46 research outputs found
The environments of Markarian galaxies
The extensively studied Markarian sample of 1500 ultraviolet excess galaxies contains many Seyfert, starburst, and peculiar galaxies. Using the 20 minute V plates obtained for the construction of the Hubble Space Telescope Guide Star Catalog, the authors investigated the morphologies of the Markarian galaxies and the environments in which they are located. The relationship between the types of nuclear activity and the morphologies and environments of the Markarian galaxies is discussed. The authors conclude that the type of nuclear activity present in the galaxies of the Markarian sample is not dependent on either the morphology or the local environment of the galaxy. This is not to imply that nuclear activity per se is not influenced by the environment in which the nucleus is located. Rather the type of nuclear activity (at least in the Markarian population) does not appear to be determined by the environment
The young stellar population of NGC 4214 as observed with HST. I. Data and methods
We present the data and methods that we have used to perform a detailed
UV-optical study of the nearby dwarf starburst galaxy NGC 4214 using
multifilter HST/WFPC2+STIS photometry. We explain the process followed to
obtain high-quality photometry and astrometry of the stellar and cluster
populations of this galaxy. We describe the procedure used to transform
magnitudes and colors into physical parameters using spectral energy
distributions. The data show the existence of both young and old stellar
populations that can be resolved at the distance of NGC 4214 (2.94 Mpc) and we
perform a general description of those populations.Comment: 33 pages, 9 figures, and 8 table
Tip of the Red Giant Branch Distances to NGC 4214, UGC 685, and UGC 5456
We have used WFPC2 VRI observations to calculate the distances to three
nearby galaxies, NGC 4214, UGC 685, and UGC 5456 using the tip of the red giant
branch method. Our values for NGC 4214 (2.94 +/- 0.18 Mpc) and UGC 685 (4.79
+/- 0.30 Mpc) are the most precise measurementes of the distances to these
objects ever made. For UGC 5456 the data do not allow us to reach a decisive
conclusion since there are two possible solutions, one leading towards a short
distance around 3.8 Mpc and another one towards a long distance of 5.6 Mpc or
more.Comment: To appear in the March 2002 issue of the Astronomical Journal. 19
pages, including 4 tables and 8 figures. Due to the size limitations of the
astro-ph service, a reduced resolution version of figures 1-3 is provided
here. For a full resolution version, please go to http://www.stsci.edu/~jmai
Sub-percent Photometry: Faint DA White Dwarf Spectophotometric Standards for Astrophysical Observatories
We have established a network of 19 faint (16.5 mag 19 mag) northern
and equatorial DA white dwarfs as spectrophotometric standards for present and
future wide-field observatories. Our analysis infers SED models for the stars
that are tied to the three CALSPEC primary standards. Our SED models are
consistent with panchromatic Hubble Space Telescope () photometry to
better than 1%. The excellent agreement between observations and models
validates the use of non-local-thermodynamic-equilibrium (NLTE) DA white dwarf
atmospheres extinguished by interstellar dust as accurate spectrophotometric
references. Our standards are accessible from both hemispheres and suitable for
ground and space-based observatories covering the ultraviolet to the near
infrared. The high-precision of these faint sources make our network of
standards ideally suited for any experiment that has very stringent
requirements on flux calibration, such as studies of dark energy using the
Large Synoptic Survey Telescope (LSST) and the Wide-Field Infrared Survey
Telescope ().Comment: 46 pages, 23 figures, 8 tables, accepted for publication in ApJ
The young stellar population of NGC 4214 as observed with HST. II. Results
We present the results of a detailed UV-optical study of the nearby dwarf
starburst galaxy NGC 4214 using multifilter HST/WFPC2+STIS photometry. The
stellar extinction is found to be quite patchy, with some areas having values
of E(4405-5495)< 0.1 mag and others, associated with star forming regions, much
more heavily obscured, a result which is consistent with previous studies of
the nebular extinction. We determined the ratio of blue-to-red supergiants and
found it to be consistent with theoretical models for the metallicity of the
SMC. The stellar IMF of the field in the range 20-100 solar masses is found to
be steeper than Salpeter. A number of massive clusters and associations with
ages between a few and 200 million years are detected and their properties are
discussed.Comment: 49 pages, 12 figures, 6 table
A massive cluster of Red Supergiants at the base of the Scutum-Crux arm
We report on the unprecedented Red Supergiant (RSG) population of a massive
young cluster, located at the base of the Scutum-Crux Galactic arm. We identify
candidate cluster RSGs based on {\it 2MASS} photometry and medium resolution
spectroscopy. With follow-up high-resolution spectroscopy, we use CO-bandhead
equivalent width and high-precision radial velocity measurements to identify a
core grouping of 26 physically-associated RSGs -- the largest such cluster
known to-date. Using the stars' velocity dispersion, and their inferred
luminosities in conjuction with evolutionary models, we argue that the cluster
has an initial mass of 40,000\msun, and is therefore among the most
massive in the galaxy. Further, the cluster is only a few hundred parsecs away
from the cluster of 14 RSGs recently reported by Figer et al (2006). These two
RSG clusters represent 20% of all known RSGs in the Galaxy, and now offer the
unique opportunity to study the pre-supernova evolution of massive stars, and
the Blue- to Red-Supergiant ratio at uniform metallicity. We use GLIMPSE,
MIPSGAL and MAGPIS survey data to identify several objects in the field of the
larger cluster which seem to be indicative of recent region-wide starburst
activity at the point where the Scutum-Crux arm intercepts the Galactic bulge.
Future abundance studies of these clusters will therefore permit the study of
the chemical evolution and metallicity gradient of the Galaxy in the region
where the disk meets the bulge.Comment: 49 pages, 22 figures. Accepted for publication in ApJ. Version with
hi-res figures can be found at http://www.cis.rit.edu/~bxdpci/RSGC2.pd
All-Sky Faint DA White Dwarf Spectrophotometric Standards for Astrophysical Observatories: The Complete Sample
Hot DA white dwarfs have fully radiative pure hydrogen atmospheres that are
the least complicated to model. Pulsationally stable, they are fully
characterized by their effective temperature Teff, and surface gravity log g,
which can be deduced from their optical spectra and used in model atmospheres
to predict their spectral energy distribution (SED). Based on this, three
bright DAWDs have defined the spectrophotometric flux scale of the CALSPEC
system of HST. In this paper we add 32 new fainter (16.5 < V < 19.5) DAWDs
spread over the whole sky and within the dynamic range of large telescopes.
Using ground based spectra and panchromatic photometry with HST/WFC3, a new
hierarchical analysis process demonstrates consistency between model and
observed fluxes above the terrestrial atmosphere to < 0.004 mag rms from 2700
{\AA} to 7750 {\AA} and to 0.008 mag rms at 1.6{\mu}m for the total set of 35
DAWDs. These DAWDs are thus established as spectrophotometric standards with
unprecedented accuracy from the near ultraviolet to the near-infrared, suitable
for both ground and space based observatories. They are embedded in existing
surveys like SDSS, PanSTARRS and GAIA, and will be naturally included in the
LSST survey by Rubin Observatory. With additional data and analysis to extend
the validity of their SEDs further into the IR, these spectrophotometric
standard stars could be used for JWST, as well as for the Roman and Euclid
observatories.Comment: Accepted for publication in Astrophysical Journal. Corrected error in
Table 10 and associated Fig 7 in which RP and BP values for the 3 CALSPEC
standards had been transpose
Discovery of an Extraordinarily Massive Cluster of Red Supergiants
We report the discovery of an extraordinarily massive young cluster of stars
in the Galaxy, having an inferred total initial cluster mass comparable to the
most massive young clusters in the Galaxy. Using {\it IRMOS}, {\it 2MASS}, and
{\it Spitzer} observations, we conclude that there are 14 red supergiants in
the cluster, compared with five, in what was previously thought to be the
richest Galactic cluster of such stars. We infer spectral types from
near-infrared spectra that reveal deep CO bandhead absorption that can only be
fit by red supergiants. We identify a gap of {\it K}4
magnitudes between the stars and the bulk of the other stars in the region that
can only be fit by models if the brightest stars in the cluster are red
supergiants. We estimate a distance of 5.8~\kpc to the cluster by associating
an OH maser with the envelope of one of the stars. We also identify a
``yellow'' supergiant of G6~I type in the cluster. Assuming a Salpeter IMF, we
infer an initial cluster mass of 20,000 to 40,000~\Msun for cluster ages of
7-12~\Myr. Continuing with these assumptions, we find 80% of the intial mass
and 99% of the number of stars remain at the present time. We associate the
cluster with an x-ray source (detected by {\it ASCA} and {\it Einstein}), a
recently discovered very high energy -ray source (detected by {\it
INTEGRAL} and {\it HESS}), and several non-thermal radio sources, finding that
these objects are likely related to recent supernovae in the cluster. In
particular, we claim that the cluster has produced at least one recent
supernova remnant with properties similar to the Crab nebula. It is not
unlikely to find such a source in this cluster, given our estimated supernova
rate of one per 40,000 to 80,000~{\it yr}.Comment: ApJ, accepte