45 research outputs found
VV124 (UGC4879): A new transitional dwarf galaxy in the periphery of the Local Group
We present the first resolved-star photometry of VV124 (UGC4879) and find
that this is the most isolated dwarf galaxy in the periphery of the Local
Group. Based on imaging and spectroscopic follow up observations with the 6m
BTA telescope, we resolve VV124 into 1560 stars down to the limiting magnitude
levels of V~25.6 and I~23.9. The young blue stellar populations and emission
gas are found near the core, but noticeably displaced from the center of the
galaxy as traced by dominant evolved red stars. The mean radial velocity
derived from the spectra of two Blue Supergiant stars, an HII region and
unresolved continuum sources is -80+/-10 km/s. The evolved ``red tangle''
stellar populations, which contains the red giant branch (RGB), are identified
at large galactocentric radii. We use the I-band luminosity function to
determine the distance based on the Tip of RGB method, 1.1+/-0.1 Mpc. This is
~10 times closer than the values usually assumed in the literature, and we
provide revised distance dependent parameters. From the mean (V-I) color of the
RGB, we estimate the mean metallicity as [Fe/H]~-1.37 dex. Despite of its
isolated location, the properties of VV124 are clearly not those of a galaxy in
formation, but rather similar to a transitional dIrr/dSph type.Comment: 4 pages, submitted to MNRAS Letter
The Stellar Structures around Disk Galaxies
We present a brief summary of our current results on the stellar distribution
and population gradients of the resolved stars in the surroundings of ~50
nearby disk galaxies, observed with space- (Hubble & Spitzer) and ground-based
telescopes (Subaru, VLT, BTA, Palomar, CFHT & INT). We examine the radial
(in-plane) and vertical (extraplanar) distributions of resolved stars as a
function of stellar age and metallicity by tracking changes in the
color-magnitude diagram of face-on and edge-on galaxies. Our data show, that
the scale length and height of a stellar population increases with age, with
the oldest detected stellar populations identified at a large galactocentric
radius or extraplanar height, out to typically a few kpc. In the most massive
of the studied galaxies there is evidence for a break in number density and
color gradients of evolved stars, which plausibly correspond to the thick disk
and halo components of the galaxies. The ratio of intermediate-age to old stars
in the outermost fields correlate with the gas fraction, while relative sizes
of the thick-to-thin disks anticorrelate with galactic circular velocity.Comment: To appear in the proceedings for the IAUS 241 'Stellar Populations as
Building Blocks of Galaxies' held in La Palma, Spain, December 10-16 200
The Stellar Content of the Polar Rings in the Galaxies NGC 2685 and NGC 4650A
We present the results of stellar photometry of polar-ring galaxies NGC 2685
and NGC 4650A, using the archival data obtained with the Hubble Space
Telescope's Wide Field Planetary Camera 2. Polar rings of these galaxies were
resolved into ~800 and ~430 stellar objects in the B, V and Ic bands,
considerable part of which are blue supergiants located in the young stellar
complexes. The stellar features in the CM-diagrams are best represented by
isochrones with metallicity Z = 0.008. The process of star formation in the
polar rings of both galaxies was continuous and the age of the youngest
detected stars is about 9 Myr for NGC 2685 and 6.5 Myr for NGC 4650A.Comment: 21 pages, 9 figures, AJ 2004 February, accepte
The ACS LCID Project: On the origin of dwarf galaxy types: a manifestation of the halo assembly bias?
We discuss how knowledge of the whole evolutionary history of dwarf galaxies,
including details on the early star formation events, can provide insight on
the origin of the different dwarf galaxy types. We suggest that these types may
be imprinted by the early conditions of formation rather than being only the
result of a recent morphological transformation driven by environmental
effects. We present precise star formation histories of a sample of Local Group
dwarf galaxies, derived from colour-magnitude diagrams reaching the oldest
main-sequence turnoffs. We argue that these galaxies can be assigned to two
basic types: fast dwarfs that started their evolution with a dominant and short
star formation event, and slow dwarfs that formed a small fraction of their
stars early and have continued forming stars until the present time (or
almost). These two different evolutionary paths do not map directly onto the
present-day morphology (dwarf spheroidal vs dwarf irregular). Slow and fast
dwarfs also differ in their inferred past location relative to the Milky Way
and/or M31, which hints that slow dwarfs were generally assembled in lower
density environments than fast dwarfs. We propose that the distinction between
a fast and slow dwarf galaxy reflects primarily the characteristic density of
the environment where they form. At a later stage, interaction with a large
host galaxy may play a role in the final gas removal and ultimate termination
of star formation.Comment: 7 pages, 3 figures, ApJ Letters, submitted. Comments welcom
Characterization of Extragalactic 24micron Sources in the Spitzer First Look Survey
In this Letter, we present the initial characterization of extragalactic 24um
sources in the Spitzer First Look Survey (FLS) by examining their counterparts
at 8um and R-band. The color-color diagram of 24-to-8 vs. 24-to-0.7um is
populated with 18,734 sources brighter than the 3sigma flux limit of 110uJy,
over an area of 3.7sq.degrees. The 24-to-0.7um colors of these sources span
almost 4 orders of magnitudes, while the 24-to-8um colors distribute at least
over 2 orders of magnitudes. In addition to identifying ~30% of the total
sample with infrared quiescent, mostly low redshift galaxies, we also found
that: (1) 23% of the 24um sources (~1200/sq.degrees) have very red 24-to-8 and
24-to-0.7 colors and are probably infrared luminous starbursts with
L(IR)>3x10^(11)Lsun at z>1. In particular, 13% of the sample (660/sq.degrees)
are 24um detected only, with no detectable emission in either 8um or R-band.
These sources are the candidates for being ULIRGs at z>2. (2) 2% of the sample
(85/sq.degrees) have colors similar to dust reddened AGNs, like Mrk231 at
z~0.6-3. (3) We anticipate that some of these sources with extremely red colors
may be new types of sources, since they can not be modelled with any familiar
type of spectral energy distribution. We find that 17% of the 24um sources have
no detectable optical counterparts brighter than R limit of 25.5mag. Optical
spectroscopy of these optical extremely faint 24um sources would be very
difficult, and mid-infrared spectroscopy from the Spitzer would be critical for
understanding their physical nature (Abridged).Comment: Accepted for publication in ApJ (Spitzer Special Issue
On the distance and reddening of the starburst galaxy IC10
We present deep and accurate optical photometry of the Local Group starburst
galaxy IC10. The photometry is based on two sets of images collected with the
Advanced Camera for Surveys and with the Wide Field Planetary Camera 2 on board
the Hubble Space Telescope. We provide new estimates of the Red Giant Branch
tip (TRGB) magnitude, m_{F814W}^{TRGB}=21.90+-0.03, and of the reddening,
E(B-V)=0.78+-0.06, using field stars in the Small Magellanic Cloud (SMC) as a
reference. Adopting the SMC and two globulars, Omega Centauri and 47 Tucanae,
as references we estimate the distance modulus to IC10: independent
calibrations give weighted average distances of mu=24.51+-0.08 (TRGB) and
mu=24.56+-0.08 (RR Lyrae). We also provide a new theoretical calibration for
the TRGB luminosity, and using these predictions we find a very similar
distance to IC10 (mu~24.60+-0.15). These results suggest that IC10 is a likely
member of the M31 subgroup.Comment: 4 pages, 4 figures, ApJ Letters accepte
On the stellar content of the starburst galaxy IC10
We investigate the stellar content of the starburst dwarf galaxy IC10 using
accurate and deep optical data collected with the Advanced Camera for Surveys
and with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope.
The comparison between theory and observations indicates a clear change in age
distribution when moving from the center toward the external regions. Moreover,
empirical calibrators and evolutionary predictions suggest the presence of a
spread in heavy element abundance of the order of one-half dex. The comparison
between old and intermediate-age core He-burning models with a well defined
overdensity in the color-magnitude diagram indicates the presence of both
intermediate-age, red clump stars and of old, red horizontal branch stars.Comment: 7 pages, 4 figures, 1 table. Accepted for publication in ApJ
The Stellar Content of NGC 6789, A Blue Compact Dwarf Galaxy in the Local Void
We find that NGC6789 is the most nearby example of a Blue Compact Dwarf
galaxy known to date. With the help of WFPC2 aboard the Hubble Space Telescope,
we resolve NGC6789 into over 15,000 point sources in the V and I bands. The
young stars of NGC6789 are found exclusively near the center of the galaxy. The
red giant population identified at large galacticentric 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 NGC6789 are
clearly not those of a galaxy in formation.Comment: 8 pages, 4 figures, ApJL Accepte
Multiwavelength study of the star formation in the bar of NGC 2903
NGC 2903 is a nearby barred spiral with an active starburst in the center and
Hii regions distributed along its bar. We aim to analyse the star formation
properties in the bar region of NGC 2903 and study the links with the typical
bar morphological features. A combination of space and ground-based data from
the far-ultraviolet to the sub-millimeter spectral ranges is used to create a
panchromatic view of the NGC 2903 bar. We produce two catalogues: one for the
current star formation regions, as traced by the halpha compact emission, and a
second one for the ultraviolet (UV) emitting knots, containing positions and
luminosities. From them we have obtained ultraviolet colours, star formation
rates, dust attenuation and halpha EWs, and their spatial distribution have
been analysed. Stellar cluster ages have been estimated using stellar
population synthesis models (Starburst99). NGC 2903 is a complex galaxy, with a
very different morphology on each spectral band. The CO(J=1-0) and the 3.6
micron emission trace each other in a clear barred structure, while the halpha
leads both components and it has an s-shape distribution. The UV emission is
patchy and does not resemble a bar. The UV emission is also characterised by a
number of regions located symmetrically with respect to the galaxy center,
almost perpendicular to the bar, in a spiral shape covering the inner ~2.5 kpc.
These regions do not show a significant halpha nor 24 micron emission. We have
estimated ages for these regions ranging from 150 to 320 Myr, being older than
the rest of the UV knots, which have ages lower than 10 Myr. The SFR calculated
from the UV emission is ~0.4 M/yr, compatible with the SFR as derived
from halpha calibrations (M/yr).Comment: Accepted for publication in A&