20 research outputs found
Mapping the Galactic Halo. V. Sgr dSph Tidal Debris 60 degrees from the Main Body
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.Comment: 10 pages, 3 figures accepted in Astrophysical Journal Letter
Deep HST 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 (; 8 orbits), F814W (; 16 orbits), and
F656N (H; 1 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 magnitudes 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 (MS) 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. Our interpretation of
the pattern of star formation is that it is an orderly stochastic process.Comment: 27 pages, 14 figures, 2 mpeg movies, accepted in the Astronomical
Journa
A WFPC2 Study of the Resolved Stellar Population of the Pegasus Dwarf Irregular Galaxy (DDO 216)
The stellar population of the Pegasus dwarf irregular galaxy is investigated
in images taken in the F439W (B), F555W (V), and F814W (I) bands with WFPC2.
These and ground-based data are combined to produce color-magnitude diagrams
which show the complex nature of the stellar population in this small galaxy. A
young (< 0.5 Gyr) main sequence stellar component is present and clustered in
two centrally-located clumps, while older stars form a more extended disk or
halo. The colors of the main sequence require a relatively large extinction of
A_V = 0.47 mag. The mean color of the well-populated red giant branch is
relatively blue, consistent with a moderate metallicity young, or older,
metal-poor stellar population. The red giant branch also has significant width
in color, implying a range of stellar ages and/or metallicities. A small number
of extended asymptotic giant branch stars are found beyond the red giant branch
tip. Near the faint limits of our data is a populous red clump superimposed on
the red giant branch. Efforts to fit self-consistent stellar population models
based on the Geneva stellar evolution tracks yield a revised distance of 760
kpc. Quantitative fits to the stellar population are explored as a means to
constrain the star formation history. The numbers of main sequence and core
helium-burning blue loop stars require that the star formation rate was higher
in the recent past, by a factor of 3-4 about 1 Gyr ago. Unique results cannot
be obtained for the star formation history over longer time baselines without
better information on stellar metallicities and deeper photometry. The youngest
model consistent with the data contains stars with constant metallicity of Z =
0.001 which mainly formed 2-4 Gyr ago. Even at its peak of star forming
activity, the Pegasus dwarf most likely remained relatively dim with M_V ~ -14.Comment: 46 pages, 16 figures, 1 tabl
The Recent Evolution of the Dwarf Starburst Galaxy NGC 625 from Hubble Space Telescope Imaging
New HST/WFPC2 imaging of the dwarf starburst galaxy NGC 625 is presented.
These data, 80% complete to V and I magnitudes of 26.0 and 25.3, respectively,
allow us to study the recent star formation history of NGC 625. We derive a tip
of the red giant branch (TRGB) distance modulus of 27.95+/-0.07, corresponding
to a distance of 3.89+/-0.22 Mpc, and a location on the far side of the
Sculptor Group. NGC 625 has a well-defined radial stellar population gradient,
evidenced by a central concentration of young MS stars and an RGB/AGB ratio
that increases with galactocentric distance. The prominent AGB is very red, and
RGB stars are detected far from the central star forming regions. Using H Alpha
and H Beta narrow band imaging and previous optical spectroscopy we identify
substantial and varying internal extinction (A_V = 0.0 to 0.6 mag) associated
with the central active star formation regions. To better understand the
effects of internal extinction on the analysis of young stellar populations,
synthetic models are presented which, for the first time, examine and account
for this effect. Using the luminous blue helium burning stars, we construct a
simple model of the recent (< 100 Myr) star formation in which an elevated but
declining star formation rate has been present over this entire period. This is
at odds with the presence of spectroscopic W-R features in the major star
formation region which imply a short duration (<= 5 Myr) for the recent
starburst. This suggests that starbursts displaying W-R features are not
necessarily all of a short duration. Finally, we speculate on the possible
causes of the present burst of star formation in this apparently isolated
galaxy, and compare it to other nearby, well-studied dwarf starburst systems.Comment: 56 pages, including 15 figures (2 in color). Scheduled to appear in
AJ, December, 2003. Full-resolution version may be obtained at
http://www.astro.umn.edu/~Cannon/n625.p
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 i:he 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