377 research outputs found
A Search for Environmental Effects on Type Ia Supernovae
We use integrated colors and B and V absolute magnitudes of Type Ia supernova
(SN) host galaxies in order to search for environmental effects on the SN
optical properties. With the new sample of 44 SNe we confirm the conclusion by
Hamuy et al. (1996a) that bright events occur preferentially in young stellar
environments. We find also that the brightest SNe occur in the least luminous
galaxies, a possible indication that metal-poorer neighbourhoods produce the
more luminous events. The interpretation of these results is made difficult,
however, due to the fact that galaxies with younger stellar populations are
also lower in luminosity. In an attempt to remove this ambiguity we use models
for the line strengths in the absorption spectrum of five early-type galaxies,
in order to estimate metallicities and ages of the SN host galaxies. With the
addition of abundance estimates from nebular analysis of the emission spectra
of three spiral galaxies, we find possible further evidence that luminous SNe
are produced in metal-poor neighborhoods. Further spectroscopic observations of
the SN host galaxies will be necessary to test these results and assist in
disentangling the age/metallicity effects on Type Ia SNe.Comment: 14 pages, 5 figures, to appear in the September 2000 issue of The
Astronomical Journa
PMN J1632-0033: A new gravitationally lensed quasar
We report the discovery of a gravitationally lensed quasar resulting from our
survey for lenses in the southern sky. Radio images of PMN J1632-0033 with the
VLA and ATCA exhibit two compact, flat-spectrum components with separation
1.47" and flux density ratio 13.2. Images with the HST reveal the optical
counterparts to the radio components and also the lens galaxy. An optical
spectrum of the bright component, obtained with the first Magellan telescope,
reveals quasar emission lines at redshift 3.42. Deeper radio images with MERLIN
and the VLBA reveal a faint third radio component located near the center of
the lens galaxy, which is either a third image of the background quasar or
faint emission from the lens galaxy.Comment: 21 pp., including 4 figures; thoroughly revised in light of new
MERLIN/HST data; accepted for publication in A
The Distance to SN 1999em from the Expanding Photosphere Method
We present optical and IR spectroscopy of the first two months of evolution
of the Type II SN 1999em. We combine these data with high-quality optical/IR
photometry beginning only three days after shock breakout, in order to study
the performance of the ``Expanding Photosphere Method'' (EPM) in the
determination of distances. With this purpose we develop a technique to measure
accurate photospheric velocities by cross-correlating observed and model
spectra. The application of this technique to SN 1999em shows that we can reach
an average uncertainty of 11% in velocity from an individual spectrum. Our
analysis shows that EPM is quite robust to the effects of dust. In particular,
the distances derived from the VI filters change by only 7% when the adopted
visual extinction in the host galaxy is varied by 0.45 mag. The superb time
sampling of the BVIZJHK light-curves of SN 1999em permits us to study the
internal consistency of EPM and test the dilution factors computed from
atmosphere models for Type II plateau supernovae. We find that, in the first
week since explosion, the EPM distances are up to 50% lower than the average,
possibly due the presence of circumstellar material. Over the following 65
days, on the other hand, our tests lend strong credence to the atmosphere
models, and confirm previous claims that EPM can produce consistent distances
without having to craft specific models to each supernova. This is particularly
true for the VI filters which yield distances with an internal consistency of
4%. From the whole set of BVIZJHK photometry, we obtain an average distance of
7.5+/-0.5 Mpc, where the quoted uncertainty (7%) is a conservative estimate of
the internal precision of the method obtained from the analysis of the first 70
days of the supernova evolution.Comment: 68 pages, 15 tables, 22 figures, to appear in Ap
Long slit spectroscopy of a sample of isolated spirals with and without an AGN
We present the kinematical data obtained for a sample of active (Seyfert) and
non active isolated spiral galaxies, based on long slit spectra along several
position angles in the Halpha line region and, in some cases, in the Ca triplet
region as well. Gas velocity distributions are presented, together with a
simple circular rotation model that allows to determine the kinematical major
axes. Stellar velocity distributions are also shown. The main result is that
active and control galaxies seem to be equivalent in all kinematical aspects.
For both subsamples, the departure from pure circular rotation in some galaxies
can be explained by the presence of a bar and/or of a spiral arm. They also
present the same kind of peculiarities, in particular, S-shape structures are
quite common near the nuclear regions. They define very similar Tully-Fisher
relations. Emission line ratios are given for all the detected HII regions; the
analysis of the [NII]/Halpha metallicity indicator shows that active and
non-active galaxies have indistinguishable disk metallicities. These results
argue in favour of active and non-active isolated spiral galaxies having
essentially the same properties, in agreement with our previous results based
on the analysis of near infrared images. It appears now necessary to confirm
these results on a larger sample.Comment: 35 pages, 54 figures, Accepted for publication in Astronomy &
Astrophysics The full paper with its figures is available on the anonymous
account of ftp.iap.fr in /home/ftp/pub/from_users/durret/marquez.ps.gz (999
kb
HST Colour-Magnitude Diagrams of Six Old Globular Clusters in the LMC
We report on HST observations of six candidate old globular clusters in the
Large Magellanic Cloud: NGC 1754, NGC 1835, NGC 1898, NGC 1916, NGC 2005 and
NGC 2019. Deep exposures with the F555W and F814W filters provide us with
colour-magnitude diagrams that reach to an apparent magnitude in V of ~25, well
below the main sequence turnoff. These particular clusters are involved with
significantly high LMC field star densities and care was taken to subtract the
field stars from the cluster colour-magnitude diagrams accurately. In two cases
there is significant variable reddening across at least part of the image, but
only for NGC 1916 does the differential reddening preclude accurate
measurements of the CMD characteristics. The morphologies of the colour-
magnitude diagrams match well those of Galactic globular clusters of similar
metallicity. All six have well-developed horizontal branches, while four
clearly have stars on both sides of the RR Lyrae gap. The abundances obtained
from measurements of the height of the red giant branch above the level of the
horizontal branch are 0.3 dex higher, on average, than previously measured
spectroscopic abundances. Detailed comparisons with Galactic globular cluster
fiducials show that all six clusters are old objects, very similar in age to
classical Galactic globulars such as M5, with little age spread among the
clusters. This result is consistent with ages derived by measuring the
magnitude difference between the horizontal branch and main sequence turnoff.
We also find a similar chronology by comparing the horizontal branch
morphologies and abundances with the horizontal branch evolutionary tracks of
Lee, Demarque, & Zinn (1994). Our results imply that the LMC formed at the same
time as the Milky Way Galaxy.Comment: 23 pages, 18 PostScript figures, LaTeX, accepted by MNRAS. Uses
mn.sty and epsf.sty. Requires ols.sty (included
The Deep Lens Survey Transient Search I : Short Timescale and Astrometric Variability
We report on the methodology and first results from the Deep Lens Survey
transient search. We utilize image subtraction on survey data to yield all
sources of optical variability down to 24th magnitude. Images are analyzed
immediately after acquisition, at the telescope and in near-real time, to allow
for followup in the case of time-critical events. All classes of transients are
posted to the web upon detection. Our observing strategy allows sensitivity to
variability over several decades in timescale. The DLS is the first survey to
classify and report all types of photometric and astrometric variability
detected, including solar system objects, variable stars, supernovae, and short
timescale phenomena. Three unusual optical transient events were detected,
flaring on thousand-second timescales. All three events were seen in the B
passband, suggesting blue color indices for the phenomena. One event (OT
20020115) is determined to be from a flaring Galactic dwarf star of spectral
type dM4. From the remaining two events, we find an overall rate of \eta = 1.4
events deg-2 day-1 on thousand-second timescales, with a 95% confidence limit
of \eta < 4.3. One of these events (OT 20010326) originated from a compact
precursor in the field of galaxy cluster Abell 1836, and its nature is
uncertain. For the second (OT 20030305) we find strong evidence for an extended
extragalactic host. A dearth of such events in the R passband yields an upper
95% confidence limit on short timescale astronomical variability between 19.5 <
R < 23.4 of \eta_R < 5.2. We report also on our ensemble of astrometrically
variable objects, as well as an example of photometric variability with an
undetected precursor.Comment: 24 pages, 12 figures, 3 tables. Accepted for publication in ApJ.
Variability data available at http://dls.bell-labs.com/transients.htm
Supernova Limits on the Cosmic Equation of State
We use Type Ia supernovae studied by the High-Z Supernova Search Team to
constrain the properties of an energy component which may have contributed to
accelerating the cosmic expansion. We find that for a flat geometry the
equation of state parameter for the unknown component, alpha_x=P_x/rho_x, must
be less than -0.55 (95% confidence) for any value of Omega_m and is further
limited to alpha_x<-0.60 (95%) if Omega_m is assumed to be greater than 0.1 .
These values are inconsistent with the unknown component being topological
defects such as domain walls, strings, or textures. The supernova data are
consistent with a cosmological constant (alpha_x=-1) or a scalar field which
has had, on average, an equation of state parameter similar to the cosmological
constant value of -1 over the redshift range of z=1 to the present. Supernova
and cosmic microwave background observations give complementary constraints on
the densities of matter and the unknown component. If only matter and vacuum
energy are considered, then the current combined data sets provide direct
evidence for a spatially flat Universe with Omega_tot=Omega_m+Omega_Lambda =
0.94 +/- 0.26 (1-sigma).Comment: Accepted for publication in ApJ, 3 figure
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