T. J. Sodroski

Detection and Characterization of Cold Interstellar Dust and Polycyclic Aromatic Hydrocarbon Emission, from COBE Observations

Using data obtained by the DIRBE instrument on the COBE spacecraft, we present the mean 3.5-240 μm spectrum of high-latitude dust. Combined with a spectrum obtained by the FIRAS instrument, these data represent the most comprehensive wavelength coverage of dust in the diffuse interstellar medium, spanning the 3.5-1000 μm wavelength regime. At wavelengths shorter than ~60 μm the spectrum shows an excess of emission over that expected from dust heated by the local interstellar radiation field and radiating at an equilibrium temperature. The DIRBE data thus extend the observations of this excess, first detected by the IRAS satellite at 25 and 12 μm, to shorter wavelengths. The excess emission arises from very small dust particles undergoing temperature fluctuations. However, the 3.5-4.9 μm intensity ratio cannot be reproduced by very small silicate or graphite grains. The DIRBE data strongly suggest that the 3.5-12 μm emission is produced by carriers of the ubiquitous 3.3, 6.2, 7.7, 8.6, and 11.3 μm solid state emission features that have been detected in a wide variety of astrophysical objects. The carriers of these features have been widely identified with polycyclic aromatic hydrocarbons (PAHs). Our dust model consists of a mixture of PAH molecules and bare astronomical silicate and graphite grains with optical properties given by Draine & Lee. We obtain a very good fit to the DIRBE spectrum, deriving the size distribution, abundances relative to the total hydrogen column density, and relative contribution of each dust component to the observed IR emission. At wavelengths above 140 μm the model is dominated by emission from T ≈ 17-20 K graphite and 15-18 K silicate grains. The model provides a good fit to the FIRAS spectrum in the 140-500 μm wavelength regime but leaves an excess Galactic emission component at 500-1000 μm. The nature of this component is still unresolved. We find that (C/H) is equal to (7.3 ± 2.2) × 10-5 for PAHs and equal to (2.5 ± 0.8) × 10-4 for graphite grains, requiring about 20% of the cosmic abundance of carbon to be locked up in PAHs, and about 70% in graphite grains [we adopt (C/H)☉ = 3.6 × 10-4]. The model also requires all of the available magnesium, silicon, and iron to be locked up in silicates. The power emitted by PAHs is 1.6 × 10-31 W per H atom, by graphite grains 3.0 × 10-31 W per H atom, and by silicates 1.4 × 10-31 W per H atom, adding up to a total infrared intensity of 6.0 × 10-31 W per H atom, or ~2 L☉ M. The [C II] 158 μm line emission detected by the FIRAS provides important information on the gas phase abundance of carbon in the diffuse ISM. The 158 μm line arises predominantly from the cold neutral medium (CNM) and shows that for typical CNM densities and temperatures C+/H = (0.5-1.0) × 10-4, which is ~14%-28% of the cosmic carbon abundance. The remaining carbon abundance in the CNM, which must be locked up in dust, is about equal to that required to provide the observed IR emission, consistent with notion that most (75%) of this emission arises from the neutral component of the diffuse ISM. The model provides a good fit to the general interstellar extinction curve. However, at UV wavelengths it predicts a larger extinction. The excess extinction may be the result of the UV properties adopted for the PAHs. If real, the excess UV extinction may be accounted for by changes in the relative abundances of PAHs and carriers of the 2200 A extinction bump.

A Three-dimensional Decomposition of the Infrared Emission from Dust in the Milky Way

We have constructed a three-dimensional model of the Galactic large-scale infrared emission from dust associated with the molecular neutral atomic (H I), and extended low-density cm~3) (H 2 ), (n e D 1E100 ionized (H II) gas phases of the interstellar medium. The model incorporates a three-dimensional map of the molecular and neutral atomic hydrogen gas distributions, derived from available 12CO and H I surveys by using the radial velocity information in the spectral lines as a distance indicator, and available 5 and 19 GHz radio continuum surveys to trace the column density of ionized gas. We use the model to decompose the Di†use Infrared Background Experiment (DIRBE) 12E240 km obserCOBE5 vations of the Galactic plane region ( o b o „ 5i), from which the zodiacal light and stellar emission have been subtracted, into distinct emission components associated with each gas phase within selected ranges of Galactocentric distance. An interstellar dust model is ‐tted to the resulting infrared spectra to derive the following quantities within each Galactocentric distance interval: (1) the abundance and equilibrium temperature of the large dust grain component within each gas phase; (2) estimates of the abundance of very small (\200 transiently heated dust grains and polycyclic aromatic hydrocarbon (PAH) mol”) ecules; and (3) constraints on various model parameters, such as the energy density of the ambient interstellar radiation ‐eld, which heats the dust within the H I gas phase. Our results show steep negative Galactocentric gradients in the equilibrium temperature of the large dust grain component within the H I, and H II gas phases, the GalaxyIs ambient interstellar radiH 2 , ation ‐eld, and the dust-to-gas mass ratio for each gas phase. The intensity of the ambient interstellar radiation ‐eld increases by a factor of D3 between the solar circle (8.5 kpc) and the molecular ring at a Galactocentric distance of D5 kpc. The dust abundance gradient of ([0.05 ^ 0.03) dex kpc~1 is equivalent, within the uncertainties, to the metallicity gradient in the Galactic disk. The derived emission spectra are consistent with a model in which very small transiently heated dust grains and PAHs are abundant and the dominant contributors to the mid-infrared (5 km \j\ 40 km) luminosity from a Galactocentric distance of 2 kpc out to a Galactocentric distance of at least 12 kpc, and indicate that the relative abundance of the PAHs is signi‐cantly higher in the outer region of the Galactic disk than inside the solar circle. We combine the results of our decomposition algorithm with the results of a study of optical extinction at high Galactic latitude to derive the radial distribution of optical opacity in the Galactic disk and ‐nd that our Galaxy would be e†ectively transparent Galaxy) \ 0.2 mag] to an external obser[A B (total ver viewing it at a low inclination (i \ 30i). All of the Galactic infrared emission observed by the DIRBE can be accounted for by dust associated with gas that is detected by current radio surveys, refuting the recent suggestion that a large fraction of the dynamically inferred hidden mass in spiral galaxies may be due to unseen gas and stars in the disk of the galaxies. Subject headings: di†use radiation E dust, extinction E Galaxy: structure E infrared: ISM: continuum E ISM: abundances E ISM: molecules

COBE diffuse infrared background experiment observations of the galactic bulge

Low angular resolution maps of the Galactic bulge at 1.25, 2.2, 3.5, and 4.9 micrometers obtained by the Diffuse Infrared Background Experiment (DIRBE) onboard NASAs Cosmic Background Explorer (COBE) are presented. After correction for extinction and subtraction of an empirical model for the Galactic disk, the surface brightness distribution of the bulge resembles a flattened ellipse with a minor-to-major axis ratio of approximately 0.6. The bulge minor axis scale height is found to be 2.1 deg +/- 0.2 deg for all four near-infrared wavelengths. Asymmetries in the longitudinal distribution of bulge brightness contours are qualitatively consistent with those expected for a triaxial bar with its near end in the first Galactic quadrant (0 deg less than l less than 90 deg). There is no evidence for an out-of-plane tilt of such a bar.

Large-scale characteristics of interstellar dust from COBE DIRBE observations

Observations from the COBE Diffuse Infrared Background Experiment of the 140 and 240 micrometer emissions from the Galatic plane region (absolute value of b less than 10 deg) are combined with radio surveys that trace the molecular (H2), neutral atomic (H I), and extended low-density (n(sub e) approximately 10 to 100/cm(exp 3)) ionized (H II) gas phases of the interstellar medium to derive physical conditions such as the dust temperature, dust-to-gas mass ratio, and far-infrared emissivity (1) averaged over these gas phases along each line of sight and (2) within each of these three gas phases. This analysis shows large-scale longitudinal and latitudinal gradients in the dust temperature and a decrease in dust temperature with increasing Galactocentric distance. The derived dust temperatures are significantly different from those derived in similar analyses using the Infrared Astronomical Satellite (IRAS) 60 and 100 micrometer data, suggesting that small (5 A approximately less than radius approximately less than 200 A) transiently heated dust particles contribute significantly o the Galactic 60 micrometer emission. It is found that 60% to 75% of the far-infrared luminosity arises from cold (approximately 17 to 22 K) dust associated with diffuse H I clouds, 15% to 30% from cold (approximately 19 K) dust associated with molecular gas, and less than 10% from warm (approximately 29 K) dust in extended low-density H II regions, consistent with the results of the IRAS analyses of the Galactic 60 and 100 micrometer emission. Within 2 deg of longitude of the Galactic center, the derived gas-to-dust mass ratio along the line of sight, G(sub d), reverses its general trend of decreasing G(sub d) toward the inner Galaxy and increases by a factor of approximately 2 to 3 toward the Galactic center. One possible explanation for this result is that the ratio of H2 column density to (12)CO intensity is lower in the Galactic center region than in the Galactic disk.

Dirbe evidence for a wrap in the interstellar dust layer and stellar disk of the galaxy

The Diffuse Infrared Background Experiment (DIRBE) of the Cosmic Background Explorer (COBE) has mapped the surface brightness distributions of the Galactic plane at wavelengths from 1.25 to 240 micrometers. In these maps the latitude of peak brightness, as a function of longitude, traces a roughly sinusoidal curve of period approximately 360 deg. In the far-infrared, where emission by interstellar dust dominates the surface brightness, this curve agrees well with that derived from maps of the velocity-integrated H 1, suggesting that the layers of dust and neutral atomic hydrogen are similarly displaced from the Galactic plane. In the near-infrared (lambda less than 5 micrometers), where old disk stars dominate the emission, the brightness crest exhibits the same phase but roughly half the amplitude. The reduced amplitude of the warp in stellar light could result from a lesser warping of the stellar disk, or from a more rapid falloff of the density of stars relative to the density of gas, possibly due to a radial truncation of the disk.

Dust energetics in the gas phases of the interstellar medium - The origin of the Galactic large-scale far-infrared emission observed by IRAS

The large-scale Galactic properties within the most massive gas phases of the ISM are derived along with the distribution of total Galactic FIR luminosity among the three phases. Most of the Galaxys total FIR luminosity is found to be emitted by cold dust associated with diffuse H I clouds and molecular gas. The total FIR luminosity of dust associated with extended low-density H II regions accounts for less than 10 percent of the Galaxys total FIR output. The absence of a significant variation with longitude in the observed temperature of the dust associated with H I gas suggests that diffuse neutral clouds contain very small dust grains that are stochastically heated by the interstellar radiation field. The results are consistent with a model in which most of the FIR luminosity of molecular clouds arises from dust that is associated with giant molecular clouds and heated by embedded and nearby OB stars. H II regions in the inner Galaxy have a mean IR excess ratio of about 2.5, suggesting that the dust in these regions is heated primarily by directly absorbed stellar photons.

COBE diffuse infrared background experiment observations of Galactic reddening and stellar populations

This Letter describes the results of an initial study of Galactic extinction and the colors of Galactic stellar populations in the near-IR using the Diffuse Infrared Background Experiment (DIRBE) aboard the Cosmic Background Explorer (COBE) spacecraft. The near-IR reddening observed by DIRBE is consistent with the extinction law tabulated by Rieke & Lebofsky (1985). The distribution of dust and stars in most of the first and fourth quadrants of the Galactic plane (0 deg less than l less than 90 deg, and 270 deg less than l less than 360 deg, respectively) can be modeled as a stellar background source seen through up to approximately 4 mag of extinction at 1.25 micrometers. The unreddened near-IR colors of the Galactic disk are similar to those of late-K and M giants. The Galactic bulge exhibits slightly bluer colors in the 2.2-3.5 micrometers range, as noted by Terndrup et al. (1991). Star-forming regions exhibit colors that indicate the presence of a approximately 900 K continuum produced by hot dust or polycyclic aromatic hydrocarbons (PAHs) contributing at wavelengths as short as 3.5 micrometers.

Large-scale Galactic dust morphology and physical conditions from IRAS observations

In this paper, the zodiacal component is subtracted from the 60 and 100 micron Galactic plane emission by applying an empirical model derived from IRAS data in regions of the sky not dominated by the Galaxy. The corrected observations are used to derive the large-scale physical conditions such as temperature, optical depth, and total FIR brightness of the dust residing in the Galactic disk. (C-12)O, H I, and 5 GHz radio continuum observations are also used to compare the large-scale properties of the gas and dust distributions in the Galaxy. Possible scenarios to explain the findings are suggested. 48 references.

The interstellar dust contribution to the diffuse infrared sky brightness

We use COBE/DIRBE data from which zodiacal light and Galactic stellar foregrounds have been removed in order to derive an all‐sky view of the Galactic cirrus. The dust is seen in emission from 3.5 to 240 μm; the scattered contribution at 1.25 and 2.2 μm is not readily discernible. Similar spatial structure allows the correlation of the dust signal in each DIRBE band with a DIRBE 100 μm cirrus template. The slopes of these correlations are used to derive an average dust spectrum at high Galactic latitudes. The effectiveness of using this average spectrum to remove the contribution of the dust to the diffuse infrared sky brightness is discussed. We also place the DIRBE‐derived average dust spectrum in context with observations in the submillimeter (from COBE/FIRAS) and optical.

Large‐scale physical conditions in the interstellar medium from DIRBE observations

Observations from the COBE Diffuse Infrared Background Experiment (DIRBE) of the 140 and 240 μm emission from the Galactic plane region (‖b‖<10°) are analyzed. Assuming an isothermal dust distribution along each line of sight, maps of dust temperature, optical depth, and total far‐infrared (FIR) brightness are derived. These quantities are combined with available 12CO and HI data to produce maps of the gas‐to‐dust mass ratio and the FIR luminosity per hydrogen mass. The results differ from those of a similar analysis using IRAS 60 and 100 μm data (Sodroski et al. 1987), at least in part because emission from small, transiently heated grains does not contribute significantly at 140 or 240 μm.A linear correlation analysis is used to decompose the 140 and 240 μm maps into components associated with the neutral atomic (HI), molecular (H2), and extended low‐density ionized (HII) gas phases of the interstellar medium. From the resulting emission components the large‐scale physical conditions within each gas pha...