arXiv subjects

Henry A. Kobulnicky

Publications and source records attributed to Henry A. Kobulnicky.

76 records · Page 5Linked to original sources

arXiv1998

Chemical Properties of Star-Forming Emission Line Galaxies at z=0.1 - 0.5

We measure oxygen and nitrogen abundances for 14 star-forming emission line galaxies (ELGs) at 0.11 -1.4 like local spirals. These chemical properties are inconsistent with unevolved objects undergoing a first burst of star formation. The majority of the ELGs are presently ~4 magnitudes brighter and ~0.5 dex more metal-rich than the bulk of the stars in well-known metal-poor dwarf spheroidals such as NGC 205 and NGC 185, making an evolution between some ELGs and metal-poor dwarf spheroidals improbable. However, the data are consistent with the hypothesis that more luminous and metal-rich spheroidal galaxies like NGC 3605 may become the evolutionary endpoints of some ELGs. [abridged]

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arXiv1997

Abundance Profiles in Low-Mass Galaxies

The nitrogen and oxygen abundances in the warm ionized gas of low-mass, metal-poor galaxies appear surprisingly homogeneous considering the prevalence of large HII regions, which contain hundreds of massive stars. Of the six galaxies with extensive optical spectroscopy, only the largest and most metal-rich, the LMC, shows evidence for a chemical gradient akin to those commonly seen in spirals. Furthermore, no significant localized chemical fluctuations are found in the vicinity of young star clusters, despite large expected chemical yields of massive stars. An ad-hoc fine-tuning of the release, dispersal and mixing rates could give rise to the observed homogeneity, but a more probable explanation is that fresh ejecta reside in a hard-to-observe hot or cold phase. In any case, the observations indicate that heavy elements which have already mixed with the warm interstellar medium are homogeneously dispersed. Mixing of fresh ejecta with the surrounding warm ISM apparently requires longer than the lifetimes of typical HII regions (>10^7 yrs). The lack of observed localized chemical enrichments is consistent with a scenario whereby freshly-synthesized metals are expelled into the halos of galaxies in a hot, 10^6 K phase by supernova-driven winds before they cool and ``rain'' back down upon the galaxy, creating gradual enrichments on spatial scales >1 kpc.

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arXiv1997

Testing CNO Enrichment Scenarios in Metal-poor Galaxies with HST Spectroscopy

Using Hubble Space Telescope ultraviolet and ground-based optical spectroscopy, we measure the C/O and N/O ratios of three metal-poor galaxies with similar metallicity but differing N/O. The C/O ratios estimated from C III] lambda1909 and [O III] lambda5007 lines are consistent with those measured from C II] lambda2326 and [O II] lambda3727 lines. We develop the use of the C^+/O^+ ratio as a reasonable substitute diagnostic of the carbon abundance in HII regions. The proximity of the C II] lambda2326 multiplet to the [O II] lambda3726,3729 lines makes these transitions a potential tool for measuring the carbon abundance in high-redshift objects. Derived chemical properties are consistent with a significant correlation between N and C abundances in metal-poor extragalactic HII regions. This result is unexpected if the dispersion in N/O among galaxies of similar metallicity is caused by localized, temporary chemical enrichments from massive stars. The presence of a correlation suggests that the majority of N and C production is coupled, as expected from chemical evolution models where C is produced predominantly by low mass stars and N is produced predominantly by intermediate mass stars. Since the occurance of localized chemical "pollution" in star-forming galaxies appears to be low, the relative overabundance of N in some galaxies compared to others at similar metallicity is most plausibly interpreted as an indicator of the global, secular chemical enrichment history. The N/O and perhaps the C/O ratios can be used as a ``clock'' to estimate the time since the last major episode of star formation.

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arXiv1997

Chemical Enrichment from Massive Stars in Starbursts II: NGC 1569

We present a longslit optical spectrophotometric survey in the irregular "post-starburst" galaxy NGC 1569 to search for chemical gradients and inhomogeneities in the interstellar medium. Despite the presence of two massive evolved starclusters and numerous HII regions, we find no evidence for chemical gradients or inhomogeneities that may be attributed to enrichment from the recent star formation activity. The chemical properties are consistent with 12+log(O/H)=8.19+/-0.04 and log(N/O)=-1.40+/-0.05 at all locations. No localized chemical self-enrichment ("pollution") from massive star evolution is found, even though the data are sensitive to the chemical yields from as few as two or three massive stars. Flat chemical abundance profiles appear to be the rule rather than the exception in low-mass galaxies. Strong chemical signatures in the surrounding interstellar material should be detected unless one or more of the following are true: 1) Different star forming regions throughout the studied galaxies ``conspire'' to keep star formation rates and global abundances uniform at all times, 2) ejecta from stellar winds and supernovae are transported to all corners of the galaxy on timescales of <10^7 yr, and are mixed instantaneously and uniformly, or 3) freshly synthesized elements remain unmixed with the surrounding interstellar medium and reside in a hard-to-observe hot 10^6 K phase or a cold, dusty, molecular phase. We advance the third scenario as the most plausible, and we suggest ways to locate the chemical products of massive star formation in starburst galaxies.

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