arXiv subjects

B. Y. Welsh

Publications and source records attributed to B. Y. Welsh.

30 records · Page 2Linked to original sources

arXiv2004

A comparative study of the spatial distribution of ultraviolet and far-infrared fluxes from M 101

The total ultraviolet (UV) flux (from 1412 to 2718 AA) of M 101 is compared on a pixel-to-pixel basis with the total far-infrared (FIR) flux (from 60 to 170 micron), using the maps of the galaxy taken by GALEX in the near-UV and far-UV and by ISOPHOT at 60, 100 and 170 micron. The main result of this investigation is the discovery of a tight dependence of the FIR/UV ratio on radius, with values monotonically decreasing from around 4 in the nuclear region to nearly zero towards the edge of the optical disk. Although the tightness of this dependence is in part attributable to resolution effects, the result is consistent with the presence of a large-scale distribution of diffuse dust having a face-on optical depth which decreases with radius and which dominates over the more localized variations in opacity between the arm and interarm regions. We also find a trend for the FIR/UV ratio to take on higher values in the regions of diffuse interarm emission than in the spiral-arm regions, at a given radius. This is interpreted quantitatively in terms of the escape probability of UV photons from spiral arms and their subsequent scattering in the interarm regions, and in terms of the larger relative contribution of optical photons to the heating of the dust in the interarm regions.

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arXiv2004

New Constraints on the Star Formation Histories and Dust Attenuation of Galaxies in the Local Universe from GALEX

We derive a variety of physical parameters including star formation rates (SFRs), dust attenuation and burst mass fractions for 6472 galaxies observed by the Galaxy Evolution Explorer (GALEX) and present in the SDSS DR1 main spectroscopic sample. Parameters are estimated in a statistical way by comparing each observed broad-band SED (two GALEX and five SDSS bands) with an extensive library of model galaxy SEDs, which cover a wide range of star formation histories and include stochastic starbursts. We compare the constraints derived using SDSS bands only with those derived using the combination of SDSS and GALEX photometry. We find that the addition of the GALEX bands leads to significant improvement in the estimation of both the dust optical depth and the star formation rate over timescales of 100 Myr to 1 Gyr in a galaxy. We are sensitive to SFRs as low as 10^{-3} M_sun/yr, and we find that low levels of star formation (SF) are mostly associated with early-type, red galaxies. The least massive galaxies have ratios of current to past-averaged SF rates (b-parameter) consistent with constant SF over a Hubble time. For late-type galaxies, this ratio on average decreases with mass. We find that b correlates tightly with NUV-r color, implying that the SF history of a galaxy can be constrained on the basis of the NUV-r color alone. The fraction of galaxies that have undergone a significant starburst episode within the last 1 Gyr steeply declines with mass-from ~20% for galaxies with ~10^8 M_sun to ~5% for ~10^11 M_sun galaxies.

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arXiv2004

Systematics of the Ultraviolet Rising flux in a GALEX/SDSS sample of Early-type Galaxies

We present ultraviolet photometry for a sample of morphologically early-type galaxies selected by matching the Sloan Digital Sky Survey Data Release 1 with the GALEX Medium and All-sky Imaging Surveys. We obtain a working sample of 1032 early-type galaxies with GALEX FUV detections, SDSS spectroscopy, and $z<0.2$. Using the SDSS spectra to identify galaxies with even weak star formation or evidence of AGN, and further removing galaxies with any evidence of non early-type morphology, we derive a final sample of 172 red quiescent early-type galaxies. We find that the $FUV-r$ color has a full range of 5 mag. Plotting against the $FUV-r$ color the metallicity sensitive Lick $\rm Mg_2$ and D4000 indices, and the stellar velocity dispersion, we find no correlation between our measurement of UV rising flux, and any parameter sensitive to metallicity.

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arXiv2004

GALEX observations of the UV surface brightness and color profiles of the Local Group elliptical galaxy M32 (NGC221)

M32, the compact elliptical-galaxy companion to the Andromeda spiral galaxy has been imaged by the Galaxy Evolution Explorer (GALEX) in two ultraviolet bands, centered at ~1500 (FUV) and 2300 Angstroms (NUV). The imaging data have been carefully decomposed so as to properly account for the complicated background contamination from the disk of M31. We have derived the surface brightness and color profiles finding a slightly positive color gradient of Delta(FUV-B)/Delta log(r)=+0.15+/-0.03 within one effective radius. Earlier data from the Ultraviolet Imaging Telescope suggested that M32 had an extremely large (negative) FUV-optical color gradient (Delta(FUV-B)/Delta log(r)<-2), inverted with respect to the majority of gradients seen in giant elliptical galaxies. Our new results show that, despite of its very low UV-upturn, M32 has similar UV properties to those observed in luminous elliptical galaxies.

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arXiv2004

GALEX Observations of the Ultraviolet Halos of NGC 253 and M82

We present Galaxy Evolution Explorer (GALEX) images of the prototypical edge-on starburst galaxies M82 and NGC253. Our initial analysis is restricted to the complex of ultraviolet (UV) filaments in the starburst-driven outflows in the galaxy halos. The UV luminosities in the halo are too high to be provided by shock-heated or photoionized gas except perhaps in the brightest filaments in M82, suggesting that most of the UV light is the stellar continuum of the starburst scattered into our line of sight by dust in the outflow. This interpretation agrees with previous results from optical imaging polarimetry in M82. The morphology of the UV filaments in both galaxies shows a high degree of spatial correlation with H-alpha and X-ray emission. This indicates that these outflows contain cold gas and dust, some of which may be vented into the intergalactic medium (IGM). UV light is seen in the ``H-alpha cap'' 11 kpc North of M82. If this cap is a result of the wind fluid running into a pre-existing gas cloud, the gas cloud contains dust and is not primordial in nature but was probably stripped from M82 or M81. If starburst winds efficiently expel dust into the IGM, this could have significant consequences for the observation of cosmologically distant objects.

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arXiv2004

The On-Orbit Performance of the Galaxy Evolution Explorer

We report the first year on-orbit performance results for the Galaxy Evolution Explorer (GALEX), a NASA Small Explorer that is performing a survey of the sky in two ultraviolet bands. The instrument comprises a 50 cm diameter modified Ritchey-Chretien telescope with a 1.25 degree field of view, selectable imaging and objective grism spectroscopic modes, and an innovative optical system with a thin-film multilayer dichroic beam splitter that enables simultaneous imaging by a pair of photon counting, microchannel plate, delay line readout detectors. Initial measurements demonstrate that GALEX is performing well, meeting its requirements for resolution, efficiency, astrometry, bandpass definition and survey sensitivity.

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arXiv2004

Panoramic GALEX FUV and NUV imaging of M31 and M33

We present Galaxy Evolution Explorer (GALEX) far-UV and near-UV mosaic observations covering the entirety of M31 and M33. For both targets, we measure the decline of surface brightness (in FUV and NUV) and changes in FUV--NUV color as a function of galactocentric radius. These UV radial profiles are compared to the distribution of ionized gas traced by H-alpha emission. We find that the extent of the UV emission, in both targets, is greater than the extent of the observed HII regions and diffuse ionized gas. We determine the ultraviolet diffuse fraction in M33 using our FUV observations and compare it to the H-alpha diffuse fraction obtained from wide-field narrow-band imaging. The FUV diffuse fraction appears to be remarkably constant near 0.65 over a large range in galactocentric radius, with departures to higher values in circumnuclear regions and, most notably, at the limit of the H-alpha disk. We suggest that the increase in FUV diffuse fraction at large galactocentric radii could indicate that a substantial portion of the diffuse emission beyond this point is not generated in situ but rather scattered from dust, after originating in the vicinity of the disk's outermost HII regions. Radial variation of the H-alpha diffuse fraction was also measured. We found the H-alpha diffuse fraction generally near 0.4 but rising toward the galaxy center, up to 0.6. We made no attempt to correct our diffuse fraction measurements for position-dependent extinction, so the quoted values are best interpreted as upper limits given the plausibly higher extinction for stellar clusters relative to their surroundings.

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arXiv2004

Recent Star Formation in the Extreme Outer Disk of M83

Ultraviolet imaging with the Galaxy Evolution Explorer (GALEX) has revealed an extensive sample of UV-bright stellar complexes in the extreme outer disk of M83, extending to about four times the radius where the majority of HII regions are detected (R_HII = 5.1' or 6.6 kpc). These sources are typically associated with large-scale filamentary HI structures in the warped outer disk of M83, and are distributed beyond the galactocentric radii at which molecular ISM has yet been detected. We present measured properties of these stellar complexes, including FUV and NUV magnitudes and local gas surface density. Only a subset of the outer disk UV sources have corresponding HII regions detected in H-alpha imaging, consistent with a sample of mixed age in which some sources are a few Myr old and others are much more evolved (~ 10^8 yr).

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arXiv2004

Large-Amplitude Ultraviolet Variations in the RR Lyrae Star ROTSE-I J143753.84+345924.8

The NASA Galaxy Evolution Explorer (GALEX) satellite has obtained simultaneous near and far ultraviolet light curves of the ROTSE-I Catalog RR Lyrae ab-type variable star J143753.84+345924.8. A series of 38 GALEX Deep Imaging Survey observations well distributed in phase within the star's 0.56432d period shows an AB=4.9mag variation in the far UV (1350-1750A) band and an AB=1.8mag variation in the near UV (1750-2750A) band, compared with only a 0.8mag variation in the broad, unfiltered ROTSE-I (4500-10000A) band. These GALEX UV observations are the first to reveal a large RR Lyrae amplitude variation at wavelengths below 1800A. We compare the GALEX and ROTSE-I observations to predictions made by recent Kurucz stellar atmosphere models. We use published physical parameters for the comparable period (0.57433d), well-observed RR Lyrae star WY Antliae to compute predicted FUV, NUV, and ROTSE-I light curves for J143753.84+345924.8. The observed light curves agree with the Kurucz predictions for [Fe/H]=-1.25 to within AB=0.2mag in the GALEX NUV and ROTSE-I bands, and within 0.5mag in the FUV. At all metallicities between solar and one hundredth solar, the Kurucz models predict 6-8mag of variation at wavelengths between 1000-1700A. Other variable stars with similar temperature variations, such as Cepheids, should also have large-amplitude FUV light curves, observable during the ongoing GALEX imaging surveys.

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arXiv2004

Intermediate-velocity gas observed towards the Shajn 147 SNR

(Abridged) We present high-resolution spectra of the interstellar NaI and CaII absorption lines for 3 early-type stars along the line-of-sight towards the Shajn 147 SNR. These are supplemented with far-UV aborption spectra of HD 36665 and HD 37318 recorded with the NASA FUSE satellite. The observations reveal intermediate-velocity (IV) absorption features at V_helio = +92 km/s towards HD 37318 and -65 & -52 km/s towards HD 36665. These IV components can be associated with the expansion of the SNR. The observations suggest that the IV component at +92 km/s towards HD 37318 is composed mainly of warm and ionized gas. The two IV components observed towards HD 36665 are likely composed of both neutral and ionized gas shells. Highly ionized gas was detected in the OVI (1032Å) absorption line at +40 km/s towards both stars. This hot gas component is characterized by a columnn density ratio (N(CIV)/N(OVI) < 0.27) consistent with that predicted by current models of evolved SNRs, but we cannot preclude its origin in the ISM. Column-density ratios have been derived for the IV gas components detected towards S147, as well as for fast-moving gas observed towards two other SNRs in order to gain some insight into the behavior of element abundances in the disturbed interstellar gas associated with these regions. In all cases except for Na and Ca, these elements appear to be present with near-solar abundance ratios. (Abridged)

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arXiv2001

Abundances of Deuterium, Oxygen, and Nitrogen in the Local Interstellar Medium: Overview of First Results from the Far Ultraviolet Spectroscopic Explorer Mission

Observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) have been used to determine the column densities of D I, O I, and N I along seven sight lines that probe the local interstellar medium (LISM) at distances from 37 pc to 179 pc. Five of the sight lines are within the Local Bubble and two penetrate the surrounding H I wall. Reliable values of N(H I) were determined for five of the sight lines from HST data, IUE data, and published EUVE measurements. The weighted mean of D I/H I for these five sight lines is (1.52 +/- 0.08) x10-5 (1 sigma uncertainty in the mean). It is likely that the D I/H I ratio in the Local Bubble has a single value. The D I/O I ratio for the five sight lines within the Local Bubble is (3.76 +/- 0.20) x10-2. It is likely that the O I column densities can serve as a proxy for H I in the Local Bubble. The weighted mean for O I/H I for the seven FUSE sight lines is (3.03 +/-0.21) x10-4, comparable to the weighted mean (3.43 +/- 0.15) x10-4 reported for 13 sight lines probing larger distances and higher column densities (Meyer et al. 1998, Meyer 2001). The FUSE weighted mean of N I/H I for the five sight lines is half that reported by Meyer et al. (1997) for seven sight lines with larger distances and higher column densities. This result combined with the variability of O I/N I (six sight lines) indicates that at the low column densities found in the LISM, nitrogen ionization balance is important. Thus, unlike O I, N I cannot be used as a proxy for H I or as a metallicity indicator in the LISM. Subject Headings: cosmology: observations- ISM: abundances- ISM: evolution - Galaxy:abundances-Ultraviolet:ISM

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arXiv2001

Observations of O VI Emission from the Diffuse Interstellar Medium

We report the first Far Ultraviolet Spectroscopic Explorer (FUSE) measurements of diffuse O VI (lambda,lambda 1032,1038) emission from the general diffuse interstellar medium outside of supernova remnants or superbubbles. We observed a 30arcsec x 30arcsec region of the sky centered at l = 315 and b = -41. From the observed intensities (2930+/-290(random)+/-410(systematic) and 1790+/-260(random)+/-250(systematic) photons/cm/cm/s/sr in the 1032 and 1038 Angstrom emission lines, respectively), derived equations, and assumptions about the source location, we calculate the intrinsic intensity, electron density, thermal pressure, and emitting depth. The intensities are too large for the emission to originate solely in the Local Bubble. Thus, we conclude that the Galactic thick disk and lower halo also contribute. High velocity clouds are ruled out because there are none near the pointing direction. The calculated emitting depth is small, indicating that the O VI-bearing gas fills a small volume. The observations can also be used to estimate the cooling rate of the hot interstellar medium and constrain models. The data also yield the first intensity measurement of the C II 3s2 S1/2 to 2p2 P3/2 emission line at 1037 Angstroms and place upper limits on the intensities of ultraviolet line emission from C I, C III, Si II, S III, S IV, S VI, and Fe III.

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