Daniela Calzetti

SINGS: The SIRTF Nearby Galaxies Survey

The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging and spectroscopic survey of 75 nearby galaxies. Its primary goal is to characterize the infrared emission of galaxies and their principal infrared-emitting components, across a broad range of galaxy properties and star formation environments. SINGS will provide new insights into the physical processes connecting star formation to the interstellar medium properties of galaxies and provide a vital foundation for understanding infrared observations of the distant universe and ultraluminous and active galaxies. The galaxy sample and observing strategy have been designed to maximize the scientific and archival value of the data set for the SIRTF user community at large. The SIRTF images and spectra will be supplemented by a comprehensive multiwavelength library of ancillary and complementary observations, including radio continuum, H i, CO, submillimeter, BVRIJHK ,H a ,P aa, ultraviolet, and X-ray data. This paper describes the main astrophysical issues to be addressed by SINGS, the galaxy sample and the observing strategy, and the SIRTF and other ancillary data products.

Dust Absorption and the Ultraviolet Luminosity Density at z 3 as Calibrated by Local Starburst Galaxies

We refine a technique to measure the absorption-corrected ultraviolet (UV) luminosity of starburst galaxies using rest-frame UV quantities alone and apply it to Lyman-limit U dropouts at z ? 3 found in the Hubble Deep Field (HDF). The method is based on an observed correlation between the ratio of far-infrared (FIR) to UV fluxes with spectral slope ? (a UV color). A simple fit to this relation allows the UV flux absorbed by dust and reprocessed to the FIR to be calculated, and hence the dust-free UV luminosity to be determined. International Ultraviolet Explorer spectra and Infrared Astronomical Satellite fluxes of local starbursts are used to calibrate the FFIR/F1600 versus ? relation in terms of A1600 (the dust absorption at 1600 ?) and the transformation from broadband photometric color to ?. Both calibrations are almost completely independent of theoretical stellar-population models. We show that the recent marginal and nondetections of HDF U dropouts at radio and submillimeter wavelengths are consistent with their assumed starburst nature and our calculated A1600. This is also true of recent observations of the ratio of optical emission-line flux to UV flux density in the brightest U dropouts. This latter ratio turns out not to be a good indicator of dust extinction. In U dropouts, absolute magnitude M1600,0 correlates with ?: brighter galaxies are redder, as is observed to be the case for local starburst galaxies. This suggests that a mass-metallicity relationship is already in place at z ? 3. The absorption-corrected UV luminosity function of U dropouts extends up to M1600,0 ? -24 AB mag, corresponding to a star formation rate ~200 ? yr-1 (H0 = 50 km s-1 Mpc-3 and q0 = 0.5 are assumed throughout). The absorption-corrected UV luminosity density at z ? 3 is ?1600,0 ? 1.4 ? 1027 ergs-1 Hz-1 Mpc-1. It is still a lower limit since completeness corrections have not been done and because only galaxies with A1600 3.6 mag are blue enough in the UV to be selected as U dropouts. The luminosity-weighted mean dust-absorption factor of our sample is 5.4 ? 0.9 at 1600 ?.

THE MID-INFRARED SPECTRUM OF STAR-FORMING GALAXIES: GLOBAL PROPERTIES OF POLYCYCLIC AROMATIC HYDROCARBON EMISSION

We present a sample of low-resolution 5-38 μm Spitzer IRS spectra of the inner few square kiloparsecs of 59 nearby galaxies spanning a large range of star formation properties. A robust method for decomposing mid-infrared galaxy spectra is described and used to explore the behavior of PAH emission and the prevalence of silicate dust extinction. Evidence for silicate extinction is found in ~1/8 of the sample, at strengths that indicate that most normal galaxies undergo A_V ≲ 3 mag averaged over their centers. The contribution of PAH emission to the total infrared power is found to peak near 10% and extend up to ~20% and is suppressed at metallicities Z ≲ Z_☉/4, as well as in low-luminosity AGN environments. Strong interband PAH feature strength variations (2-5 times) are observed, with the presence of a weak AGN and, to a lesser degree, increasing metallicity shifting power to the longer wavelength bands. A peculiar PAH emission spectrum with markedly diminished 5-8 μm features arises among the sample solely in systems with relatively hard radiation fields harboring low-luminosity AGNs. The AGNs may modify the emitting grain distribution and provide the direct excitation source of the unusual PAH emission, which cautions against using absolute PAH strength to estimate star formation rates in systems harboring active nuclei. Alternatively, the low star formation intensity often associated with weak AGNs may affect the spectrum. The effect of variations in the mid-infrared spectrum on broadband infrared surveys is modeled and points to more than a factor of 2 uncertainty in results that assume a fixed PAH emission spectrum, for redshifts z = 0-2.5.

The Dust Opacity of Star‐forming Galaxies

ABSTRACT The presence of dust in galaxies removes one‐half or more of the stellar energy from the UV‐optical budget of the universe and has a profound impact on our understanding of how galaxies evolve. Measures of opacity in local galaxies are reviewed together with widely used theoretical and empirical methods for quantifying its effects. Existing evidence shows that the dust content of nearby galaxies depends not only on their morphology but also on their luminosity and activity level. A digression is devoted to starbursts in view of their potential relevance for measures of opacity in distant galaxies. Scarcity of coherent multiwavelength data sets hampers our ability to derive reliable obscuration estimates in intermediate‐ and high‐redshift galaxies. This, in turn, limits the reliability of inferred physical quantities, such as star formation rates, stellar population ages, galaxy luminosity functions, and others.

Star Formation in NGC 5194 (M51a): The Panchromatic View from GALEX to Spitzer*

(Abridged) Far ultraviolet to far infrared images of the nearby galaxy NGC5194, from Spitzer, GALEX, Hubble Space Telescope and ground--based data, are used to investigate local and global star formation, and the impact of dust extinction in HII-emitting knots. In the IR/UV-UV color plane, the NGC5194 HII knots show the same trend observed for normal star-forming galaxies, having a much larger dispersion than starburst galaxies. We identify the dispersion as due to the UV emission predominantly tracing the evolved, non-ionizing stellar population, up to ages 50-100 Myr. While in starbursts the UV light traces the current SFR, in NGC5194 it traces a combination of current and recent-past SFR. Unlike the UV emission, the monochromatic 24 micron luminosity is an accurate local SFR tracer for the HII knots in NGC5194; this suggests that the 24 micron emission carriers are mainly heated by the young, ionizing stars. However, preliminary results show that the ratio of the 24 micron emission to the SFR varies by a factor of a few from galaxy to galaxy. While also correlated with star formation, the 8 micron emission is not directly proportional to the number of ionizing photons. This confirms earlier suggestions that the carriers of the 8 micron emission are heated by more than one mechanism.

An Ultraviolet-to-Radio Broadband Spectral Atlas of Nearby Galaxies

The ultraviolet-to-radio continuum spectral energy distributions are presented for all 75 galaxies in the Spitzer Infrared Nearby Galaxies Survey (SINGS). A principal component analysis of the sample shows that most of the samples spectral variations stem from two underlying components, one representative of a galaxy with a low infrared-to-ultraviolet ratio and one representative of a galaxy with a high infrared-to-ultraviolet ratio. The influence of several parameters on the infrared-to-ultraviolet ratio is studied (e.g., optical morphology, disk inclination, far-infrared color, ultraviolet spectral slope, and star formation history). Consistent with our understanding of normal star-forming galaxies, the SINGS sample of galaxies in comparison to more actively star-forming galaxies exhibits a larger dispersion in the infrared-to-ultraviolet versus ultraviolet spectral slope correlation. Early-type galaxies, exhibiting low star formation rates and high optical surface brightnesses, have the most discrepant infrared-to-ultraviolet correlation. These results suggest that the star formation history may be the dominant regulator of the broadband spectral variations between galaxies. Finally, a new discovery shows that the 24 μm morphology can be a useful tool for parameterizing the global dust temperature and ultraviolet extinction in nearby galaxies. The dust emission in dwarf/irregular galaxies is clumpy and warm accompanied by low ultraviolet extinction, while in spiral galaxies there is typically a much larger diffuse component of cooler dust and average ultraviolet extinction. For galaxies with nuclear 24 μm emission, the dust temperature and ultraviolet extinction are relatively high compared to disk galaxies.

The COSMOS Survey: Hubble Space Telescope Advanced Camera for Surveys Observations and Data Processing

We describe the details of the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) observations of the COSMOS field, including the data calibration and processing procedures. We obtained a total of 583 orbits of HST ACS/WFC imaging in the F814W filter, covering a field that is 1.64 deg^2 in area, the largest contiguous field ever imaged with HST. The median exposure depth across the field is 2028 s (one HST orbit), achieving a limiting point-source depth AB(F814W) = 27.2 (5 σ). We also present details of the astrometric image registration and distortion removal and image combination using MultiDrizzle, motivating the choice of our final pixel scale (30 mas pixel^(-1)), based on the requirements for weak-lensing science. The final set of images are publicly available through the archive sites at IPAC and STScI, along with further documentation on how they were produced.

An atlas of ultraviolet spectra of star-forming galaxies

A systematic study is presented of the UV spectra of star-forming galaxies of different morphological type and activity class using a sample drawn from a uniformly reduced IUE data set. The spectra for a wide variety of galaxies, including normal spiral, LINER, starburst, blue compact, blue compact dwarf, and Seyfert 2 galaxies, are presented in the form of spectral energy distributions to demonstrate the overall characteristics according to morphology and activity class and in the form of absolute flux distributions to better show the absorption and emission features of individual objects. The data support the picture based on UV spectra of the Orbiting Astronomical Observatory and of the Astronautical Netherlands Satellite that spiral galaxies of later Hubble class have more flux at the shortest UV wavelengths than do spiral galaxies of earlier Hubble class.

Dust in Starburst Galaxies

To investigate the nature of starburst dust, we constructed a model of the stars and dust in starburst galaxies and applied it to 30 observed starburst spectral energy distributions (SEDs). The starburst model was constructed by combining two stellar evolutionary synthesis models with a model describing the radiative transfer of stellar photons through dust. The stellar evolutionary synthesis models were used to compute the dust-free SEDs for stellar populations with ages between 1 ? 106 and 15 ? 109 yr. Using a Monte Carlo radiative transfer model, the effects of dust were computed for average Milky Way (MW) and Small Magellanic Cloud (SMC) dust, two different star/dust geometries, and locally homogeneous or clumpy dust. Using color-color plots, the starburst model was used to interpret the behavior of 30 starbursts with aperture-matched UV and optical SEDs (and IR for 19 of the 30) from previous studies. From the color-color plots, it was evident that the dust in starbursts has an extinction curve lacking a 2175 ? bump, like the SMC curve, and a steep far-UV rise, intermediate between the MW and SMC curves. The star/dust geometry that is able to explain the distribution of the 30 starbursts in various color-color plots has an inner dust-free sphere of stars surrounded by an outer star-free shell of clumpy dust. When combined with other work from the literature on the Orion region and the 30 Dor region of the Large Magellanic Cloud, this work implies a trend in dust properties with star formation intensity.

Far-Infrared Galaxies in the Far-Ultraviolet*

In an effort to better understand the UV properties of ultraluminous infrared galaxies (ULIGs), and compare them to the rest-frame UV properties of high redshift submillimeter and Lyman-break galaxies, we have obtained far- and near-UV imaging observations (λeff = 1457 and 2364 A, respectively) of two luminous infrared galaxies (LIGs-VV 114 and IC 883) and five ULIGs (IRAS 08572+3915, Mrk 273, IRAS 15250+3609, Arp 220, and IRAS 19254-7245) using the Hubble Space Telescope. All the galaxies were detected in both channels. UV light, both diffuse and from star clusters, can be traced to within the inner kiloparsec of the dominant near-IR nuclei. However, in general, the brightest UV sources are clearly displaced from the I-band and near-IR peaks by at least hundreds of parsecs. Furthermore, only 0.07%-7.3% of the total near-UV light is projected within the inner 500 pc radius, even though this is the same region where most of the bolometric energy is generated. All nuclei are highly obscured by dust. Even after correction for dust reddening, the global UV emission fails to account for the total bolometric luminosities of these systems by factors of 3-75. The discrepancy is much worse if only the central regions, where the bolometric luminosities are generated, are included. In two cases (VV 114 and IRAS 08572+3915), the merging companion galaxies are more prominent in the UV than the more IR luminous member. While all our galaxies show possible signatures of active galactic nucleus (AGN) activity, only IRAS 19254-7245 yields even a possible detection of an AGN in our UV images. Simple calculations show that all but one of our galaxies would be expected to drop below the detection thresholds of, e.g., the Hubble Deep Fields at redshifts between 1.5 and 3, and we find that ~2 of our five ULIGs would be selected as extremely red objects in this redshift range. A typical ULIG in our sample would be too faint to be detected at high redshift in the deepest current optical or submillimeter deep surveys. Only VV 114 has UV luminosity and color similar to Lyman-break galaxies at z ~ 3; the other galaxies would be too faint and/or red to be selected by current surveys. The low UV brightnesses of our ULIGs mean that they would not appear as optically bright (or bright ERO) submillimeter galaxy counterparts, although they might be similar to the fainter submillimeter galaxy counterparts.