Search

We present results of a search for extremely metal-poor (EMP) stars in the Large Magellanic Cloud, which can provide crucial information about the properties of the first stars as well as on the formation conditions prevalent during the earliest stages of star formation in dwarf galaxies. Our search utilised SkyMapper photometry, together with parallax and proper motion cuts (from Gaia), colour-magnitude cuts (by selecting the red giant branch region) and finally a metallicity-sensitive cut. Low-resolution spectra of a sample of photometric candidates were taken using the ANU 2.3m telescope/WiFeS spectrograph, from which 7 stars with [Fe/H] $\leq$ -2.75 were identified, two of which have [Fe/H] $\leq$ -3. Radial velocities, derived from the CaII triplet lines, closely match the outer rotation curve of the LMC for the majority of the candidates in our sample. Therefore, our targets are robustly members of the LMC based on their 6D phase-space information (coordinates, spectrophotometric distance, proper motions and radial velocities), and they constitute the most metal-poor stars so far discovered in this galaxy., Accepted for publication in MNRAS

The origin of multiple stellar populations in Globular Clusters (GCs) is one of the greatest mysteries of modern stellar astrophysics. N-body simulations suggest that the present-day dynamics of GC stars can constrain the events that occurred at high redshift and led to the formation of multiple populations. Here, we combine multi-band photometry from the Hubble Space Telescope (HST) and ground-based facilities with HST and Gaia Data Release 2 proper motions to investigate the spatial distributions and the motions in the plane of the sky of multiple populations in the type II GCs NGC 5139 ($��\,$Centauri) and NGC 6656 (M 22). We first analyzed stellar populations with different metallicities. Fe-poor and Fe-rich stars in M 22 share similar spatial distributions and rotation patterns and exhibit similar isotropic motions. Similarly, the two main populations with different iron abundance in $��\,$Centauri share similar ellipticities and rotation patterns. When analyzing different radial regions, we find that the rotation amplitude decreases from the center towards the external regions. Fe-poor and Fe-rich stars of $��\,$Centauri are radially anisotropic in the central region and show similar degrees of anisotropy. We also investigate the stellar populations with different light-element abundances and find that their N-rich stars exhibit higher ellipticity than N-poor stars. In $��\,$Centauri Centauri both stellar groups are radially anisotropic. Interestingly, N-rich, Fe-rich stars exhibit different rotation patterns than N-poor stars with similar metallicities. The stellar populations with different nitrogen of M 22 exhibit similar rotation patterns and isotropic motions. We discuss these findings in the context of the formation of multiple populations., 24 pages, 13 figures, accepted for publication in ApJ

We introduce the Southern Stellar Stream Spectroscopy Survey (S⁵), an on-going program to map the kinematics and chemistry of stellar streams in the Southern Hemisphere. The initial focus of S⁵ has been spectroscopic observations of recently identified streams within the footprint of the Dark Energy Survey (DES), with the eventual goal of surveying streams across the entire southern sky. Stellar streams are composed of material that has been tidally striped from dwarf galaxies and globular clusters and hence are excellent dynamical probes of the gravitational potential of the Milky Way, as well as providing a detailed snapshot of its accretion history. Observing with the 3.9-m Anglo-Australian Telescope’s 2-degree-Field fibre positioner and AAOmega spectrograph, and combining the precise photometry of DES DR1 with the superb proper motions from Gaia DR2, allows us to conduct an efficient spectroscopic survey to map these stellar streams. So far S⁵ has mapped 9 DES streams and 3 streams outside of DES; the former are the first spectroscopic observations of these recently discovered streams. In addition to the stream survey, we use spare fibres to undertake a Milky Way halo survey and a low-redshift galaxy survey. This paper presents an overview of the S⁵ program, describing the scientific motivation for the survey, target selection, observation strategy, data reduction and survey validation. Finally, we describe early science results on stellar streams and Milky Way halo stars drawn from the survey. Updates on S⁵, including future public data releases, can be found at http://s5collab.github.io.

We present deep stellar photometry of the Bo��tes I dwarf spheroidal galaxy in g and i band filters, taken with the Dark Energy Camera at Cerro Tololo in Chile. Our analysis reveals a large, extended region of stellar substructure surrounding the dwarf, as well as a distinct over-density encroaching on its tidal radius. A radial profile of the Bo��tes I stellar distribution shows a break radius indicating the presence of extra-tidal stars. These observations strongly suggest that Bo��tes I is experiencing tidal disruption, although not as extreme as that exhibited by the Hercules dwarf spheroidal. Combined with revised velocity dispersion measurements from the literature, we see evidence suggesting the need to review previous theoretical models of the Bo��tes I dwarf spheroidal galaxy., 13 pages, 13 figures, accepted for publication in MNRAS

The characteristics of the cluster systems of the Magellanic Clouds, as inferred from integrated properties, are compared with those from individual cluster studies and from the field population. The agreement is generally satisfactory though in the case of the LMC, the lack of clusters older than ~3 Gyr is not reflected in the field population. The possible origin(s) for this cluster ``age-gap'' are discussed. The SMC cluster age-metallicity relation is also presented and discussed., 11 pages, 4 figures. To appear in the Proceedings of IAU Symposium 207, "Extragalactic Star Clusters", eds E. Grebel, D. Geisler and D. Minniti

The stellar system omega Centauri is well known for the large range in abundance among its member stars. Recent work has indicated that the globular cluster M22 (NGC 6656) also possesses an internal abundance range, albeit substantially smaller than that in omega Cen. Here we compare, as a function of [Fe/H], element-to-iron ratios in the two systems for a number of different elements using data from abundance analyses of red giant branch stars. It appears that the nucleosynthetic enrichment processes were very similar in these two systems despite the substantial difference in total mass., 12 pages, 7 figures, accepted for publication in Publications of the Astronomical Society of Australia (PASA)

g', r', and i' images obtained with the Gemini Multi-Object Spectrograph (GMOS) on Gemini North are used to investigate the metallicity and stellar content of the M31 dwarf spheroidal companion galaxy And V. Red giant branch (RGB) stars are traced out to radii in excess of 126 arcsec from the galaxy center, indicating that And V extends over a diameter approaching 1 kpc. The mean g'-i' color of the RGB does not change with radius. Based on the slope of the RGB in the (i', g'-i') color-magnitude diagram we conclude that [Fe/H] = -2.2 +/- 0.1. This metallicity is lower than earlier estimates, and places And V squarely on the relation between metallicity and integrated brightness defined by other dwarf spheroidal and dwarf elliptical galaxies. We also fail to find a statistically significant population of luminous asymptotic giant branch stars near the center of the galaxy., 17 pages of text and 7 postscript figures; to appear in the Astronomical Journal

Holwerda, B W; Allen, R J; Van der Kruit, P C; ( Kapteyn Institute Groningen ) ( Space Telescope Science Institute, ) 29 Jan 2002 . - 4 p Abstract: We have automated the ``Synthetic Field Method'' developed by Gonzalez et al.(1998) and used it to measure the opacity of the ISM in the Local Group dwarf galaxy Sextans A by using the changes in counts of background galaxies seen through the foreground system. The Sextans A results are consistent with the observational relation found by Cuillandre et al. (2001) between dust opacity and HI column density in the outer parts of M31.

We have carried out Wide Field Planetary Camera 2 F160BW, F555W and F656N imaging of four young populous clusters: NGC 330 in the Small Magellanic Cloud and NGC 1818, NGC 2004 and NGC 2100 in the Large Magellanic Cloud. We report photometric results for these four clusters, including identification using photometric colours of the cluster Be star population. We present theoretical WFPC2 and broad band colours and bolometric corrections for LMC and SMC metallicities. The use of the far-UV F160BW filter enables accurate determination of the effective temperatures for stars in the vicinity of the main sequence turnoff and on the unevolved main sequence., 33 pages, 14 figures, AJ accepted

In the first part of this contribution the observed velocity dispersions in Galactic halo dwarf spheroidal (dSph) galaxies are reviewed, and the consequences for dark matter content outlined. The results are suggestive of a dSph dark matter mass of ~2x10^7 solar masses, independent of the luminosity of the dSph. Alternatives to the dark matter interpretation are also briefly discussed. In the second part of the contribution the emphasis is on the stellar populations of Galactic dSphs. Recent results for the ages of the oldest populations are presented. These data, together with similar recent results for Galactic halo and LMC globular clusters, indicate that regardless of the subsequent star formation history, the initial epoch of star formation was well synchronized throughout the entire proto-Galactic halo. The implications of the first high dispersion studies of element abundance ratios in Galactic dSph red giants are also discussed. For the Sagittarius dSph in particular, the observed abundance ratios show good agreement with expectations for an episodic star formation history, and such a history is in fact deduced from colour-magnitude diagrams studies., 14 pages, 2 Figs. To be published in The Third Stromlo Symposium - The Galactic Halo: Bright Stars and Dark Matter, eds B K Gibson, T S Axelrod and M E Putman, ASP Conference Series, in press

We report on the chemical abundances of stars in the Carina dwarf spheroidal galaxy (dSph) derived from low-resolution spectra. We have determined values of [Fe/H] for 52 stars from the reduced equivalent width of the Ca II infrared triplet lines. The Carina dSph has a mean metallicity of [Fe/H] = -1.99 +/- 0.08 and an intrinsic metallicity dispersion 0.25 dex (1 sigma). By directly determining the chemical abundances of Carina stars through spectroscopy, we can overcome the age-metallicity degeneracy inherent in color-magnitude diagrams (CMDs) and determine its star-formation history with unprecedented accuracy., Comment: 4 pages, 2 figures, to appear in in "Spectro-Photometric Dating of Stars and Galaxies", eds. I. Hubeny, S. Heap & R. Cornett, (ASP: San Francisco), ASP Conf. Series, in press

We have obtained spectra at the Ca II triplet of individual red giants in seven SMC star clusters whose ages range from ~4 to 12 Gyr. The spectra have been used to determine mean abundances for six of the star clusters to a typical precision of 0.12 dex. When combined with existing data for other objects, the resulting SMC age-metallicity relation is generally consistent with that for a simple model of chemical evolution, scaled to the present-day SMC mean abundance and gas mass fraction. Two of the clusters (Lindsay 113 and NGC 339), however, have abundances that ~0.5 dex lower than that expected from the mean age-metallicity relation. It is suggested that the formation of these clusters, which have ages of ~5 Gyr, may have involved the infall of uneriched gas, perhaps from the Magellanic Stream. The spectra also yield radial velocities for the seven clusters. The resulting velocity dispersion is 16 +/- 4 km/sec, consistent with those of the SMC planetary nebula and carbon star populations., Comment: 28 pages including 4 figures

Images have been obtained with the Hubble Space Telescope WFPC2 camera of Andromeda I, a dwarf spheroidal (dSph) galaxy that lies in the outer halo of M31. The resulting color-magnitude diagrams reveal for the first time the morphology of the horizontal branch in this system. We find that, in a similar fashion to many of the galactic dSph companions, the horizontal branch (HB) of And~I is predominantly red. Combined with the metal abundance of this dSph, this red HB morphology indicates that And I can be classified as a ``second parameter'' system in the outer halo of M31. This result then supports the hypothesis that the outer halo of M31 formed in the same extended chaotic manner as is postulated for the outer halo of the Galaxy., 26 pages using aas2pp4.sty, including 2 tables and 7 figures, to be published in AJ. Figure 1 is in gif form. To include in main ps file, use xv to create a ps file called Da_Costa.fig1.ps and uncomment appropriate lines in .tex file

We observed six fields of the Small Magellanic Cloud (SMC) with the Advanced Camera for Survey on board the Hubble Space Telescope in the F555W and F814W filters. These fields sample regions characterized by very different star and gas densities, and, possibly, by different evolutionary histories. We find that the SMC was already forming stars ~12 Gyr ago, even if the lack of a clear horizontal branch suggests that in the first few billion years the star formation activity was low. Within the uncertainties of our two-band photometry, we find evidence of a radial variation in chemical enrichment, with the SMC outskirts characterized by lower metallicity than the central zones. From our CMDs we also infer that the SMC formed stars over a long interval of time until ~2-3 Gyr ago. After a period of modest activity, star formation increased again in the recent past, especially in the bar and the wing of the SMC, where we see an enhancement in the star-formation activity starting from ~500 Myr ago. The inhomogeneous distribution of stars younger than ~100 Myr indicates that recent star formation has mainly developed locally., 38 pages, 12 figures, to be published on ApJ (accepted)

The color-magnitude (CM) diagram of the Draco dwarf spheroidal was first studied by Baade & Swope (1961). In addition to finding a general similarity to the CM-diagrams of metal-poor globular clusters, they also detected the anomalous Cepheids and broad giant branch which are characteristic of dwarf spheroidals. The best ground-based CCD CM diagrams of the Draco dwarf spheroidal to date have been presented by Stetson, Vandenberg, & McClure (1985) and Carney & Seitzer (1986, hereafter CS86). Here we describe the first results of an HST program to investigate the stellar population of this galaxy.