arXiv subjects

Eline Tolstoy

Publications and source records attributed to Eline Tolstoy.

At least 73 records · Page 4Linked to original sources

arXiv2008

The ACS LCID Project: RR Lyrae stars as tracers of old population gradients in the isolated dwarf spheroidal galaxy Tucana

We present a study of the radial distribution of RR Lyrae variables, which present a range of photometric and pulsational properties, in the dwarf spheroidal galaxy Tucana. We find that the fainter RR Lyrae stars, having a shorter period, are more centrally concentrated than the more luminous, longer period RR Lyrae variables. Through comparison with the predictions of theoretical models of stellar evolution and stellar pulsation, we interpret the fainter RR Lyrae stars as a more metal-rich subsample. In addition, we show that they must be older than about 10 Gyr. Therefore, the metallicity gradient must have appeared very early on in the history of this galaxy.

astro-ph↗

arXiv2006

The stellar content of the isolated transition dwarf galaxy DDO210

We use Subaru Suprime-Cam and VLT FORS1 photometry of the dwarf galaxy DDO210 to study the global stellar content and structural properties of a transition-type galaxy (with properties intermediate between dwarf irregular and dwarf spheroidal systems). This galaxy is sufficiently isolated that tidal interactions are not likely to have affected its evolution in any way. The colour-magnitude diagrams of DDO210 show a red giant branch (RGB) population (with an RGB bump), a bright asymptotic giant branch population, a red clump, young main sequence stars and blue-loop stars. The youngest stars formed within the last 60 Myrs and have a distinct radial distribution compared to the main population. Whereas the overall stellar spatial distribution and HI spatial distribution are concentric, the young stars are offset from the center of DDO210 and are coincident with a `dent' in the HI distribution. The implied recent star formation rate required to form the young population is significantly higher than the derived current star formation rate, by a factor of > 10. Most of the stars in DDO210 are found in a red clump, and its mean I-band magnitude suggests that the majority of stars in DDO210 have an average age of 4^{+2}_{-1} Gyr. Given this age, the colour of the RGB implies a mean metallicity of [Fe/H] ~ -1.3. By comparing the shape of the red clump with models for a variety of star formation histories, we estimate that an old (> 10 Gyr) stellar population can contribute ~ 20 - 30% of the stars in DDO210 at most. The unusual star formation history of DDO210, its low mass estimate and its isolated nature, provide insight into how star formation proceeds in the lowest mass, unperturbed, dwarf galaxy haloes.

astro-ph↗

arXiv2006

Resolved Stellar Populations at the Distance of Virgo

Top of the wish list of any astronomer who wants to understand galaxy formation and evolution is to resolve the stellar populations of a sample of giant elliptical galaxies: to take spectra of the stars and make Colour-Magnitude Diagrams going down to the oldest main sequence turn-offs. It is only by measuring the relative numbers of stars on Main Sequence Turnoffs at ages ranging back to the time of the earliest star formation in the Universe that we can obtain unambiguous star formation histories. Understanding star formation histories of individual galaxies underpins all our theories of galaxy formation and evolution. To date we only have detailed star formation histories for the nearest-by objects in the Local Group, namely galaxies within 700kpc of our own. This means predominantly small diffuse dwarf galaxies in a poor group environment. To sample the full range of galaxy types and to consider galaxies in a high density environment (where much mass in the Universe resides) we need to be able to resolve stars at the distance of the Virgo (~17Mpc) or Fornax (~18Mpc) clusters. This ambitious goal requires an Extremely Large Telescope (ELT), with a diameter of 50-150m, operating in the optical/near-IR at its diffraction limit.

astro-ph↗

arXiv2006

VLT/UVES Spectroscopy of Individual Stars in Three Globular Clusters in the Fornax Dwarf Spheroidal Galaxy

We present a high resolution (R ~ 43000) abundance analysis of a total of nine stars in three of the five globular clusters associated with the nearby Fornax dwarf spheroidal galaxy. These three clusters (1, 2 and 3) trace the oldest, most metal-poor stellar populations in Fornax. We determine abundances of O, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y, Ba, La, Nd and Eu in most of these stars, and for some stars also Mn and La. We demonstrate that classical indirect methods (isochrone fitting and integrated spectra) of metallicity determination lead to values of [Fe/H] which are 0.3 to 0.5 dex too high, and that this is primarily due to the underlying reference calibration typically used by these studies. We show that Cluster 1, with [Fe /H] = -2.5, now holds the record for the lowest metallicity globular cluster. We also measure an over-abundance of Eu in Cluster 3 stars that has only been previously detected in a subgroup of stars in M15. We find that the Fornax globular cluster properties are a global match to what is found in their Galactic counterparts; including deep mixing abundance patterns in two stars. We conclude that at the epoch of formation of globular clusters both the Milky Way and the Fornax dwarf spheroidal galaxy shared the same initial conditions, presumably pre-enriched by the same processes, with identical nucleosynthesis patterns.

astro-ph↗

arXiv2005

The Chemical Composition of Local Group Dwarf Spheroidals

I will review the progress of VLT spectroscopy of large numbers of individual stars in nearby dwarf spheroidal galaxies. This spectroscopy has allowed us to obtain detailed insights into the chemical and dynamical properties of the resolved stellar population in these nearby systems.

astro-ph↗

arXiv2004

Abundances as Tracers of the Formation and Evolution of (Dwarf) Galaxies

This aims to be an overview of what detailed observations of individual stars in nearby dwarf galaxies may teach us about galaxy evolution. This includes some early results from the DART (Dwarf Abundances and Radial velocity Team) Large Programme at ESO. This project has used 2.2m/WFI and VLT/FLAMES to obtain spectra of large samples of individual stars in nearby dwarf spheroidal galaxies and determine accurate abundances and kinematics. These results can be used to trace the formation and evolution of nearby galaxies from the earliest times to the present.

astro-ph↗

arXiv2004

Two distinct ancient components in the Sculptor Dwarf Spheroidal Galaxy: First Results from DART

We have found evidence for the presence of two distinct ancient stellar components (both geq 10 Gyr old) in the Sculptor dwarf spheroidal galaxy. We used the ESO Wide Field Imager (WFI) in conjunction with the VLT/FLAMES spectrograph to study the properties of the resolved stellar population of Sculptor out to and beyond the tidal radius. We find that two components are discernible in the spatial distribution of Horizontal Branch stars in our imaging, and in the [Fe/H] and v_hel distributions for our large sample of spectroscopic measurements. They can be generally described as a ``metal-poor'' component ([Fe/H]< -1.7) and a ``metal-rich'' component ([Fe/H]> -1.7). The metal-poor stars are more spatially extended than the metal-rich stars, and they also appear to be kinematically distinct. These results provide an important insight into the formation processes of small systems in the early universe and the conditions found there. Even this simplest of galaxies appears to have had a surprisingly complex early evolution.

astro-ph↗

arXiv2004

Stellar Abundances in Local Group Galaxies with the VLT

Here we describe some of our latest results from measuring detailed abundances in Local Group dwarf galaxies with VLT. Combining spectroscopic abundances with Colour-Magnitude diagrams allows the effective measurement of detailed chemical evolution with time in these galaxies. Although there are not yet significant numbers of individual stars observed in local group dwarf galaxies the uniformity of the abundance patterns of the majority of stars in galaxies with very different star formation histories must hint at a general properties of all star formation in these small systems.

astro-ph↗

arXiv2004

First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A

We present the abundance analyses of three isolated A-type supergiant stars in the dwarf irregular galaxy Sextans A from high-resolution spectra the UVES spectrograph at the VLT. Detailed model atmosphere analyses have been used to determine the stellar atmospheric parameters and the elemental abundances of the stars. The mean iron group abundance was determined from these three stars to be [(FeII,CrII)/H]=-0.99+/-0.04+/-0.06. This is the first determination of the present-day iron group abundances in Sextans A. These three stars now represent the most metal-poor massive stars for which detailed abundance analyses have been carried out. The mean stellar alpha element abundance was determined from the alpha element magnesium as [alpha(MgI)/H]=-1.09+/-0.02+/-0.19. This is in excellent agreement with the nebular alpha element abundances as determined from oxygen in the H II regions. These results are consistent from star-to-star with no significant spatial variations over a length of 0.8 kpc in Sextans A. This supports the nebular abundance studies of dwarf irregular galaxies, where homogeneous oxygen abundances are found throughout, and argues against in situ enrichment. The alpha/Fe abundance ratio is [alpha(MgI)/FeII,CrII]=-0.11+/-0.02+/-0.10, which is consistent with the solar ratio. This is consistent with the results from A-supergiant analyses in other Local Group dwarf irregular galaxies but in stark contrast with the high [alpha/Fe] results from metal-poor stars in the Galaxy, and is most clearly seen from these three stars in Sextans A because of their lower metallicities. The low [alpha/Fe] ratios are consistent with the slow chemical evolution expected for dwarf galaxies from analyses of their stellar populations.

astro-ph↗

arXiv2003

Anomalous Evolution of the Dwarf Galaxy HIPASS J1321-31

We present HST/WFPC2 observations of the dwarf galaxy HIPASS J1321-31. This unusual galaxy lies in the direction of the Centaurus A group of galaxies, and has a color-magnitude diagram with a distinctive red plume of luminous stars. This feature could arise from (a) a red giant branch if the galaxy were much nearer than previously recognized, (b) a peculiar asymptotic giant branch, or, (c) an ~1 Gigayear old population of intermediate mass red supergiants, which we find to be the most likely explanation. However, the lack of equally luminous blue stars requires that the star formation has dropped substantially since these stars were formed. Evidently HIPASS J1321-31 experienced an episode of enhanced star formation rather recently in its star formation history followed by a period of relative quiescence which has led to the evolution of the main sequence stars into the red supergiant branch. The stellar populations in HIPASS J1321-31 reflect a star formation history that is uncommon in star forming dwarf galaxies. This is the first time such a star formation history has been noted, although the literature contains a small number of other dwarf galaxies with similar color-magnitude diagrams. Therefore, HIPASS J1321-31 and these other galaxies represent a different path of dwarf galaxy evolution that has not been well-explored and an important probe into how dwarf galaxies evolve.

astro-ph↗

arXiv2003

Deep Hubble Space Telescope Imaging of IC 1613 II. The Star Formation History

We present deep HST WFPC2 imaging of the Local Group dwarf irregular galaxy IC 1613. The photometry is the deepest to date for an isolated dwarf irregular galaxy. The resulting color-magnitude diagram (CMD) is analyzed using three different methods to derive a star formation history (SFH). All three find an enhanced star formation rate (SFR), from 3 to 6 Gyr ago, and similar age-metallicity relationships (AMR). A comparison of the newly observed outer field with an earlier studied central field of IC 1613 shows that the SFR in the outer field has been significantly depressed during the last Gyr. This implies that the optical scale length of the galaxy has been decreasing with time and that comparison of galaxies at intermediate redshift with present day galaxies should take this effect into account. We find strong similarities between IC 1613 and the more distant Milky Way dSph companions in that all are dominated by star formation at intermediate ages. In particular, the SFH and AMR for IC 1613 and Leo I are indistinguishable. This implies that dIrr galaxies cannot be distinguished from dSphs by their intermediate age stellar populations. This type of a SFH may also be evidence for slower or suppressed early star formation in dwarf galaxies due to photoionization after the reionization of the universe by background radiation. Assuming that IC 1613 is typical of a dIrr evolving in isolation, since most of the star formation occurs at intermediate ages, these dwarf systems cannot be responsible for the fast chemical enrichment of the IGM which is seen at high redshift. There is no evidence for any large amplitude bursts of star formation in IC 1613, and we find it highly unlikely that analogs of IC 1613 have contributed to the excess of faint blue galaxies in existing galaxy redshift surveys.

astro-ph↗

arXiv2003

Stellar Abundances in Dwarf Irregular Galaxies

Dwarf irregular galaxies appear to have undergone very slow chemical evolution since they have low nebular abundances, but have had ongoing star formation over the past 15 Gyr. They are too distant for red giant abundance analyses to examine the details of their chemical evolution, however the isolated, bright blue supergiants do allow us to determine their present-day iron abundances to compare with both stellar and nebular alpha-element results. The [alpha/Fe] ratios in four Local Group dwarf irregular galaxies (NGC6822, WLM, Sextans A, and GR 8) all appear to have solar ratios regardless of the differences in their metallicities and star formation histories. Surprisingly, WLM's stellar metallicity is three times higher than the nebular oxygen abundance. We compare the [alpha/Fe] ratios in the dwarf irregulars to those from recent analyses of red giant branch stars in dwarf spheroidal galaxies, and also to damped Ly-alpha systems, and discuss these in the context of model predictions.

astro-ph↗

arXiv2003

Deep HST Imaging of Sextans A. III. The Star Formation History

We present a measurement of the star formation history of Sextans A, based on WFPC2 photometry that is 50% complete to V=27.5 (M_V ~+ 1.9) and I=27.0. The star formation history and chemical enrichment history have been measured through modeling of the CMD. We find evidence for increased reddening in the youngest stellar populations and an intrinsic metallicity spread at all ages. Sextans A has been actively forming stars at a high rate for ~ 2.5 Gyr ago, with an increased rate beginning ~ 0.1 Gyr ago. We find a non-zero number of stars older than 2.5 Gyr, due to the limited depth of the photometry, a detailed star formation history at intermediate and older ages has considerable uncertainties. The mean metallicity was found to be [M/H] ~- 1.4 over the measured history of the galaxy, with most of the enrichment happening at ages of at least 10 Gyr. We also find that an rms metallicity spread of 0.15 dex at all ages allows the best fits to the observed CMD. We revisit our determination of the recent star formation history (age <= 0.7 Gyr) using BHeB stars and find good agreement for all but the last 25 Myr, a discrepancy resulting primarily from different distances used in the two analyses and the differential extinction in the youngest populations. This indicates that star formation histories determined solely from BHeB stars should be confined to CMD regions where no contamination from reddened MS stars is present.

astro-ph↗

arXiv2002

Deep HST Imaging of Sextans A. II. Cepheids and Distance

We have identified 82 short-period variable stars in Sextans A from deep WFPC2 observations. All of the periodic variables appear to be short-period Cepheids, with periods as small as 0.8 days for fundamental-mode Cepheids and 0.5 days for first-overtone Cepheids. These objects have been used, along with measurements of the RGB tip and red clump, to measure a true distance modulus to Sextans A of (m-M)_0 = 25.61 +/- 0.07, corresponding to a distance of d = 1.32 +/- 0.04 Mpc. Comparing distances calculated by these techniques, we find that short-period Cepheids (P < 2 days) are accurate distance indicators for objects at or below the metallicity of the SMC. As these objects are quite numerous in low-metallicity star-forming galaxies, they have the potential for providing extremely precise distances throughout the Local Group. We have also compared the relative distances produced by other distance indicators. We conclude that calibrations of RR Lyraes, the RGB tip, and the red clump are self-consistent, but that there appears to be a small dependence of long-period Cepheid distances on metallicity. Finally, we present relative distances of Sextans A, Leo A, IC 1613, and the Magellanic Clouds.

astro-ph↗

arXiv2002

VLT/UVES Abundances in Four Nearby Dwarf Spheroidal Galaxies: II. Implications for Understanding Galaxy Evolution

We have used UVES on VLT-UT2 to take spectra of 15 individual red giant stars in the centers of four nearby dwarf spheroidal galaxies: Sculptor, Fornax, Carina and Leo I. We measure the abundance variations of numerous elements in these low mass stars with a range of ages (1-15Gyr old). This means that we can effectively measure the chemical evolution of these galaxies WITH TIME. Our results show a significant spread in metallicity with age, but an overall trend consistent with what might be expected from a closed (or perhaps leaky) box chemical evolution scenario over the last 10-15Gyr. We notice that each of these galaxies show broadly similar abundance patterns for all elements measured. This suggests a fairly uniform progression of chemical evolution with time, despite quite a large range of star formation histories. It seems likely that these galaxies had similar initial conditions, and evolve in a similar manner with star formation occurring at a uniformly low rate, even if at different times. With our accurate measurements we find evidence for small variations in abundances which are correlated to variations in star formation histories. The alpha-elements suggest that dSph chemical evolution has not been affected by very high mass stars (>15-20 Msun). The abundance patterns we measure for stars in dwarf spheroidal galaxies are significantly different from those typically observed in the disk, bulge and inner-halo of our Galaxy. This suggests that it is NOT possible to construct a significant fraction of our Galaxy from STARS formed in these dwarf spheroidal galaxies which subsequently merged into our own. Any merger scenario involving dSph has to occur in the very early Universe whilst they are still gas rich, so the majority of mass transfer is gas, and few stars.

astro-ph↗

arXiv2002

Dwarf Galaxies: Important Clues to Galaxy Formation

The smallest dwarf galaxies are the most straight forward objects in which to study star formation processes on a galactic scale. They are typically single cell star forming entities, and as small potentials in orbit around a much larger one they are unlikely to accrete much (if any) extraneous matter during their lifetime (either intergalactic gas, or galaxies) because they will typically lose the competition with the much larger galaxy. We can utilise observations of stars of a range of ages to measure star formation and enrichment histories back to the earliest epochs. The most ancient objects we have ever observed in the Universe are stars found in and around our Galaxy. Their proximity allows us to extract from their properties detailed information about the time in the early Universe into which they were born. A currently fashionable conjecture is that the earliest star formation in the Universe occurred in the smallest dwarf galaxy sized objects. Here I will review some recent observational highlights in the study of dwarf galaxies in the Local Group and the implications for understanding galaxy formation and evolution.

astro-ph↗

arXiv2002

UVES Abundances of Stars in Nearby Galaxies: How Good are Theoretical Isochrones?

Here we report some results from an ESO-VLT programme to observe individual stars in nearby dwarf galaxies at high resolution with the UVES spectrograph (Tolstoy, Venn, Shetrone, Primas, Hill, Kaufer & Szeifert 2002, submitted to AJ). We mainly concentrate on illustrating the issues and uncertainties surrounding our efforts to determine the ages of stars for which we have accurately measured [Fe/H] and [alpha/Fe].

astro-ph↗

arXiv2002

An Optical Velocity for the Phoenix Dwarf Galaxy

We present the results of a VLT observing program carried out in service mode using FORS1 on ANTU in Long Slit mode to determine the optical velocities of nearby low surface brightness galaxies. As part of our program of service observations we obtained long-slit spectra of several members of the Phoenix dwarf galaxy from which we derive an optical helio-centric radial velocity of -13 +/- 9km/s. This agrees very well with the velocity of the most promising of the HI clouds seen around Phoenix, which has a helio-centric velocity of -23 km/s, but is significantly different to the recently published optical heliocentric velocity of Phoenix of -52 +/- 6 km/s of Gallart et al. (2001).

astro-ph↗