Cen) and NGC 362 – all of which are shown to have ∣RV∣ > 100 km s−1. The high radial velocity of cluster members compared to the bulk of surrounding disc stars enables us to identify members using their measured radial velocities, supplemented by proper motion information and location relative to the tidal radius of each cluster. The identification of globular cluster stars in RAVE DR4 data offers a unique opportunity to test the precision and accuracy of the stellar parameters determined with the currently available Stellar Parameter Pipelines used in the survey, as globular clusters are ideal test-beds for the validation of stellar atmospheric parameters, abundances, distances and ages. For both NGC 3201 and ω Cen, there is compelling evidence for numerous members (>10) in the RAVE data base; in the case of NGC 362 the evidence is more ambiguous, and there may be significant foreground and/or background contamination in our kinematically selected sample. A comparison of the RAVE-derived stellar parameters and abundances with published values for each cluster and with BASTI isochrones for ages and metallicities from the literature reveals overall good agreement, with the exception of the apparent underestimation of surface gravities for giants, in particular for the most metal-poor stars. Moreover, if the selected members are part of the main body of each cluster our results would also suggest that the distances from Binney et al., where only isochrones more metal rich than −0.9 dex were used, are typically underestimated by ∼40 per cent with respect to the published distances for the clusters, while the distances from Zwitter et al. show stars ranging from 1 to ∼6.5 kpc – with indications of a trend towards higher distances at lower metallicities – for the three clusters analysed in this study.
Filamentary star formation in NGC 1275 Canning, R. E. A; Ryon, J. E; Gallagher, J. S ...
Monthly notices of the Royal Astronomical Society,
10/2014, Letnik:
444, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We examine the star formation in the outer halo of NGC 1275, the central galaxy in the Perseus cluster (Abell 426), using far-ultraviolet and optical images obtained with the Hubble Space Telescope. ...We have identified a population of very young, compact star clusters with typical ages of a few Myr. The star clusters are organized on multiple kiloparsec scales. Many of these star clusters are associated with ‘streaks’ of young stars, the combination of which has a cometary appearance. We perform photometry on the star clusters and diffuse stellar streaks, and fit their spectral energy distributions to obtain ages and masses. These young stellar populations appear to be normal in terms of their masses, luminosities and cluster formation efficiency; <10 per cent of the young stellar mass is located in star clusters. Our data suggest star formation is associated with the evolution of some of the giant gas filaments in NGC 1275 that become gravitationally unstable on reaching and possibly stalling in the outer galaxy. The stellar streaks then could represent stars moving on ballistic orbits in the potential well of the galaxy cluster. We propose a model where star-forming filaments, switched on ∼50 Myr ago and are currently feeding the growth of the NGC 1275 stellar halo at a rate of ≈−2 to 3 M⊙ yr−1. This type of process may also build stellar haloes and form isolated star clusters in the outskirts of youthful galaxies.
In the thick of it: metal-poor disc stars in RAVE Kordopatis, G.; Gilmore, G.; Wyse, R. F. G. ...
Monthly notices of the Royal Astronomical Society,
12/2013, Letnik:
436, Številka:
4
Journal Article
Recenzirano
Odprti dostop
By selecting in the Radial Velocity Experiment-fourth data release (RAVE-DR4) survey the stars located between 1 and 2 kpc above the Galactic plane, we question the consistency of the simplest ...three-component model (thin disc, thick disc and halo) for the Milky Way. We confirm that the metallicity and azimuthal velocity distribution functions of the thick disc are not Gaussian. In particular, we find that the thick disc has an extended metallicity tail going at least down to M/H = -2 dex, contributing roughly 3 per cent of the entire thick disc population and having a shorter scalelength compared to the canonical thick disc. The mean azimuthal velocity of these metal-poor stars allows us to estimate the correlation between the metallicity (M/H) and the orbital velocity (V...), which is an important constraint on the formation mechanisms of the Galactic thick disc. Given our simple approach, we find ... 50 km s... dex..., which is in very good agreement with previous literature values. We complete the study with a brief discussion on the implications of the formation scenarios for the thick disc and suggest that given the above-mentioned characteristics, a thick disc mainly formed by radial migration mechanisms seems unlikely. (ProQuest: ... denotes formulae/symbols omitted.)
We report on the discovery of new members of nearby young moving groups, exploiting the full power of combining the Radial Velocity Experiment (RAVE) survey with several stellar age diagnostic ...methods and follow-up high-resolution optical spectroscopy. The results include the identification of one new and five likely members of the β Pictoris moving group, ranging from spectral types F9 to M4 with the majority being M dwarfs, one K7 likely member of the ε Cha group and two stars in the Tucana-Horologium association. Based on the positive identifications, we foreshadow a great potential of the RAVE data base in progressing towards a full census of young moving groups in the solar neighbourhood.
We present the first data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities and stellar atmosphere parameters (temperature, metallicity, ...and surface gravity) of up to one million stars using the Six Degree Field multiobject spectrograph on the 1.2 m UK Schmidt Telescope of the Anglo-Australian Observatory. The RAVE program started in 2003, obtaining medium-resolution spectra (median R = 7500) in the Ca-triplet region (8410-8795 A) for southern hemisphere stars drawn from the Tycho-2 and SuperCOSMOS catalogs, in the magnitude range 9 < I < 12. The first data release is described in this paper and contains radial velocities for 24,748 individual stars (25,274 measurements when including reobservations). Those data were obtained on 67 nights between 2003 April 11 and 2004 April 3. The total sky coverage within this data release is ~4760 deg2. The average signal-to-noise ratio of the observed spectra is 29.5, and 80% of the radial velocities have uncertainties better than 3.4 km s-1. Combining internal errors and zero-point errors, the mode is found to be 2 km s-1. Repeat observations are used to assess the stability of our radial velocity solution, resulting in a variance of 2.8 km s-1. We demonstrate that the radial velocities derived for the first data set do not show any systematic trend with color or signal-to-noise ratio. The RAVE radial velocities are complemented in the data release with proper motions from Starnet 2.0, Tycho-2, and SuperCOSMOS, in addition to photometric data from the major optical and infrared catalogs (Tycho-2, USNO-B, DENIS, and the Two Micron All Sky Survey). The data release can be accessed via the RAVE Web site.
The overlap between the spectroscopic Galactic Archaeology with HERMES (GALAH) survey and 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, and sub-giant stars. These stars have spectra from the GALAH survey and high parallax precision from the Gaia DR1 Tycho-Gaia Astrometric Solution. We investigate correlations between chemical compositions, ages, and kinematics for this sample. Stellar parameters and elemental abundances are derived from the GALAH spectra with the spectral synthesis code SPECTROSCOPY MADE EASY. We determine kinematics and dynamics, including action angles, from the Gaia astrometry and GALAH radial velocities. Stellar masses and ages are determined with Bayesian isochrone matching, using our derived stellar parameters and absolute magnitudes. We report measurements of Li, C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, as well as Ba and we note that we have employed non-LTE calculations for Li, O, Al, and Fe. We show that the use of astrometric and photometric data improves the accuracy of the derived spectroscopic parameters, especially log g. Focusing our investigation on the correlations between stellar age, iron abundance Fe/H, and mean alpha-enhancement α/Fe of the magnitude-selected sample, we recover the result that stars of the high-α sequence are typically older than stars in the low-α sequence, the latter spanning iron abundances of −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 and the low-α sequence even in this regime. When dissecting the sample by stellar age, we find that the old stars (>8 Gyr) have lower angular momenta Lz than the Sun, which implies that they are on eccentric orbits and originate from the inner disc. 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 and show a gradient in Lz from low Fe/H (Lz > Lz, ⊙) towards higher Fe/H (Lz < Lz, ⊙), which implies that the stars at the ends of this sequence are likely not originating from the close solar vicinity.
The Carina dwarf spheroidal galaxy is the only galaxy of this type that shows clearly episodic star formation separated by long pauses. Here we present metallicities for 437 radial velocity members ...of this Galactic satellite. The metallicities and radial velocities were measured as part of a Large Programme with the Very Large Telescope at the European Southern Observatory, Chile. We obtained medium-resolution spectroscopy with the multiobject spectrograph FLAMES. Our target red giants cover the entire projected surface area of Carina. Our spectra are centered at the near-infrared Ca II triplet, which is a well-established metallicity indicator for old and intermediate-age red giants. The resulting data sample provides the largest collection of spectroscopically derived metallicities for a Local Group dwarf spheroidal galaxy to date. Four of our likely radial velocity members of Carina lie outside this galaxy's nominal tidal radius, supporting earlier claims of the possible existence of such stars beyond the main body of Carina. We find a mean metallicity of Fe/H ~ -1.7 dex on the 1997 metallicity scale of Carretta and Gratton for Carina. The formal FWHM of the metallicity distribution function is 0.92 dex, while the full range of metallicities is found to span approximately -3.0 dex < Fe/H < 0.0 dex. The metallicity distribution function might be indicative of several subpopulations distinct in metallicity. There appears to be a mild radial gradient such that more metal-rich populations are more centrally concentrated, matching a similar trend for an increasing fraction of intermediate-age stars (see the 2001 work of Harbeck and coworkers). This, as well as the photometric colors of the more metal-rich red giants, suggests that Carina exhibits an age-metallicity relation. Indeed, the age-metallicity degeneracy seems to conspire to form a narrow red giant branch despite the considerable spread in metallicity and wide range of ages. The metallicity distribution function is not well matched by a simple closed-box model of chemical evolution. Qualitatively better matches are obtained by chemical models that also take into account infall and outflows. A G dwarf problem remains for all these models.
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