D. Kunze

Massive Star Formation and Evolution in Starburst Galaxies: Mid-infrared Spectroscopy with the ISO Short Wavelength Spectrometer*

We present new Infrared Space Observatory Short Wavelength Spectrometer data for a sample of 27 starburst galaxies, and with these data we examine the issues of formation and evolution of the most massive stars in starburst galaxies. Using starburst models which incorporate time evolution, new stellar atmosphere models for massive stars, and a starburst model geometry derived from observations of the prototypical starburst M82, we model the integrated mid-infrared line ratio (Ne III)(15.6 km)/(Ne II)(12.8 km). This line ratio is sensitive to the hardness of the stellar energy distribution and therefore to the most massive stars present. We conclude from our models, with consideration of recent determinations of the stellar census in local, high-mass star-forming regions, that the (Ne III)/(Ne II) ratios we measure are consistent with the formation of massive (D50¨100 stars in most starbursts. In this framework, M _ ) the low nebular excitation inferred from the measured line ratios can be attributed to aging eUects. By including estimates of the ratio of infrared-to-Lyman continuum luminosity for the galaxies in our sample, we further —nd that most starbursts are relatively short-lived (106¨107 yr), only a few O star lifetimes. We discuss a possible cause of such short events: the eUectiveness of stellar winds and super- novae in destroying the starburst environment. Subject headings: galaxies: starburstinfrared: galaxiesstars: atmospheresstars: evolution ¨ stars: formationtechniques: spectroscopic

Near-Infrared Integral Field Spectroscopy and Mid-Infrared Spectroscopy of the Starburst Galaxy M82*

We present new infrared observations of the central regions of the starburst galaxy M82. The observations consist of near-infrared integral field spectroscopy in the H and K bands obtained with the MPE 3D instrument and of λ = 2.4-45 μm spectroscopy from the Short Wavelength Spectrometer (SWS) onboard the Infrared Space Observatory. These measurements are used, together with data from the literature, to (1) reexamine the controversial issue of extinction, (2) determine the physical conditions of the interstellar medium (ISM) within the star-forming regions, and (3) characterize the composition of the stellar populations. Our results provide a set of constraints for detailed starburst modeling, which we present in a companion paper. We find that purely foreground extinction cannot reproduce the global relative intensities of H recombination lines from optical to radio wavelengths. A good fit is provided by a homogeneous mixture of dust and sources, and with a visual extinction of AV = 52 mag. The SWS data provide evidence for deviations from commonly assumed extinction laws between 3 and 10 μm. The fine-structure lines of Ne, Ar, and S detected with SWS imply an electron density of ≈300 cm-3, and abundance ratios Ne/H and Ar/H nearly solar and S/H about one-fourth solar. The excitation of the ionized gas indicates an average effective temperature for the OB stars of 37,400 K, with little spatial variation across the starburst regions. We find that a random distribution of closely packed gas clouds and ionizing clusters and an ionization parameter of ≈10-2.3 represent well the star-forming regions on spatial scales ranging from a few tens to a few hundreds of parsecs. From detailed population synthesis and the mass-to-K-light ratio, we conclude that the near-infrared continuum emission across the starburst regions is dominated by red supergiants with average effective temperatures ranging from 3600 to 4500 K and roughly solar metallicity. Our data rule out significant contributions from older, metal-rich giants in the central few tens of parsecs of M82.

ISO Spectroscopy of Ultraluminous Infrared Galaxies

Spectroscopy from the Infrared Space Observatory ISO has for the first timeprovided the sensitivity to exploit the diagnostic power ofmid-infrared fine structure lines and PAH features for the study ofultraluminous infrared galaxies (LIR >1012L⊙). We report on observations obtainedwith SWS, ISOPHOT-S, and the CVF option of ISOCAM. From both fine structure lines and PAH features, we find that the majority of ULIRGs is predominantlypowered by star formation. Our total sample of about 75 ULIRGs allows tosearch for trends within the class of ULIRGs: The fraction of AGNs increaseswith luminosity above ∼ 3 × 1012L⊙ but there is no obvioustrend for ULIRGs to be more AGN-like with more advanced merger phase.

[ITAL]Infrared Space Observatory[/ITAL] Observations toward the Reflection Nebula NGC 7023: A Nonequilibrium Ortho–to–Para-H[TINF]2[/TINF] Ratio

We have observed the S(0), S(1), S(2), S(3), S(4), and S(5) rotational lines of molecular hydrogen (H2) toward the peak of the photodissociation region (PDR) associated with the reflection nebula NGC 7023. The observed H2 line ratios show that they arise in warm gas with kinetic temperatures ~300-700 K. However, the data cannot be fitted by an ortho-to-para-H2 ratio of 3. An ortho-to-para (OTP) ratio in the range ~1.5-2 is necessary to explain our observations. This is the first detection of a nonequilibrium OTP ratio measured from the H2 pure rotational lines in a PDR. The existence of a dynamical PDR is discussed as the most likely explanation for this low OTP ratio.

Infrared Space Observatory Observations toward the Reflection Nebula NGC 7023: A Nonequilibrium Ortho-to-Para-H2 Ratio*

We have observed the S(0), S(1), S(2), S(3), S(4), and S(5) rotational lines of molecular hydrogen (H2) toward the peak of the photodissociation region (PDR) associated with the reflection nebula NGC 7023. The observed H2 line ratios show that they arise in warm gas with kinetic temperatures ~300-700 K. However, the data cannot be fitted by an ortho-to-para-H2 ratio of 3. An ortho-to-para (OTP) ratio in the range ~1.5-2 is necessary to explain our observations. This is the first detection of a nonequilibrium OTP ratio measured from the H2 pure rotational lines in a PDR. The existence of a dynamical PDR is discussed as the most likely explanation for this low OTP ratio.

ISO spectroscopy of nearby starburst galaxies

We present spectroscopic mid-IR observations of prominent starburst galaxies obtained with the Short Wavelength Spectrometer onboard the Infrared Space Observatory . The wavelength range accessible by ISO -SWS (2.5–45 μm) contains a large number of emission lines of atomic and molecular hydrogen and several other atomic species. In this paper we discuss the interpretation of the [Ne III] 15.5 μm/[Ne II] 12.8 μm line-ratio, the faint [OIV]25.9 μm line and the pure rotational lines of molecular hydrogen observed in our target galaxies.

Star Formation Histories of Starbursts

We present results of a mid-infrared ISO spectroscopic survey of 27 starburst galaxies, and of near-infrared integral field spectroscopy and mid-infrared ISO spectroscopy of the starburst galaxy M 82. Together with the application of starburst models, the data are consistent with the formation of massive stars (≳50 – 100 M⊙) in starburst environments, and support short decay timescales of starburst episodes (~ 106 – 107 yr) indicating important negative feedback effects of starburst activity.

ISO observations toward the reflection nebula NGC 7023: A nonequilibrium ortho- to para-H2 ratio

We have observed the S(0), S(1), S(2), S(3), S(4), and S(5) rotational lines of molecular hydrogen (H2) toward the peak of the photodissociation region (PDR) associated with the reflection nebula NGC 7023. The observed H2 line ratios show that they arise in warm gas with kinetic temperatures ~300-700 K. However, the data cannot be fitted by an ortho-to-para-H2 ratio of 3. An ortho-to-para (OTP) ratio in the range ~1.5-2 is necessary to explain our observations. This is the first detection of a nonequilibrium OTP ratio measured from the H2 pure rotational lines in a PDR. The existence of a dynamical PDR is discussed as the most likely explanation for this low OTP ratio.