468 research outputs found
The blue compact dwarf galaxy I Zw 18: a comparative study of its low-surface-brightness component
(abridged): Using HST and ground-based optical and NIR imaging data, we
investigate whether the blue compact dwarf (BCD) galaxy I Zw 18 possesses an
extended low-surface-brightness (LSB) old stellar population underlying its
star-forming (SF) regions. We show that the exponential intensity decrease
observed in the filamentary LSB envelope of the BCD out to 18 arcsec (1.3 kpc
at a distance of 15 Mpc) is not due to an evolved stellar disc, but rather due
to extended ionized gas emission. Broad-band images reveal, after subtraction
of nebular line emission, a compact stellar LSB component extending slightly
beyond the SF regions. This stellar host, being blue over a radius range of 5
exponential scale lengths and showing little colour contrast to the SF
component, differs strikingly from the red LSB host of standard BCDs. This
fact, in connection with the blue colors of component I Zw 18 C, suggests that
most of the stellar mass in I Zw 18 has formed within the last 0.5 Gyr.
Furthermore, we show that the exponential intensity fall-off in the filamentary
ionized envelope of I Zw 18 is not particular to this system but a common
property of the ionized halo of many SF dwarf galaxies on galactocentric
distances of several kpc. In the absence of an appreciable underlying stellar
background, extended ionized gas emission dominates in the periphery of I Zw
18, mimicking an exponential stellar disc on optical surface brightness
profiles.Comment: 24 pages, 16 figures; accepted for publication in Astronomy &
Astrophysics; higher resolution images available at
http://alpha.uni-sw.gwdg.de/~papade/IZw18
New insights to the photometric structure of Blue Compact Dwarf Galaxies from deep Near-Infrared studies I. Observations, surface photometry and decomposition of surface brightness profiles
(shortened) We analyze deep Near Infrared (NIR) broad band images for a
sample of Blue Compact Dwarf Galaxies (BCDs), allowing for the quantitative
study of their extended stellar low-surface brightness (LSB) host galaxies. NIR
surface brightness profiles (SBPs) of the LSB hosts agree at large
galactocentric radii with those from optical studies. At small to intermediate
radii, however, the NIR data reveal for more than half of our sample a
significant flattening of the exponential SBP of the LSB host. Such SBPs ("type
V" SBPs, Binggeli & Cameron 1991) have rarely been detected in LSB hosts of
BCDs at optical wavelengths, where the relative flux contribution of the
starburst is stronger than in the NIR and can hide such central intensity
depressions of the LSB host. The structural properties, frequency and physical
origin of type V LSB SBPs in BCDs and other dwarf galaxies have not yet been
systematically studied. Nevertheless, their occurrence in a significant
fraction of BCDs would impose important new constraints to the radial
distribution of their stellar mass, and to the photometric fading of BCDs after
the termination of star-forming activity. Both a modified exponential
(Papaderos et al. 1996a) and the Sersic law give satisfactory empirical
descriptions for type V SBPs. However, we argue that the practical
applicability of Sersic fits to LSB SBPs of BCDs is limited by, e.g., the
extreme sensitivity of the solutions to small SBP uncertainties. Most stellar
LSB host galaxies in the sample show optical-NIR colors indicative of evolved
stellar populations with subsolar metallicity. Unsharp-masked NIR images and
optical-NIR maps reveal numerous morphological details, and cases of
non-uniform dust absorption on spatial scales up to ~1 kpc.Comment: 29 pages, 17 figures; accepted for publication in Astronomy &
Astrophysics; 1 typo in Table 2 corrected; higher resolution images are
available at
http://www.uni-sw.gwdg.de/~knoeske/PUB_LIST/noeske_BCDs_NIR.ps.g
HST observations of the cometary blue compact dwarf galaxy UGC 4483: a relatively young galaxy?
We present V and I photometry of the resolved stars in the cometary blue
compact dwarf galaxy UGC 4483 using Hubble Space Telescope Wide Field Planetary
Camera 2 (WFPC2) images. The resulting I vs. (V-I) color-magnitude diagram
(CMD) reaches limiting magnitudes V = 27.5 mag and I = 26.5 mag for photometric
errors less than 0.2 mag. It reveals not only a young stellar population of
blue main-sequence stars and blue and red supergiants, but also an older
evolved population of red giant and asymptotic giant branch stars. The measured
magnitude I = 23.65 +/- 0.10 mag of the red giant branch tip results in a
distance modulus (m-M) = 27.63 +/- 0.12, corresponding to a distance of 3.4 +/-
0.2 Mpc. The youngest stars are associated with the bright H II region at the
northern tip of the galaxy. The population of older stars is found throughout
the low-surface-brightness body of the galaxy and is considerably more spread
out than the young stellar population, suggesting stellar diffusion. The most
striking characteristics of the CMD of UGC 4483 are the very blue colors of the
red giant stars and the high luminosity of the asymptotic giant branch stars.
Both of these characteristics are consistent with either: 1) a very low
metallicity ([Fe/H] = -2.4 like the most metal-deficient globular clusters) and
an old age of 10 Gyr, or 2) a higher metallicity ([Fe/H] = -1.4 as derived from
the ionized gas emission lines) and a relatively young age of the oldest
stellar population in UGC 4483, not exceeding ~ 2 Gyr. Thus our data do not
exclude the possibility that UGC 4483 is a relatively young galaxy having
formed its first stars only ~ 2 Gyr ago.Comment: 37 pages, 15 PS figures, to appear in Ap
A HST study of the stellar populations in the cometary dwarf irregular galaxy NGC 2366
We present V and I photometry of the resolved stars in the cometary dwarf
irregular galaxy NGC 2366, using Wide Field Planetary Camera 2 images obtained
with the Hubble Space Telescope. The resulting color-magnitude diagram reaches
down to I~26.0 mag. It reveals not only a young population of blue
main-sequence stars (age <30 Myr) but also an intermediate-age population of
blue and red supergiants (20 Myr<age<100 Myr), and an older evolved populations
of asymptotic giant branch (AGB) stars (age >100 Myr) and red giant branch
(RGB) stars (age >1 Gyr). The measured magnitude I=23.65+/-0.10 mag of the RGB
tip results in a distance modulus m-M=27.67+/-0.10, which corresponds to a
distance of 3.42+/-0.15 Mpc, in agreement with previous distance
determinations. The youngest stars are associated with the bright complex of
HII regions NGC 2363=Mrk 71 in the southwest extremity of the galaxy. As a
consequence of the diffusion and relaxation processes of stellar ensembles, the
older the stellar population is, the smoother and more extended is its spatial
distribution. An underlying population of older stars is found throughout the
body of NGC 2366. The most notable feature of this older population is the
presence of numerous relatively bright AGB stars. The number ratio of AGB to
RGB stars and the average absolute brightness of AGB stars in NGC 2366 are
appreciably higher than in the BCD VII Zw 403, indicating a younger age of the
AGB stars in NGC 2366. In addition to the present burst of age <100 Myr, there
has been strong star formation activity in the past of NGC 2366, from ~100 Myr
to <3 Gyr ago.Comment: 32 pages, 15 figures, accepted for publication in the Astrophysical
Journa
AEGIS: Extinction and Star Formation Tracers from Line Emission
Strong nebular emission lines are a sensitive probe of star formation and
extinction in galaxies, and the [O II] line detects star forming populations
out to z>1. However, star formation rates from emission lines depend on
calibration of extinction and the [O II]/H-alpha line ratio, and separating
star formation from AGN emission. We use calibrated line luminosities from the
DEEP2 survey and Palomar K magnitudes to show that the behavior of emission
line ratios depends on galaxy magnitude and color. For galaxies on the blue
side of the color bimodality, the vast majority show emission signatures of
star formation, and there are strong correlations of extinction and [O
II]/H-alpha with restframe H magnitude. The conversion of [O II] to
extinction-corrected H-alpha and thus to star formation rate has a significant
slope with M_H, 0.23 dex/mag. Red galaxies with emission lines have a much
higher scatter in their line ratios, and more than half show AGN signatures. We
use 24 micron fluxes from Spitzer/MIPS to demonstrate the differing populations
probed by nebular emission and by mid-IR luminosity. Although extinction is
correlated with luminosity, 98% of IR-luminous galaxies at z~1 are still
detected in the [O II] line. Mid-IR detected galaxies are mostly bright and
intermediate color, while fainter, bluer galaxies with high [O II] luminosity
are rarely detected at 24 microns.Comment: 4 pages, 3 figures. Accepted for publication in ApJ Letters AEGIS
special editio
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