586 research outputs found
A low-absorption disk zone at low Galactic latitude in Centaurus
We investigate the properties of two stellar concentrations in a
low-absorption disk zone in Centaurus, located respectively at
, , and , . The present analysis is based mostly on 2MASS photometry, as well as
optical photometry. Based on colour-magnitude diagrams and stellar radial
density profiles, we show that these concentrations are not open star clusters.
Instead, they appear to be field stars seen through a differentially-reddened
window. We estimate that the bulk of the stars in both stellar concentrations
is located at kpc from the Sun, a distance consistent with that of
the Sgr-Car arm in that direction. This low-absorption window allows one to
probe into distant parts of the disk besides the Sgr-Car arm, probably the
tangent part of the Sct-Cru arm, and/or the far side of the Sgr-Car arm in that
direction. The main sequence associated to the Sgr-Car arm is reddened by
\ebv\sim0.5, so that this window through the disk is comparable in reddening
to Baade's window to the bulge. We also investigate the nature of the open
cluster candidate Ru 166. The presently available data do not allow us to
conclude whether Ru 166 is an actual open cluster or field stars seen through a
small-scale low-absorption window
Balmer and Metal Absorption Feature Gradients in M32
Spectra from MDM Observatory are used to assess Lick/IDS feature strength
gradients inside the half-light radius of the compact Local Group elliptical
galaxy M32. All but a few (of 24 measured) indices show a statistically
significant gradient. Comparing with models, the index gradients indicate a
mean age and abundance gradient in the sense that the nucleus is a factor of
2.5 younger and a factor of 0.3 dex more metal-rich than at 1 effective radius.
This conclusion is only weakly dependent on which index combinations are used
and is robust to high accuracy. Stars near the M32 nucleus have a mean age and
heavy element abundance [M/H] of (4.7 Gyr, +0.02), judging from models by
Worthey with variable abundance ratios. This result has very small formal
random errors, although, of course, there is significant age-metallicity
degeneracy along an (age, abundance) line segment from (5.0 Gyr, 0.00) to (4.5
Gyr, +0.05). An abundance pattern of [C/M]=+0.077, [N/M]=-0.13, [Mg/M]=-0.18,
[Fe/M]~0.0, and [Na/M]=+0.12 is required to fit the feature data, with a
fitting precision of about 0.01 dex. Model uncertainties make the accuracies of
these values at least twice the magnitude of the precision. Forcing
scaled-solar abundances does not change the age very much, but it increases the
rms goodness of model-data fit by a factor of 3 and broadens the allowed range
of age to Gyr. The overall abundance pattern contrasts with larger
elliptical galaxies, in which all measurable lighter elements are enhanced
relative to iron and calcium.Comment: 23 pages, 9 figures, Astronomical Journal, in pres
A Chandra Observation of the Nearby Lenticular Galaxy NGC 5102: Where are the X-ray Binaries?
We present results from a 34 ks Chandra/ACIS-S observation of the nearby
(d=3.1 Mpc) lenticular galaxy NGC 5102, previously shown to have an unusually
low X-ray luminosity. We detect eleven X-ray point sources within the the
optical boundary of the galaxy (93% of the light), one third to one
half of which are likely to be background AGN. One source is coincident with
the optical nucleus and may be a low-luminosity AGN. Only two sources with an
X-ray luminosity greater than 10 ergs s in the 0.5-5.0 keV band
were detected, one of which is statistically likely to be a background AGN. We
expected to detect 6 such luminous sources if the XRB population scales
linearly with optical magnitude of the host galaxy. NGC 5102 has an unusually
low number of XRBs. NGC 5102 is unusually blue for its morphological type, and
has undergone at least two recent bursts of star formation. We present the
results of optical/UV spectral synthesis analysis and demonstrate that a
significant fraction (50%) of the stars in this galaxy are comparatively
young ( years old). If the lack of X-ray binaries is related to
the relative youth of most of the stars, this would support models of LMXB
formation and evolution that require wide binaries to shed angular momentum on
a timescale of Gyrs. We find that NGC 5102 has an unusually low specific
frequency of globular clusters (0.4), which could also explain the
lack of LMXBs. We also detect diffuse X-ray emission in the central 1 kpc
of the galaxy. This hot gas is most likely a superbubble created by multiple
supernovae of massive stars born during the most recent star burst, and is
driving the shock into the ISM which was inferred from optical observations.Comment: 33 pages, 7 figures, 6 tables - Accepted for publication in the
Astrophysical Journa
Synthetic spectra of H Balmer and HeI absorption lines. II: Evolutionary synthesis models for starburst and post-starburst galaxies
We present evolutionary stellar population synthesis models to predict the
spectrum of a single-metallicity stellar population, with a spectral sampling
of 0.3 A in five spectral regions between 3700 and 5000 A. The models, which
are optimized for galaxies with active star formation, synthesize the profiles
of the hydrogen Balmer series (Hb, Hg, Hd, H8, H9, H10, H11, H12 and H13) and
the neutral helium absorption lines (HeI 4922, HeI 4471, HeI 4388, HeI 4144,
HeI 4121, HeI 4026, HeI 4009 and HeI 3819) for a burst with an age ranging from
1 to 1000 Myr, and different assumptions about the stellar initial mass
function. Continuous star formation models lasting for 1 Gyr are also
presented. The input stellar library includes NLTE absorption profiles for
stars hotter than 25000 K and LTE profiles for lower temperatures. The
temperature and gravity coverage is 4000 K <Teff< 50000 K and 0.0< log g$< 5.0,
respectively.
The models can be used to date starburst and post-starburst galaxies until 1
Gyr. They have been tested on data for clusters in the LMC, the super-star
cluster B in the starburst galaxy NGC 1569, the nucleus of the dwarf elliptical
NGC 205 and a luminous "E+A" galaxy. The full data set is available for
retrieval at http://www.iaa.es/ae/e2.html and at
http://www.stsci.edu/science/starburst/, or on request from the authors at
[email protected]: To be published in ApJS. 48 pages and 20 figure
Compact Nuclei in Moderately Redshifted Galaxies
The Hubble Space Telescope WFPC2 is being used to obtain high-resolution
images in the V and I bands for several thousand distant galaxies as part of
the Medium Deep Survey (MDS). An important scientific aim of the MDS is to
identify possible AGN candidates from these images in order to measure the
faint end of the AGN luminosity function as well as to study the host galaxies
of AGNs and nuclear starburst systems. We are able to identify candidate
objects based on morphology. Candidates are selected by fitting bulge+disk
models and bulge+disk+point source nuclei models to HST imaged galaxies and
determining the best model fit to the galaxy light profile. We present results
from a sample of MDS galaxies with I less than 21.5 mag that have been searched
for AGN/starburst nuclei in this manner. We identify 84 candidates with
unresolved nuclei in a sample of 825 galaxies. For the expected range of galaxy
redshifts, all normal bulges are resolved. Most of the candidates are found in
galaxies displaying exponential disks with some containing an additional bulge
component. 5% of the hosts are dominated by an r^-1/4 bulge. The V-I color
distribution of the nuclei is consistent with a dominant population of
Seyfert-type nuclei combined with an additional population of starbursts. Our
results suggest that 10% +/- 1% of field galaxies at z less than 0.6 may
contain AGN/starburst nuclei that are 1 to 5 magnitudes fainter than the host
galaxies.Comment: 12 pages AASTeX manuscript, 3 separate Postscript figures, to be
published in ApJ Letter
Structures in surface-brightness profiles of LMC and SMC star clusters: evidence of mergers?
The LMC and SMC are rich in binary star clusters, and some mergers are
expected. It is important to characterize single clusters, binary clusters and
candidates to mergers. We selected a sample of star clusters in each Cloud with
this aim. Surface photometry of 25 SMC and 22 LMC star clusters was carried
with the ESO Danish 1.54 m telescope. 23 clusters were observed for the first
time for these purposes. We fitted Elson, Fall and Freeman (1987, EFF) profiles
to the data, deriving structural parameters, luminosities and masses. We also
use isophotal maps to constrain candidates to cluster interactions.} {The
structural parameters, luminosities and masses presented good agreement with
those in the literature. Three binary clusters in the sample have a double
profile. Four clusters (NGC 376, K 50, K 54 and NGC 1810) do not have
companions and present as well important deviations from EFF profiles. The
present sample contains blue and red Magellanic clusters. Extended EFF profiles
were detected in some blue clusters. We find evidence that important deviations
from the body of EFF profiles might be used as a tool to detect cluster
mergers.Comment: 16 pages and 8 figures. Accepted by A&
Bridge over troubled gas: clusters and associations under the SMC and LMC tidal stresses
We obtained SOAR telescope B and V photometry of 14 star clusters and 2
associations in the Bridge tidal structure connecting the LMC and SMC. These
objects are used to study the formation and evolution of star clusters and
associations under tidal stresses from the Clouds. Typical star clusters in the
Bridge are not richly populated and have in general relatively large diameters
(~30-35 pc), being larger than Galactic counterparts of similar age. Ages and
other fundamental parameters are determined with field-star decontaminated
photometry. A self-consistent approach is used to derive parameters for the
most-populated sample cluster NGC 796 and two young CMD templates built with
the remaining Bridge clusters. We find that the clusters are not coeval in the
Bridge. They range from approximately a few Myr (still related to optical HII
regions and WISE and Spitzer dust emission measurements) to about 100-200 Myr.
The derived distance moduli for the Bridge objects suggests that the Bridge is
a structure connecting the LMC far-side in the East to the foreground of the
SMC to the West. Most of the present clusters are part of the tidal dwarf
candidate D 1, which is associated with an H I overdensity. We find further
evidence that the studied part of the Bridge is evolving into a tidal dwarf
galaxy, decoupling from the Bridge.Comment: 15 pages, 15 figures, MNRAS, Accepted 2015 July 2
The frequency of nuclear star-formation in Seyfert 2 galaxies
We investigate the detectability of starburst signatures in the nuclear
spectrum of Seyfert 2 galaxies by constructing spectral models in the
wavelength range 3500-4100A, combining the spectrum of a bulge population (of
age ~10Gyr) with that of younger stellar populations, spanning ages from ~3 Myr
to 1 Gyr. We also construct models combining the bulge template with a
power-law (PL) continuum, which is observed in some Seyfert 2's in polarized
light, contributing with typically 10-40% of the flux at 4020A. We conclude
that such continuum cannot be distinguished from that of a very young stellar
population (age < 10 Myr), contributing with less than ~0.02% of the mass of
the bulge. The models are compared with nuclear spectra - corresponding to a
radius of 200-300 pc at the galaxy - of 20 Seyfert 2 galaxies, in which we
specifically look for the signatures above of young to intermediate age stellar
populations. We find them in ten galaxies, thus 50% of the sample. But only in
six cases (30% of the sample) they can be attributed to young stars (age < 500
Myr): Mrk 1210, ESO 362-G8, NGC 5135, NGC 5643, NGC 7130 and NGC 7582. In the
remaining four cases, the signatures are due to intermediate age stars (~1
Gyr). We find a tendency for the young stars to be found more frequently among
the late type Seyfert's. This tendency is supported by a comparison between the
equivalent widths (W) of absorption lines of the nuclear spectra of the Seyfert
2's with those of normal galaxies of the same Hubble type.Comment: 18 figures, revised version published in ApJ, December 2000, vol.
544, p. 74
Near Infrared properties of 12 Globular Clusters toward the inner Bulge of the Galaxy
We present near-IR Colour-Magnitude diagrams and physical parameters for a
sample of 12 galactic globular clusters located toward the inner Bulge region.
For each cluster we provide measurements of the reddening, distance,
photometric metallicity, luminosity of the horizontal branch red clump, and of
the red giant branch bump and tip. The sample discussed here together with that
presented in Valenti, Ferraro & Origlia (2007) represent the largest
homogeneous catalog of Bulge globular clusters (comprising ~ 80% of the entire
Bulge cluster population) ever studied. The compilation is available in
electronic form on the World Wide Web (http://www.bo.astro.it/~GC/ir_archive)Comment: 2 pages, 13 figures, 4 tables, accepted for publication in MNRA
Spectral Classification of Galaxies
We investigate the integrated spectra of a sample of 24 normal galaxies. A
principal component analysis suggests that most of the variance present in the
spectra is due to the differences in morphology of the galaxies in the sample.
We show that spectroscopic parameters extracted from the spectra, like the
amplitude of the 4000 \AA~ break or of the CN band, correlate well with Hubble
types and are useful for quantitative classification.Comment: 7 pages uuencoded compressed PostScript file. To appear in Vistas in
Astronomy, special issue on Artificial Neural Networks in Astronom
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