Samir Salim has a lot of space to fit into a new NASA-funded
database; about 11 million galaxies of it would be a ballpark estimate based on
the number of galaxies for which distances can be estimated to about 3.5
billion light years, what astronomers still refer to as the relatively
“local” universe. But the Indiana University astronomer and research
scientist believes the vast archives, produced by NASA space telescopes and
ground-based observatories, hold the right information to create the largest
resource ever for the study of how star formation proceeds in galaxies.
By integrating the measurements of the electromagnetic
radiation of galaxies from four projects—the two space missions Galaxy
Evolution Explorer (GALEX) and Wide-Field Infrared Survey Explorer (WISE), and
two ground-based projects, the Sloan Digital Sky Survey (SDSS) and the Two Micron
All-Sky Survey (2MASS)—an international team led by Salim hopes to use novel
galaxy modeling techniques to produce a single database of the physical properties
of millions of galaxies.
That database will involve constructing spectral energy
distributions (SEDs) for 11 million galaxies, a resource that will use
wide-field, multi-wavelength observations at a scale that does not currently
exist. The SED of a galaxy is the quantification of the electromagnetic
radiation it distributes over the full range of frequency and wavelength, from
ultraviolet to far infrared, and analysis of that multi-wavelength radiation is
a primary means of learning about star formation and evolution.
The team may have selfish reasons for developing such a huge
database: They want to focus on so-called transitional galaxies that have
subdued star formation activity and that may be transforming from what are
known as spiral galaxies to more bland-looking elliptical galaxies. They argue
that the study of star formation regulation requires large galaxy samples in
order to identify and analyze trends for robust statistical significance.
“And in order to reveal the physical processes behind
star formation regulation, these large samples require reliable,
well-calibrated sets of fundamental galaxy physical properties like star
formation rates, stellar mass, dust content, stellar age and stellar
metallicity,” Salim says. “For this purpose we proposed building a
database of galaxies in the local universe combing the data from GALEX, 2MASS,
WISE, and SDSS, based on which we will derive these physical properties using
the techniques of modeling galaxy evolution. Both the SEDs and the physical
parameters will be included in the database that will be made publicly
Each of the four studies provides a unique type of data to
the creation of the SEDs. The GALEX photometry, for example, used a telescope
that swept the sky for ultraviolet light sources, or frequencies higher than
those humans identify as the color violet. It provided the necessary
sensitivity to low levels of star formation that will be needed to separate
transitional galaxies from true “red and dead” passive galaxies.
Salim, who joined IU’s Astronomy Department in September
2009, is no stranger to GALEX and its primary science mission of studying
galaxy evolution. After earning his PhD from Ohio
in 2002, he went on to work as a GALEX postdoctoral researcher at University of California,
until 2006. Before coming to IU, he was at the National Optical Astronomy
Observatory (NOAO) in Tucson
as a research associate, where he compared and contrasted star-forming
properties of galaxies derived from GALEX with mid-infrared emissions recorded
by the Spitzer Space Telescope.
Work on the database will begin next year and is expected to
run into 2015, with NASA expected to provide about $330,000 for the work.
Recent results from Salim’s continuing work employing multi-wavelength
observations to study star formation have included the discovery of
observational evidence for active galactic nuclei feedback in transitional
galaxies; establishing of the robust SED fitting technique to derive galaxy’s
star formation rates and stellar masses; identification of intermediate-age
populations as the significant source of mid-infrared dust heating; and the
discovery of early-type galaxies with extended star forming regions.
SOURCE – Indiana University