1. Understanding Local Luminous Infrared Galaxies in the Herschel Era
Jason K. Chu
Institute for Astronomy, University of Hawai`i
Collaborators: David Sanders (IfA), Kirsten Larson (Caltech/IPAC), Joe Mazzarella (Caltech/IPAC)

2. Why Study Luminous IR Galaxies?
Hundreds of luminous infrared galaxies (LIRGs) first discovered in the 1980s.
They emit the bulk of their intrinsic luminosities in the infrared: LIR= L( μm).
Many are interacting/merging!
LIRGs are much more common in
the early universe!
Discovered by IRAS. It was very interesting because prior to their discovery, we had no idea this class of exotic objects even existed!
Very bright, these galaxies or galaxy systems can emit luminosities of up to a trillion times or more that of the Sun!
Many of these objects are found to be interacting and/or merging. This happens when the mutual gravity between two galaxies is strong enough that they collide and significantly changes their appearance.
These objects are also much more common in the early universe simply because back then the universe was smaller and so galaxies were closer together, and more likely to run into another galaxy resulting in a merger event. So our understanding of how they work in the local universe significantly affects our understanding of galaxy evolution in the distant universe, where LIRGs are a thousand times more common!

3. Infrared radiation originates from dust in galaxies.
GOALS is a complete, nearby sample of infrared luminous galaxies.
They represent a complete picture of galaxy evolution.
Very high star formation rates.
Rapid feeding of the central supermassive black holes.
Infrared radiation originates from dust in galaxies.
 Critical to study these galaxies in high resolution in the far-infrared, where they emit the bulk of their intrinsic luminosity.
GOALS selected from RBGS sample of 629 objects, which were selected by their IRAS S60 flux densities. GOALS sample are all LIRGs + ULIRGs up to a redshift of or D<400 Mpc. Observed in 21 cm, submillimeter, infrared, optical, as well as space based IR, optical, UV, and X-ray observations
They represent galaxies going through all the major interaction stages as well as infrared luminosity evolution. As galaxies merge the process can induce a huge amount of star formation very quickly, as well as the sudden feeding of the supermassive black holes in the centers of both galaxies. Both of these effects produce a huge amount of dust and dust heating, and as a result the elevated far-infrared luminosity is largely due to the intense heating of dust in merging galaxies. This is what produces the intense infrared signature that we associate with LIRGs.
Since the majority of the radiation is emitted in the FIR, it is paramount that we directly measure their infrared brightnesses. Before Herschel, our ability to probe the FIR brightnesses was limited to very low resolution studies. An alternative is to use NIR data, which is much easier to obtain, as an estimator of FIR luminosities, but they aren’t always correlated, especially for extreme objects such as LIRGs.

4. Herschel-GOALS Observations
Entire GOALS sample imaged by PACS and SPIRE instruments on board the Herschel Space Observatory.
Imaging obtained between 70 – 500 μm in wavelength.
Herschel has better resolution than all previous far-infrared space missions.
Observations have to be made from space because Earth’s atmosphere blocks FIR light.
Extends wavelength coverage into the never-before-seen regime between FIR and sub-mm light. Previously the longest FIR wavelength is 160, and the shortest sub-mm wavelength is 450.
3) Herschel is the last in a long line of famous infrared missions and it has allowed us to better characterize the energy output of LIRGs in the FIR, where they emit the bulk of their radiation.
Chu+ 2017

5. Results: The Herschel-GOALS Atlas
Maps of all ~200 GOALS systems have been published for all six Herschel bands.
1) The published paper also includes the brightnesses for each system at all six different wavelengths. Here we show an optical image from the Pan-STARRS telescope on Haleakala in Maui, of a merging system in our sample called Arp 84. You can see some obvious spiral structures in both galaxies although the top left one is clearly disturbed and warped. However in the Herschel far-infrared image the view is completely different! The dark dust lanes in the optical image of the larger spiral galaxy now show up as a prominent ring of hot dust emission due to intense star formation that is hidden from view in the optical, while the bright optical spiral features are largely invisible. The nucleus of the smaller galaxy shines brightly in the far-infrared due to dust heated by both intense star formation and a growing massive black hole.

6. Results: Infrared Spectra of (U)LIRGs
Here we will show the power of our new Herschel data. We divided our sample into 6 different levels of infrared brightness. On the x-axis is the wavelength from microns, which is a huge range. Generally NIR is about 3-10 microns, MIR is microns, FIR is microns, and submillimeter is microns. For a sense of scale, human hair is about 0.1mm or 100 microns thick.

7. Results: Infrared Spectra of (U)LIRGs
IRAS

8. Results: Infrared Spectra of (U)LIRGs
IRAS + Spitzer + WISE

9. Results: Infrared Spectra of (U)LIRGs
IRAS + Spitzer + WISE + Herschel
Now that we have the full infrared picture of what LIRGs look like, we can accurately calculate many different physical properties such as the mass of the dust emitting the FIR light, the temperature of that dust, and also star formation rates on scales smaller than the galaxies. These data can also help decipher the phases in which the central supermassive black holes grow the fastest, by looking at the amount of dust heating in the galaxy’s nucleus.

10. Summary
Our Herschel observations of a complete sample of nearby LIRGs represent the highest resolution far-infrared study of these colliding, exotic galaxies.
We can now accurately calculate many important far-infrared galaxy properties that was previously impossible.
Or, the Tweet-able version:
We obtain the best far-infrared maps of local luminous infrared galaxies allowing us to study galaxy properties previously impossible to do.
Contact: Jason Chu
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