Eline Tolstoy

The role of stellar feedback and dark matter in the evolution of dwarf galaxies

Supernova and multiple supernova events regulate several structural properties of dwarf galaxies. In particular, they govern the metal enrichment and the energy budget of the ISM; they might induce partial (blowout) or total (blowaway) gas removal from the galaxy; they also regulate the pressure of the ISM, and consequently the morphology of the galactic gaseous body. Significant amounts of dark matter may play an equally important role: the dark matter gravitational potential tends to concentrate baryons towards the centre, thus enhancing both the star formation rate and the metal production. Also, the dynamical properties of the ISM, and the occurrence of a blowout or blowaway, are shown to be determined by the dark matter content. We present detailed analytical/numerical models describing the evolution of dwarf irregular galaxies (dIs), including the above and other effects. The main results are: (i) dwarfs with total masses M & 5 � 10 6 M( are blown away; those with gas masses up to .10 9 M( lose mass in an outflow; (ii) metallicities are found to correlate tightly with dark matter content, and are consistent with a range of dark-to-visible mass ratios f < 0±30; with about 65per cent of the dwarfs in the sample having f < 0±10; (iii) we predict a lower limit to the oxygen abundance in dIs of 12 1 log…O=H† < 7:2; (iv) outflows are not particularly important for the metallicity evolution of dwarf galaxies, and certainly less important than star formation for gas consumption; however, dwarfs with gas masses of a few �10 8 M( are shown to be the major pollutants of the IGM; (v) the ISM Hi velocity dispersion correlates with metallicity and, independently of dark matter, scales as Z 3.5 . Specific comparisons with well-studied dIs, such as for example Leo A, yield excellent agreement with the data. Based on our results, we discuss a scenario in which late-type and early-type dwarfs had common progenitors in the past, but differences in their total mass forced these objects to follow different evolutionary paths. We therefore consider dI ! dE transitions occurring at present cosmic times as very unlikely.

Deep Hubble Space Telescope Imaging of IC 1613. I. Variable Stars and Distance

We present WFPC2 VI photometry of a field in the halo of IC 1613, finding 13 RR Lyrae stars and 11 Cepheids. Our photometry of the red giant branch tip and red clump is used to derive distances to IC 1613, which are consistent with each other and with distances based on the variable stars. We compare these values with similarly measured distances for the Magellanic Clouds, and are able to measure metallicity dependencies of the RR Lyrae and Cepheid distances by requiring consistent relative distance measurements from the four techniques. For metallicities of [Fe/H] = -1.3 (RR Lyrae stars) and -1.0 (Cepheids), we find a relatively steep slope of 0.34 ± 0.20 mag per dex for the RR Lyrae stars and a shallow slope of -0.07 ± 0.16 mag per dex for the Cepheids, both values within the range of theoretical and empirical results in the literature. We find that a dependence of the red clump absolute magnitude on age, in addition to metallicity, is required to produce self-consistent relative distances between IC 1613 and the Magellanic Clouds. Adopting such a red clump calibration and self-consistent calibrations for the other three distance indicators, we find that the distances to all three objects are in excellent agreement. Our best distance modulus to IC 1613 is μ0 = 24.31 ± 0.06, corresponding to a distance of 730 ± 20 kpc. This distance produces an RR Lyrae absolute magnitude of 0.61 ± 0.08.

The Recent Star Formation History of GR 8 from Hubble Space Telescope Photometry of the Resolved Stars

?????We have used the Hubble Space Telescope to observe the resolved stars in the dwarf irregular galaxy GR 8 (DDO 155, UGC 8091). The data consisted of dithered Wide Field Planetary Camera 2 images in three bands: F439W (1 hr), F555W (30 minutes), and F814W (30 minutes). The stellar photometry was extracted with a modified version of DoPHOT. Artificial star tests showed the data to be 50% complete to V = 26.3, B = 25.4, and I = 25.2. The color-magnitude diagrams contain well-defined populations, including a very young main sequence (MS) (<10 Myr), and a red giant branch as old as several Gyr. These features align well with stellar evolution models of the appropriate metallicity. The distance based on the tip of the red giant branch is in excellent agreement with the Cepheid determination of ? = 26.75 ? 0.35 (2.2 Mpc), which we adopted. An extended stellar halo was discovered well beyond the H I. Based on the MS and blue HeB luminosity function, we calculated the star formation rate (SFR) over the past 500 Myr. The SFR has been fairly constant, at 400 M? Myr-1 kpc-2 with up to 60% variations. The blue HeB stars were used as a tracer for the location of star formation over this time period. The star formation occurred in superassociation size regions (100?200 pc), which lasted ~100 Myr. These regions come and go with no obvious pattern, except that they seem to concentrate in the current locations of H I clumps. This suggested that the H I clumps are long-lived features that support several star-forming events over time. The most likely explanation is that the star-forming regions are gravitationally bound. We estimated the gas-to-star conversion efficiency to be 6%. We compared our results with those of three other dI galaxies: Sextans A, Pegasus DIG, and Leo A. There is a trend of higher SFR per area with larger MH I/LB. Also, the star formation pattern is similar in all four galaxies. Finally, none of the four dI galaxies contained a large starburst, comparable to those in BCD galaxies. Combining the histories of all four galaxies, this implies that less than 5% of dI galaxies are hosting a strong burst of star formation at any given time. Observations of more galaxies are needed to improve this statistic.

Using the Ca II triplet to trace abundance variations in individual red giant branch stars in three nearby galaxies

Spectroscopic abundance determinations for stars spanning a Hubble time in age are necessary in order to determine unambiguously the evolutionary histories of galaxies. Using FORS I in multi-object spectroscopy mode on ANTU (UT1) at the ESO VLT on Paranal, we have obtained near-infrared spectra from which we have measured the equivalent widths of the two strongest Ca II triplet lines to determine metal abundances for a sample of red giant branch stars, selected from ESO NTT optical (I, V - I) photometry of three nearby Local Group galaxies: the Sculptor dwarf spheroidal, the Fornax dwarf spheroidal and the dwarf irregular NGC 6822. The summed equivalent width of the two strongest lines in the Ca it triplet absorption-line feature, centred at 8500 Angstrom, can be readily converted into an [Fe/H] abundance using the previously established calibrations by Armandroff & Da Costa and Rutledge, Hesser & Stetson. We have measured metallicities for 37 stars in Sculptor, 32 stars in Fornax and 23 stars in NGC 6822, yielding more precise estimates of the metallicity distribution functions for these galaxies than it is possible to obtain photometrically. In the case of NGC 6822, this is the first direct measurement of the abundances of the intermediate-age and old stellar populations. We find metallicity spreads in each galaxy which are broadly consistent with the photometric width of the red giant branch, although the abundances of individual stars do not always appear to correspond to their colour. This is almost certainly predominantly due to a highly variable star formation rate with time in these galaxies, which results in a non-uniform, non-globular-cluster-like evolution of the Ca/Fe ratio.

Star Formation in and Evolution of the Blue Compact Dwarf Galaxy UGC 6456 Determined from Hubble Space Telescope Images

Photometry on the UVI system has been performed on the resolved stellar content of the blue compact dwarf galaxy UGC 6456 using Wide Field Planetary Camera 2 (WFPC2) images obtained with the Hubble Space Telescope. The resulting color-magnitude diagram (CMD) goes to about V = 27.5 and reveals not only a young population of blue main-sequence stars and blue and red supergiants, but also an older evolved population of red giants and a fairly well represented asymptotic giant branch. The distance to the galaxy is estimated from the tip of the red giant branch to be 4.5 Mpc, placing it about 1.5 Mpc farther away than the major members of the M81 Group, with which it is usually associated. The youngest stars are generally associated with H II regions shown on our Hα image and are largely confined to the 745 pc field of our PC images. A comparison of their distribution in the CMD with theoretical isochrones suggests ages from 4 to 10 Myr. The population of older stars is found throughout all WFPC2 camera fields and seems to show an elliptical distribution with an aspect ratio of about 2.4 and an exponential falloff in surface density with distance from a center of symmetry that is not far from the centroid of the youngest stars. Theoretical modeling of the CMD at a metallicity of Z = 0.001 suggests star formation in the age interval 1–2 Gyr, a strong burst in the interval 600–800 Myr, and a lower rate of star formation up to the present. The evidence is compatible with a scenario beginning with the formation of a population of low-metallicity stars, enriching a major residual of prestellar material that subsequently fueled an active episode of star formation. That burst of star formation must have been particularly spectacular and may be related to the activity we now see in the distant blue dwarf galaxies revealed in deep imaging.

Stellar Populations at the Center of IC 1613

We have observed the center of the Local Group dwarf irregular galaxy IC 1613 with the WFPC2 aboard the Hubble Space Telescope in the F439W, F555W, and F814W filters. We analyze the resulting color-magnitude diagrams (CMDs) using the main-sequence and giant-branch luminosity functions and comparisons with theoretical stellar models to derive a preliminary star formation history for this galaxy. We find a dominant old stellar population (age ≈7 Gyr), identifiable by the strong red giant branch (RGB) and red clump populations. From the (V-I) color of the RGB, we estimate a mean metallicity of the intermediate-age stellar population [Fe/H] = -1.38 ± 0.31. We confirm a distance of 715 ± 40 kpc using the I magnitude of the RGB tip. The main-sequence luminosity function down to I ≈ 25 provides evidence for a roughly constant star formation rate (SFR) of approximately 3.5 × 10-4 M⊙ yr-1 across the WFPC2 field of view (0.22 kpc2) during the past 250–350 Myr. Structure in the blue loop luminosity function implies that the SFR was ≈50% higher 400–900 Myr ago than today. The mean heavy-element abundance of these young stars is around one-tenth solar. The best explanation for a red spur on the main sequence at I ≈ 24.7 is the blue horizontal branch component of a very old stellar population at the center of IC 1613. We have also imaged a broader area of IC 1613 using the 3.5 m WIYN telescope under excellent seeing conditions. The WIYN CMD reveals a prominent sequence of asymptotic giant branch stars and red supergiants that is less prominent in the WFPC2 CMD because of the smaller field of view. The asymptotic giant branch star luminosity function is consistent with a period of continuous star formation over at least the age range 2–10 Gyr. We present an approximate age-metallicity relation for IC 1613, which appears to be similar to that of the Small Magellanic Cloud. We compare the Hess diagram of IC 1613 with similar data for three other Local Group dwarf galaxies and find that IC 1613 most closely resembles the nearby, transition-type dwarf galaxy Pegasus (DDO 216).

Deep Hubble Space Telescope Imaging of Sextans A. I. The Spatially Resolved Recent Star Formation History

We have measured stellar photometry from deep Cycle 7 Hubble Space Telescope/WFPC2 imaging of the dwarf irregular galaxy Sextans A. The imaging was taken in three filters: F555W (V; eight orbits), F814W (I; 16 orbits), and F656N (Hα; one orbit). Combining these data with Cycle 5 WFPC2 observations provides nearly complete coverage of the optically visible portion of the galaxy. The Cycle 7 observations are nearly 2 mag more sensitive than the Cycle 5 observations, which provides unambiguous separation of the faint blue helium-burning stars (BHeB stars) from contaminant populations. The depth of the photometry allows us to compare recent star formation histories recovered from both the main-sequence stars and the BHeB stars for the last 300 Myr. The excellent agreement between these independent star formation rate (SFR) calculations is a resounding confirmation for the legitimacy of using the BHeB stars to calculate the recent SFR. Using the BHeB stars we have calculated the global star formation history over the past 700 Myr. The history calculated from the Cycle 7 data is remarkably identical to that calculated from the Cycle 5 data, implying that both halves of the galaxy formed stars in concert. We have also calculated the spatially resolved star formation history, combining the fields from the Cycle 5 and Cycle 7 data. The star-forming regions are found in three major zones of the galaxy. One of these zones is extremely young, consisting of only a single star-forming region that is less than 20 Myr old. Two of these zones are associated with high column density neutral gas, while the third, and oldest, is not. Our interpretation of this pattern of star formation is that it is an orderly stochastic process. Star formation begins on the edge of a gas structure and progressively eats away at the cloud, breaking it up and inducing further star formation. A more quantitative analysis of the star formation process must await a larger sample of galaxies with spatially resolved star formation histories to allow correlation studies with the physical properties of the galaxy.

The STIS Parallel Survey: Introduction and First Results.

The installation of the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) allows for the first time two-dimensional optical and ultraviolet slitless spectroscopy of faint objects from space. The STIS Parallel Survey (SPS) routinely obtains broadband images and slitless spectra of random fields in parallel with HST observations using other instruments. The SPS is designed to study a wide variety of astrophysical phenomena, including the rate of star formation in galaxies at intermediate to high redshift through the detection of emission-line galaxies. We present the first results of the SPS, which demonstrate the capability of STIS slitless spectroscopy to detect and identify high-redshift galaxies.

Wide field planetary camera 2 observations of Leo A: A predominantly young galaxy within the Local Group

The unprecedented detail of the WFPC2 colour-magnitude diagrams of the resolved stellar population of Leo A presented here allows us to determine a new distance and an accurate star formation history for this extremely metal-poor Local Group dwarf irregular galaxy. From the position of the red clump, the helium-burning blue loops and the tip of the red giant branch, we obtain a distance modulus, m-M=24.2+/-0.2, or 690 +/- 60 kpc, which places Leo A firmly within the Local Group. Our interpretation of these features in the WFPC2 CMDs at this new distance based upon extremely low metallicity (Z=0.0004) theoretical stellar evolution models suggests that this galaxy is predominantly young, i.e. <2 Gyr old. A major episode of star formation 900 - 1500 Gyr ago can explain the red clump luminosity and also fits in with our interpretation of the number of anomalous Cepheid variable stars seen in this galaxy. We cannot rule out the presence of an older, underlying globular cluster age stellar population with these data. However, using the currently available stellar evolution models, it would appear that such an older population is limited to no more than 10% of the total star formation to have occured in this galaxy. Leo A provides a nearby laboratory for studying young metal poor stars and investigations of metal-poor galaxy evolution, such as is supposed to occur for larger systems at intermediate and high redshifts.

Stellar chemo-kinematics of the Cetus dwarf spheroidal galaxy

In order to minimize environmental effects and gain an insight into the internal mechanisms that shape the properties of the early-type dwarf systems, we study one of the few isolated dwarf spheroidal galaxies (dSphs) of the Local Group (LG): Cetus. We obtained VLT/FORS2 spectra (