arXiv subjects

Andrew E. Dolphin

Publications and source records attributed to Andrew E. Dolphin.

160 records · Page 9Linked to original sources

arXiv2007

The Aptly Named Phoenix Dwarf Galaxy

The Local Group galaxy Phoenix has the properties of a dwarf spheroidal galaxy, but an adjacent HI cloud has been found to be at the same radial velocity as the stars. The proximity suggests that this cloud is associated with the most recent ($\le 100$ Myr) star formation in Phoenix. We have obtained relatively high sensitivity and high resolution HI imaging with the VLA with the goal of distinguishing between different processes for displacing the gas from the galaxy. Due to the outer curvature of the HI cloud, it appears that expulsion from the galaxy by winds from supernovae is more likely than ram-pressure stripping. The isolation of the galaxy makes tidal stripping highly unlikely. Using a star formation history constructed from HST imaging, we construct a simple kinematic model which implies that the HI cloud is still gravitationally bound to the galaxy. Gas which is expelled from the centers of dwarf galaxies but which remains gravitationally bound may explain the episodic star formation observed in several dwarfs. In the specific case of Phoenix, there may be future star formation in this currently dSph-like galaxy.

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arXiv2006

Tip of the Red Giant Branch Distances. I. Optimization of a Maximum Likelihood Algorithm

Accurate distances to galaxies can be determined from the luminosities of stars at the Tip of the Red Giant Branch (TRGB). We use a Maximum Likelihood algorithm to locate the TRGB in galaxy color-magnitude diagrams. The algorithm is optimized by introducing reliable photometric errors and a completeness characterization determined with artificial star experiments. The program is extensively tested using Monte-Carlo simulations, artificial galaxies, and a sample of nearby dwarf galaxies observed with HST/WFPC2 and ACS. Our procedure is shown to be reliable, to have good accuracy, and to not introduce any systematic errors. The methodology is especially useful in cases where the TRGB approaches the photometric limit and/or the RGB is poorly populated.

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arXiv2005

Star Formation Histories of Local Group Dwarf Galaxies

We report preliminary results of a project to homogeneously measure star formation histories for the majority of Local Group dwarf galaxies, based on archival WFPC2 observations. For the dwarf spheroidal and elliptical galaxies, we find that the more luminous systems all show extended star formation episodes, while the less luminous systems typically formed most of their stars over ten Gyr ago. Irregular galaxies show no comparable trend. We find that a galaxy's classification as an irregular or spheroidal is entirely explained by its star formation within the past Gyr or less. Several elliptical/spheroidal systems actively formed stars within the past two Gyr, and would have been indistinguishable from present-day irregulars if observed two Gyr ago.

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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.

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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.

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arXiv2002

WFPC2 CTE Characterization

The limiting factor of the accuracy of WFPC2 photometry is the CTE loss, which has increased to the level of 50% or more for faint stars at the top of the chips. I describe recent work on characterizing this effect, and provide improved equations for CTE correction. I also examine issues affecting background measurement, which if not done correctly can introduce artificial nonlinearities into photometry.

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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.

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arXiv2002

HST Survey of Clusters in Nearby Galaxies. II. Statistical Analysis of Cluster Populations

We present a statistical system that can be used in the study of cluster populations. The basis of our approach is the construction of synthetic cluster color-magnitude-radius diagrams (CMRDs), which we compare with the observed data using a maximum likelihood calculation. This approach permits a relatively easy incorporation of incompleteness (a function of not only magnitude and color, but also radius), photometry errors and biases, and a variety of other complex effects into the calculation, instead of the more common procedure of attempting to correct for those effects. We then apply this procedure to our NGC 3627 data from Paper I. We find that we are able to successfully model the observed CMRD and constrain a number of parameters of the cluster population. We measure a power law mass function slope of alpha = -1.50 +/- 0.07, and a distribution of core radii centered at r_c = 1.53 +/- 0.15 pc. Although the extinction distribution is less constrained, we measured a value for the mean extinction consistent with that determined in Paper I from the Cepheids.

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arXiv2001

Numerical Methods of Star Formation History Measurement and Applications to Seven Dwarf Spheroidals

A comprehensive study of the measurement of star formation histories from colour-magnitude diagrams (CMDs) is presented, with an emphasis on a variety of subtle issues involved in the generation of model CMDs and maximum likelihood solution. Among these are the need for a complete sampling of the synthetic CMD, the use of of proper statistics for dealing with Poisson-distributed data (and a demonstration of why chi-square must not be used), measuring full uncertainties in all reported parameters, quantifying the goodness-of-fit, and questions of binning the CMD and incorporating outside information. Several example star formation history measurements are given. Two examples involve synthetic data, in which the input and recovered parameters can be compared to locate possible flaws in the methodology (none were apparent) and measure the accuracy with which ages, metallicities, and star formation rates can be recovered. Solutions of the histories of seven Galactic dwarf spheroidal companions (Carina, Draco, Leo I, Leo II, Sagittarius, Sculptor, and Ursa Minor) illustrate the ability to measure star formation histories given a variety conditions -- numbers of stars, complexity of star formation history, and amount of foreground contamination. Significant measurements of ancient >8 Gyr star formation are made in all seven galaxies. Sculptor, Draco, and Ursa Minor appear entirely ancient, while the other systems show varying amounts of younger stars.

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arXiv2001

HST Survey of Clusters in Nearby Galaxies. I. Detection and Photometry

We have developed photometric techniques that can be applied to images with highly-variable backgrounds, as well as to slightly-extended objects (object size comparable to or smaller than PSF size). We have shown that ordinary stellar PSF-fitting photometry can be applied to slightly-extended objects, provided that one applies a systematic correction to the photometry that is a function primarily of the observed sharpness. Applying these techniques to the Cepheid target NGC 3627, we find that we are successfully able to photometer the stars and clusters, as well as discriminating the cluster population with a negligible number of false detections.

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arXiv2000

Deep HST 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 Lyraes 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 Lyraes) and -1.0 (Cepheids), we find a relatively steep slope of 0.34 +/- 0.20 magnitudes per dex for the RR Lyraes and a shallow slope of -0.07 +/- 0.16 mag/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 mu_0 = 24.31 +/- 0.06, corresponding to a distance of 730 +/- 20 kpc. This distance produces an RR Lyrae absolute magnitude of MV = 0.61 +/- 0.08.

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arXiv2000

Hubble Space Telescope Photometry of the Dwarf Spheroidal Galaxy ESO 410-G005

We present HST WFPC2 imaging of the nearby low-surface-brightness dwarf spheroidal galaxy ESO 410-G005, which has been resolved into stars for the first time. The resulting color-magnitude diagram for about 2500 stars shows a red giant branch branch with a tip at I=(22.4+-0.15), which yields a distance of D=(1.9+-0.2) Mpc. ESO 410-G005 is found to be metal-poor with a mean metallicity of (-1.8+-0.4) dex estimated from its red giant branch. Upper asymptotic giant branch stars appear to be present near the center of the galaxy, indicative of a substantial, centrally concentrated intermediate-age population, unless these objects are artifacts of crowding. Previous studies did not detect ESO 410-G005 in H alpha or in HI. ESO 410-G005 is a probable member of the Sculptor group. Its linear separation from the nearest spiral, NGC 55, is 230 kpc on the sky. The deprojected separation ranges from 340 to 615 kpc depending on the assumed distance of NGC 55. ESO 410 G005 appears to be a relatively isolated dSph within the Sculptor group. Its absolute magnitude, Mv = (-12.1+-0.2) mag, its central surface brightness, mu_V = (22.7+-0.1) mag/arcsec^2, and its mean metallicity, [Fe/H] = (-1.8+-0.4) dex, follow the trend observed for dwarf galaxies in the Local Group. (abridged)

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arXiv2000

The Charge Transfer Efficiency and Calibration of WFPC2

A new determination of WFPC2 photometric corrections is presented, using HSTphot reduction of the WFPC2 Omega Centauri and NGC 2419 observations from January 1994 through March 2000 and a comparison with ground-based photometry. No evidence is seen for any position-independent photometric offsets (the "long-short anomaly"); all systematic errors appear to be corrected with the CTE and zero point solution. The CTE loss time dependence is determined to be very significant in the Y direction, causing time-independent CTE solutions (Stetson 1998; Saha, Lambert, & Prosser 2000) to be valid only for a small range of times. On average, the present solution produces corrections similar to Whitmore, Heyer, & Casertano (1999), although with an improved functional form that produces less scatter in the residuals and determined with roughly a year of additional data. In addition to the CTE loss characterization, zero point corrections are also determined as functions of chip, gain, filter, and temperature. Of interest, there are chip-to-chip differences of order 0.01-0.02 magnitudes relative to the Holtzman et al. (1995) calibrations, and the present study provides empirical zero point determinations for the non-standard filters such as the frequently-used F450W, F606W, and F702W.

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arXiv2000

WFPC2 Stellar Photometry with HSTphot

HSTphot, a photometry package designed to handle the undersampled PSFs found in WFPC2 images, is introduced and described, as well as some of the considerations that have to be made in order to obtain accurate PSF-fitting stellar photometry with WFPC2 data. Tests of HSTphot's internal reliability are made using multiple observations of the same field, and tests of external reliability are made by comparing with DoPHOT reductions of the same data.

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arXiv1999

{HST Studies of the WLM Galaxy. II. The Star Formation History from Field Stars

HST F555W and F814W photometry of a portion of the WLM galaxy are presented. The distance modulus is determined via fitting of the entire color-magnitude diagram to be (m-M)_0 = 24.88 +/- 0.09, which is consistent with the RGB tip distance. The galaxy's measurable star formation history appears to have begun no more than 12 Gyr ago, with about half of WLM's total star formation (by mass) formed before 9 Gyr ago. The star formation rate gradually decreased, until a recent increase in activity starting between 1 and 2.5 Gyr ago. This is still continuing to the present time, and is concentrated in the bar of the galaxy, as shown by the difference in recent star formation rates in the three WF chips.

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arXiv1999

HST Studies of the WLM Galaxy. I. The Age and Metallicity of the Globular Cluster

We have obtained V and I images of the lone globular cluster that belongs to the dwarf Local Group irregular galaxy known as WLM. The color-magnitude diagram of the cluster shows that it is a normal old globular cluster with a well-defined giant branch reaching to M_V=-2.5, a horizontal branch at M_V=+0.5, and a sub-giant branch extending to our photometry limit of M_V=+2.0. A best fit to theoretical isochrones indicates that this cluster has a metallicity of [Fe/H]=-1.52\pm0.08 and an age of 14.8\pm0.6 Gyr, thus indicating that it is similar to normal old halo globulars in our Galaxy. From the fit we also find that the distance modulus of the cluster is 24.73\pm0.07 and the extinction is A_V=0.07\pm0.06, both values that agree within the errors with data obtained for the galaxy itself by others. We conclude that this normal massive cluster was able to form during the formation of WLM, despite the parent galaxy's very small intrinsic mass and size.

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