Introduction

The study of galactic evolution has matured enormously over the past 15 years, producing a paradigm which integrates the theory of galaxy formation and evolution with the larger theory of cosmology and structure formation. However, while the broad outlines of galaxy evolution have taken shape, the details have yet to be filled in. Our understanding of the exact physical processes that drive the growth of galaxies — most notably star formation and its interaction with the ISM — are acutely limited.

Spitzer has the power to provide critical insight into these mechanisms, via its ability to (1) probe the component of the UV and optical light that is reprocessed into the infrared; (2) characterize the physical state of the dusty ISM; and (3) map the structure of old, red stellar populations. Spitzer observations already have begun to achieve this objective, thanks to the Spitzer Infrared Nearby Galaxies Survey (SINGS ; Kennicutt et al. 2003) and other programs. However, these Spitzer studies are incomplete, and show the common observational bias toward massive, metal-rich, and high surface brightness galaxies. What sampling does exist for lower mass systems is sparse, and is far from representative, despite the fact that this population offers the greatest diversity of properties, the best-measured star formation histories, and hence optimal leverage for elucidating the processes that underlie star formation and shape the properties of galaxies. Gaining this optimal leverage depends on (1) maximizing the range of metallicities, masses, star formation rates (SFRs), star formation histories, dust contents, ISM properties, and internal kinematics in a given sample, and (2) gaining access to a large and representative number of galaxies, to provide the statistical power needed to separate dependencies among multiple variables.

Luckily, ideal samples on which this leverage can be built now exist. Superb, volume-limited studies of galaxies, such as our 11 Mpc Hα Ultraviolet Galaxy Survey (11HUGS) and ACS Nearby Galaxy Survey Treasury (ANGST) programs, provide robust datasets for probing the properties of star formation and the ISM. The galaxies in these samples span the entire range of star formation modes and host galaxy properties, and provide a complete and statistically unbiased view of the Local Volume population. Further, the galaxies, being among our nearest neighbors, offer the best spatial resolution and faintest absolute detection limits possible. Moreover, 11HUGS and ANGST supply a rich suite of multi-wavelength data including Hα and GALEX UV imaging, stellar population mapping with HST, HI mapping with the VLA B, C & D arrays, and broad-band optical and NIR imaging. The addition of Spitzer observations would complete the full SED coverage for these galaxies, and finally secure a true Legacy dataset for the Local Volume.

Science Goals: Demographics of Stars and Dust in Galaxies

The multi-wavelength UV-to-FIR SED census provided by the Local Volume Legacy will be the definitive core dataset on the Galactic neighborhood for at least the next decade, and will enable the community to make progress on a wide range of astrophysical problems. Here, we outline some of the principal science issues to be addressed by our team, with emphasis on those that exploit the unique properties of a volume-limited dataset.

Diagnostics and Demographics of Star Formation and Starbursts

The inventory of UV-to-FIR SEDs produced by the Local Volume Legacy will contain essential information about the SFR. The combination of UV, Hα and infrared observations encompasses all the light emitted by short-lived massive stars; the first two of these components trace light emitted by O&B stars, whereas the FIR luminosity captures the light absorbed and re-radiated by dust. We will use these SED's to derive accurate spatially-resolved SFRs that are independent of extinction (which can be substantial even in low-luminosity galaxies; e.g., Houck et al. 2004, Cannon et al. 2005; 2006a, b). Members of our team have used this technique to calibrate extinction-corrected SFR indices for HII regions and galaxies (e.g. Calzetti et al. 2005; Kennicutt et al. 2007a, b). The resulting star formation rates are vastly superior to the constant-factor extinction corrections (or guesses) that have traditionally been applied to Hα and UV-based SFRs (Kennicutt 1998).

As the first application of these data we will extend the calibration of the suite of SFR diagnostics (UV continuum, Hα, 8μm emission, 24μm and FIR continuum, radio continuum) initiated by SINGS and previous programs, over a more complete range of host galaxy metallicities and star formation properties. A full understanding of the dependences of the zeropoints, scatter, and systematic errors of these methods is critical, if we are to apply these SFR and extinction estimators to high-redshift objects, whose metallicities, physical conditions, and star formation modes can differ markedly from those at the present day.

A second major program will assess the demographics of star formation and starbursts in the local galaxy population. First, we will construct and analyze volume-averaged UV-to-FIR SEDs of star-forming galaxies as a function of mass, SFR, and morphological type. Second, we will apply the unique power of a volume-complete sample, and use the observed distribution of current SFRs to calculate the frequencies of starbursts in a given mass range, providing a direct measurement of the duty cycle. We have performed an initial duty cycle analysis using the Hα data from 11HUGS (Lee 2006), but major uncertainties remain due to the lack of complete extinction information. These undertakings demand the LVL unbiased, complete sample — they are simply impossible with datasets such as SINGS alone, which only contain a representative sampling of galaxies, rather than all star-forming galaxies in a given volume. Finally, we will produce a spatially resolved temporal characterization of the different modes of star formation (e.g., bursts vs. continuous) by exploiting the range of timescales for different SFR tracers, such as the emission from the FUV, NUV and optical recombination lines. Dust extinction strongly biases each of these tracers, however, so observations of the re-radiated light in the mid- and far-infrared are crucial.

FIR Emission and Dust in Galaxies Across the Luminosity Function

The proposed MIPS observations of the LVL sample will provide us with the first comprehensive inventory of the dusty ISM in galaxies of all masses, gas contents, stellar surface densities, metallicities, and star formation histories. These data will have a notable impact on our understanding of the dust contents and FIR SEDs of dwarf irregular galaxies, only a third of which were detected with IRAS, but most of which we expect to be detected with MIPS based on our past experience with SINGS and other GTO projects.

These limited observations show that dust emission is not connected in any simple way to galaxy mass, gas content, or metallicity (e.g., Dale et al. 2007, Walter et al. 2007). For example the second most metal-poor dIrr galaxy known, SBS0335-052, has an infrared luminosity of more than 10⁹ L_sun and embedded super star clusters with visual extinctions of at least 12 mag (e.g., Houck et al. 2004). Given that the dependence of dust content on metallicity and gas mass is non-trivial, the other factors controlling the dust-to-gas ratio must be identified. To do so, we will characterize the infrared SEDs, warm dust masses, and temperature distributions across the extensive range of physical conditions covered by our local sample, and identify the processes that drive changes in the dust-to-gas ratio. This work will benefit from high-resolution HI maps available or being obtained with the VLA, GMRT and ATCA for more than 100 of our galaxies.

Beyond constructing integrated galaxy SEDs, the proximity of the LVL sample allows us to create spatially-resolved maps of the dust mass and total SFR. The resulting maps offer the best opportunity outside the Local Group to cleanly separate individual dusty regions, many of which will have local star formation histories measured from color-magnitude diagrams (CMDs) constructed from HST imaging. These maps will also constrain the physical nature of the dust heating in galaxies, and the lifetimes of the heating populations in particular. By correlating the ages of the underlying stellar populations (from ANGST) with the observed infrared emission, we can empirically calibrate the duration over which FIR emission is detectable. These measurements can be made for hundreds of different regions within the sample galaxies, for a wide variety of metallicities and starburst amplitudes.

PAH Emission and Star Formation in Galaxies

Emission from small "PAH" molecules/grains dominates the mid-infrared radiation of most nearby galaxies, and has become a primary SFR indicator for high-redshift dusty galaxies. Understanding the physical factors that influence the strength of the PAH emission, and establishing its reliability as a SFR tracer is thus of paramount importance. ISO and Spitzer have shown that PAH emission is generally well-correlated with independent measures of the SFR in luminous and metal-rich galaxies (e.g., Roussel et al. 2001, Forster-Schreiber et al. 2004, Dale et al. 2005, 2007), but weakens or disappears altogether in low-luminosity galaxies with metallicities below 0.3–0.5 solar (e.g., Engelbracht et al. 2005, Rosenberg et al. 2006, Wu et al. 2006). However, other factors such as the strength and hardness of the local radiation field also influence the PAH band strengths (Madden et al. 2006 and refs above). Separating these effects has proven difficult, due to limited numbers of observations and a strong bias toward compact starburst galaxies.

Our IRAC imaging will make significant inroads to this important problem. Our sample spans the metallicity range over which the PAH emission changes, and covers large dynamic ranges (10⁵) in the total SFR, the SFR/area, and the UV-radiation intensity. Our proposed observations will include sufficient numbers of galaxies to separate the effects of metallicity, the hardness of the radiation field, and the star formation rate. Specifically, we will use the IRAC 8.0μm, IRAC 4.5μm, and MIPS 24μm images to generate continuum-subtracted maps of the 7.7μm PAH emission. We will compare these spatially well-resolved maps to the local strength and hardness of the background UV radiation field (as measured from our GALEX, Hα, and HST imaging) and with metallicity (as measured from optical spectra). We are well aware from our experience with SINGS and the MIPS and IRS GTO projects that continuum-subtracted 8μm band maps are no substitute for spectra, and need to be interpreted conservatively when the PAH emission is weak. However with proper allowance for uncertainties these data will significantly increase the available constraints on PAH emission strengths in unusual environments, and complement the many ongoing spectroscopic studies of PAH emission in galaxies.

Stellar Structure and Populations of Dwarf Galaxies

Infrared starlight is a critical probe of the underlying stellar mass of a galaxy. At IRAC's 3.6 and 4.5 μm wavelengths, galaxies are dominated by a scattering of bright AGB stars and M supergiants, superimposed on a diffuse sheet of red giant branch stars (Cannon et al. 2006). These smoothly distributed older stellar populations are one of the most robust tracers of the stellar mass of the galaxy, but they are largely undetectable from the ground for galaxies with low masses and low surface brightnesses. What little near-infrared imaging has been done has been directed directed at Local Group galaxies or blue compact starburst galaxies (e.g., Noeske et al. 2005 and references therein).

LVL will bring the unprecedented infrared surface brightness sensitivity of IRAC to bear on measuring the stellar masses of a truly unbiased set of the nearest galaxies. From space, the IR sky background is dramatically reduced, such that our imaging strategy produces S/N ratios of >10 at the optical (R₂₅) radii of the disks, far beyond the radius where NIR observations are effective from the ground. We will use IRACs 3.6 and 4.5 μm bands to map the locations of AGB and M-supergiants and to trace the spatial distribution of underlying stellar mass. These stellar masses will be an important parameter in the dust and star formation studies discussed above, and their spatial distribution will provide essential information about the background density field in which the interaction between star formation and the ISM is taking place. In addition, we will use the subset of the LVL sample with resolved stellar populations from HST to calibrate the relationship between IRAC colors & luminosities and the star formation history and metallicity of the underlying stellar populations. Only the LVL sample is sufficiently large, close, and diverse to provide the volume of data necessary for such a calibration.

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