Mary M. Crone

The cosmological dependence of cluster density profiles

We use N-body simulations to study the shape of mean cluster density and velocity profiles in the nonlinear regime formed via gravitational instability. The dependence of the final structure on both cosmology and initial density field is examined, using a grid of cosmologies and scale-free initial power spectra P(k) varies as k(exp n). Einstein-de Sitter, open (Omega(sub 0) = 0.2 and 0.1) and flat, low density (Omega(sub 0) = 0.2 lambda(sub 0) = 0.8) models are examined, with initial spectral indices n = -2, -1 and 0. For each model, we stack clusters in an appropriately scaled manner to define an average density profile in the nonlinear regime. The profiles are well fit by a power law rho(r) varies as r(exp -alpha) for radii whereat the local density contrast is between 100 and 3000. This covers 99% of the cluster volume. We find a clear trend toward steeper slopes (larger alphas) with both increasing n and decreasing Omega(sub 0). The Omega(sub 0) dependence is partially masked by the n dependence; there is degeneracy in the values of alpha between the Einstein-de Sitter and flat, low-density cosmologies. However, the profile slopes in the open models are consistently higher than the Omega = 1 values for the range of n examined. Cluster density profiles are thus potentially useful cosmological diagnostics. We find no evidence for a constant density core in any of the models, although the density profiles do tend to flatten at small radii. Much of the flattening is due to the force softening required by the simulations. An attempt is made to recover the unsoftened profiles assuming angular momentum invariance. The recovered profiles in Einstein-de Sitter cosmologies are consistent with a pure power law up to the highest density contrasts (10(exp 6)) accessible with our resolution. The low-density models show significant deviation from a power law above density contrasts approximately 10(exp 5). We interpret this curvature as reflecting the non-scale-invariant nature of the background cosmology in these models. These results are at the limit of our resolution and so should be tested in the future using simulations with larger numbers of particles. Such simulations will also provide insight on the broader problem of understanding, in a statistical sense, the full phase space structure of collapsed, cosmological halos.

A Near-Infrared Stellar Census of Blue Compact Dwarf Galaxies: The Wolf-Rayet Galaxy I Zw 36*

We report the results of near-IR imaging in J and H, of I Zw 36 (≈Z⊙/14) with the Hubble Space Telescope. Whereas imaging with the pre-COSTAR Faint Object Camera (FOC) previously resolved hot and massive stars in the near-UV, the NICMOS data furnish a census of the cool, intermediate- and low-mass stars. There clearly was star formation in I Zw 36 prior to the activity which earned it its blue compact dwarf/Wolf-Rayet galaxy classification. The detection of luminous, asymptotic giant branch stars requires that stars formed vigorously several hundred megayears ago. The well-populated red giant branch indicates stars with ages of at least 1–2 Gyr (and possibly older than 10 Gyr). We use the tip-of-the-red-giant-branch method to derive a distance of ≥5.8 Mpc. This is the third in a series of papers on near-IR–resolved blue compact dwarf galaxies. We notice that the color-magnitude diagrams of VII Zw 403, Mrk 178, and I Zw 36 do not exhibit the gaps expected from an episodic mode of star formation. Using simulated color-magnitude diagrams we demonstrate for I Zw 36 that star formation did not stop for more than a few 108 yr over the past 109 yr, and we discuss the implications of this result.

A Near-Infrared Stellar Census of Blue Compact Dwarf Galaxies: NICMOS* Detection of Red Giant Stars in the Wolf-Rayet Galaxy Markarian 178

We observed the blue compact dwarf/Wolf-Rayet galaxy Mrk 178 with the NICMOS camera aboard Hubble Space Telescope. The galaxy is well resolved into individual stars in the near-IR; photometry in J and H yields color-magnitude diagrams containing 791 individual point sources. We discuss the stellar content, drawing particular attention to the intermediate-age and/or old stars. Mrk 178 is only the second blue compact dwarf galaxy in which the red giant branch has been resolved, indicating stars with ages of at least 1–2 Gyr. This allows us to derive a distance of ≥4.2(±0.5) Mpc. The near-IR color-magnitude diagram also exhibits an abundance of luminous, asymptotic giant branch stars. We find that this requires vigorous star formation several hundred Myr ago. Some candidate carbon stars are identified via their extreme near-IR color. We argue that Mrk 178 is fundamentally an old galaxy, based on the NICMOS detection of red giants underlying the blue, starburst core, and its extended, faint halo of redder color.

The Stellar Content of NGC 6789, A Blue Compact Dwarf Galaxy in the Local Void*

We find that NGC 6789 is the most nearby example of a blue compact dwarf galaxy known to date. With the help of the Wide Field Planetary Camera 2 aboard the Hubble Space Telescope, we resolve NGC 6789 into over 15,000 point sources in the V and I bands. The young stars of NGC 6789 are found exclusively near the center of the galaxy. The red giant population identified at large galactocentric radii yields a distance of about 3.6 Mpc, a stellar metallicity [Fe/H] of about -2, and a minimum age of about 1 Gyr. Despite its isolated location in the Local Void, its low metallicity, and its active star formation, the properties of NGC 6789 are clearly not those of a galaxy in formation.

A Near-Infrared Stellar Census of the Blue Compact Dwarf Galaxy VII Zw 403*

We present near-infrared single-star photometry for the low-metallicity blue compact dwarf galaxy VII Zw 403. We achieve limiting magnitudes of F110W ≈ 25.5 and F160W ≈ 24.5 using one of the NICMOS cameras with the Hubble Space Telescope equivalents of the ground-based J and H filters. The data have a high photometric precision (0.1 mag) and are more than 95% complete down to magnitudes of about 23, far deeper than previous ground-based studies in the near-IR. The color-magnitude diagram contains about 1000 point sources. We provide a preliminary transformation of the near-IR photometry into the ground system. We investigate the tip of the red giant branch method in the J and H bands to provide an empirical distance calibration. Combining our result with globular cluster data as well as stellar-evolution models, we recommend MH,TRGB = -5.5(±0.1) for -2.3 < [Fe/H] < -1.5. We proceed to discuss the stellar content of VII Zw 403 using evolutionary tracks as well as a classification scheme based on optical and near-IR colors, and we comment on the detection of asymptotic giant branch stars and the blue Hertzprung gap. We use MH as an indicator of Mbol for red stars after evaluating BCH at low metallicity. We calculate the fractional contribution of individual stars from our color-magnitude diagram to the integrated light of VII Zw 403 and determine which red stellar population dominates the integrated colors. We find that young red supergiants, and young and intermediate-age asymptotic giants, together provide about 50% of the light in I, J, and H bands, whereas the old red giant stars contribute less than 15%. Young, main-sequence stars and blue supergiants account for the remaining light and dominate in V. This explains the difficulties in discerning the nature of blue compact dwarf galaxies when only integrated photometry is available.

The Oldest Stars of the Extremely Metal-Poor Local Group Dwarf Irregular Galaxy Leo A

We present deep Hubble Space Telescope (HST) single-star photometry of Leo A in B, V, and I. Our new field of view is offset from the centrally located field observed by Tolstoy et al. in order to expose the halo population of this galaxy. We report the detection of metal-poor red horizontal branch stars, which demonstrate that Leo A is not a young galaxy. In fact, Leo A is as least as old as metal-poor Galactic Globular Clusters that exhibit red horizontal branches and are considered to have a minimum age of about 9 Gyr. We discuss the distance to Leo A and perform an extensive comparison of the data with stellar isochrones. For a distance modulus of 24.5, the data are better than 50% complete down to absolute magnitudes of 2 or more. We can easily identify stars with metallicities between 0.0001 and 0.0004, and ages between about 5 and 10 Gyr, in their post–main-sequence phases, but we lack the detection of main-sequence turnoffs that would provide unambiguous proof of ancient (>10 Gyr) stellar generations. Blue horizontal branch stars are above the detection limits but difficult to distinguish from young stars with similar colors and magnitudes. Synthetic color-magnitude diagrams show it is possible to populate the blue horizontal branch in the halo of Leo A. The models also suggest ≈50% of the total astrated mass in our pointing to be attributed to an ancient (>10 Gyr) stellar population. We conclude that Leo A started to form stars at least about 9 Gyr ago. Leo A exhibits an extremely low oxygen abundance, only 3% of solar, in its ionized interstellar medium. The existence of old stars in this very oxygen-deficient galaxy illustrates that a low oxygen abundance does not preclude a history of early star formation.

A Compact Population of Red Giants in the Blue Compact Dwarf Galaxy UGCA 290

We present Hubble Space Telescope/Wide Field Planetary Camera 2 single-star photometry for the blue dwarf galaxy UGCA 290, whose morphology is intermediate between classic iE blue compact dwarf galaxies and blue dwarf galaxies that exhibit no red background sheet of older stars. The color-magnitude diagram of this galaxy in V and I, extending over 6 mag, is remarkably similar to that of the star-forming region in the iE blue compact dwarf galaxy VII Zw 403. There is no evidence for gaps in its star formation history over the last billion years, and the color of its red giant branch indicates a very metal-poor stellar population. From the magnitude of the tip of the red giant branch, we derive a distance of 6.7 Mpc, more than twice the distance estimated from the brightest blue supergiants.

The environmental impact of vacuum decay

In the standard model of particle physics, the force laws changed discontinuously as the Universe cooled below the critical temperature for electroweak gauge symmetry breaking. In a recently proposed model, the gauge symmetry of electrodynamics, which is responsible for the photon being massless, will also be broken as the Universe cools below some critical temperature. As a result, the photon will suddenly acquire a mass that is of the order of 10−4 eV/c2, corresponding to a Compton wavelength of a centimeter. The effects of such a transition on our civilization are examined. All long‐range electric and magnetic fields would vanish, as would all low‐frequency electromagnetic waves. Atomic structure and solar radiation would not be appreciably affected. Effects on neural transmission might be serious, depending on the precise value of the photon mass. It is concluded that the impact on society would be severe, but probably not fatal. Life would continue, and our civilization could be restructured to accom...

The blue compact dwarf galaxy UGC 6456 resolved into single stars with HST

We present single-star photometry for the blue compact dwarf galaxy UGC 6456 (=VII Zw 403), imaged with the HST/WFPC2 in F336W, F555W, F814W, and F656N (i.e., U, V, I, and Hα). Whereas ground-based, large telescope images of UGC 6456 resolved a few tens of objects in this galaxy, DAOPHOT/ALLSTAR finds several thousand pointlike sources with signals larger than 3 σ above the background noise in the V-band HST/PC image alone. As expected, a young population of massive stars is indicated from the Hα as well as the broad-band images. However, more interesting is the evidence for a much older stellar population in this galaxy; the color-magnitude diagrams show the presence of blue supergiants, main-sequence stars, and blue-loop stars, but also of red supergiants, asymptotic giant branch and red giant stars. The features in the V-I color-magnitude diagram known as the “red-tail” and the brightest part of the “red-tangle” specifically suggest intermediate-age (and possibly old) stars. Therefore, UGC 6456 is clea...