Ultraviolet Colors and Extinctions of H ii Regions in the Whirlpool Galaxy (M51) (original) (raw)
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Low surface brightness Hα observations of local intergalactic hydrogen clouds
The Astrophysical Journal …, 1995
We present upper limits on the local ionizing background based on a search for extended H␣ emission from three nearby intergalactic H I clouds: the Leo Ring (M96 group), both the NE and SW lobes of the Haynes-Giovanelli Virgo Cloud (H I 1225ϩ01), and the H I tidal tails associated with the NGC 4631/4656 group. These clouds were chosen to have 21 cm emission that is extended (10 -100 kpc) and distant from any associated galaxy. Deep, wide-field CCD images were acquired through narrow-(131 Å) and broadband R filters with the Burrell Schmidt telescope on Kitt Peak. We set a 95% confidence upper limit on the H␣ surface brightness for the areas of the clouds detected in H I of 1.6 ϫ 10 Ϫ19 ergs s Ϫ1 cm Ϫ2 arcsec Ϫ2 in Leo and of 3.7 ϫ 10 Ϫ19 ergs s Ϫ1 cm Ϫ2 arcsec Ϫ2 in Virgo. We limit the local ionizing background to ⌽ 0 Ͻ 5.0 ϫ 10 3 photons s Ϫ1 cm Ϫ2 sr Ϫ1 (95%) at the location of the Leo Ring cloud and ⌽ 0 Ͻ 1.1 ϫ 10 4 photons s Ϫ1 cm Ϫ2 sr Ϫ1 at the Virgo cloud, assuming spherical clouds. (Limits are a factor of 2 higher for a thin face-on slab.) The limits correspond to J 0 Ͻ 3.3 ϫ 10 Ϫ23 and 7.6 ϫ 10 Ϫ23 ergs s Ϫ1 cm Ϫ2 sr Ϫ1 Hz Ϫ1 for a Ϫ1/ 2 spectrum (1.6 times higher for a Ϫ1.4 spectrum) between 1 and 4 ryd. Such low limits suggest that quasar light, and not galactic light, dominates the ionizing background at low redshift. The H␣ limit on the Leo cloud is significantly below a previously reported detection. In the field of the edge-on galaxies NGC 4631 and NGC 4656, we detect H␣ from ionized gas extending nearly 16 kpc above N4631, which could have been blown out by starburst activity in the plane, and a low surface brightness companion or stellar tidal tail. This companion lies between N4631's H I tidal tails and may have played a role in creating the H I tidal tails, or it may represent star formation within the tidal tails. We also report the tentative detection of an ultrafaint ''sheet'' of H␣ emission extending from NGC 4631 to NGC 4656.
The Astrophysical Journal, 2004
The 905 to 1180 Å spectral range of the Far Ultraviolet Spectroscopic Explorer (FUSE) includes numerous transitions of molecular hydrogen, making it possible to study H 2 in diffuse interstellar environments directly through absorption measurements. We have searched for H 2 absorption in five starburst galaxies: NGC 1705, NGC 3310, NGC 4214, M83 (NGC 5236), and NGC 5253. We tentatively detect weak absorption by H 2 in M83 and NGC 5253, and set upper limits on the H 2 column density in the other galaxies. Conservative upper limits on the mass of molecular gas detected with FUSE are many orders of magnitude lower than the H 2 mass inferred from CO emission measurements for the four galaxies in our sample in which CO has been detected. This indicates that almost all of the H 2 is in the form of clouds with N(H 2 ) 10 20 cm −2 that are opaque to far-UV light and therefore cannot be probed with far-UV absorption measurements. The far-UV continuum visible in the FUSE spectra passes between the dense clouds, which have a covering factor < 1. The complex observational biases related to varying extinction across the extended UV emission in the FUSE apertures prevent an unambiguous characterization of the diffuse H 2 in these starbursts. However, the evidence is suggestive that there is less H 2 in the diffuse interstellar medium between the dense clouds compared to similarly reddened sight lines in the Milky Way. This holds with the expectation that the destructive UV radiation field is stronger in starbursts. However, previous UV observations of these starbursts have shown that there is reddening caused by the diffuse interstellar medium. This suggests that while diffuse H 2 may be destroyed in the starburst, dust still exists.
Astrophysical Journal, 2000
We study the e †ects of the ionizing and dissociating photons produced by Population III objects on the surrounding intergalactic medium. We Ðnd that the typical size of an photodissociated region, H 2 kpc, is smaller than the mean distance between sources at z B 20È30 but larger than the R d B 1È5 ionized region by a factor depending on the detailed properties of the emission spectrum. This implies that clearing of intergalactic occurs before reionization of the universe is complete. In the same red-H 2 shift range, the soft-UV background in the Lyman-Werner bands, when the intergalactic H and H 2 opacity is included, is found to be to 10~27 ergs cm~2 s~1 Hz~1. This value is well below J LW B 10~30 the threshold required for the negative feedback of Population III objects on the subsequent galaxy formation to be e †ective in that redshift range.
2008
The 905 to 1180 Å spectral range of the Far Ultraviolet Spectroscopic Explorer (FUSE) includes numerous transitions of molecular hydrogen, making it possible to study H2 in diffuse interstellar environments directly through absorption measurements. We have searched for H2 absorption in five starburst galaxies: NGC 1705, NGC 3310, NGC 4214, M83 (NGC 5236), and NGC 5253. We tentatively detect weak absorption by H2 in M83 and NGC 5253, and set upper limits on the H2 column density in the other galaxies. Conservative upper limits on the mass of molecular gas detected with FUSE are many orders of magnitude lower than the H2 mass inferred from CO emission measurements for the four galaxies in our sample in which CO has been detected. This indicates that almost all of the H2 is in the form of clouds with N(H2) � 10 20 cm −2 that are opaque to far-UV light and therefore cannot be probed with far-UV absorption measurements. The far-UV continuum visible in the FUSE spectra passes between the ...
The evolution of emission lines in H II galaxies
Astronomy & Astrophysics, 2001
We constructed diagnostic diagrams using emission line ratios and equivalent widths observed in several independent samples of H ii galaxies. Significant trends are seen, both in the line ratio diagrams, and in diagrams relating line ratios to the equivalent width of Hβ. The diagrams are compared to predictions from photoionization models for evolving starbursts. This study extends the work of Stasińska & Leitherer (1996) by including objects with no direct determination of the metallicities, and by using updated synthesis models with more recent stellar tracks and atmospheres. We find that H ii galaxies from objective-prism surveys are not satisfactorily reproduced by simple models of instantaneous starbursts surrounded by constant density, ionization bounded H ii regions. The observed relations between emission line ratios and Hβ equivalent width can be understood if older stellar populations generally contribute to the observed optical continuum in H ii galaxies. In addition, different dust obscuration for stars and gas and leakage of Lyman continuum photons from the observed H ii regions can be important. As a result, H ii galaxies selected from objective-prism surveys are not likely to contain significant numbers of objects in which the most recent starburst is older than about 5 Myr. This explains the success of the strong line method to derive oxygen abundances, at least in metal poor H ii galaxies. The observed increase of [O i]/Hβ with decreasing Hβ equivalent width can result from the dynamical effects of winds and supernovae. This interpretation provides at the same time a natural explanation of the small range of ionization parameters in giant H ii regions. The classical diagnostic diagram [O iii]/Hβ vs. [O ii]/Hβ cannot be fully understood in terms of pure photoionization models. The largest observed [O ii]/Hβ ratios require additional heating. The [N ii]/[O ii] ratio is shown to increase as the Hβ equivalent width decreases. A possible explanation is an N/O increase due to gradual enrichment by winds from Wolf-Rayet stars on a time scale of ∼5 Myr. Alternatively, the relation between N/O and O/H could be steeper than N/O ∝ O/H 0.5 , with a previous stellar generation more important at higher metallicities.
Astrophys J, 2004
The 905-1180 Å spectral range of FUSE includes numerous transitions of molecular hydrogen, making it possible to study H2 in diffuse interstellar environments directly through absorption measurements. We have searched for H2 absorption in five starburst galaxies: NGC 1705, NGC 3310, NGC 4214, M83 (NGC 5236), and NGC 5253. We tentatively detect weak absorption by H2 in M83 and NGC 5253 and set upper limits on the H2 column density in the other galaxies. Conservative upper limits on the mass of molecular gas detected with FUSE are many orders of magnitude lower than those of the H2 mass inferred from CO emission measurements for the four galaxies in our sample in which CO has been detected. This indicates that almost all the H2 is in the form of clouds with N(H2)>~1020 cm-2 that are opaque to far-UV light and therefore cannot be probed with far-UV absorption measurements. The far-UV continuum visible in the FUSE spectra passes between the dense clouds, which have a covering factor less than 1. The complex observational biases related to varying extinction across the extended UV emission in the FUSE apertures prevent an unambiguous characterization of the diffuse H2 in these starbursts. However, the evidence suggests that there is less H2 in the diffuse interstellar medium between the dense clouds compared with similarly reddened sight lines in the Milky Way. This holds with the expectation that the destructive UV radiation field is stronger in starbursts. However, previous UV observations of these starbursts have shown that there is reddening caused by the diffuse interstellar medium. This suggests that, while diffuse H2 may be destroyed in the starburst, dust still exists. Based on observations made with the NASA-CNES-CSA Far-Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985.
Lyman‐Alpha Absorption and the D/H Ratio in the Local Interstellar Medium
The Astrophysical Journal, 1997
Using the Goddard High Resolution Spectrograph onboard the Hubble Space T elescope, we have observed Lya absorption against stellar chromospheres along six lines of sight, with additional observations of the Mg II H and K lines along Ðve of the lines of sight and Fe II absorption along four of the lines of sight. We found absorption near the projected velocity of the local interstellar cloud (LIC) along three lines of sight. The velocity toward the stars b Gem and p Gem was only marginally consistent with the LIC. The single interstellar component toward 31 Com had a velocity that was inconsistent with the projected LIC velocity. Three of the lines of sight showed a multicomponent velocity structure. For the star v Eri we required an additional hot, low-density component which we have interpreted as a stellar hydrogen wall. The LIC temperatures derived from our data range from 7800 to 9700 K with values of the microturbulence parameter less than 2.0 km s~1. The measured D/H ratio for the LIC along every sight line is consistent with a value of 1.6 ] 10~5, the best determined value being the b Cas line of sight with D/H \ 1.7^0.3 ] 10~5.
The Astrophysical Journal, 1998
We present ultra-high resolution (0.32 km s −1 ) spectra obtained with the 3.9m Anglo-Australian Telescope (AAT) and Ultra-High-Resolution Facility (UHRF), of interstellar Na i D 1 , Na i D 2 , Ca ii K, K i and CH absorption toward two high galactic latitude stars HD 141569 and HD 157841. We have compared our data with 21-cm observations obtained from the Leiden/Dwingeloo H i survey. We derive the velocity structure, column densities of the clouds represented by the various components and identify the clouds with ISM structures seen in the region at other wavelengths. We further derive abundances, linear depletions and H 2 fractional abundances for these clouds, wherever possible.
The Astrophysical Journal
This is the first in a series of three papers describing a project with the Cosmic Origins Spectrograph on the Hubble Space Telescope to measure abundances of the neutral interstellar medium (ISM) in a sample of 9 nearby star-forming galaxies. The goal is to assess the (in)homogeneities of the multiphase ISM in galaxies where the bulk of metals can be hidden in the neutral phase, yet the metallicity is inferred from the ionized gas in the H ii regions. The sample, spanning a wide range in physical properties, is to date the best suited to investigate the metallicity behavior of the neutral gas at redshift z = 0. ISM absorption lines were detected against the far-ultraviolet spectra of the brightest star-forming region(s) within each galaxy. Here we report on the observations, data reduction, and analysis of these spectra. Column densities were measured by a multi-component line-profile fitting technique, and neutral-gas abundances were obtained for a wide range of elements. Several caveats were considered including line saturation, ionization corrections, and dust depletion. Ionization effects were quantified with 'ad-hoc' CLOUDY models reproducing the complex photoionization structure of the ionized and neutral gas surrounding the UV-bright sources. An 'average spectrum of a redshift z = 0 star-forming galaxy' was obtained from the average column densities of unsaturated profiles of neutral-gas species. This template can be used as a powerful tool for studies of the neutral ISM at both low and high redshift.