583 research outputs found
The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda I
Images have been obtained with the Hubble Space Telescope WFPC2 camera of
Andromeda I, a dwarf spheroidal (dSph) galaxy that lies in the outer halo of
M31. The resulting color-magnitude diagrams reveal for the first time the
morphology of the horizontal branch in this system. We find that, in a similar
fashion to many of the galactic dSph companions, the horizontal branch (HB) of
And~I is predominantly red. Combined with the metal abundance of this dSph,
this red HB morphology indicates that And I can be classified as a ``second
parameter'' system in the outer halo of M31. This result then supports the
hypothesis that the outer halo of M31 formed in the same extended chaotic
manner as is postulated for the outer halo of the Galaxy.Comment: 26 pages using aas2pp4.sty, including 2 tables and 7 figures, to be
published in AJ. Figure 1 is in gif form. To include in main ps file, use xv
to create a ps file called Da_Costa.fig1.ps and uncomment appropriate lines
in .tex fil
The optical counterpart of an Ultra-luminous X-Ray Source in NGC 5204
Ultra-luminous X-Ray sources are extra-nuclear point sources in external
galaxies with -- erg/s and are among the most poorly
understood X-ray sources. To help understand their nature, we are trying to
identify their optical counterparts by combining images from the Hubble Space
Telescope and the Chandra Observatory. Here we report upon the optical
counterpart for the ULX in NGC 5204, which has average X-ray luminosity of
erg/s and has varied by a factor of 50% over the last 10
years. A unique optical counterpart to this ULX is found by carefully comparing
the Chandra ACIS images and HST WFPC2 and ACS/HRC images. The spectral energy
distribution and the HST/STIS FUV spectrum of this object show that it is a B0
Ib supergiant star with peculiarities, including the 1240 N V emission
line that is uncommon in B stellar spectra but has been predicted for X-ray
illuminated accretion disks and seen in some X-ray binaries. Study of its FUV
spectrum leads to a binary model for this ULX in which the B0 Ib supergiant is
overflowing its Roche Lobe and accreting onto the compact primary, probably a
black hole. This picture predicts an orbital period of days for
different black hole mass, which can be tested by future observations
Keck Spectroscopy of Two Young Globular Clusters in the Merger Remnant NGC 3921
Low-resolution UV-to-visual spectra of two candidate globular clusters in the
merger remnant NGC 3921 are presented. These two clusters of apparent magnitude
V = 22.2 (Mv = -12.5) lie at projected distances of ~5 kpc from the center and
move with halo-type radial velocities relative to the local galaxy background.
Their spectra show strong Balmer absorption lines indicative of main-sequence
turnoffs dominated by A-type stars. Comparisons with model-cluster spectra
computed by Bruzual & Charlot and others yield cluster ages in the range of
200-530 Myr, and metallicities about solar to within a factor of three. Given
their small half-light radii (Reff < 5 pc) and ages corresponding to ~100 core-
crossing times, these clusters are gravitationally bound and, hence, indeed
young globulars. Assuming that they had Chabrier-type initial mass functions,
their estimated current masses are 2.3(+-0.1)x10^6 Msun and 1.5(+-0.1)x10^6
Msun, respectively, or roughly half the mass of omegaCen. Since NGC 3921 itself
shows many signs of being a 0.7(+-0.3) Gyr old protoelliptical, these two young
globulars of roughly solar metallicity and their many counterparts observed
with the Hubble Space Telescope provide supporting evidence that, in the
process of forming elliptical-like remnants, major mergers of gas-rich disks
can also increase the number of metal-rich globular clusters. (Abridged)Comment: 22 pages, 6 figures, accepted for publication in AJ, July 200
- âŠ