M. W. Werner

Metallicity Effects on Dust Properties in Starbursting Galaxies

We present infrared observations of 66 starburst galaxies over the full range of oxygen abundances observed in local star-forming galaxies, from -->12 + log (O/H) = 7.1 to 8.9. The data include imaging and spectroscopy from the Spitzer Space Telescope, supplemented by ground-based near-infrared imaging. We confirm a strong correlation of aromatic emission with metallicity, with a threshold at -->12 + log (O/H) ~ 8. We show that the far-infrared color temperature of the large dust grains increases toward lower metallicity, peaking at a metallicity of 8 before turning over. We compute dust masses and compare them to H I masses from the literature to derive the ratio of atomic gas to dust, which increases by nearly 3 orders of magnitude between solar metallicity and a metallicity of 8, below which it flattens out. The abrupt change in aromatic emission at mid-infrared wavelengths thus appears to be reflected in the far-infrared properties, indicating that metallicity changes affect the composition of the full range of dust grain sizes that dominate the infrared emission. Although the great majority of galaxies show similar patterns of behavior as described above, there are three exceptions, SBS 0335?052E, Haro 11, and SHOC 391. Their infrared SEDs are dominated energetically by the mid-IR near 24 ?m rather than by the 60-200 ?m region. In addition, they have very weak near-infrared outputs and their SEDs are dominated by emission by dust at wavelengths as short as 1.8 ?m. The latter behavior indicates that the dominant star-forming episodes in them are extremely young. The component of the ISM responsible for the usual far-infrared emission appears to be either missing or inefficiently heated in these three galaxies.

Metallicity Effects on Mid-Infrared Colors and the 8 μm PAH Emission in Galaxies

We examine colors from 3.6 to 24 μm as a function of metallicity (O/H) for a sample of 34 galaxies. The galaxies range over 2 orders of magnitude in metallicity. They display an abrupt shift in the 8 μm-to-24 μm color for metallicities between one-third and one-fifth of the solar value. The mean 8-to-24 μm flux density ratio below and above 12 + log (O/H) = 8.2 is 0.08 ± 0.04 and 0.70 ± 0.53, respectively. We use mid-IR colors and spectroscopy to demonstrate that the shift is primarily due to a decrease in the 8 μm flux density, as opposed to an increase in the 24 μm flux density. This result is most simply interpreted as being due to a weakening at low metallicity of the mid-IR emission bands usually attributed to PAHs (polycyclic aromatic hydrocarbons) relative to the small-grain dust emission. However, existing empirical spectral energy distribution models cannot account for the observed short-wavelength (below 8 μm) colors of the low-metallicity galaxies merely by reducing the strength of the PAH features; some other emission source (e.g., hot dust) is required.

Infrared Space Observatory Measurements of [C II] Line Variations in Galaxies

We report measurements of the [C II] fine-structure line at 157.714 ?m in 30 normal star-forming galaxies with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). The ratio of the line to total far-infrared (FIR) luminosity, LC II/LFIR, measures the ratio of the cooling of gas to that of dust, and thus the efficiency of the grain photoelectric heating process. This ratio varies by more than a factor of 40 in the current sample. About two-thirds of the galaxies have LC II/LFIR ratios in the narrow range of (2-7) ? 10 -->?3. The other one-third show trends of decreasing LC II/LFIR with increasing dust temperature, as measured by the flux ratio of infrared emission at 60 and 100 ?m, F?(60 ?m)/F?(100 ?m), and with increasing star formation activity, measured by the ratio of FIR and blue-band luminosity, LFIR/L -->B. We also find three FIR-bright galaxies that are deficient in the [C II] line, which is undetected with 3 ? upper limits of LC II/LFIR ?4. The trend in the LC II/LFIR ratio with the temperature of dust and with star formation activity may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively charged, decreasing the yield and the energy of the photoelectrons. The three galaxies with no observed photodissociation region lines have among the highest LFIR/L -->B and F?(60 ?m)/F?(100 ?m) ratios. Their lack of [C II] lines may be due to a continuing trend of decreasing LC II/LFIR with increasing star formation activity and dust temperature seen in one-third of the sample with warm IRAS colors. In that case, the upper limits on LC II/LFIR imply a ratio of UV flux to gas density of G -->0/n>10 cm -->3 (where G -->0 is in units of the local average interstellar field). The low LC II/LFIR ratio could also be due to either weak [C II], owing to self-absorption, or a strong FIR continuum from regions weak in [C II], such as dense H II regions or plasma ionized by hard radiation of active galactic nuclei. The mid-infrared and radio images of these galaxies show that most of the emission comes from a compact nucleus. CO and H I are detected in these galaxies, with H I seen in absorption toward the nucleus.

New Infrared Emission Features and Spectral Variations in Ngc 7023

We observed the reflection nebula NGC 7023, with the Short-High module and the long-slit Short-Low and Long-Low modules of the Infrared Spectrograph on the Spitzer Space Telescope. We also present Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) images of NGC 7023 at 3.6, 4.5, 8.0, and 24 μm. We observe the aromatic emission features (AEFs) at 6.2, 7.7, 8.6, 11.3, and 12.7 μm, plus a wealth of weaker features. We find new unidentified interstellar emission features at 6.7, 10.1, 15.8, 17.4, and 19.0 μm. Possible identifications include aromatic hydrocarbons or nanoparticles of unknown mineralogy. We see variations in relative feature strengths, central wavelengths, and feature widths, in the AEFs and weaker emission features, depending on both distance from the star and nebular position (southeast vs. northwest).

Infrared Space Observatory Mid-Infrared Spectra of Reflection Nebulae*

We present 5-15 μm imaging spectroscopy of the reflection nebulae vdB 17 (NGC 1333), vdB 59 (NGC 2068), vdB 101, vdB 111, vdB 133, and vdB 135, obtained with the infrared camera and circular variable filter wheel on the Infrared Space Observatory (ISO). These nebulae are illuminated by stars with Teff = 3,600-19,000 K, implying UV (λ < 400 nm) to total stellar flux ratios of F(λ<400 nm)/Ftotal = 0.01-0.87. We detect the infrared emission features (IEFs) at 6.2, 7.7, 8.6, 11.3, and 12.7 μm, broad emission features at 6-9 μm and 11-13 μm, and 5-15 μm continuum emission, from the interstellar medium in vdB 17, vdB 59, and vdB 133 (F(λ<400 nm)/Ftotal = 0.22-0.87), and place upper limits on the emission from the interstellar medium in vdB 101, vdB 111, and vdB 135 (F(λ<400 nm)/Ftotal = 0.01-0.20). Our goal is to test predictions of models attributing the IEFs to polycyclic aromatic hydrocarbons (PAHs). Interstellar models predict PAHs change from singly ionized to neutral as the UV intensity, G0, decreases. The ratio of PAH emission at 6-10 μm to PAH emission at 10-14 μm is expected to be 10 times higher in ionized than in neutral PAHs. We observe no spectroscopic differences with varying Teff. We analyze the spectra of vdB 17 and vdB 59 as a function of distance from the star to see how the spectra depend on G0 within each source. The only quantitative difference we find is a broadening of the 7.7 μm IEF at G0 = 20-60 within vdB 17. We observe only a 40% change in the 6-10 μm to 10-14 μm flux ratio over G0 = 20-6 × 104.

Extended near infrared emission from visual reflection nebulae

Extended near infrared (2 to 5 microns) emission was observed from three visual reflection nebulae, NGC 7023, 2023, and 2068. The emission from each nebula consists of a smooth continuum, which can be described by a greybody with a color temperature of 1000 K, and emission features at 3.3 and 3.4 microns. The continuum emission cannot be explained by free-free emission, reflected light, or field stars, or by thermal emission from grains, with commonly accepted ratios of infrared to ultraviolet emissivities, which are in equilibrium with the stellar radiation field. A possible explanation is thermal emission from grains with extremely low ratios of infrared to ultraviolet emissivities, or from grains with a temperature determined by mechanisms other than equilibrium radiative heating. Another possibility is continuum fluorescence. Previously announced in STAR N83-25629

Far-infrared observations of M17SW - The clumpy structure of the photodissociation region

Forbidden O I 63-micron and forbidden Si II 35-micron fine-structure line emission in M17SW was mapped, and the intensities of the forbidden O I 63 and 146 microns, forbidden Si II 35 microns, and forbidden C II 158 microns were measured at four positions. New 50- and 100-micron continuum maps of the M17SW cloud at comparable resolution to the FIR line observations are presented. Analysis in terms of a homogeneous model yields an incident UV field of 56,000 habings, a density of 30,000 cu cm, and a temperature of about 300 K for the atomic gas. It is concluded that the M17SW photodissociation region is clumpy in nature. The observed forbidden Si II and high-J CO imply the presence of high-density clumps. The clumps dominate the emission in the forbidden O I, Si II, and high-level CO lines, while the forbidden C II, C I, and low-level CO arise mostly in the interclump gas. The extended (about 15 pc) forbidden C II and forbidden C I emission is attributed to the halo gas.

The Dust and Gas Around β Pictoris

We have obtained Spitzer IRS 5.5-35 ?m spectroscopy of the debris disk around ? Pictoris. In addition to the 10 ?m silicate emission feature originally observed from the ground, we also detect the crystalline silicate emission bands at 28 and 33.5 ?m. This is the first time that the silicate bands at wavelengths longer than 10 ?m have ever been seen in the ? Pictoris disk. The observed dust emission is well reproduced by a dust model consisting of fluffy cometary and crystalline olivine aggregates. We searched for line emission from molecular hydrogen and atomic [S I], Fe II, and Si II gas but detected none. We place a 3 ? upper limit of <17 M? on the H2 S(1) gas mass, assuming an excitation temperature of Tex = 100 K. This suggests that there is less gas in this system than is required to form the envelope of Jupiter. We hypothesize that some of the atomic Na I gas observed in Keplerian rotation around ? Pictoris may be produced by photon-stimulated desorption from circumstellar dust grains.

Do the Infrared Emission Features Need Ultraviolet Excitation

We present the results of imaging spectroscopy of the reflection nebula vdB 133, obtained with the infrared camera and circular variable filter wheel on the Infrared Space Observatory. Our observations reveal the infrared emission features (IEFs), at 6.2, 7.7, 8.7, 11.3, and 12.7 μm, and associated 5-15 μm continuum emission. The stellar system illuminating vdB 133 has the lowest ratio of ultraviolet (shortward of 0.4 μm) to total flux of any stars demonstrated to date to excite the IEFs and associated continuum emission from adjacent interstellar dust, as opposed to circumstellar dust. The low fraction of UV flux from this system poses a problem for existing models for the emission mechanism and emitting material, which all require substantial UV radiation for the excitation of the IEFs and associated continuum.

The Relationship between the Optical Depth of the 9.7 μm Silicate Absorption Feature and Infrared Differential Extinction in Dense Clouds

We have examined the relationship between the optical depth of the 9.7 μm silicate absorption feature (τ_(9.7)) and the near-infrared color excess, E(J - K_s), in the Serpens, Taurus, IC 5146, Chameleon I, Barnard 59, and Barnard 68 dense clouds/cores. Our data set, based largely on Spitzer IRS spectra, spans E(J - K_s) = 0.3-10 mag (corresponding to visual extinction between about 2 and 60 mag). All lines of sight show the 9.7 μm silicate feature. Unlike in the diffuse ISM where a tight linear correlation between the 9.7 μm silicate feature optical depth and the extinction (A_V) is observed, we find that the silicate feature in dense clouds does not show a monotonic increase with extinction. Thus, in dense clouds, τ_(9.7) is not a good measure of total dust column density. With few exceptions, the measured τ_(9.7) values fall well below the diffuse ISM correlation line for E(J - K_s) > 2 mag (A_V > 12 mag). Grain growth via coagulation is a likely cause of this effect.