R. C. Dohm-Palmer

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.

Mapping the Galactic Halo. V. Sagittarius Dwarf Spheroidal Tidal Debris 60 from the Main Body

As part of the Spaghetti Project Survey, we have detected a concentration of giant stars well above expectations for a smooth halo model. The position (l ~ 350°, b ~ 50°) and distance (~50 kpc) of this concentration match those of the northern overdensity detected by the Sloan Digital Sky Survey. We find additional evidence for structure at ~80 kpc in the same direction. We present radial velocities for many of these stars, including the first published results from the 6.5 m Magellan telescope. The radial velocities for stars in these structures are in excellent agreement with models of the dynamical evolution of the Sagittarius dwarf tidal debris, whose center is 60° away. The metallicity of stars in these streams is lower than that of the main body of the Sgr dwarf, which may indicate a radial metallicity gradient prior to disruption.As part of the Spaghetti Project Survey (SPS) we have detected a concentration of giant stars well above expectations for a smooth halo model. The position (l~350, b~50) and distance (~50 kpc) of this concentration match those of the Northern over-density detected by SDSS (Yanny et al. 2000, Ivezic et al. 2000). We find additional evidence for structure at ~80 kpc in the same direction. We present radial velocities for many of these stars, including the first published results from the 6.5m Magellan telescope. The radial velocities for stars in these structures are in excellent agreement with models of the dynamical evolution of the Sgr dwarf tidal debris, whose center is 60 degrees away. The metallicity of stars in these streams is lower than that of the main body of the Sgr dwarf, which may indicate a radial metallicity gradient prior to disruption.

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 Ratio of Blue to Red Supergiants in Sextans A from Hubble Space Telescope Imaging

We have examined the ratio of blue to red supergiants (B/R) in the dwarf irregular galaxy Sextans A. The supergiants were identified in previously published stellar photometry measured from Hubble Space Telescope imaging. The high-resolution imaging and low-dust environment provided high photometric accuracy such that the main sequence and blue helium-burning supergiants are clearly separated. This allows us to isolate the He-burning phase at both the red and blue ends of the so-called blue loops. The supergiant B/R provides an observational constraint on the relative lifetimes of these two phases that is a sensitive test for convection, mass-loss, and rotation parameters. These parameters have direct implications for the period-luminosity relationship for Cepheid variable stars. Previous studies have used a single number to represent this ratio. However, since the B/R is a fairly strong function of mass for a single-age stellar population, both changes in recent star formation rate and choice of luminosity cutoff can dramatically affect the result. We have analyzed the ratio as a function of age, or, equivalently, mass. This method eliminates the confusion of unknown star formation histories so that the B/R can be a more reliable diagnostic tool. We compare the result with a model based on stellar evolution tracks of an appropriate metallicity. The functional form of the observed ratio matches the model extremely well. However, the observed B/R is lower than the model by a factor of 2. This result suggests that stellar rotation is an important effect in the evolution of these stars.

Mapping the Galactic Halo. III. Simulated Observations of Tidal Streams

We have simulated the evolution of tidal debris in the Galactic halo in order to guide our ongoing survey to determine the fraction of halo mass accreted via satellite infall. Contrary to naive expectations that the satellite debris will produce a single narrow velocity peak on a smooth distribution, there are many different signatures of substructure, including multiple peaks and broad but asymmetrical velocity distributions. Observations of the simulations show that there is a high probability of detecting the presence of tidal debris with a pencil-beam survey of 100 deg2. In the limiting case of a single 107 M⊙ satellite contributing 1% of the luminous halo mass the detection probability is a few percent using just the velocities of 100 halo stars in a single 1 deg2 field. The detection probabilities scale with the accreted fraction of the halo and the number of fields surveyed. There is also surprisingly little dependence of the detection probabilities on the time since the satellite became tidally disrupted, or on the initial orbit of the satellite, except for the time spent in the survey volume.

Mapping the Galactic Halo. VI. Spectroscopic Measures of Luminosity and Metallicity

We present our calibration of spectroscopic measures of luminosity and metallicity for halo giant candidates and give metallicities and distances for our first sample of spectroscopically confirmed giants. These giants have distances ranging from 15 to 83 kpc. As surveys reach farther into the Galaxys halo with K giant samples, identification of giants becomes more difficult. This is because the numbers of foreground halo K dwarfs rise for V magnitudes of 19–20, typical for halo giants at ~100 kpc. Our photometric survey uses the strength of the Mg b/H feature near 5170 A to weed K dwarfs out of the disk and thick disk, but we need spectroscopic measures of the strength of the Ca II K, Ca I λ4227, and Mg b/H features to distinguish between the very metal-poor dwarfs and halo giants. Using a full error analysis of our spectroscopic measures, we show why a signal-to-noise ratio of ~15 pixel-1 at Ca I λ4227 and ~10 at Ca II K is needed for reliable luminosity discrimination. We use the Ca II K and Mg b features to measure metallicity in our halo giants, with typical errors (random plus systematic) of 0.3 dex for [Fe/H] values from -0.8 to -3.0.

Mapping the Galactic Halo. IV. Finding Distant Giants Reliably with the Washington System

We critically examine the use of the Washington photometric system (with the DDO51 filter) for identifying distant halo giants. While this is the most powerful photometric technique for isolating G and K giant stars, spectroscopic follow-up of giant candidates is vital. There are two situations in which interlopers outnumber genuine giants in the diagnostic M-51/M-T2 plot and are indistinguishable photometrically from the giants. (1) In deep surveys covering tens of square degrees, very metal-poor halo dwarfs are a significant contaminant. An example is our survey of the outer halo, where these metal-poor dwarfs dominate the number of photometric giant candidates at magnitudes fainter than V = 18 and cannot be isolated photometrically. (2) In deep surveys of smaller areas with low photometric precision, most objects in the giant region of the color-color plot are dwarfs whose photometric errors have moved them there. Color errors in M-51 and M-T2 need to be smaller than 0.03 mag to avoid this problem. An example of a survey whose photometric errors place the giant identifications under question is the survey for extratidal giants around the Carina dwarf spheroidal galaxy of Majewski et al. Accurate photometry and spectroscopic follow-up of giant candidates are essential when using the Washington system to identify the rare outer halo giants.

Mapping the Galactic Halo. II. Photometric Survey

We present imaging results from a high Galactic latitude survey designed to examine the structure of the Galactic halo. The objective of the survey is to identify candidate halo stars which can be observed spectroscopically to obtain radial velocities and confirm halo membership. The Washington filter system is used for its ability to distinguish between dwarfs and giants, as well as provide a metallicity indicator. Our most successful imaging run used the BTC camera on the CTIO 4 m telescope in 1999 April. Photometric conditions during these observations provided superb photometry, with average errors for a star at M = 18.5 of 0.009, 0.008, 0.011, and 0.009 for C, M, DDO51, and T2, respectively. We use these data as a template to describe the details of our photometric reduction process. It is designed to perform CCD reductions and stellar photometry automatically during the observation run, without the aid of external packages, such as IRAF and IDL. We describe necessary deviations from this procedure for other instruments used in the survey up to 2000 June. Preliminary results from spectroscopic observations indicate a 97% efficiency in eliminating normal dwarfs from halo giant candidates for M < 18.5. Unfortunately, low-metallicity subdwarfs cannot be photometrically distinguished from giants using the Washington filters. These major contaminants unavoidably reduced the overall giant identification efficiency to 66% for M < 18.5. Our improved knowledge of these stars will increase this efficiency for future spectroscopic observations.