Gas In M33 Research Articles

Ci](1–0) observations in M33 A study of CO dark H2gas in M33

Based on [Ci] (1–0) observations and complementary Hi, 12/13 CO (1–0), 12 CO (2–1) and [Cii] data we estimated the column density and fraction of molecular hydrogen in GMCs along the major axis of the galaxy M33. We found that on average 75%± 14% of the hydrogen is in molecular form. Roughly 40–45% of the H 2 is both traced by [Cii] and CO, while ∼ 15% by the [Ci] lines. The found CO-dark H 2 gas fractions are within the predictions by Wolfire et al. (2010) for a half solar metallicity, Z = 0.5 Z ⊙ , as in M33.

Optical Spectroscopy and Ionization Models of the Diffuse Ionized Gas in M33, M51/NGC 5195, and M81

We report the results of a spectroscopic investigation of the diffuse ionized gas (DIG) in M33, M81, and M51. All three galaxies have enhanced [S II]/Hα and [N II]/Hα ratios in the DIG compared to H II regions, in agreement with the predictions of photoionization models. The DIG in all three galaxies also has [O III]/Hβ ratios equal to or greater than that in neighboring H II regions. The [S II]/[N II] ratio does not vary with Hα surface brightness and is similar in both H II regions and DIG. We have tentatively detected the He I λ5876 line in the bright DIG near NGC 604, indicating a contribution from O7 and hotter stars to the ionizing spectrum in the DIG at that location. The DIG in M33 has lower [N II]/Hα ratios than M51 or the Galaxy, most likely because of low N abundance in M33. We used CLOUDY models to test the feasibility of two photoionization sources for the DIG: (1) the diluted ionizing continua of massive stars (standard models) and (2) the transmitted ionizing continua of density-bounded H II regions (leaky H II models). The leaky H II region models produce higher [S II]/Hα, [N II]/Hα, and [O III]/Hβ ratios than standard DIG models for the same stellar temperatures. This alleviates somewhat the problem of producing high [O III]/Hβ ratios in the DIG. However, no single model can simultaneously reproduce all three line ratios. We conclude that either a combination of models is necessary or other ionization or heating mechanisms play a role in powering the DIG. Finally, we report on the spectrum of ionized gas associated with the companion to M51, NGC 5195. The gas in and around NGC 5195 has very high forbidden-line ratios and broad line widths (~ 500 km s-1 FWHM), indicative of shock-ionized gas. At some locations the emission lines split into two velocity components, suggesting an expanding structure or outflow. The line ratios, morphology, and kinematics show that the interstellar medium in NGC 5195 is very disturbed.

The Contribution of Field OB Stars to the Ionization of the Diffuse Ionized Gas in M33

We present a study of the ionizing stars associated with the diffuse ionized gas (DIG) and H II regions in the nearby spiral galaxy M33. We compare our Schmidt Hα image to the far-ultraviolet (FUV, 1520 A) image from the Ultraviolet Imaging Telescope (UIT). The Hα/FUV ratio is higher in H II regions than in the DIG, suggesting an older population of ionizing stars in the DIG. Assuming ionization equilibrium, we convert the Hα flux to the number of Lyman continuum photons NLyc. When compared to models of evolving stellar populations, the NLyc/FUV ratio in H II 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, which is probably a more accurate description of the stellar population in the field. The UIT data is complemented with archival FUV and optical images of a small portion of the disk of M33 obtained with WFPC2 on board the Hubble Space Telescope (HST). These images overlap low- and mid-luminosity H II regions as well as DIG, so we can investigate the stellar population in these environments. Using the HST FUV and optical photometry, we assign spectral types to the stars observed in DIG and H II 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 Hα emission, and we find that field OB stars in the HST images can account for 40% ± 12% of the ionization of the DIG, while the stars in H II regions can provide 107% ± 26% of the Hα luminosity of the H II regions. Due to the limited coverage of the HST data, we cannot determine if stars outside the HST fields ionize some of the DIG located in the HST fields, nor can we determine if photons from stars inside the HST fields leak out of the area covered by the HST fields. We do not find any correlation between leakage of ionizing photons and Hα luminosity for the H II regions in our HST fields. However, the HST fields do not include very luminous H II regions, and it would be worthwhile to see if there is any trend at higher luminosities. 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.

Optical emission-line properties of M33 supernova remnants

We have obtained spectra of the 42 supernova remnants (SNRs) and remnant candidates identified in the recent survey of the inner portion of M33. Relative fluxes for the important diagnostic emission lines in the red portion of the optical spectrum (6200-7500 A) are reported. We have confirmed that all the candidate SNRs have [S II]/Hα ratios >= 0.4, the canonical dividing line between SNR emission and emission from the photoionized gas in H II regions and planetary nebulae. Significant [O I] emission, another distinguishing characteristic of shocked gas, is also seen in the majority of the remnants. We find little evidence of emission line variation with SNR diameter, unlike that reported in previous SNR samples. Using published grids of shock models, we derive abundances and abundance gradients in the interstellar gas in M33. While the oxygen abundances and gradient derived agree (to within the rather large errors) with those derived from H II region studies, the nitrogen abundances and gradient show the same offset as previously noted in studies of other galaxies. Subject headings : galaxies : abundances galaxies : individual (M33) - H ii regions - nebulae: supernova remnants