37 research outputs found
A Compact Population of Red Giants in the Blue Compact Dwarf Galaxy UGCA 290
We present HST/WFPC2 single-star photometry for the blue dwarf galaxy UGCA
290, whose morphology is intermediate between classic iE Blue Compact Dwarfs
and blue dwarfs which exhibit no red background sheet of older stars. The
color-magnitude diagram of this galaxy in V and I, extending over six
magnitudes, is remarkably similar to that of the star-forming region in the iE
Blue Compact Dwarf 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.Comment: 10 pages, 3 color figures, LaTeX2e. Accepted for publication in ApJ
Letter
Hubble Space Telescope Observations of the Gravitationally Lensed Cloverleaf Broad Absorption Line QSO H1413+1143: Imaging Polarimetry and Evidence for Microlensing of a Scattering Region
We report the results of HST WFPC2 broadband F555W and F702W photometric and
F555W polarimetric observations of the "Cloverleaf" QSO H1413+1143. This is a
four-component gravitationally-lensed broad absorption line (BAL) QSO.
Observations were obtained at two epochs in March 1999 and June 1999 separated
by about 100 days. The goal of our program was to detect any relative changes
among the components and between the two epochs. Over this time baseline we
detected an approximately 0.07 mag dimming in component D of the lensed image,
which we interpret as evidence for microlensing. In March 1999 we find
significant evidence for a difference in the relative linear polarization of
component D in comparison to the other three components; in June 1999 the
combined polarization of the Cloverleaf components was lower. In March 1999 the
apparently microlensed component D has a rotated polarization position angle
and a somewhat higher degree of polarization than the other three components.
We suggest that this difference in polarization is due to microlensing
magnification of part of a scattered-light (i.e. polarized) continuum-producing
region. The results indicate that in the Cloverleaf the size-scale of the
polarized scattered-light region exceeds about 10^{16} cm but lies interior to
the region producing the broad emission lines (< 10^{18} cm).Comment: 21 pages, accepted for publication in Ap
The Oldest Stars of the Extremely Metal-Poor Local Group Dwarf Irregular Galaxy Leo A
We present deep Hubble Space Telescope 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. (1998) 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 which 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 lack the detection of
main-sequence turnoffs which 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, of 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.Comment: 44 pages, 18 figures, accepted for publication in the August 2002
issue of AJ. High resolution figures is available at
http://www.astro.spbu.ru/staff/dio/preprints.htm