587 research outputs found
Stellar abundances and molecular hydrogen in high-redshift galaxies -the far-ultraviolet view
FUSE spectra of star-forming regions in nearby galaxies are compared to
composite spectra of Lyman-break galaxies (LBGs), binned by strength of Lyman
alpha emission and by mid-UV luminosity. Several far-UV spectral features,
including lines dominated by stellar wind and by photospheric components, are
very sensitive to stellar abundances. Their measurement in Lyman-break galaxies
is compromised by the strong interstellar absorption features, allowing in some
cases only upper limits. The derived C and N abundances in the LBGs are no
higher than half solar (scaled to oxygen abundance for comparison with
emission-line analyses), independent of the strength of Lyman alpha emission. P
V absorption indicates abundances as low as 0.1 solar, with an upper limit near
0.4 solar in the reddest and weakest-emission galaxies. Unresolved interstellar
absorption components would further lower the derived abundances. Trends of
line strength, and derived abundances, are stronger with mid-UV luminosity than
with Lyman-alpha strength. H2 absorption in the Lyman and Werner bands is very
weak in the LBGs. Template H2 absorption spectra convolved to appropriate
resolution show that strict upper limits N(H2)< 10^18 cm^-2 apply in all cases,
with more stringent values appropriate for the stronger-emission composites and
for mixes of H2 level populations like those on Milky Way sight lines. Since
the UV-bright regions are likely to be widespread in these galaxies, these
results rule out massive diffuse reservoirs of H2, and suggest that the
dust/gas ratio is already fairly large at z~3.Comment: Astron J., in press (June 2006
Far-Ultraviolet Spectroscopy of Star-Forming Regions in Nearby Galaxies: Stellar Populations and Abundance Indicators
We present FUSE spectroscopy and supporting data for star-forming regions in
nearby galaxies, to examine their massive-star content and explore the use of
abundance and population indicators in this spectral range for high-redshift
galaxies. New far-ultraviolet spectra are shown for four bright H II regions in
M33 (NGC 588, 592, 595, and 604), the H II region NGC 5461 in M101, and the
starburst nucleus of NGC 7714, supplemented by the very-low-metallicity galaxy
I Zw 18. In each case, we see strong Milky Way absorption systems from H2, but
intrinsic absorption within each galaxy is weak or undetectable, perhaps
because of the "UV bias" in which reddened stars which lie behind
molecular-rich areas are also heavily reddened. We see striking changes in the
stellar-wind lines from these populations with metallicity, suggesting that C
II, C III, C IV, N II, N III, and P V lines are potential tracers of stellar
metallicity in star-forming galaxies. Three of these relations - involving N
IV, C III, and P V - are nearly linear over the range from O/H=0.05--0.8 solar.
The major difference in continuum shapes among these systems is that the giant
H II complex NGC 604 has a stronger continuum shortward of 950 A than any other
object in this sample. Small-number statistics would likely go in the other
direction; we favor this as the result of a discrete star-forming event ~3 Myr
ago, as suggested by previous studies of its stellar population. (Supported by
NASA grant NAG5-8959)Comment: Astronomical Journal, in press (July 2004). 8 figures; before
publication, full-resolution figures are available as a single PDF file from
http://www.astr.ua.edu/keel/fusefigs.pd
Properties of H II Regions in the Centers of Nearby Galaxies
As part of an optical spectroscopic survey of nearby, bright galaxies, we
have identified a sample of over 200 emission-line nuclei having optical
spectra resembling those of giant extragalactic H II regions. Such "H II
nuclei," powered by young, massive stars, are found in a substantial fraction
of nearby galaxies, especially those of late Hubble type. This paper summarizes
the observational characteristics of H II nuclei, contrasts the variation of
their properties with Hubble type, and compares the nuclear H II regions with
those found in galaxy disks. Similarities and differences between H II nuclei
and luminous starburst nuclei are additionally noted.Comment: To appear in the Astrophysical Journal. LaTex, 37 pages including 15
postscript figures. AAStex macros include
Properties of five z~0.3-0.4 confirmed LyC leakers: VLT/XShooter observations
Using new VLT/XShooter spectral observations we analyse the physical
properties of five z~0.3-0.4 confirmed LyC leakers. Strong resonant MgII
2796,2803 emission lines (I(2796,2803)/I(Hbeta)=10-38 per cent) and
non-resonant FeII* 2612,2626 emission lines are observed in spectra of five and
three galaxies, respectively. We find high electron densities Ne~400cm-3,
significantly higher than in typical low-z, but comparable to those measured in
z~2-3 star-forming galaxies. The galaxies have a mean value of logN/O=-1.16,
close to the maximum values found for star-forming (SF) galaxies in the
metallicity range of 12+logO/H=7.7-8.1. All 11 low-z LyC emitting galaxies
found by Izotov et al. (2016, 2018), including the ones considered in the
present study, are characterised by high EW(Hbeta)~200-400A, high ionisation
parameter (log(U)=-2.5 to -1.7), high average ionising photon production
efficiency \xi= 10^{25.54} Hz erg-1 and hard ionising radiation. On the BPT
diagram we find the same offset of our leakers from low- main-sequence SFGs
as that for local analogues of LBGs and extreme SF galaxies at z~2-3. We
confirm the effectiveness of the HeI emission lines diagnostics proposed by
Izotov et al. (2017) in searching for LyC leaker candidates and find that their
intensity ratios correspond to those in a median with low neutral hydrogen
column density N(HI)=10^{17}-5x10^{17} cm-2 that permit leakage of LyC
radiation, likely due to their density-bounded HII regions.Comment: 20 pages, 10 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
An imaging and spectroscopic study of the very metal-deficient blue compact dwarf galaxy Tol 1214--277
We present a spectrophotometric study based on VLT/FORS I observations of one
of the most metal-deficient blue compact dwarf (BCD) galaxies known, Tol
1214-277 (Z ~ Zsun/25). The data show that roughly half of the total luminosity
of the BCD originates from a bright and compact starburst region located at the
northeastern tip of a faint dwarf galaxy with cometary appearance. The
starburst has ignited less than 4 Myr ago and its emission is powered by
several thousands O7V stars and ~ 170 late-type nitrogen Wolf-Rayet stars
located within a compact region with < 500 pc in diameter. For the first time
in a BCD, a relatively strong [Fe V] 4227 emission line is seen which together
with intense He II 4686 emission indicates the presence of a very hard
radiation field in Tol 1214-277. We argue that this extraordinarily hard
radiation originates from both Wolf--Rayet stars and radiative shocks in the
starburst region. The structural properties of the low-surface-brightness (LSB)
component underlying the starburst have been investigated by means of surface
photometry down to 28 B mag/sq.arcsec. We find that, for a surface brightness
level fainter than ~ 24.5 B mag/sq.arcsec, an exponential fitting law provides
an adequate approximation to its radial intensity distribution. The broad-band
colors in the outskirts of the LSB component of Tol 1214-277 are nearly
constant and are consistent with an age below one Gyr. This conclusion is
supported by the comparison of the observed spectral energy distribution (SED)
of the LSB host with theoretical SEDs.Comment: 17 pages, 11 Postscript figures, uses emulateapj.sty, to appear in
Astronomical Journa
Peculiar motions of the gas at the centre of the barred galaxy UGC 4056
We derive the circular velocity curves of the gaseous and stellar discs of
UGC 4056, a giant barred galaxy with an active galactic nucleus (AGN). We
analyse UGC 4056 using the 2D spectroscopy obtained within the framework of the
Mapping Nearby Galaxies at APO (MaNGA) survey. Using images and the colour
index g-r from the Sloan Digital Sky Survey (SDSS), we determined the tilt of
the galaxy, which allows us to conclude that the galaxy rotates clockwise with
trailing spiral arms. We found that the gas motion at the central part of the
UGC 4056 shows peculiar features. The rotation velocity of the gaseous disc
shows a bump within around three kiloparsecs while the rotation velocity of the
stellar disc falls smoothly to zero with decreasing galactocentric distance. We
demonstrate that the peculiar radial velocities in the central part of the
galaxy may be caused by the inflow of the gas towards the nucleus of the
galaxy. The unusual motion of the gas takes place at the region with the
AGN-like radiation and can be explained by the gas response to the bar
potential.Comment: 11 pages, 10 figures, 1 table, accepted for publication in A&
Stellar Iron Abundances at the Galactic Center
We present measurements of [Fe/H] for six M supergiant stars and three giant
stars within 0.5 pc of the Galactic Center (GC) and one M supergiant star
within 30 pc of the GC. The results are based on high-resolution (lambda /
Delta lambda =40,000) K-band spectra, taken with CSHELL at the NASA Infrared
Telescope Facility.We determine the iron abundance by detailed abundance
analysis,performed with the spectral synthesis program MOOG.The mean [Fe/H] of
the GC stars is determined to be near solar,[Fe/H] = +0.12 0.22. Our
analysis is a differential analysis, as we have observed and applied the same
analysis technique to eleven cool, luminous stars in the solar neighborhood
with similar temperatures and luminosities as the GC stars. The mean [Fe/H] of
the solar neighborhood comparison stars, [Fe/H] = +0.03 0.16, is similar
to that of the GC stars. The width of the GC [Fe/H] distribution is found to be
narrower than the width of the [Fe/H] distribution of Baade's Window in the
bulge but consistent with the width of the [Fe/H] distribution of giant and
supergiant stars in the solar neighborhood.Comment: 41 pages, 9 figures, ApJ, in pres
Role of simian virus 40 in cancer incidence in solid organ transplant patients
Transplant recipients have an increased risk of developing cancer in comparison with the general population. We present here data on cancer development in transplanted subjects who received organs from donors whose DNA was previously examined for the genomic insertion of Simian Virus 40 (SV40). Active follow-up of 387 recipients of solid organs donated by 134 donors, not clinically affected by cancer, was performed through the National Transplant Center (NTC). The average length of follow-up after transplant was 671±219 days (range 0–1085 days). Out of 134 proposed donors, 120 were utilised for organ donation. Of these, 12 (10%) were classified as positive for SV40 genomic insertion. None of the 41 recipients of organs from SV40 positive donors developed a tumour during the follow-up. In all, 11 recipients of organs given by SV40 negative donors developed a tumour (cancer incidence: 0.015 per year). In conclusion, cancer rates observed in our study are comparable to what reported by the literature in transplanted patients. Recipients of solid organs from SV40 positive donors do not have an increased risk of cancer after transplant. The role of SV40 in carcinogenesis in transplanted patients may be minimal
Intracluster Planetary Nebulae in the Virgo Cluster I. Initial Results
We report the initial results of a survey for intracluster planetary nebulae
in the Virgo Cluster. In two 16' x 16' fields, we identify 69 and 16
intracluster planetary nebula candidates, respectively. In a third 16' x 16'
field near the central elliptical galaxy M87, we detect 75 planetary nebula
candidates, of which a substantial fraction are intracluster in nature. By
examining the number of the planetaries detected in each field and the shape of
the planetary nebula luminosity function, we show that 1) the intracluster
starlight of Virgo is distributed non-uniformly, and varies between subclumps A
and B, 2) the Virgo Cluster core extends ~3 Mpc in front of M87, and thus is
elongated along the line-of-sight, and 3) a minimum of 22% of Virgo's stellar
luminosity resides between the galaxies in our fields, and that the true number
may be considerably larger. We also use our planetary nebula data to argue that
the intracluster stars in Virgo are likely derived from a population that is of
moderate age and metallicity.Comment: 20 pages, 3 figures, accepted for publication in the Astrophysical
Journal. Paper is also available at
http://www.astro.psu.edu/users/johnf/Text/research.htm
Using Cepheids to determine the galactic abundance gradient I. The solar neighbourhood
A number of studies of abundance gradients in the galactic disk have been
performed in recent years. The results obtained are rather disparate: from no
detectable gradient to a rather significant slope of about -0.1 dex kpc -1. The
present study concerns the abundance gradient based on the spectroscopic
analysis of a sample of classical Cepheids. These stars enable one to obtain
reliable abundances of a variety of chemical elements. Additionally, they have
well determined distances which allow an accurate determination of abundance
distributions in the galactic disc. Using 236 high resolution spectra of 77
galactic Cepheids, the radial elemental distribution in the galactic disc
between galactocentric distances in the range 6-11 kpc has been investigated.
Gradients for 25 chemical elements (from carbon to gadolinium) are derived...Comment: 28 pages, 14 postscript figures, LaTeX, uses Astronomy and
Astrophysics macro aa.cls, graphicx package, to be published in Astronomy and
Astrophysics (2002) also available at
http://www.iagusp.usp.br/~maciel/index.htm
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