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

Bar imprints on the inner gas kinematics of M33

We present measurements of the stellar and gaseous velocities in the central 5' of the Local Group spiral M33. The data were obtained with the ARC 3.5m telescope. Blue and red spectra with resolutions from 2 to 4\AA covering the principal gaseous emission and stellar absorption lines were obtained along the major and minor axes and six other position angles. The observed radial velocities of the ionized gas along the photometric major axis of M33 remain flat at ~22 km s^{-1} all the way into the center, while the stellar velocities show a gradual rise from zero to 22 km s^{-1} over that same region. The central star cluster is at or very close to the dynamical center, with a velocity that is in accordance with M33's systemic velocity to within our uncertainties. Velocities on the minor axis are non-zero out to about 1' from the center in both the stars and gas. Together with the major axis velocities, they point at significant deviations from circular rotation. The most likely explanation for the bulk of the velocity patterns are streaming motions along a weak inner bar with a PA close to that of the minor axis, as suggested by previously published IR photometric images. The presence of bar imprints in M33 implies that all major Local Group galaxies are barred. The non-circular motions over the inner 200 pc make it difficult to constrain the shape of M33's inner dark matter halo profile. If the non-circular motions we find in this nearby Sc galaxy are present in other more distant late-type galaxies, they might be difficult to recognize.Comment: 20 pages, 12 figures, ApJ in pres

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

Star formation in the small magellanic cloud: The youngest star clusters

AbstractWe recently launched a comprehensive ground based (ESO/VLT/NTT) and space (HST & SST) study of the present and past star formation in the Small Magellanic Cloud (SMC), in clusters and in the field, with the goal of understanding how star and cluster formation occur and propagate in an environment of low metallicity, with a gas and dust content that is significantly lower than in the Milky Way. In this paper, we present some preliminary results of the "young cluster" program, where we acquired deep F555W (~V), and F814W (~I) HST/ACS images of the four young and massive SMC star clusters: NGC 346, NGC 602, NGC 299, and NGC 376

Far Ultraviolet and H-alpha Imaging of Nearby Spirals: The OB Stellar Population in the Diffuse Ionized Gas

(Abridged) We have compared H-alpha and far ultraviolet (FUV) images of 10 nearby spirals, with the goal of understanding the contribution of field OB stars to the ionization of the diffuse ionized gas (DIG) in spiral galaxies. The FUV images were obtained by the Ultraviolet Imaging Telescope (UIT) and the H-alpha images were obtained using various ground-based telescopes. In all of the galaxies, the F_H-alpha/F_UIT flux ratio is lower in the DIG than in the HII regions. This is likely an indication that the mean spectral type for OB stars in the field is later than that in HII regions. Comparison of the N_Lyc/L_UIT ratio with models of evolving stellar populations shows that the stellar population in the DIG is consistent with either an older single burst population or a steady state model with constant star formation and an initial mass function (IMF) slope steeper than alpha=2.35. We compared the F_H\alpha/F_UIT ratio in the DIG of these galaxies with that in M33. If the mean spectral types of stars in HII regions and in the DIG are the same as in M33, and the difference in extinction between DIG and HII regions is constant among galaxies, then the analysis suggests that field stars are important sources of ionization in most galaxies, and may be the dominant source in some galaxies.Comment: 29 pages, 8 figures, 6 tables. Accepted for publication in Ap

A New Giant Stellar Structure in the Outer Halo of M31

The Sloan Digital Sky Survey has revealed an overdensity of luminous red giant stars ~ 3 degrees (40 projected kpc) to the northeast of M31, which we have called Andromeda NE. The line-of-sight distance to Andromeda NE is within approximately 50 kpc of M31; Andromeda NE is not a physically unrelated projection. Andromeda NE has a g-band absolute magnitude of ~ -11.6 and central surface brightness of ~ 29 mag/sq.arcsec, making it nearly two orders of magnitude more diffuse than any known Local Group dwarf galaxy at that luminosity. Based on its distance and morphology, Andromeda NE is likely undergoing tidal disruption. Andromeda NE's red giant branch color is unlike that of M31's present-day outer disk or the stellar stream reported by Ibata et al. (2001), arguing against a direct link between Andromeda NE and these structures. However, Andromeda NE has a red giant branch color similar to that of the G1 clump; it is possible that these structures are both material torn off of M31's disk in the distant past, or that these are both part of one ancient stellar stream.Comment: 11 pages, 3 figures; ApJ Letters accepted versio