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
H 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
H
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
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, H
As the first application of these data we will extend the calibration
of the suite of SFR diagnostics (UV continuum, H
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 H
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.
,
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 H
and UV-based SFRs (Kennicutt 1998).
, 8
m
emission, 24
m 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.
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.
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
8.0 m, IRAC 4.5
m, and MIPS 24
m
images to generate continuum-subtracted
maps of the 7.7
m 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, H
,
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 8
m 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 m 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 m 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.
REFERENCES
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Cannon, J.M. et al. 2006a, ApJ, 647, 293
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