61 research outputs found
Far Ultraviolet Spectroscopic Explorer Observations of a Supernova Remnant in the Line of Sight to HD 5980 in the Small Magellanic Cloud
We report a detection of far ultraviolet absorption from the supernova
remnant SNR 0057 - 7226 in the Small Magellanic Cloud (SMC). The absorption is
seen in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the
LBV/WR star HD 5980. Absorption from O VI 1032 and C III 977 is seen at a
velocity of +300 km/s with respect to the Galactic absorption lines, +170 km/s
with respect to the SMC absorption. The O VI 1038 line is contaminated by H_2
absorption, but is present. These lines are not seen in the FUSE spectrum of
Sk80, only ~1' (~17 pc) away from HD 5980. No blue-shifted O VI 1032 absorption
from the SNR is seen in the FUSE spectrum. The O VI 1032 line in the SNR is
well described by a Gaussian with FWHM=75 km/s. We find log N(O
VI)=14.33-14.43, which is roughly 50% of the rest of the O VI column in the SMC
(excluding the SNR) and greater than the O VI column in the Milky Way halo
along this sight line. The N(C IV)/N(O VI) ratio for the SNR absorption is in
the range of 0.12-0.17, similar to the value seen in the Milky Way disk, and
lower than the halo value, supporting models in which SNRs produce the highly
ionized gas close to the plane of the Galaxy, while other mechanisms occur in
the halo. The N(C IV)/N(O VI) ratio is also lower than the SMC ratio along this
sight line, suggesting that other mechanisms contribute to the creation of the
global hot ionized medium in the SMC. The O VI, C IV, and Si IV apparent column
density profiles suggest the presence of a multi-phase shell followed by a
region of higher temperature gas.Comment: 7 pages, 3 figures, 2 tables, uses emulateapj5.sty. Accepted for
publication in ApJ Letter
The Massive Stellar Population in the Diffuse Ionized Gas of M33
We compare Far-UV, H alpha, and optical broadband images of the nearby spiral galaxy M33, to investigate the massive stars associated with the diffuse ionized gas. The H-alpha/FUV ratio is higher in HII regions than in the DIG, possibly indicating that an older population ionizes the DIG. The broad-band colors support this conclusion. The HII region population is consistent with a young burst, while the DIG colors resemble an older population with constant star formation. Our results indicate that there may be enough massive field stars to ionize the DIG, without the need for photon leakage from HII regions
Diffuse Ionized Gas in Three Sculptor Group Galaxies
We present a study of the diffuse ionized gas (DIG) in three Sculptor group
galaxies: NGC 55, NGC 253, and NGC 300. The study is based on narrow band
imagery in H-alpha+[NII](6548+6583A) and [SII] (6717+6731A). We find that DIG
contributes 33 to 58% of the total H-alpha luminosity in these galaxies, or 30
to 54% after correcting for scattered light. We find that NGC 300 has a higher
fractional DIG luminosity than the other galaxies in our sample, but it is not
clear whether this is a significant difference or an effect of the high
inclination of the other galaxies. The diffuse emission, averaged over the
optical extent of the disk, has a face-on emission measure of 5 to 10 pc
cm^{-6}. The DIG is concentrated near HII regions, although significant
emission is seen at large distances from HII regions, up to 0.5 to 1 kpc. The
[SII]/(H-alpha+[NII]) ratio is enhanced in the DIG, typically around 0.3 to
0.5, compared to 0.2 for the HII regions in these galaxies. These properties
are similar to those measured for the DIG in the Milky Way and in other nearby
spirals. The line ratios, large-scale distribution, and energy requirement
suggest that photoionization is the dominant ionization mechanism.Comment: 19 pages, LaTex, 3 LaTex tables, 4 Postscript figures, 3 jpeg plates.
Plates also available at ftp://astro.nmsu.edu/pub/choopes/ , accepted for
publication in the Astronomical Journal. Replaced version includes plate
Deuterium Toward Two Milky Way Disk Stars: Probing Extended Sight Lines with the Far Ultraviolet Spectroscopic Explorer
We have carried out an investigation of the abundance of deuterium along two
extended sight lines through the interstellar medium (ISM) of the Galactic
disk. The data include Far Ultraviolet Spectroscopic Explorer (FUSE)
observations of HD 195965 (B1Ib) and HD 191877 (B0V), as well as Space
Telescope Imaging Spectrograph (STIS) observations of HD 195965. The distances
to HD 195965 and HD 191877, derived from spectroscopic parallax, are 794+/-200
pc and 2200+/-550 pc, respectively, making these the longest Galactic disk
sight lines in which deuterium has been investigated with FUSE. The higher
Lyman lines clearly show the presence of deuterium. We use a combination of
curve of growth analyses and line profile fitting to determine the DI abundance
toward each object. We also present column densities for OI and NI toward both
stars, and HI measured from Ly-alpha absorption in the STIS spectrum of HD
195965. The D/H ratios along these sight lines are lower than the average value
found with FUSE for the local interstellar medium (37 to 179 pc from the Sun).
These observations lend support to earlier detections of variation in D/H over
distances greater than a few hundred pc. The D/H and O/H values measured along
these sight lines support the expectation that the ISM is not well mixed on
distances of ~1000 pc.Comment: 32 pages, 18 figures. Abridged abstract. Accepted for publication in
ApJ. Uses emulateapj5.st
Ongoing Formation of Bulges and Black Holes in the Local Universe: New Insights from GALEX
We analyze a volume-limited sample of massive bulge-dominated galaxies with
data from both the Sloan Digital Sky Survey and the Galaxy Evolution Explorer
(GALEX) satellite. The galaxies have central velocity dispersions greater than
100 km/s and stellar surface mass densities that lie above the value where
galaxies transition from actively star forming to passive systems. The sample
is limited to redshifts 0.03<z<0.07. At these distances, the SDSS spectra
sample the light from the bulge-dominated central regions of the galaxies. The
GALEX NUV data provide high sensitivity to low rates of global star formation
in these systems. Our sample of bulge-dominated galaxies exhibits a much larger
dispersion in NUV-r colour than in optical g-r colour. Nearly all of the
galaxies with bluer NUV-r colours are AGN. Both GALEX images and SDSS colour
profiles demonstrate that the excess UV light is associated with an extended
disk. We find that galaxies with red outer regions almost never have a young
bulge or a strong AGN. Galaxies with blue outer regions have bulges and black
holes that span a wide range in age and accretion rate. Galaxies with young
bulges and strongly accreting black holes almost always have blue outer disks.
Our suggested scenario is one in which the source of gas that builds the bulge
and black hole is a low mass reservoir of cold gas in the disk.The presence of
this gas is a necessary, but not sufficient condition for bulge and black hole
growth. Some mechanism must transport this gas inwards in a time variable way.
As the gas in the disk is converted into stars, the galaxies will turn red, but
further gas infall can bring them back into the blue NUV-r sequence.(Abridged)Comment: 34 pages, 16 figures. Accepted for the GALEX special issue of ApJ
The Contribution of Field OB Stars to the Ionization of the Diffuse Ionized Gas in M33
(Abridged) We present a study of the ionizing stars associated with the
diffuse ionized gas (DIG) and HII regions in the nearby spiral galaxy M33. We
compare our Schmidt H-alpha image to the far-ultraviolet (FUV, 1520A) image
from the Ultraviolet Imaging Telescope (UIT). The H-alpha/FUV ratio is higher
in HII regions than in the DIG, suggesting an older population of ionizing
stars in the DIG. When compared to models of evolving stellar populations, the
N(Lyc)/FUV ratio in HII regions is consistent with a young burst, while the DIG
ratio resembles an older burst population, or a steady state population built
up by constant star formation. The UIT data is complimented with archival FUV
and optical images of a small portion of the disk of M33 obtained with WFPC2 on
HST. Using the HST FUV and optical photometry, we assign spectral types to the
stars observed in DIG and HII regions. The photometry indicates that ionizing
stars are present in the DIG. We compare the predicted ionizing flux with the
amount required to produce the observed H-alpha emission, and find that field
OB stars in the HST images can account for 40% +/- 12% of the ionization of the
DIG, while the stars in HII regions can provide 107% +/- 26% of the H-alpha
luminosity of the HII regions. We do not find any correlation between leakage
of ionizing photons and H-alpha luminosity for the HII regions in these HST
fields. If stellar photons alone are responsible for ionizing the DIG, the
current results are consistent with no or few ionizing photons escaping from
the galaxy.Comment: 30 pages, 7 figures, accepted for publication in Ap
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