arXiv subjects

Rene A. M. Walterbos

Publications and source records attributed to Rene A. M. Walterbos.

23 records · Page 2Linked to original sources

arXiv2000

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.

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arXiv2000

HIIphot: Automated Photometry of HII Regions Applied to M51

We have developed a robust, automated method, hereafter designated HIIphot, which enables accurate photometric characterization of HII regions while permitting genuine adaptivity to irregular source morphology. HIIphot utilizes object-recognition techniques to make a first guess at the shapes of all sources then allows for departure from such idealized ``seeds'' through an iterative growing procedure. Photometric corrections for spatially coincident diffuse emission are derived from a low-order surface fit to the background after exclusion of all detected sources. We present results for the well-studied, nearby spiral M51 in which 1229 HII regions are detected above the 5-sigma level. A simple, weighted power-law fit to the measured H-alpha luminosity function (HII LF) above log L_H-alpha = 37.6 gives alpha = -1.75+/-0.06, despite a conspicuous break in the HII LF observed near L_H-alpha = 10^38.9. Our best- fit slope is marginally steeper than measured by Rand (1992), perhaps reflecting our increased sensitivity at low luminosities and to notably diffuse objects. HII regions located in interarm gaps are preferentially less luminous than counterparts which constitute M51's grand-design spiral arms and are best fit with a power-law slope of alpha = -1.96+/-0.15. We assign arm/interarm status for HII regions based upon the varying surface brightness of diffuse emission as a function of position throughout the image. Using our measurement of the integrated flux contributed by resolved HII regions in M51, we estimate the diffuse fraction to be approximately 0.45 -- in agreement with the determination of Greenawalt et al. (1998). Automated processing of degraded datasets is undertaken to gauge systematic effects associated with limiting spatial resolution and sensitivity.

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arXiv1997

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.

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arXiv1997

The Warm Ionized Medium in Spiral Galaxies: A View from Above

The Warm Ionized Medium (WIM), also referred to as Diffuse Ionized Gas, contains most of the mass of interstellar medium in ionized form, contributing as much as 30% of the total atomic gas mass in the solar neighborhood. The advent of CCDs has enabled unprecedented study of this medium in external galaxies, probing a variety of environments. In particular, we can derive the morphology of the WIM, its distribution across disks, and the correlation with other Population I material. Spectroscopy of the WIM makes it possible to test various ionization models. I will review here our current understanding of the properties of the WIM in spiral galaxies. A perhaps unexpected result is that the H-alpha emission from the WIM contributes about 40% of the total observed H-alpha luminosity from spirals. This places severe constraints on possible sources of ionization, since only photo ionization by OB stars meets this requirement. Spectroscopic measurements of forbidden line strengths appear in reasonable agreement with photo ionization models. It is not yet clear if the Lyman continuum photons that ionize the WIM are mostly from OB stars located inside traditional HII regions, or from field OB stars.

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arXiv1996

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.

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