arXiv subjects

J. S. Gallagher

Publications and source records attributed to J. S. Gallagher.

122 records · Page 7Linked to original sources

arXiv1999

Dwarf Elliptical Galaxies in the Perseus Cluster

Dwarf ellipticals have the lowest stellar densities of any galaxies, but paradoxically are most common in the densest regions of the universe, and are especially frequent in rich clusters of galaxies. Simple estimates suggest that low luminosity dwarf ellipticals should be subject to substantial tidal heating in rich clusters if most of their mass is in the form of normal stars. We have therefore undertaken a program to image dwarfs in clusters with the 3.5-m high performance WIYN, ARC and smaller, wide field telescopes. Our objectives include testing for evidence of tidal disruption in the form of asymmetries and searching for evidence of recent star formation which might be associated with the production of elliptical dwarfs from in-falling field galaxies.

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arXiv1999

Panchromatic Study of Nearby UV-Bright Starburst Galaxies: Implications for Massive Star Formation and High Redshift Galaxies

We present a panchromatic study of nearby starburst galaxies from the ultraviolet to the visible, including narrow band H(alpha) using WIYN and HST data, to determine how star formation processes affect the morphology and integrated fluxes of nearby starbursts. We find the UV/H(alpha) morphology of starbursts tend to differ, although not in a standard or predictable manner. From our sample of six nearby starbursts, three systems show a good correlation between UV and H(alpha) fluxes, but we find differences in UV and H(alpha) morphology between the other three. Occasionally we find systems with well defined H II regions without the corresponding brightness in the UV, and vice-versa. We discuss the likely mechanisms behind these differences which include: starburst ages, dust absorption, stellar energy ejecta through SNe and winds, as well as leakage of UV photons from stellar clusters. We conclude that the large scale morphological features in starbursts are primarily due to both age and absorption from a `picket fence' dust distribution. We further demonstrate the similarity and differences between these nearby starbursts and high redshift star forming galaxies. The overall morphology of our sample of starbursts changes little between UV and visible wavelengths. If high redshift galaxies are similar to these starbursts, their morphologies should change little between rest-frame UV and optical. We also show that FIR and UV spectral energy distributions and slopes can be used to determine large scale morphological features for extreme starbursts, with the steepest FIR slopes correlating with the most disturbed galaxies.

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arXiv1999

The Extraordinary `Superthin' Spiral Galaxy UGC7321. I. Disk Color Gradients and Global Properties from Multiwavelength Observations

We present B- and R-band imaging and photometry, H-alpha narrow-band imaging, NIR H-band imaging, and HI 21-cm spectroscopy of the nearby Sd spiral galaxy UGC7321. UGC7321 exhibits a remarkably thin stellar disk with no bulge component. The galaxy has a very diffuse, low surface brightness disk, which appears to suffer little internal extinction in spite of its edge-on geometry. The UGC7321 disk shows significant B-R color gradients in both the radial and vertical directions. These color gradients cannot be explained solely by dust and are indicative of changes in the mix of stellar ages and/or metallicity as a function of both radius and height above the galaxy plane. The outer regions of the UGC7321 disk are too blue to be explained by low metallicity alone (B-R<0.6), and must be relatively young. However, the galaxy also contains stellar populations with B-R>1.1, indicating it is not a young or recently-formed galaxy. The disk of UGC7321 is not a simple exponential, but exhibits a light excess at small radii, as well as distinct surface brightness zones. Together the properties of UGC7321 imply that it is an under-evolved galaxy in both a dynamical and in a star-formation sense. (Abridged)

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arXiv1999

The Structure and Morphology of the Ionized Gas in Starburst Galaxies: NGC5253/5236

(Abridged) We investigate the interplay between starbursts and host galaxies by studying the structure and physical characteristics of the ionized gas surrounding the central starbursts in the two nearby galaxies NGC5253 and NGC5236. The two systems represent very different galactic environments, NGC5253 being a metal-poor dwarf, and NGC5236 being a metal-rich, massive spiral. We present images of the starburst regions in these two galaxies in the line emission [OIII], H-alpha, and [SII], and in continuum U, V, R. For NGC5253, line ratio maps [OIII]/H-beta and [SII]/H-alpha show that in the outer regions the diffuse ionized gas (DIG) is partially excited by a non-photoionization process (`shocks'). The `shocked' gas is mostly concentrated south-west of the galaxy's center, in coincidence with the position of H-alpha bubbles and with extended soft X-ray emission. The H-alpha emission from the shock-excited gas is ~1-2% of the total. About 80-90% of the DIG is consistent with being photoionized, requiring that about 10% of the ionizing photons escape from the starburst site. The starburst in NGC5253 appears to be fed by gas infalling along the galaxy's optical minor axis, while hot gas expanding from the starburst has a preferential direction along the major axis. The results for NGC5236 are less clear than for NGC5253, as the images are not as deep. The emission line ratio maps show very little or no evidence for presence of shock excitation and very little or no ionized gas appears expanding from the center of the galaxy outward along the disk plane. The starburst in NGC5236 is thus more strongly confined than that in NGC5253, probably due to the deeper gravitational potential well of the more massive galaxy.

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arXiv1998

The Extreme Outer Regions of Disk Galaxies: Star Formation and Metal Abundances

The extreme outer regions of disk galaxies, lying at or beyond the classical optical radius defined by R25, present an opportunity to study star formation and chemical evolution under unique physical conditions, possibly reminscent of those which existed during the early stages of disk evolution. We present here some of the first results from a large study to measure star formation rates and metallicities in the extreme outer limits of a sample of nearby spiral galaxies. Despite their low gas column densities, massive star formation is often observed in these outer parts, but at an azimuthally--averaged rate much lower than that seen in the inner disk. Gas-phase O/H abundances of roughly 10% solar characterize the gas at 1.5--2 R25. The implications of our results for star formation `laws' and models of disk evolution are discussed.

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arXiv1998

An Exploration of the Tully-Fisher Relation for Extreme Late-Type Spiral Galaxies

This paper explores the adherence of 47 extreme late-type galaxies to the B- and V-band Tully-Fisher relations defined by a sample of local calibrators. In both bands we find the mean luminosity at a given line width for extreme late-type spirals to lie below that predicted by standard Tully-Fisher relations. While many of the extreme late-type spirals do follow the Tully-Fisher relation to within our observational uncertainties, most of these galaxies lie below the normal, linear Tully-Fisher relation, and some are underluminous by more than 2 sigma (i.e. >1.16 magnitudes in V). This suggests a possible downward curvature of the Tully-Fisher relation for some of the smallest and faintest rotationally supported disk galaxies. This may be a consequence of the increasing prevalence of dark matter in these systems. We find the deviation from the Tully-Fisher relation to increase with decreasing luminosity and decreasing optical linear size in our sample, implying that the physically smallest and faintest spirals may be a structurally and kinematically distinct class of objects.

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arXiv1998

The Exceptionally Soft X-ray Spectrum of the Low-mass Starburst Galaxy NGC 1705

NGC 1705 is one of the optically brightest and best studied dwarf galaxies. It appears to be in the late stage of a major starburst and contains a young super star cluster. Type II supernovae are therefore likely to have been a major effect in the recent evolution of this galaxy and are likely to have produced a superbubble whose affects on the low-density ambient interstellar medium can be ideally studied. ROSAT PSPC observations of this galaxy reveal two striking blobs of X-ray emission embedded in \Ha loops which can be interpreted as both sides of the upper plumes of the same superbubble. These sources are a surprise. They are much softer than those observed from other starburst dwarf galaxies, and are so soft that they should have been blocked if the observed Galactic HI column density were uniformly distributed across NGC 1705 or if the sources were embedded in the HI disk of NGC 1705. In addition, the total X-ray luminosity in the ROSAT energy band of 1.2x10^{38} erg s^{-1} is low in comparison to similar objects. We discuss possible models for the two X-ray peaks in NGC 1705 and find that the sources most likely originate from relatively cool gas of one single superbubble in NGC 1705. The implications of the exceptional softness of these sources are addressed in terms of intrinsic properties of NGC 1705 and the nature of the foreground Galactic absorption.

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arXiv1998

A WFPC2 Study of the Resolved Stellar Population of the Pegasus Dwarf Irregular Galaxy (DDO 216)

The stellar population of the Pegasus dwarf irregular galaxy is investigated in images taken in the F439W (B), F555W (V), and F814W (I) bands with WFPC2. These and ground-based data are combined to produce color-magnitude diagrams which show the complex nature of the stellar population in this small galaxy. A young (< 0.5 Gyr) main sequence stellar component is present and clustered in two centrally-located clumps, while older stars form a more extended disk or halo. The colors of the main sequence require a relatively large extinction of A_V = 0.47 mag. The mean color of the well-populated red giant branch is relatively blue, consistent with a moderate metallicity young, or older, metal-poor stellar population. The red giant branch also has significant width in color, implying a range of stellar ages and/or metallicities. A small number of extended asymptotic giant branch stars are found beyond the red giant branch tip. Near the faint limits of our data is a populous red clump superimposed on the red giant branch. Efforts to fit self-consistent stellar population models based on the Geneva stellar evolution tracks yield a revised distance of 760 kpc. Quantitative fits to the stellar population are explored as a means to constrain the star formation history. The numbers of main sequence and core helium-burning blue loop stars require that the star formation rate was higher in the recent past, by a factor of 3-4 about 1 Gyr ago. Unique results cannot be obtained for the star formation history over longer time baselines without better information on stellar metallicities and deeper photometry. The youngest model consistent with the data contains stars with constant metallicity of Z = 0.001 which mainly formed 2-4 Gyr ago. Even at its peak of star forming activity, the Pegasus dwarf most likely remained relatively dim with M_V ~ -14.

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arXiv1998

WFPC2 Observations of Leo A: A Predominantly Young Galaxy within the Local Group

The unprecedented detail of the WFPC2 colour-magnitude diagrams of the resolved stellar population of Leo A presented here allows us to determine a new distance and an accurate star formation history for this extremely metal-poor Local Group dwarf irregular galaxy. From the position of the red clump, the helium-burning blue loops and the tip of the red giant branch, we obtain a distance modulus, m-M=24.2+/-0.2, or 690 +/- 60 kpc, which places Leo A firmly within the Local Group. Our interpretation of these features in the WFPC2 CMDs at this new distance based upon extremely low metallicity (Z=0.0004) theoretical stellar evolution models suggests that this galaxy is predominantly young, i.e. <2 Gyr old. A major episode of star formation 900 - 1500 Gyr ago can explain the red clump luminosity and also fits in with our interpretation of the number of anomalous Cepheid variable stars seen in this galaxy. We cannot rule out the presence of an older, underlying globular cluster age stellar population with these data. However, using the currently available stellar evolution models, it would appear that such an older population is limited to no more than 10% of the total star formation to have occured in this galaxy. Leo A provides a nearby laboratory for studying young metal poor stars and investigations of metal-poor galaxy evolution, such as is supposed to occur for larger systems at intermediate and high redshifts.

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arXiv1998

Far-Ultraviolet and Visible Imaging of the Nucleus of M32

We have imaged the nucleus of M32 at 1600 Angstroms (FUV) and 5500 Angstroms (V) using the Wide-Field/Planetary Camera 2 (WFPC2) aboard HST. We detected the nucleus at 1600 Angstroms using the redleak-free Woods filter on WFPC2. The FUV light profile can be fit with a Gaussian of FWHM 0.46" (4.6 pixels), but cannot be resolved into individual stars; no UV-bright nuclear structure was detected. The (FUV-V) color of the nucleus is 4.9 +/- 0.3, consistent with earlier observations. We are unable to confirm any radial variation in (FUV-V) within 0.8" of the nucleus; beyond that radius the FUV surface brightness drops below our detection threshhold. We also performed surface photometry in V and found our results to be in excellent agreement with deconvolved, WFPC1 results. M32's light profile continues to rise in a nuclear cusp even within 0.1" of its center. No intermediate-age stellar population is required by evolutionary population synthesis models to reproduce the (FUV-V) color of the nucleus, although these data and current models are insufficient to resolve this issue.

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arXiv1998

Ionization Structure in the 30 Doradus Nebula as seen with HST/WFPC-2

Using the Hubble Space Telescope and WFPC2 we have imaged the central 20pc of the giant H II region 30 Doradus nebula in three different emission lines. The images allow us to study the nebula with a physical resolution that is within a factor of two of that typical of ground based observations of Galactic H II regions. Most of the emission within 30 Dor is confined to a thin zone located between the hot interior of the nebula and surrounding dense molecular material. This zone appears to be directly analogous to the photoionized photoevaporative flows that dominate emission from small, nearby H II regions. The dynamical effects of the photoevaporative flow can be seen. The ram pressure in the photoevaporative flow, derived from thermal pressure at the surface of the ionization front, is found to balance with the pressure in the interior of the nebula derived from previous x-ray observations. By analogy with the comparison of ground and HST images of M16 we infer that the same sharply stratified structure seen in HST images of M16 almost certainly underlies the observed structure in 30 Dor. 30 Doradus is a crucial case because it allows us to bridge the gap between nearby H II regions and the giant H II regions seen in distant galaxies. The real significance of this result is that it demonstrates that the physical understanding gained from detailed study of photoevaporative interfaces in nearby H II regions can be applied directly to interpretation of giant H II regions.

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arXiv1997

Hubble Space Telescope Observations of the Draco Dwarf Spheroidal

We present an F606W-F814W color-magnitude diagram for the Draco dwarf spheroidal galaxy based on Hubble Space Telescope WFPC2 images. The luminosity function is well-sampled to 3 magnitudes below the turn-off. We see no evidence for multiple turnoffs and conclude that, at least over the field of the view of the WFPC2, star formation was primarily single-epoch. If the observed number of blue stragglers is due to extended star formation, then roughly 6% (upper limit) of the stars could be half as old as the bulk of the galaxy. The color difference between the red giant branch and the turnoff is consistent with an old population and is very similar to that observed in the old, metal-poor Galactic globular clusters M68 and M92. Despite its red horizontal branch, Draco appears to be older than M68 and M92 by 1.6 +/- 2.5 Gyrs, lending support to the argument that the ``second parameter'' which governs horizontal branch morphology must be something other than age. Draco's observed luminosity function is very similar to that of M68, and the derived initial mass function is consistent with that of the solar neighborhood.

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arXiv1996

The Spectacular Ionized Interstellar Medium of NGC55

We present deep Halpha+[NII], [SII] (6716,6731A) and [OII] (3726,3729A) images of the highly inclined, actively star--forming SBm galaxy NGC 55, located in the nearby Sculptor Group. Due to its proximity, NGC 55 provides a unique opportunity to study the disk--halo interface in a late--type galaxy with unprecedented spatial resolution. Our images reveal a spectacular variety of ionized gas features, ranging from giant HII region complexes, to supergiant filamentary and shell features, to patches of very faint diffuse emission. Many of these features protrude well above the plane of the galaxy, including a very faint fragmented shell of emission which is visible at 2.6 kpc above the disk. We identify candidate `chimneys' extending out of the disk, which could be the conduits into the halo for hot gas around disk star-forming regions, and could also provide low-density paths for the passage of UV photons from the disk to the halo. Several of the identified chimneys are `capped' with clumps of ionized gas, one of which, located at 1.5 kpc above the disk plane, appears to be the site of recent star formation. Emission--line ratios ([OII]/Halpha+[NII], [SII]/Halpha+[NII]) constrain the ionization mechanism of the gas, and our images allow the first measurement of [OII]/Halpha+[NII] in extra-planar diffuse ionized gas. The diffuse gas is characterized by emission--line ratios which are enhanced on average by a factor of two compared to those of bright HII regions. Each line ratio increases in value smoothly from the cores of HII regions, through the haloes of HII regions, into the diffuse ionized gas. Such a continuous trend is predicted by models in which the diffuse gas is ionized by photons produced by massive stars in HII regions.

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arXiv1996

Diffuse Ionized Gas in Spiral Galaxies: Probing Photon Leakage from HII Regions?

As part of a large study to map the distribution of star formation across galactic disks, we have obtained deep Halpha images of the nearby Sculptor Group spirals NGC 247 and NGC 7793. These images are of sufficiently high quality that they allow identification and analysis of diffuse Halpha emission at surface brightness levels ranging from those of extremely low density HII regions to those of the local Galactic disk diffuse emission. This paper presents a study of the large scale distribution and global energetics of diffuse ionized gas (DIG) in these galaxies and investigates the association between DIG and discrete HII regions. Our results support the hypothesis that the DIG is photoionized by Lyc photons which leak out of traditional HII regions, and suggest that the local HI column density plays a role in regulating the amount of leakage which can occur. This interpretation has profound implications for the derivation of star-formation rates based on Halpha emission--line fluxes since HII region counts alone will lead to significant underestimates of the true rate. The contribution of the diffuse Halpha component to the total Halpha emission, ie. the diffuse fraction, in these galaxies is found to be similar to values found in other disk galaxies with differing Hubble types and star formation rates. The constancy of the diffuse fraction is rather unexpected and implies that the overall fraction of photons which can leak out of HII regions and ionize the ISM over large scales is relatively invariant from one galaxy to another.

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