28 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
The Complex Star Formation History of NGC 1569
We present new results on the star formation history of the dwarf irregular
galaxy NGC 1569. The data were obtained with Hubble Space Telescope's
NICMOS/NIC2 in the F110W (J) and F160W (H) near-infrared (NIR) filters and
interpreted with the synthetic color-magnitude diagram method. The galaxy
experienced a complex star formation (SF) activity. The best fit to the data is
found by assuming three episodes of activity in the last 1-2 Gyr. The most
recent and strong episode constrained by these NIR data started ~37 Myr ago and
ended ~13 Myr ago, although we cannot exclude the possibility that up to three
SF episodes occurred in this time interval. The average star-formation rate
(SFR) of the episode is 3.2 Msun yr-1 kpc-2, in agreement with literature data.
A previous episode produced stars between 150 Myr and 40 Myr ago, with a mean
SFR about 2/3 lower than the mean SFR of the youngest episode. An older SF
episode occurred about 1 Gyr ago. All these SFRs are 2-3 orders of magnitude
higher than those derived for late-type dwarfs of the Local Group. In all cases
an initial mass function similar to Salpeter's allows for a good reproduction
of the data, but we cannot exclude flatter mass functions. These results have
been obtained adopting a distance of 2.2 Mpc and a reddening E(B-V)=0.56. A
larger distance would require younger episodes and higher SFRs. We have
explored some possible scenarios using the astrated mass in the best fit model,
in order to constrain the past star formation history. We cannot rule out a low
rate in the past SF but we can safely conclude that the last 1-2 Gyr have been
peculiar.Comment: Accepted by AJ, 2005, vol 129, 14 pages, 9 figure
A Near-Infrared Stellar Census of the Blue Compact Dwarf Galaxy VII~Zw~403
We present near-infrared single-star photometry for the low-metallicity Blue
Compact Dwarf galaxy VII~Zw~403. We achieve limiting magnitudes of
F110W~~25.5 and F160W~~24.5 using one of the NICMOS cameras
with the HST equivalents of the ground-based J and H filters. The data have a
high photometric precision (0.1 mag) and are % complete down to magnitudes
of about 23, far deeper than previous ground-based studies in the near-IR. The
color-magnitude diagram contains about 1000 point sources. We provide a
preliminary transformation of the near-IR photometry into the ground system...Comment: Accepted for publication by the AJ, preprint has 49 pages, 2 tables,
and 16 figure
Exploring Cluster Ellipticals as Cosmological Standard Rods
We explore the possibility to calibrate massive cluster ellipticals as
cosmological standard rods using the Fundamental Plane relation combined with a
correction for luminosity evolution. Though cluster ellipticals certainly
formed in a complex way, their passive evolution out to redshifts of about 1
indicates that basically all major merging and accretion events took place at
higher redshifts. Therefore, a calibration of their luminosity evolution can be
attempted. We propose to use the Mg relation for that purpose because
it is independent of distance and cosmology. We discuss a variety of possible
caveats, ranging from dynamical evolution to uncertainties in stellar
population models and evolution corrections to the presence of age spread.
Sources of major random and systematic errors are analysed as well. We apply
the described procedure to nine elliptical galaxies in two clusters at
and derive constraints on the cosmological model. For the best
fitting -free cosmological model we obtain: , with
90% confidence limits being (the lower limit being due to the
presence of matter in the Universe). If the inflationary scenario applies (i.e.
the Universe has flat geometry), then, for the best fitting model, matter and
contribute about equally to the critical cosmic density (i.e.
). With 90% confidence
should be smaller than 0.9.Comment: 21 pages, including 5 eps-figures, Latex, uses aasms4.sty, accepted
by ApJ main journa
The stellar content of the super star clusters in NGC 1569
We discuss HST FOS ultraviolet spectroscopy and NICMOS near-infrared
photometry of four young super star clusters in the central region of the
irregular starburst galaxy NGC 1569. The new observations coupled with previous
HST WFPC2 photometry and ground-based optical spectroscopy allow us to isolate
and age-date the hot and cool stellar components of these massive clusters. We
analyze the two components A1 and A2 of the brightest super star cluster NGC
1569-A. This cluster received previous attention due to the simultaneous
presence of Wolf-Rayet stars and red supergiants. The FOS spectra provide the
first evidence for O-stars in NGC 1569-A, indicating a young (5 Myr) stellar
component in A1 and/or A2. Comparison with other high-mass star-forming regions
suggests that the O- and Wolf-Rayet stars are spatially coincident. If so,
cluster A2 could be the host of the very young O- and Wolf-Rayet population,
and the somewhat older red supergiants could be predominantly located in A1.
The mass-to-light ratio of NGC 1569-A1 is analyzed in five optical and infrared
photometric bands and compared to evolutionary synthesis models. No indications
for an anomalous initial mass function are found, consistent with a scenario
where this cluster is the progenitor of present-day globular clusters. The
clusters A1 and A2 are compared to clusters B and #30. The latter two clusters
are older and fully dominated by red supergiants. All four super star clusters
provide a significant fraction (20 - 25%) of the total optical and
near-infrared light in the central region of the galaxy. The photometric
properties of the super star cluster population in NGC 1569 resemble those of
the populous clusters in the Magellanic Clouds
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
VLT observations of the z=6.28 quasar SDSS 1030+0524
We present new VLT spectroscopic observations of the most distant quasar
known, SDSS J1030+0524 at z=6.28 which was recently discovered by the Sloan
Digital Sky Survey. We confirm the presence of a complete Gunn-Peterson trough
caused by neutral hydrogen in the intergalactic medium. There is no detectable
flux over the wavelength range from 8450 to 8710 A. We set an improved limit on
the drop of the flux level blueward of the Ly alpha line: a factor of
> 200. Below 8450 A the spectrum shows a rise in flux, with a large fraction
(> 60 %) of the total emission produced by few narrow features of transmitted
flux. We discuss the obvious proximity effect around this quasar, with the
presence of transmitted flux with many absorption features in a region of about
23h^{-1} comoving Mpc. If assuming the surrounding medium were completely
neutral, the size of this region would imply a quasar lifetime of \~1.3x10^7
years.
We also present near-IR spectroscopy of both SDSS J1030+0524 and of SDSS
J1306+05, the second most distant quasar known at redshift 6.0. We combine
measurements of the CIV line and limits on the HeII emission with the NV line
measurements from the optical spectra to derive line ratios, and by implication
the abundances of these early quasar environments. The results are
indistinguishable from those of lower redshift quasars and indicate little or
no evolution in the abundances from z ~ 6 to z ~ 2. The line ratios suggest
supersolar metallicities, implying that the first stars around the quasars must
have formed at least a few hundreds of Myrs prior to the observation, i.e. at
redshift higher than 8.Comment: Accepted by the Astronomical Journal (scheduled for May 2002).
Proximity effect section revise
The Dwarf Irregular/Wolf-Rayet Galaxy NGC4214: I. A New Distance, Stellar Content, and Global Parameters
We present the results of a detailed optical and near-IR study of the nearby
star-forming dwarf galaxy NGC4214. We discuss the stellar content, drawing
particular attention to the intermediate-age and/or old field stars, which are
used as a distance indicator. On images obtained with the Hubble Space
Telescope WFPC2 and NICMOS instruments in the equivalents of the V, R, I, J and
H bands, the galaxy is well resolved into stars. We achieve limiting magnitudes
of F814W ~27 in the WF chips and F110W ~25 in the NIC2. The optical and
near-infrared color-magnitude diagrams confirm a core-halo galaxy morphology:
an inner high surface-brightness young population within ~1.5' (~1 kpc) from
the center of the galaxy, where the stars are concentrated in bright complexes
along a bar-like structure; and a relatively low-surface-brightness, field-star
population extending out to at least 8' (7 kpc). The color-magnitude diagrams
of the core region show evidence of blue and red supergiants, main-sequence
stars, asymptotic giant branch stars and blue loop stars. We identify some
candidate carbon stars from their extreme near-IR color. The field-star
population is dominated by the "red tangle", which contains the red giant
branch. We use the I-band luminosity function to determine the distance based
on the tip-of-the-red-giant-branch method: 2.7\pm0.3 Mpc. This is much closer
than the values usually assumed in the literature, and we provide revised
distance dependent parameters such as physical size, luminosity, HI mass and
star-formation rate.Comment: 24 pages, 19 figures, accepted for publication in the July 2002 issue
of AJ. Version with high resolution figures is available at
http://www.astro.spbu.ru/staff/dio/preprints.htm
The Resolved Stellar Population of the Post-Starburst Galaxy NGC 1569
We present WFPC2-HST photometry (in the F439W and F555W bands) of the
resolved stars in NGC 1569. The derived color-magnitude diagram (CMD) contains
2800 objects down to the 26th mag, and is complete down to the 23rd. Adopting
the literature distance modulus and reddening, the CMD samples stars more
massive than 4 Mo, corresponding to a look-back time of 0.15 Gyr approximately.
The data are compared to synthetic CMDs to derive the recent star formation
history in NGC 1569. It is found that the observed field has experienced a
global burst of star formation (SF) lasting at least 0.1 Gyr, and ended a few
Myr ago. During the burst, the SF rate was approximately constant, and, if
quiescent periods occurred, they lasted less than 10 Myr. The level of the SF
rate was very high: 0.5 Mo/yr for a Salpeter IMF, in stars with mass ranging
from 0.1 to 120 Mo. When scaled for the surveyed area, the SF rate is
approximately 100 times larger than found in the most active dwarf irregulars
in the Local Group. The data are consistent with a Salpeter IMF, or slightly
steeper exponents. We discuss the implications of our results in the general
context of the evolution of dwarf galaxies.Comment: 63 pages including 2 tables and 12 figures, to be published in The
Astrophysical Journa
Efficient Allele-Specific Targeting of LRRK2 R1441 Mutations Mediated by RNAi
Since RNA interference (RNAi) has the potential to discriminate between single nucleotide changes, there is growing interest in the use of RNAi as a promising therapeutical approach to target dominant disease-associated alleles. Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene have been linked to dominantly inherited Parkinson's disease (PD). We focused on three LRRK2 mutations (R1441G/C and the more prevalent G2109S) hoping to identify shRNAs that would both recognize and efficiently silence the mutated alleles preferentially over the wild-type alleles. Using a luciferase-based reporter system, we identified shRNAs that were able to specifically target the R1441G and R1441C alleles with 80% silencing efficiency. The same shRNAs were able to silence specifically mRNAs encoding either partial or full-length mutant LRRK2 fusion proteins, while having a minimal effect on endogenous wild-type LRRK2 expression when transfected in 293FT cells. Shifting of the mutant recognition site (MRS) from position 11 to other sites (4 and 16, within the 19-mer window of our shRNA design) reduced specificity and overall silencing efficiency. Developing an allele-specific RNAi of G2019S was problematic. Placement of the MRS at position 10 resulted in efficient silencing of reporters (75–80%), but failed to discriminate between mutant and wild-type alleles. Shifting of the MRS to positions 4, 5, 15, 16 increased the specificity of the shRNAs, but reduced the overall silencing efficiency. Consistent with previous reports, these data confirm that MRS placement influences both allele-specificity and silencing strength of shRNAs, while further modification to hairpin design or MRS position may lead to the development of effective G2019S shRNAs. In summary, the effective shRNA against LRRK2 R1441 alleles described herein suggests that RNAi-based therapy of inherited Parkinson's disease is a viable approach towards developing effective therapeutic interventions for this serious neurodegenerative disease