104 research outputs found
Searching for Stars in Compact High-Velocity Clouds. II
We address the hypothesis that High Velocity Clouds correspond to the
"missing" dwarf galaxies of the Local Group predicted by cosmological
simulations. To this end, we present optical and near-infrared photometry of
five additional High Velocity Clouds, one of which produces Lyman series
absorption on the sight line towards the Quasar Ton S210, with sufficient
resolution and sensitivity to enable the detection of an associated stellar
content. We do not detect significant stellar populations intrinsic to any of
the five clouds. In combination with the results from our paper I, which had
yielded non detections of stellar content in another five cases, we find that
there is a 50% chance of getting a null result in ten trials if fewer than 7%
of all High Velocity Clouds contain stars. We conclude that the population of
High Velocity Clouds is an unlikely repository for the "missing" dwarfs of the
Local Group.Comment: 6 pages, 3 figures. submitted to MNRA
Searching for Stars in Compact High-Velocity Clouds. I First Results from VLT and 2MASS
We investigate the hypothesis that compact high-velocity clouds (CHVC) are
the "missing" dwarf galaxies of the Local Group, by searching them for
populations of resolved stars. To this end we conducted two distinct tests
based on optical and near-infrared single-star photometry. The optical and the
near-infrared experiments complement one another; the optical data help us to
rule out distant populations but they are restricted to the central regions of
the gas distributions, whereas the near-infrared photometry allows us to set
limits on nearby populations spread over the typical cloud size. First, we
discuss deep optical single-star photometry of five CHVCs in the V and I
filters, obtained with the FORS instrument at the Very Large Telecope (VLT). We
find that their optical colour-magnitude diagrams are indistinguishable from
that of a population of Galactic stars, and attribute all of the resolved stars
to Galactic foreground. We present simulations which address the question of
how much of a "normal" dwarf-galaxy type population we might hide in the data.
A Kolmogorov-Smirnov test allows us to set very stringent limits on the absence
of a resolved stellar population in CHVCs. Second, we also culled near infrared
single-star photometry in the J, H, and K_S bands for four of the CHVCs from
the Two Micron All Sky Survey (2MASS). The infrared data do not reveal any
stellar contents in the CHVCs which resembles that of nearby dwarf galaxies
either, and are explained with Galactic foreground as well. We interpret our
null detections to indicate that the five CHVCs investigated by us do not host
an associated stellar content which is similar to that of the known dwarf
galaxies of the Local Group. These CHVCs are very likely pure hydrogen clouds
in which no star formation has taken place over cosmic time.Comment: 13 pages, 10 figures, accepted for publication by MNRA
Polarimetric Evidence of Non-Spherical Winds
Polarization observations yield otherwise unobtainable information about the
geometrical structure of unresolved objects. In this talk we review the
evidences for non-spherically symmetric structures around Luminous Hot Stars
from polarimetry and what we can learn with this technique. Polarimetry has
added a new dimension to the study of the envelopes of Luminous Blue Variables,
Wolf-Rayet stars and B[e] stars, all of which are discussed in some detail.Comment: 8 pages, 2 encapsulated Postscript figures, uses lamuphys.sty.
Invited review to appear in IAU Coll. 169, Variable and Non-Spherical Stellar
Winds in Luminous Hot Stars, eds. B. Wolf, A.Fullerton and O. Stahl
(Springer
Ca II and Na I Quasar Absorption-Line Systems in an Emission-Selected Sample of SDSS DR7 Galaxy/Quasar Projections: I. Sample Selection
The aim of this project is to identify low-redshift host galaxies of quasar
absorption-line systems by selecting galaxies which are seen in projection onto
quasar sightlines. To this end, we use the Seventh Data Release of the Sloan
Digital Sky Survey (SDSS-DR7) to construct a parent sample of 97489
galaxy/quasar projections at impact parameters of up to 100 kpc to the
foreground galaxy. We then search the quasar spectra for absorption line
systems of Ca II and Na I within +- 500 km/s of the galaxy's velocity. This
yields 92 Ca II and 16 Na I absorption systems. We find that most of the Ca II
and Na I systems are sightlines through the Galactic disk, through High
Velocity Cloud complexes in our halo, or Virgo cluster sightlines. Placing
constraints on the absorption line rest equivalent width significance (>=3.0
sigma), the Local Standard of Rest velocity along the sightline (>= 345 km/s),
and the ratio of the impact parameter to the galaxy optical radius (<=5.0), we
identify 4 absorption line systems that are associated with low-redshift
galaxies at high confidence, consisting of two Ca II systems (one of which also
shows Na I), and two Na I systems. These 4 systems arise in blue, L_r^*
galaxies. Tables of the 108 absorption systems are provided to facilitate
future follow up.Comment: 13 pages, 11 figures, 6 tables; online data included in electronic
version as 1 FITS table and 2 machine readable tables; to be published in The
Astronomical Journa
Circumstellar environment of RX Puppis
The symbiotic Mira, RX Pup, shows long-term variations in its mean light
level due to variable obscuration by circumstellar dust. The last increase in
extinction towards the Mira, between 1995 and 2000, has been accompanied by
large changes in the degree of polarization in the optical and red spectral
range. The lack of any obvious associated changes in the position angle may
indicate the polarization variations are driven by changes in the properties of
the dust grains (e.g. variable quantity of dust and variable particle size
distribution, due to dust grain formation and growth) rather than changes in
the viewing geometry of the scattering region(s), e.g. due to the binary
rotation.Comment: Paper presented at Torun 2000 conference on Post-AGB objects as a
phase of stellar evolution; 8 pages, 3 figure
The Star Formation History of NGC 1705: a Post-Starburst Galaxy on the Verge of Activity
We infer the star formation history in different regions of the blue compact
dwarf NGC 1705 by comparing synthetic color-magnitude diagrams with HST optical
and near-infrared photometry. We find that NGC 1705 is not a young galaxy
because its star formation commenced at least 5 Gyr ago. On the other hand, we
confirm the existence of a recent burst of star formation between 15 and 10 Myr
ago. We also find evidence for new strong activity, which started 3 Myr ago and
is still continuing. The old population is spread across the entire galaxy,
while the young and intermediate stars are more concentrated in the central
regions. We derive an almost continuous star formation with variable rate, and
exclude the presence of long quiescent phases between the episodes during the
last ~1 Gyr. The central regions experienced an episode of star formation of
\~0.07 Msun/yr (for a Salpeter initial mass function [IMF]) 15 to 10 Myr ago.
This coincides with the strong activity in the central super star cluster. We
find a rate of ~0.3 Msun/yr for the youngest ongoing burst which started ~3 Myr
ago. This is higher than in other dwarfs and comparable to the rate of NGC
1569. The star formation rate of earlier episodes is not especially high and
falls in the range 10^{-3}-10^{-1} Msun/yr. The IMF is close to the Salpeter
value or slightly steeper.Comment: 34 pages, including 6 tables and 14 .ps figures (9 in colour), AJ in
pres
HI Observations of the Ca II absorbing galaxies Mrk 1456 and SDSS J211701.26-002633.7
In an effort to study Damped Lyman Alpha galaxies at low redshift, we have
been using the Sloan Digital Sky Survey to identify galaxies projected onto QSO
sightlines and to characterize their optical properties. For low redshift
galaxies, the HI 21cm emission line can be used as an alternate tool for
identifying possible DLA galaxies, since HI emitting galaxies typically exhibit
HI columns that are larger than the classical DLA limit. Here we report on
follow-up HI 21 cm emission line observations of two DLA candidates that are
both low-redshift spiral galaxies, Mrk 1456 and SDSS J211701.26-002633.7. The
observations were made using the Green Bank and Arecibo Telescopes,
respectively. Analysis of their HI properties reveal the galaxies to be about
one and two M_HI* galaxies, respectively, and to have average HI mass,
gas-richness, and gas mass fraction for their morphological types. We consider
Mrk 1456 and SDSS J211701.26-002633.7 to be candidate DLA systems based upon
the strength of the CaII absorption lines they cause in their QSO's spectra,
and impact parameters to the QSO that are smaller than the stellar disk.
Compared to the small numbers of other HI-detected DLA and candidate DLA
galaxies, Mrk 1456 and SDSS J211701.26-002633.7 have high HI masses. When
compared with the expected properties of low-z DLAs from an HI-detected sample
of galaxies, Mrk 1456 and SDSS J211701.26-002633.7 fall within the ranges for
impact parameter and B-band absolute magnitude; and the HI mass distribution
for the HI-detected DLAs agrees with that of the expected HI mass distribution
for low-z DLAs. Our observations support galaxy-evolution models in which high
mass galaxies make up an increasing contribution to the DLA cross-section at
lower redshifts. [abridged]Comment: 12 pages, 5 figures, 5 tables; to be published in The Astronomical
Journa
Detection of accreting gas toward HD 45677: A newly recognized, Herbig Be proto-planetary system
We report detection of high velocity, accreting gas toward the Be star with IR excess and bipolar nebula, HD 45677. High velocity (+200 to +400 km/s), variable column density gas is visible in all IUE spectra from 1979-1992 in transitions of Si II, C II, Al III, Fe III, Si IV, and C IV. Low-velocity absorption profiles from low oscillator-strength transitions of Si II, Fe II, and Zn II exhibit double-peaked absorption profiles similar to those previously reported in optical spectra of FU Orionis objects. The UV absorption data, together with previously reported analyses of the IR excess and polarization of this object, suggest that HD 45677 is a massive, Herbig Be star with an actively accreting circumstellar, proto-planetary disk
Hubble Space Telescope Observations of the Gravitationally Lensed Cloverleaf Broad Absorption Line QSO H1413+1143: Imaging Polarimetry and Evidence for Microlensing of a Scattering Region
We report the results of HST WFPC2 broadband F555W and F702W photometric and
F555W polarimetric observations of the "Cloverleaf" QSO H1413+1143. This is a
four-component gravitationally-lensed broad absorption line (BAL) QSO.
Observations were obtained at two epochs in March 1999 and June 1999 separated
by about 100 days. The goal of our program was to detect any relative changes
among the components and between the two epochs. Over this time baseline we
detected an approximately 0.07 mag dimming in component D of the lensed image,
which we interpret as evidence for microlensing. In March 1999 we find
significant evidence for a difference in the relative linear polarization of
component D in comparison to the other three components; in June 1999 the
combined polarization of the Cloverleaf components was lower. In March 1999 the
apparently microlensed component D has a rotated polarization position angle
and a somewhat higher degree of polarization than the other three components.
We suggest that this difference in polarization is due to microlensing
magnification of part of a scattered-light (i.e. polarized) continuum-producing
region. The results indicate that in the Cloverleaf the size-scale of the
polarized scattered-light region exceeds about 10^{16} cm but lies interior to
the region producing the broad emission lines (< 10^{18} cm).Comment: 21 pages, accepted for publication in Ap
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