Donald R. Garnett

Starbursts and Star Clusters in the Ultraviolet

Hubble Space Telescope ultraviolet (UV) images of nine starburst galaxies reveal them to be highly irregular, even after excluding compact sources (clusters and resolved stars). Most (7/9) are found to have a similar intrinsic effective surface brightnesses, suggesting that a negative feedback mechanism is setting an upper limit to the star formation rate per unit area. All starbursts in our sample contain UV bright star clusters indicating that cluster formation is an important mode of star formation in starbursts. On average about 20% of the UV luminosity comes from these clusters. The brightest clusters, or super star clusters (SSC), are preferentially found at the very heart of starbursts. The size of the nearest SSCs are consistent with those of Galactic globular clusters. The luminosity function of SSCs is well represented by a power law with a slope alpha ~ -2. There is a strong correlation between the far infrared excess and the UV spectral slope. The correlation is well modeled by a geometry where much of their dust is in a foreground screen near to the starburst, but not by a geometry of well mixed stars and dust.

The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale

We use high signal-to-noise ratio spectra of 20 H II regions in the giant spiral galaxy M101 to derive electron temperatures for the H II regions and robust metal abundances over radii R = 0.19-1.25R0 (6-41 kpc). We compare the consistency of electron temperatures measured from the [O III] ?4363, [N II] ?5755, [S III] ?6312, and [O II] ?7325 auroral lines. Temperatures from [O III], [S III], and [N II] are correlated with relative offsets that are consistent with expectations from nebular photoionization models. However, the temperatures derived from the [O II] ?7325 line show a large scatter and are nearly uncorrelated with temperatures derived from other ions. We tentatively attribute this result to observational and physical effects, which may introduce large random and systematic errors into abundances derived solely from [O II] temperatures. Our derived oxygen abundances are well fitted by an exponential distribution over six disk scale lengths, from approximately 1.3 (O/H)? in the center to 1/15 (O/H)? in the outermost region studied [for solar 12 + log(O/H) = 8.7]. We measure significant radial gradients in N/O and He/H abundance ratios, but relatively constant S/O and Ar/O. Our results are in approximate agreement with previously published abundances studies of M101 based on temperature measurements of a few H II regions. However, our abundances are systematically lower by 0.2-0.5 dex than those derived from the most widely used strong-line empirical abundance indicators, again consistent with previous studies based on smaller H II region samples. Independent measurements of the Galactic interstellar oxygen abundance from ultraviolet absorption lines are in good agreement with the Te-based nebular abundances. We suspect that most of the disagreement with the strong-line abundances arises from uncertainties in the nebular models that are used to calibrate the empirical scale, and that strong-line abundances derived for H II regions and emission-line galaxies are as much as a factor of 2 higher than the actual oxygen abundances. However, other explanations, such as the effects of temperature fluctuations on the auroral line based abundances, cannot be completely ruled out. These results point to the need for direct abundance determinations of a larger sample of extragalactic H II regions, especially for objects more metal-rich than solar.

The Luminosity-Metallicity Relation, Effective Yields, and Metal Loss in Spiral and Irregular Galaxies

I present results on the correlation between galaxy mass, luminosity, and metallicity for a sample of spiral and irregular galaxies having well-measured abundance profiles, distances, and rotation speeds. Additional data for low surface brightness galaxies from the literature are also included for comparison. These data are combined to study the metallicity-luminosity and metallicity-rotation speed correlations for spiral and irregular galaxies. The metallicity-luminosity correlation shows its familiar form for these galaxies, a roughly uniform change in the average present-day O/H abundance of about a factor of 100 over 11 mag in B luminosity. However, the O/H-Vrot relation shows a change in slope at a rotation speed of about 125 km s-1. At faster Vrot, there appears to be no relation between average metallicity and rotation speed. At lower Vrot, the metallicity correlates with rotation speed. This change in behavior could be the result of increasing loss of metals from the smaller galaxies in supernova-driven winds. This idea is tested by looking at the variation in effective yield, derived from observed abundances and gas fractions assuming closed box chemical evolution. The effective yields derived for spiral and irregular galaxies increase by a factor of 10-20 from Vrot ≈ 5 to 300 km s-1, asymptotically increasing to approximately constant yeff for Vrot 150 km s-1. The trend suggests that galaxies with Vrot 100-150 km s-1 may lose a large fraction of their supernova ejecta, while galaxies above this value tend to retain metals.

Electron temperature variations and the measurement of nebular abundances

In computing ionic abundances in H II regions, a two-zone model for the electron temperature, T e , is often assumed, with one temperature assigned to a « low-ionization » zone for species such as O + and N + , and another temperature assigned to a « high-ionization » zone for species such as O +2 and Ne +2 . Photoionization models, however, suggest that some ions, such as S +2 and Ar +2 , do not fit easily into such a scheme; in such cases, an intermediate value for T e is more appropriate. It is shown that this is an important consideration when computing S +2 abundances from measurements of the [S III] λ 6312 line, which has a strong temperature dependence

Abundances of Metal-rich H II Regions in M51*

We have obtained multiobject spectroscopy of H II regions in the spiral galaxy M51 with the Keck I telescope and the Low Resolution Imaging Spectrometer. For 10 objects we have detected the auroral line [N II] λ5755, while [S III] λ6312 has been measured in seven of these. This has allowed us to measure the electron temperature of the gas and to derive oxygen, sulfur, and nitrogen abundances for the 10 H II regions. Contrary to expectations from previous photoionization models of a few H II regions in M51 and from strong-line abundance indicators, the O/H abundance is below the solar value for most objects, with the most metal-rich H II regions, P203 and CCM 72, having log(O/H) = -3.16 [~1.4(O/H)☉] and log(O/H) = -3.29 [~1.0 (O/H)☉], respectively. The reduction of O/H by factors of up to 2 or 3 with respect to previous indirect determinations has important consequences for the calibration of empirical abundance indicators, such as R23, in the abundance and excitation range found in the central regions of spiral galaxies. The abundance gradients in these galaxies can therefore be considerably flatter than those determined by using such empirical calibrations. The H II regions with a measured electron temperature span the range (0.19-1.04) R0 in galactocentric radius and indicate a shallow abundance gradient for M51: -0.02 ± 0.01 dex kpc-1. The S/O abundance ratio is found to be similar to previous determinations of its value in other spiral galaxies, log(S/O) ≈ -1.6. Therefore, we find no evidence for a variation in massive-star initial mass function or nucleosynthesis at high oxygen abundance. An overabundance of nitrogen is measured, with log(N/O) -0.6. On the basis of our new abundances, we revise the effective yield for M51, now found to be almost 4 times lower than previous estimates, and we discuss this result in the context of chemical evolution in galactic disks. Features from Wolf-Rayet stars (the blue bump at 4660 A and the C III line at 5696 A) are detected in a large number of H II regions in M51, with the C III λ5696 line found preferentially in the central, most metal-rich objects.

Nitrogen in irregular galaxies

A study of nitrogen in giant H II regions located within low-abundance irregular galaxies is presented. Both observations and theoretical models are used to examine variations in the abundance of nitrogen relative to oxygen in these galaxies. Analysis of these and previously published observations suggests that the use of low spectral resolution is the main contributing factor in reported discrepancies in nitrogen abundances. Photoionization models, combined with the limited existing data on N(2+), suggest that, in nebulae having low abundances and/or ionizing stars hotter than 40,000 K, the N(+)/O(+) ratio is an accurate estimator of N/O. The detection of forbidden N II line in the extreme metal-poor galaxy I Zw 18 confirms that N/O does not exhibit a systematic variation with O/H in irregular galaxies, while the relatively high N/O suggests that the IMF in dwarf star-burst galaxies does not have a lower mass cutoff as high as 10 solar masses, but does not rule out that the IMF could be truncated below 1-3 solar mass. 99 refs.

Carbon in Spiral Galaxies from HUBBLE SPACE TELESCOPE Spectroscopy

We present measurements of the gas-phase abundance ratio C/O in six H II regions in the spiral galaxies M101 and NGC 2403, based on ultraviolet spectroscopy using the Faint Object Spectrograph on the Hubble Space Telescope. The ratios of C to O increase systematically with O/H in both galaxies, from log C/O≈-0.8 at log O/H=-4.0 to log C/O≈-0.1 at log O/H=-3.4. C/N shows no correlation with O/H. The rate of increase of C/O is somewhat uncertain because of uncertainty as to the appropriate UV reddening law and uncertainty in the metallicity dependence on grain depletions. However, the trend of increasing C/O with O/H is clear, confirming and extending the trend in C/O indicated previously from observations of irregular galaxies. Our data indicate that the radial gradients in C/H across spiral galaxies are steeper than the gradients in O/H. Comparing the data to chemical-evolution models for spiral galaxies shows that models in which the massive star yields do not vary with metallicity predict radial C/O gradients that are much flatter than the observed gradients. The most likely hypothesis at present is that stellar winds in massive stars have an important effect on the yields and thus on the evolution of carbon and oxygen abundances. C-to-O and N-to-O abundance ratios in the outer disks of spirals determined to date are very similar to those in dwarf irregular galaxies. This implies that the outer disks of spirals have average stellar-population ages much younger than those of the inner disks.

The abundance of sulfur in extragalactic H II regions

A long-slit CCD survey of forbidden S III 9069, 9532 A radiation in 13 extragalactic H II regions is presented, and the data are used to study the variation of S/O as a function of O/H. The data are consistent with the idea that S/O remains constant as O/H varies. There is no evidence that S/O is larger at low O/H. Photoionization models confirm that observations of O(+)/O(2+) and S(+)/S(2+) can be used to estimate the effective temperature of the ionizing stars in an H II region. This provides a potentially powerful new tool for studying the ionizing stars of H II regions in external galaxies. However, the models fail to reproduce the S(+)/S(2+) ratio in nebulae with large O(2+) fractions for reasonable values of T(eff). Simple chemical evolution models are calculated to compare observations with expectations from stellar neucleosynthesis calculations. 84 refs.

He II emission in extragalactic H II regions

Spectroscopic observations confirming the presence of nebular He II 4686-A emission in the SMC H II region N76 are presented, and it is demonstrated that the He II emission associated with the WO star in IC 1613 is also extended. The properties of four H II regions are discussed. The close correlation of the emission with specific stars indicates that photoionization by the stars themselves is the excitation mechanism, and it is proposed that this may be true for those nebulae ionized by WO stars and some rare high-excitation WN stars. The existence of these nebulae with He II emission increases the likelihood that the 4686-A emission frequently observed in dwarf emission-line galaxies is nebular in origin. This prompts the conclusion that the radiation field associated with star-forming regions can be harder than previously suspected, and reopens the issue of whether photoionization by stars in young galaxies can account for the ionization observed in QSO absorption-line systems. 42 refs.

Neon and Oxygen Abundances in M33

We present new spectroscopic observations of 13 H II regions in the Local Group spiral galaxy M33. The regions observed range from 1 to 7 kpc in distance from the nucleus. Of the 13 H II regions observed, the [O III] λ4363 line was detected in six regions. Electron temperatures were thus able to be determined directly from the spectra using the [O III] λλ4959, 5007/λ4363 line ratio. Based on these temperature measurements, oxygen and neon abundances and their radial gradients were calculated. For neon, a gradient of -0.016 ± 0.017 dex kpc-1 was computed, which agrees with the Ne/H gradient derived previously from ISO spectra. A gradient of -0.012 ± 0.011 dex kpc-1 was computed for O/H, much shallower than was derived in previous studies. The newly calculated O/H and Ne/H gradients are in much better agreement with each other, as expected from predictions of stellar nucleosynthesis. We examine the correlation between the WC/WN ratio and metallicity, and find that the new M33 abundances do not impact the observed correlation significantly. We also identify two new He II-emitting H II regions in M33, the first to be discovered in a spiral galaxy other than the Milky Way. In both cases the nebular He II emission is not associated with Wolf-Rayet stars. Therefore, caution is warranted in interpreting the relationship between nebular He II emission and Wolf-Rayet stars when both are observed in the integrated spectrum of an H II region.