50 research outputs found
The Hubble Diagram of the Calan/Tololo Type Ia Supernovae and the value of Ho
The Calan/Tololo supernova survey has discovered ~30 Type Ia supernovae out
to z~0.1. Using BVI data for these objects and nearby SNe Ia, we have shown
that there exists a significant dispersion in the intrinsic luminosities of
these objects. We have devised a robust chisquare minimization technique
simultaneously fitting the BVI light curves to parametrize the SN event as a
function of (tb,m, m15(B)) where tb is the time of B maximum, m is the peak BVI
magnitude corrected for luminosity variations, and m15(B) is a single parameter
describing the whole light curve morphology. When properly corrected for
m15(B), SNe Ia prove to be high precision distance indicators,yielding relative
distances with errors 7-10%. The corrected peak magnitudes are used to
construct BVI Hubble diagrams (HD), and with Cepheid distances recently
measured with the HST to four nearby SNe Ia (37C, 72E, 81B, 90N) we derive a
value of the Hubble constant of 63.1+/-3.4 (internal) km/s/Mpc. This value is
~10-15% larger than the value obtained by assuming that SNe Ia are perfect
standard candles. As we have shown in Paper V, there is now strong evidence
that galaxies with younger stellar population appear to host the
slowest-declining, and therefore most luminous SNe Ia. Hence, the use of Pop I
objects such as Cepheids to calibrate the zero point of the SNe Ia HD can
easily bias the results toward luminous SNe Ia, unless the absolute
magnitude-decline relation is taken into account.Comment: 32 pages, figures attached, all tables available, to appear in the
Astronomical Journa
The Absolute Luminosities of the Calan/Tololo Type Ia Supernovae
We examine the absolute luminosities of 29 SNe Ia in the Calan/Tololo survey.
We confirm a relation between the peak luminosity of the SNe and the decline
rate as measured by the light curve, as suggested by Phillips (1993). We derive
linear slopes to this magnitude-decline rate relation in BV(I)kc colors, using
a sample with Bmax-Vmax < 0.2 mag. The scatter around this linear relation (and
thus the ability to measure SNe Ia distances) ranges from 0.13 mag (in the I
band) to 0.17 mag (in the B band). We also find evidence for significant
correlations between the absolute magnitudes or the decline rate of the light
curve, and the morphological type of the host galaxy.Comment: 21 pages, 4 figures, to appear in the Astronomical Journa
The Morphology of Type Ia Supernovae Light Curves
We present a family of six BVI template light curves for SNe Ia for days -5
and +80, based on high-quality data gathered at CTIO. These templates display a
wide range of light curve morphologies, with initial decline rates of their B
light curves between m15(B)=0.87 mag and 1.93 mag. We use these templates to
study the general morphology of SNe Ia light curves. We find that several of
the main features of the BVI templates correlate tightly with m15(B). In
particular, the V light curves, which are probably a reasonably good
approximation of the bolometric light curves, display an orderly progression in
shapes between the most-luminous, slowest-declining events and the
least-luminous, fastest-declining SNe. This supports the idea that the observed
spectroscopic and photometric sequences of SNe Ia are due primarily to one
parameter. Nevertheless, SNe with very similar initial decline rates do show
significant differences in their light curve properties when examined in
detail, suggesting the influence of one or more secondary parameters.Comment: 32 pages, 15 figures, to appear in 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
A nearly symmetric double-image gravitational lens
We report the discovery of a new double-image gravitational lens resulting
from our search for lenses in the southern sky. Radio source PMN J2004-1349 is
composed of two compact components separated by 1.13 arcseconds in VLA, MERLIN
and VLBA images. The components have a flux ratio of 1:1 at radio frequencies
ranging from 5 GHz to 22 GHz. The I-band optical counterpart is also an equal
double, with roughly the same separation and position angle as the radio
double. Upon subtraction of the components from the I-band image, we identify a
dim pattern of residuals as the lens galaxy. While the present observations are
sufficient to establish that J2004-1349 is a gravitational lens, additional
information will be necessary (such as the redshifts of the galaxy and quasar,
and precise astrometry and photometry of the lens galaxy) before constructing
detailed mass models.Comment: 17 pp., 5 bitmapped figs. (contact [email protected] for nicer figs), in
press, A.J. (substantially revised
Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant
We present observations of 10 type Ia supernovae (SNe Ia) between 0.16 < z <
0.62. With previous data from our High-Z Supernova Search Team, this expanded
set of 16 high-redshift supernovae and 34 nearby supernovae are used to place
constraints on the Hubble constant (H_0), the mass density (Omega_M), the
cosmological constant (Omega_Lambda), the deceleration parameter (q_0), and the
dynamical age of the Universe (t_0). The distances of the high-redshift SNe Ia
are, on average, 10% to 15% farther than expected in a low mass density
(Omega_M=0.2) Universe without a cosmological constant. Different light curve
fitting methods, SN Ia subsamples, and prior constraints unanimously favor
eternally expanding models with positive cosmological constant (i.e.,
Omega_Lambda > 0) and a current acceleration of the expansion (i.e., q_0 < 0).
With no prior constraint on mass density other than Omega_M > 0, the
spectroscopically confirmed SNe Ia are consistent with q_0 <0 at the 2.8 sigma
and 3.9 sigma confidence levels, and with Omega_Lambda >0 at the 3.0 sigma and
4.0 sigma confidence levels, for two fitting methods respectively. Fixing a
``minimal'' mass density, Omega_M=0.2, results in the weakest detection,
Omega_Lambda>0 at the 3.0 sigma confidence level. For a flat-Universe prior
(Omega_M+Omega_Lambda=1), the spectroscopically confirmed SNe Ia require
Omega_Lambda >0 at 7 sigma and 9 sigma level for the two fitting methods. A
Universe closed by ordinary matter (i.e., Omega_M=1) is ruled out at the 7
sigma to 8 sigma level. We estimate the size of systematic errors, including
evolution, extinction, sample selection bias, local flows, gravitational
lensing, and sample contamination. Presently, none of these effects reconciles
the data with Omega_Lambda=0 and q_0 > 0.Comment: 36 pages, 13 figures, 3 table files Accepted to the Astronomical
Journa
The Globular Cluster Systems of the Sculptor Group
We use CTIO 4-m Mosaic II images taken with the Washington and Harris
filters to identify candidate globular clusters in the six major galaxies
of the Sculptor group: NGC 45, NGC 55, NGC 247, NGC 254, NGC 300, and NGC 7793.
From follow-up spectroscopy with Hydra-CTIO, we find 19 new globular clusters
in NGC 55, NGC 247, NGC 253, and NGC 300, bringing the total number of known
Sculptor group globular clusters to 36. The newly discovered clusters have
spectroscopic ages consistent with those of old Milky Way globular clusters,
and the majority are metal-poor. Their luminosity function closely resembles
that of the Milky Way's globular clusters; their metallicity distribution is
somewhat more metal-rich, but this may be the result of our color selection of
candidates. The mean [/Fe] ratio in the clusters is , which
is lower than the Milky Way average. The specific frequencies are similar
to those of other late-type galaxies. However, if we calculate the specific
frequency using the -band total magnitudes of the host galaxies, we find
values that are more than a factor of two higher. The kinematics of the
globular cluster systems are consistent with rotation with the \ion{H}{1} disk
in each of the four galaxies; however, only in NGC 253 is this result based on
more than seven objects. We suggest that the Sculptor group galaxies add to
evidence indicating that many of the first generation globular clusters formed
in disks, not halos.Comment: 22 pages, 13 figures, 7 Tables. Full Table 3 available electronically
at http://www.ctio.noao.edu/~olsen/tab3.tex. To appear in May 2004 issue of
the Astronomical Journa
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