34 research outputs found
Massive Stellar Content of the Galactic Supershell GSH 305+01-24
The distribution of OB stars along with that of H, CO, dust
infrared emission, and neutral hydrogen is carried out in order to provide a
more complete picture of interactions of the young massive stars and the
observed supershell GSH 305+01-24. The studied field is located between
and . The
investigation is based on nearly 700 O-B9 stars with photometry
currently available. The derived stellar physical parameters were used to
establish a homogeneous scale for the distances and extinction of light for
major apparent groups and layers of foreground and background stars in
Centaurus and study the interaction with the surrounding interstellar medium.
The distance to the entire Centaurus star-forming complex is revised and a
maximum of the OB-star distance distribution is found at 1.80.4 (r.m.s)
kpc. The massive star component of GSH 305+01-24 is identified at about 85-90 %
completeness up to 11.5-12 mag. The projected coincidence of the OB stars with
the shell and the similarities between the shell's morphology and the OB-star
distribution indicate a strong interaction of the stellar winds with the
superbubble material. We demonstrate that these stars contribute a sufficient
wind injection energy in order to explain the observed size and expansion
velocity of the supershell. The derived stellar ages suggest an age gradient
over the Coalsack Loop. A continuous star-formation might be taking place
within the shell with the youngest stars located at its periphery and the open
cluster NGC 4755 being the oldest. A layer of very young stars at 1 kpc is
detected and its connection to both GSH 305+01-24 and the foreground GSH
304-00-12 H I shells is investigated.Comment: Accepted for publication in A&A. Paper consists of 11 pages, 3 tables
and 9 figures. Table 1 and Table 3 will only be available from CD
Untangling the X-ray Emission From the Sa Galaxy NGC1291 With Chandra
We present a Chandra ACIS-S observation of the nearby bulge-dominated Sa
galaxy NGC1291. The X-ray emission from the bulge resembles the X-ray emission
from a sub-class of elliptical and S0 galaxies with low L_X/L_B luminosity
ratios. The X-ray emission is composed of a central point-like nucleus, ~50
point sources that are most likely low mass X-ray binaries (LMXBs), and diffuse
gas detectable out to a radius of 120" (5.2 kpc). The diffuse gas has a global
temperature of 0.32^{+0.04}_{-0.03} keV and metallicity of 0.06 +/- 0.02 solar,
and both quantities marginally decrease with increasing radius. The hot gas
fills the hole in the HI distribution, and the softening of the spectrum of the
X-ray gas with radius might indicate a thermal coupling of the hot and cold
phases of the interstellar medium as previously suggested. The integrated X-ray
luminosity of the LMXBs, once normalized by the optical luminosity, is a factor
of 1.4 less than in the elliptical galaxy NGC4697 or S0 galaxy NGC1553. The
difference in L_{X,stellar}/L_B between the galaxies appears to be because of a
lack of very bright sources in NGC1291. No sources above 3 x 10^38 ergs/s were
found in NGC1291 when ~7 were expected from scaling from NGC4697 and NGC1553.
The cumulative L_{X,stellar}/L_B value including only sources below 1.0 x 10^38
ergs/s is remarkably similar between NGC1291 and NGC4697, if a recent surface
brightness fluctuation-determined distance is assumed for NGC4697. If this is a
common feature of the LMXB population in early-type systems, it might be used
as a distance indicator. Finally, a bright, variable (1.6-3.1 x 10^39 ergs/s)
source was detected at the optical center of the galaxy. Its spectrum shows
excess soft emission superimposed on a highly absorbed power law component,
similar to what has been found in several other low luminosity AGN (ABRIDGED).Comment: 13 pages in emulateapj5 style with 11 embedded Postscript figures;
minor revisions since last version; accepted by Ap
UV properties of early-type galaxies in the Virgo cluster
We study the UV properties of a volume limited sample of early-type galaxies
in the Virgo cluster combining new GALEX far- (1530 A) and near-ultraviolet
(2310 A) data with spectro-photometric data available at other wavelengths. The
sample includes 264 ellipticals, lenticulars and dwarfs spanning a large range
in luminosity (M(B)<-15). While the NUV to optical or near-IR color magnitude
relations (CMR) are similar to those observed at optical wavelengths, with a
monotonic reddening of the color index with increasing luminosity, the (FUV-V)
and (FUV-H) CMRs show a discontinuity between massive and dwarf objects. An
even more pronounced dichotomy is observed in the (FUV-NUV) CMR. For
ellipticals the (FUV-NUV) color becomes bluer with increasing luminosity and
with increasing reddening of the optical or near-IR color indices. For the
dwarfs the opposite trend is observed. These observational evidences are
consistent with the idea that the UV emission is dominated by hot, evolved
stars in giant systems, while in dwarf ellipticals residual star formation
activity is more common.Comment: 5 pages, 2 figures, 1 table. Accepted for publication in
Astrophysical Journal Letter
Where is the coronal line region in active galactic nuclei?
We report the new finding that type 1 Seyfert nuclei (S1s) have excess
[FeVII]6087 emission with respect to type 2s (S2s). The S1s exhibit broad
emission lines which are attributed to ionized gas within 1 pc of the black
hole, whereas the S2s do not show such broad lines. The current unified model
of active galactic nuclei explains this difference as that the central 1 pc
region in the S2s is hidden from the line of sight by a dusty torus if we
observe it from a nearly edge-on view toward the torus. Therefore, our finding
implies that the coronal line region (CLR) traced by the [FeVII]6087 emission
resides in the inner wall of such dusty tori. On the other hand, the frequency
of occurrence of the CLR in the optical spectra is nearly the same between the
S1s and the S2s. Moreover, some Seyfert nuclei exhibit a very extended (~ 1
kpc) CLR. All these observational results can be unified if we introduce a
three-component model for the CLR; 1) the inner wall of the dusty torus, 2) the
clumpy ionized region associated with the narrow line region at distance from ~
10 to ~ 100 pc, and 3) the extended ionized region at distance ~ 1 kpc.Comment: 10 pages, 3 figures, aaspp4.sty. To appear in ApJ Letter
GALEX UV Color Relations for Nearby Early-Type Galaxies
We use GALEX/optical photometry to construct color-color relationships for
early-type galaxies sorted by morphological type. We have matched objects in
the GALEX GR1 public release and the first IR1.1 internal release, with the RC3
early-type galaxies having a morphological type -5.5<T<-1.5 with mean error in
T<1.5, and mean error on (B-V)T<0.05. After visual inspection of each match, we
are left with 130 galaxies with a reliable GALEX pipeline photometry in the
far-UV and near-UV bands. This sample is divided into Ellipticals (-5.5<T<-3.5)
and Lenticulars (-3.5<T<-1.5). After correction for the Galactic extinction,
the color-color diagrams FUV-NUV vs. (B-V)_{Tc} are plotted for the two
subsamples. We find a tight anti-correlation between the FUV-NUV and (B-V)_{Tc}
colors for Ellipticals, the UV color getting bluer when the (B-V)_{Tc} get
redder. This relationship very likely is an extension of the color-metallicity
relationship into the GALEX NUV band. We suspect that the main source of the
correlation is metal line blanketing in the NUV band. The FUV-NUV vs B-V
correlation has larger scatter for lenticular galaxies; we speculate this
reflects the presence of low level star formation. If the latter objects (i.e.
those that are blue both in FUV-NUV and B-V) are interpreted as harboring
recent star formation activity, this would be the case for a few percent (~4%)
of Ellipticals and ~15% of Lenticulars; this would make about 10% of early-type
galaxies with residual star formation in our full sample of 130 early-type
galaxies. We also plot FUV-NUV vs. the Mg_2 index and central velocity
dispersion. We find a tight anti-correlation between FUV-NUV and the Mg_2
index(...).Comment: 25 pages, 5 figures, accepted for publication in ApJS (abstract
abridged), typos corrected in section 2.
Galactic Structure Toward the Carina Tangent
This investigation presents a photometric study of the Galactic structure
toward the Carina arm tangent. The field is located between 280 deg and 286 deg
galactic longitude and -4 deg to 4 deg galactic latitude. All currently
available uvbybeta data is used to obtain homogeneous color excesses and
distances for more than 260 stars of spectral types O to G. We present revised
distances and average extinction for the open clusters and cluster candidates
NGC 3293, NGC 3114, Loden 46 and Loden 112. The cluster candidate Loden 112
appears to be a very compact group at a true distance modulus of 11.06 +\- 0.11
(s.e.) (1629 +84,-80 pc), significantly closer than previous estimates. We
found other OB stars at that same distance and, based on their proper motions,
suggest a new OB association at coordinates 282 deg < l < 285 deg, -2 deg < b <
2 deg. Utilizing BV photometry and spectral classification of the known O-type
stars in the very young open cluster Wd 2 we provide a new distance estimate of
14.13 +\-0.16 (s.e.) (6698 +512,-475 pc), in excellent agreement with recent
distance determinations to the giant molecular structures in this direction. We
also discuss a possible connection between the HII region RCW 45 and the
highly-reddened B+ star CPD -55 3036 and provide a revised distance for the
luminous blue variable HR Car.Comment: accepted to PAS
Iron is not Depleted in High-Ionization Nuclear Emission-Line Regions of Active Galactic Nuclei
In order to examine whether or not high-ionization nuclear emission-line
regions (HINERs) in narrow-line regions of active galactic nuclei are dusty, we
focus on two high-ionization forbidden emission lines, [Fe VII]6087 and [Ne
V]3426. We perform photoionization model calculations to investigate possible
dependences of the flux ratio of [Fe VII]6087/[Ne V]3426 on various gas
properties, in order to investigate how useful this flux ratio to explore the
dust abundances in HINERs. Based on our photoionization model calculations, we
show that the observed range of the flux ratio of [Fe VII]6087/[Ne V]3426 is
consistent with the dust-free models while that is hard to be explained by the
dusty models. This suggests that iron is not depleted at HINERs, which implies
that the HINERs are not dusty. This results is consistent with the idea that
the HINERs are located closer than the dust-sublimation radius (i.e., inner
radius of dusty tori) and thus can be hidden by dusty tori when seen from a
edge-on view toward the tori, which has been also suggested by the AGN-type
dependence of the visibility of high-ionization emission lines.Comment: 8 pages including 4 figures, to appear in The Astronomical Journa
Near-infrared spectroscopy of stellar populations in nearby spiral galaxies
We present high spatial resolution, medium spectral resolution near-infrared
(NIR) H- and K-band long-slit spectroscopy for a sample of 29 nearby (z < 0.01)
inactive spiral galaxies, to study the composition of their NIR stellar
populations. These spectra contain a wealth of diagnostic stellar absorption
lines, e.g. MgI 1.575 micron, SiI 1.588 micron, CO (6-3) 1.619 micron, MgI
1.711 micron, NaI 2.207 micron, CaI 2.263 micron and the 12CO and 13CO
bandheads longward of 2.29 micron. We use NIR absorption features to study the
stellar population and star formation properties of the spiral galaxies along
the Hubble sequence, and we produce the first high spatial resolution NIR
HK-band template spectra for low redshift spiral galaxies along the Hubble
sequence. These templates will find applications in a variety of galaxy
studies. The strength of the absorption lines depends on the luminosity and/or
temperature of stars and, therefore, spectral indices can be used to trace the
stellar population of galaxies. The entire sample testifies that the evolved
red stars completely dominate the NIR spectra, and that the hot young star
contribution is virtually nonexistent.Comment: 13 pages, 5 figures. Accepted to MNRAS. arXiv admin note: text
overlap with arXiv:astro-ph/040313
Design concepts for the Cherenkov Telescope Array CTA: an advanced facility for ground-based high-energy gamma-ray astronomy
Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA