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We present mid-infrared Spitzer spectra of 11 planetary nebulae in the Galactic bulge. We derive argon, neon, sulfur, and oxygen abundances for them using mainly infrared line fluxes combined with some optical line fluxes from the literature. Due to the high extinction toward the bulge, the infrared spectra allow us to determine abundances for certain elements more accurately than previously possible with optical data alone. Abundances of argon and sulfur ( and in most cases neon and oxygen) in planetary nebulae in the bulge give the abundances of the interstellar medium at the time their progenitor stars formed; thus, these abundances give information about the formation and evolution of the bulge. The abundances of bulge planetary nebulae tend to be slightly higher than those in the disk on average, but they do not follow the trend of the disk planetary nebulae, thus confirming the difference between bulge and disk evolution. In addition, the bulge planetary nebulae show peculiar dust properties compared to the disk nebulae. Oxygen-rich dust features (crystalline silicates) dominate the spectra of all of the bulge planetary nebulae; such features are more scarce in disk nebulae. In addition, carbon-rich dust features (polycyclic aromatic hydrocarbons) appear in roughly half of the bulge planetary nebulae in our sample, which is interesting in light of the fact that this dual chemistry is comparatively rare in the Milky Way as a whole.

H I features near young star clusters in M81 are identified as the photodissociated surfaces of giant molecular clouds (GMCs) from which the young stars have recently formed. The H I column densities of these features show a weak trend, from undetectable values inside R = 3.7 kpc and increasing rapidly to values around 3 x 10(21) cm(-2) near R approximate to 7.5 kpc. This trend is similar to that of the radially averaged H I distribution in this galaxy, and implies a constant area covering factor of approximate to 0.21 for GMCs throughout M81. The incident UV fluxes G(0) of our sample of candidate PDRs decrease radially. A simple equilibrium model of the photodissociation-reformation process connects the observed values of the incident UV flux, the H I column density, and the relative dust content, permitting an independent estimate to be made of the total gas density in the GMC. Within the GMC this gas will be predominantly molecular hydrogen. Volume densities of 1

We present a mid-infrared study of four H II regions based on the combination of Infrared Space Observatory (ISO) SWS observations with spatial information from SpectroCam-10 images and long-slit spectra. We report a prominent broad emission feature near 8.9 mum. The peak position and width of this feature do not resemble known polycyclic aromatic hydrocarbon ( PAH) or dust emission features. Its spatial distribution is analyzed for two H II regions and is clearly distinct from that of the PAH emission features, being more similar to that of the ionized gas and especially to the spatial distribution of the dust continuum at 15 mum. Thus, the carrier of this band likely resides within the H II region. Possible carriers of this feature are highlighted.

We present the abundance analyses of three isolated A-type supergiant stars in the dwarf irregular galaxy Sextans A (= DDO 75) from high-resolution spectra obtained with the Ultraviolet-Visual Echelle Spectrograph (UVES) on the Kueyen telescope (UT2) of the ESO Very Large Telescope (VLT). Detailed model atmosphere analyses have been used to determine the stellar atmospheric parameters and the elemental abundances of the stars. The mean iron-group abundance was determined from these three stars to be [[( Fe II, Cr II)/ H]] = - 0.99 +/- 0.04 +/- 0.06.(2) This is the first determination of the present-day iron-group abundances in Sextans A. These three stars now represent the most metal-poor massive stars for which detailed abundance analyses have been carried out. The mean stellar alpha-element abundance was determined from the alpha-element magnesium as [[alpha(Mg I)/ H]] = - 1.09 +/- 0.02 +/- 0.19. This is in excellent agreement with the nebular alpha-element abundances as determined from oxygen in the H II regions. These results are consistent from star to star, with no significant spatial variations over a length of 0.8 kpc in Sextans A. This supports the nebular abundance studies of dwarf irregular galaxies, where homogeneous oxygen abundances are found throughout, and argues against in situ ("on the spot'') enrichment. The alpha/Fe abundance ratio is [[alpha(Mg I)/ Fe II, Cr II]] = -0.11 +/- 0.02 +/- 0.10, which is slightly lower but consistent with the solar ratio. This is consistent with the results from A-supergiant analyses in other Local Group dwarf irregular galaxies, NGC 6822 and WLM. The results of near-solar [alpha/Fe] ratios in dwarf galaxies is in stark contrast with the high [alpha/Fe] results from metal-poor stars in the Galaxy ( which plateau at values near + 0.4 dex) and is most clearly seen from these three stars in Sextans A because of their lower metallicities. The low [alpha/Fe] ratios are consistent with the slow chemical evolution expected for dwarf galaxies from analyses of their stellar populations.

We present a radio continuum survey at 1400 and 350 MHz of a region of 2degrees x 2degrees centered on the Cygnus OB2 association (d = 1.7 kpc), using the Westerbork Synthesis Radio Telescope (WSRT) with angular resolutions of, respectively, 13" and 55". The resulting 5 sigma flux-density limits of, respectively, similar to2 mJy and similar to10-15 mJy are a significant improvement over previous surveys. We detected 210 discrete sources with sizes less than 1.9theta(beam) (beam size), 98 of which at both frequencies. We also detected 28 resolved sources (sizes > 1.9theta(beam)) still having well-defined peak intensities. The observed spectral index alpha(350)(1400) distribution and source count strongly suggest an excess of sources of Galactic origin in the direction of Cyg OB2.We have searched for positional coincidences of the detected sources in our list with other radio, infrared, and optical objects from various surveys by using the likelihood ratio (LR) method. Furthermore, we looked for objects that show characteristics of either optically thick stellar winds (alpha greater than or similar to +0.6), or nonthermal emission (-1 less than or similar to alpha less than or similar to +0.6) and/or variable spectral flux density. The LR method resulted in 108 identifications. Eighty unidentified sources, i.e., similar to2/3, show characteristics of sources of Galactic origin, 10 of which may be stars. The remaining unidentified sources are probably of extragalactic origin.We identified one source with the O7 star Cyg OB2-335 and consider it to be a candidate colliding-wind binary. We also identify 19 point sources with known infrared and optical objects: these have nearly. at or inverted spectral indices, and some of them show flux-density variability. Follow-up multifrequency monitoring of these sources will be important in establishing the reality of the flux variabilities and to assess the nature of these sources.

We present Infrared Space Observatory (ISO) Long-Wavelength Spectrometer (LWS) far-infrared (FIR) spectra of the nucleus and six giant H II regions in M33 (NGC 595, IC 142, NGC 592, NGC 604, NGC 588, and IC 133). The seven fine-structure lines observed in the FIR are used to model the H II and photodissociation regions (PDRs). There is no observed trend in the FIR properties, observed with the LWS, as a function of galactic radius or metallicity. The cold neutral medium (CNM) is the main reservoir for the atomic gas, containing between 60% and 95% of the gas. The FIRLWS spectral energy distribution can be fitted with a single-temperature graybody spectrum with a temperature in the range 35 K less than or equal to T less than or equal to 49 K. The [C II] 158 mum line flux is 0.2%-0.7% FIRLWS, which is typical of values seen (0.1%-1% FIR) in the nuclei of star-forming galaxies. The [C II]/FIRLWS ratio peaks at the nucleus and is fairly constant across the rest of the sample. Massive star formation is traced by the intensity of the [O III] 88 mum line. The emission from the observed FIR lines that arise solely from H II regions can be modeled as a single component with a given oxygen and nitrogen abundance, effective temperature, density, and ionizing flux. There is no need for an extended low-density component (ELDWIM). Apart from NGC 604 and NGC 595, the fractional [C II] emission that arises from the H II regions and/or PDRs is not well constrained, but typically 5%-50% arises in the H II regions, 10%-35% from the CNM, and the bulk of the emission (40%-90%) in the PDRs. The average PDR in this sample has a gas density [n] similar to 10(3.1) cm(-3), an average incident far-ultraviolet flux (in units of the local interstellar value) [G(0)] = 10(2.4), a gas temperature T similar to 200 K, and an A(V) similar to 10 through the clouds. NGC 604 has 40% of the atomic gas residing in the PDRs, while the rest have a much smaller fraction, similar to5%-15%. The PDRs are similar to those found in other star-forming galaxies such as Centaurus A. [G(0)] is at the lower end of the range observed in samples of spiral and starburst galaxies (2.2 less than or equal to log G(0) less than or equal to 5), and [log(n)] sits comfortably in the middle of the observed range (1.8 less than or equal to log n less than or equal to 4.2).

Much of the interstellar medium in disk galaxies is in the form of neutral atomic hydrogen, H I. This gas can be in thermal equilibrium at relatively low temperatures, T less than or similar to 300 K (the cold neutral medium [CNM]), or at temperatures somewhat less than 104 K (the warm neutral medium [WNM]). These two phases can coexist over a narrow range of pressures, P(min) less than or equal to P less than or equal to P(max). We determine P(min) and P(max) in the plane of the Galaxy as a function of Galactocentric radius R using recent determinations of the gas heating rate and the gas-phase abundances of interstellar gas. We provide an analytic approximation for P(min) as a function of metallicity, far-ultraviolet radiation field, and the ionization rate of atomic hydrogen. Our analytic results show that the existence of P(min), or the possibility of a two-phase equilibrium, generally requires that H(+) exceed C(+) in abundance at P(min). The abundance of H(+) is set by EUV/soft X-ray photoionization and by recombination with negatively charged polycyclic aromatic hydrocarbons. In order to assess whether thermal or pressure equilibrium is a realistic assumption, we de. ne a parameter Y = t(cool)/t(shock), where t(cool) is the gas cooling time and t(shock) is the characteristic shock time or "time between shocks in a turbulent medium.'' For Y

We describe the properties of a remarkable group of actively star-forming dwarf galaxies and H II galaxies in the A1367 cluster, which were discovered in a large-scale Halpha imaging survey of the cluster. Approximately 30 Halpha-emitting knots were identified in a region approximately 150 kpc across, in the vicinity of the spiral galaxies NGC 3860, CGCG 97-125, and CGCG 97-114. Follow-up imaging and spectroscopy reveal that some of the knots are associated with previously uncataloged dwarf galaxies (M-B = -15.8 to -16.5), while others appear to be isolated H II galaxies or intergalactic H II regions. Radial velocities obtained for several of the knots show that they are physically associated with a small group or subcluster, including CGCG 97-114 and CGCG 97-125. No comparable concentration of emission-line objects has been found elsewhere in any of the eight northern Abell clusters surveyed to date. The strong H emission in the objects and their high spatial density argue against this being a group of normal, unperturbed dwarf galaxies. Emission-line spectra of several of the knots also show some to be anomalously metal-rich relative to their luminosities. The results suggest that many of these objects were formed or triggered by tidal interactions or mergers involving CGCG 97-125 and other members of the group. A Westerbork Synthesis Radio Telescope H I map of the region shows direct evidence for tidal interactions in the group. These objects may be related to the tidal dwarf galaxies found in some interacting galaxy pairs, merger remnants, and compact groups. They could also represent evolutionary precursors to the class of isolated ultracompact dwarf galaxies that have been identified in the Fornax Cluster.

This Letter presents wide-field optical and near-IR (UBVRIHalphaK') images of the galaxy M74 that were taken between 0.6 and 8.3 yr before the discovery of the Type Ic supernova 2002ap. We have located the position of the supernova on these images with an accuracy of 0."3. We find no sign of a progenitor object on any of the images. The deepest of these images is the B-band exposure, which has a sensitivity limit corresponding to an absolute magnitude of M-B less than or equal to -6.3. From our observed limits, we rule out as the progenitor all evolved states of single stars with initial masses greater than 20 M-. unless the W-R phase has been entered. Two popular theories for the origin of Type Ic supernovae are the core collapse of massive stars when they are in the W-R phase or the core collapse of a massive star in an interacting binary that has had its envelope stripped through mass transfer. Our prediscovery images would be sensitive only to the most luminous similar to30% of W-R stars, hence leaving a substantial fraction of typical W-R stars as viable progenitors. The energetics measured from modeling the initial light curve and spectral evolution of SN 2002ap suggest an explosion of a 5 M-. C+O core. While W-R stars generally have measured final masses greater than this, the uncertainties associated with the explosion model, stellar evolutionary calculations, and mass measurements suggest we cannot definitively rule out a W-R star progenitor. The alternative scenario is that the progenitor was a star of initial mass that was part of an interacting binary and stripped of its similar to20-25 M-. that was part of an interacting binary and stripped of its hydrogen and helium envelope via mass transfer. We discuss future observations of the supernova environment that will provide further constraints on the nature of the progenitor star.

We present deep UBVJHK H alpha images and H I maps of the irregular galaxy NGC 2366. Optically, NGC 2366 is a boxy-shaped exponential disk seen at high inclination angle. The scale length and central surface brightness of the disk are normal for late-type galaxies. Although NGC 2366 has been classified as a barred Im galaxy, we do not see any unambiguous observational signature of a bar. There is an asymmetrical extension of stars along one end of the major axis of the galaxy, and this is where the furthest star-forming regions are found, at a radius of 1.3 times the Holmberg radius. The star formation activity of the galaxy is dominated by the supergiant H II complex NGC 2363, but the global star formation rate for NGC 2366 is only moderately elevated relative to other Im galaxies. The star formation activity drops off with radius approximately as the starlight in the inner part of the galaxy but it drops faster in the outer part. There are some peculiar features of the H I distribution and kinematics. First, the integrated H I shows two ridges running parallel to the major axis that when deprojected appear as a large ring. Second, the velocity field exhibits several large-scale anomalies superposed on a rotating disk; some of these may be from a weak bar that has no inner Lindblad resonance. Third, the inclination and position angles derived from the kinematics differ from those derived from the optical and H I morphology. Fourth, there are regions in the H I of unusually high velocity dispersion, but these regions are not associated with the optical galaxy nor any obvious H I feature. Instead the velocity dispersion correlates with a deficit of H I emission in a manner suggestive of long-range, turbulent pressure equilibrium. In other respects the H I is fairly normal. The azimuthally averaged surface density of H I is comparable to that of other irregulars in the inner part of the galaxy but drops off slower and extends further in the outer parts. The H I around the star-forming complex NGC 2363 is fairly unremarkable. As in other disk galaxies, the gas in NGC 2366 is lumpy and star-forming regions are associated with these H I complexes. H II regions are found where the gas densities locally exceed 6 M. pc(-2). This threshold is required to provide a cool phase of H I as a first step toward star formation. NGC 2366, like other irregulars, has low gas densities relative to the critical gas densities of gravitational instability models, so large-scale gravitational instabilities operate slowly or not at all. Considering the lack of shear in the optical part of this galaxy, the relative slowness of such instabilities may not be a problem-there is little competition to the slow gravitational contraction that follows energy dissipation. This differs from the situation in giant spiral disks where the shear time is short, comparable to the energy dissipation time, and strong self-gravity is required for a condensation to grow and dissipate its turbulent energy before it shears away. The subthreshold surface densities are also not unusual if they are viewed using the critical tidal density for gravitational self-binding of a rotating cloud, rather than the critical surface density from the usual disk instability condition. The peak densities in all regions of star formation are equal to the local tidal densities, giving an agreement between these two quantities that is much better than between the surface density and the critical value. Evidently the large-scale gas concentrations are all marginally bound against background galactic tidal forces.This condition for self-binding may be more fundamental than the instability condition because it is local, three-dimensional, and does not involve spiral arm generation as an intermediate step toward star formation.

We discuss the composition of dust and ice along the line of sight to the Galactic center (GC) based on analysis of mid-infrared spectra (2.4-13 mu m) from the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO). We have analyzed dust absorption features arising in the molecular cloud material and the diffuse interstellar medium along the lines of sight toward Sgr A* and the Quintuplet sources, GCS 3 and GCS 4. It is evident from the depth of the 3.0 mu m H(2)O and the 4.27 mu m CO(2) ice features that there is more molecular cloud material along the line of sight toward Sgr AM than toward GCS 3 and GCS 4. In fact, Sgr A* has a rich infrared ice spectrum with evidence for the presence of solid CH(4), NH(3), and possibly HCOOH. Hydrocarbon dust in the diffuse interstellar medium along the line of sight to the GC is characterized by absorption features centered at 3.4, 6.85, and 7.25 mu m. Ground-based studies have identified the 3.4 mu m feature with aliphatic hydrocarbons, and ISO has given us the first meaningful observations of the corresponding modes at longer wavelengths. The integrated strengths of these three features suggest that hydrogenated amorphous carbon is their carrier. We attribute an absorption feature centered at 3.28 mu m in the GCS 3 spectrum to the C-H stretch in aromatic hydrocarbons. This feature is not detected, and its C-C stretch counterpart appears to be weaker, in the Sgr A* spectrum. A key question now is whether or not aromatics are a widespread component of the diffuse interstellar medium, analogous to aliphatic hydrocarbons.

We present high-resolution interferometric data of the inner 30' of the H I gas of the irregular galaxy NGC 4449. In addition to the previously known two counter-rotating gas systems, the H I exhibits an unusual ring morphology and high velocity dispersions in the outer part of the optical galaxy. In addition the position angle and inclination of the outer gas are moderately different from those of the optical galaxy. The position angle and inclination of the inner gas system are approximately similar to those of the inner optical galaxy only if strong deviations from ordered motion in the center are ignored. The H I interstellar medium appears to be fragmented into clouds, which are similar to clouds observed in other galaxies. There is very little correlation between the H I surface density and H alpha surface brightness. The optical isophotes of the galaxy define three different and distinct position angles and inclinations. The inner structure is similar in properties to a bar structure that includes a large fraction of the optical galaxy. The nature of the middle structure is unclear. Exterior to these bars are unusual faint filamentary structures. The V- and J-band surface photometry are well fit by an exponential disk to low surface brightness levels, including the bar structures. However, the central surface brightness is unusually bright. The star formation activity compared over three different time scales as a function of radius is fairly consistent. In the optical there are also unusual extensions at low light levels in the outskirts of the galaxy. Many characteristics of the gas and stars are consistent with a picture in which NGC 4449 has been disturbed by an external perturber, and the gas has not yet returned to equilibrium.

We have used the Owens Valley Millimeter Array to map (CO)-C-12 (J = 1-0) along a 3.5 kpc segment of the eastern spiral arm of M83 at resolutions of 6 ".5 x 3 ".5, 10 ", and 16 ". The CO emission in most of this segment lies along the sharp dust lane demarcating the inner edge of the spiral arm, but beyond a certain point along the arm the emission shifts downstream from the dust lane to become better aligned with the young stars seen in blue and H beta images. This morphology resembles that of the western arm of M100. Three possibilities, none of which is wholly satisfactory, are considered to explain the deviation of the CO arm from the dust lane: heating of the CO by UV radiation from young stars, heating by low-energy cosmic rays, and a molecular medium consisting of two (diffuse and dense) components that react differently to the density wave. Regardless, the question of what CO emission traces along this spiral arm is a complicated one. Masses based on CO emission and the virial theorem for 10 emission features roughly agree and are in the range 1.5-16 x 10(6) M.. These are lower than the masses of giant molecular associations in M51, but the discrepancy is probably due to the much higher linear resolution of these observations. Despite the uncertainty in what CO emission is tracing, we do not require a conversion factor of CO brightness to H-2 column density much different from the standard Galactic value if these structures are bound. Surprisingly, for the two fields where we can compare with single-dish data, only 2%-5% of the single-dish flux is seen in our observations. A possible explanation is that M83 contains much smoothly distributed molecular gas that is resolved out by the interferometer. Strong tangential streaming is observed where the arm crosses the kinematic major axis of the galaxy, implying that the shear becomes locally prograde in the arms. The amplitude of the tangential streaming is used along with a low-resolution single-dish radial profile of CO emission to infer a very high gas surface density of about 230 M. pc(-2) and an arm-interarm contrast greater than 2.3 in the part of the arm near the major axis. Using two different criteria, we find that the gas at this location is well above the threshold for gravitational instability-much more clearly so than in either M51 or M100. This finding is consistent with the unusually high H alpha surface brightness and star formation efficiency in M83: star formation may be particularly active because of strong gravitational instabilities.

Line strengths and their gradients in Mg, Fe, and H beta have been determined for a sample of 20 SO galaxies in order to study the stellar populations of their bulges and disks and to investigate their relationship to elliptical galaxies. Data are also presented for the elliptical galaxy NGC 1700 and the E/S0 NGC 3585. We find that S0 galaxies generally follow a similar, though possibly steeper, relationship between central Mg-2 line strength and central velocity dispersion as found for elliptical galaxies. The S0 galaxies show no dependence between Fe line strengths and central velocity dispersion-similar to the behavior observed in elliptical galaxies. The central Mg/Fe ratios in the luminous S0 galaxies show an overabundance of Mg to Fe with respect to solar element ratios. The disks (at 1 h) and bulges (at 1 r(e)) show Mg/Fe ratios lower than those found in the central regions. The magnitudes of the Mg and Fe line-strength gradients within the galaxies are found to be correlated, i.e., objects with steep Mg gradients have correspondingly large Fe gradients.We infer bulge and disk gradients for the nine most edge-on galaxies for which we have both major and minor-axis profiles. The metal line strengths decrease with radius along the major and minor axes in the bulge-dominated central regions. At larger radii, however, the major-axis metal line strength profiles flatten while the minor-axis bulge profiles fall to lower values. Representative color maps in B-R are presented that display a separation between bulge and disk colors corresponding to the metal line strength profiles. Based on our Mg-2 profiles, the average metal gradient found in the disks of our subsample is Delta[Fe/H]/Delta(r/h) = -0.08 +/- 0.06, which corresponds to a reduction in the mean metallicity of the disk stellar population by less than or similar to 15% per disk scale length (h). These shallow metallicity gradients are approximately a factor of 2-3 smaller than those derived for the disks of late-type spiral galaxies from H II regions and are consistent with previous investigations that showed a trend for disk metallicity gradients to decrease toward earlier Hubble types. As inferred from our Mg and Fe line strengths, the mean size of the bulge metallicity gradients is Delta[Fe/H]/Delta log r = -0.7 +/- 0.4, which is steeper than typical elliptical galaxy gradients.Our findings do not support formation scenarios in which bulges formed either from heated disk material at late times after disk formation or through dissipationless stellar merging, as neither process includes mechanisms for producing the observed metallicity gradients. Our observations are better explained in terms of formation via dissipative collapse (or merging) at early times.

We present new high-resolution observations of the central region in the late-type spiral galaxy M100 (NGC 4321) supplemented by three-dimensional numerical modeling of stellar and gas dynamics, including star formation (SF). Near-infrared imaging has revealed a small bulge of 4'' effective diameter; a 60'' radial length stellar bar of moderate strength, previously inferred from optical and 21 cm observations; and an ovally shaped, ringlike structure in the plane of the disk between 10''-22'' from the center, whose major axis makes a large angle with the bar. The K isophotes become progressively elongated and skewed to the position angle of the bar both outside and inside the ''ring,'' forming an inner barlike region. The galaxy exhibits a mild circumnuclear starburst concentrated in the inner part of the K ''ring.'' This SF is prominent in Ha and the U, B, and V bands, forming an incomplete ring. In addition, two strong local maxima of K emission have been observed to lie remarkably symmetrically with respect to the galactic nucleus and equidistant from it at 7.''5, slightly leading the stellar bar. CO molecular emission is peaked in the dust lanes seen in the I - K color index image.We interpret the twists in K isophotes and the swinging of spiral arms through similar to 360 degrees inside the corotation radius as being indicative of the presence of a double inner Lindblad resonance (ILR) and test this hypothesis by modeling the gas flow in a self-consistent gas+stars disk embedded in a halo, with an overall NGC 4321-like mass distribution in the system. Both ILRs have been verified using nonlinear orbit analysis by determining the spatial extent of the family of orbits oriented along the minor axis of the bar.We have reproduced the basic morphology of the region inside corotation, including (1) the similar to 60 degrees bar; (2) the large-scale trailing shocks representing the offset dust lanes in the bar; (3) two symmetric K peaks corresponding to gas compression maxima which lie at the caustic formed by the interaction of a pair of trailing and leading shocks in the vicinity of the inner ILR, both peaks being sites of SF; and (4) two additional zones of SF corresponding to gas compression maxima at the bar's minor axis, where the large-scale shocks start to curl and which are referred in the literature as ''twin peaks.'' We argue further that the twisting of K isophotes in the neighborhood of the resonance region requires a population of red stars which are dynamically young and follow gas rather than stellar orbits, i.e., red supergiants. At the same time, a substantial contribution from OB stars to the K light is expected within the inner kiloparsec and especially in the symmetric K peaks. We also conclude that NGC 4321 hosts a single stellar bar which fuels the starburst activity within the circumnuclear ''ring'' by channeling gas there at the median rate of similar to 0.1-1 Mo yr(-1).