The Evolving Physical Processes in Interacting Galaxies Traced by Their Spectral Energy Distributions
Mergers and interactions have profound effects on the evolution of galaxies and on the various physical processes associated with star formation and the fueling of active nuclei (AGN). There remains, however, an incomplete understanding of how interactions affect such processes or how important they are in controlling the appearance of today's universe. Recent Harvard grad Lauranne Lanz for her thesis successfully completed a multi-band analyses (GALEX-Herschel) of set of 31 galaxies in 14 merger groups. This initial set contained all the Keel-Kennicutt mergers for which suitable Herschel data were in the archive when she began her project several years ago. Our conclusions, published in Lanz et al. (2013), demonstrate the power of the techniques to trace physical activity across the merger sequence. The conclusions, however, were limited by the small numbers. Today the available dataset is much larger and, with the end of Herschel, is in some sense complete.
We are now starting a study of 180 interacting galaxies in 101 systems spanning early to late stage mergers for which newly archived NASA data enable detailed analyses of their ultraviolet-to-far infrared (UV-FIR) spectral energy distributions (SEDs). Our goal is an improved understanding of how a wide range of key galaxy parameters vary across the interaction sequence. Our derived physical parameters will include the total optical infrared luminosity, star formation rate, specific star formation rate, stellar mass, dust temperatures and dust masses, compactness, photo-dissociation region (PDR) fractions, and AGN contributions to the FIR SED. Our sample is taken from the Keel-Kennicutt catalog of merging galaxies (based only on apparent galaxy separations and hence free of morphological bias) and the Surace IRAS sample of bright mergers. Our sample contains virtually all bright mergers with UV-FIR data in the archives, including (but not limited to) data from missions GALEX, Swift, Spitzer, WISE, and Herschel.
Our analysis plan emphasizes three new SED modeling tools, one of which we have recently developed. Nearly all of the sources also have Spitzer IRS spectral data (primarily of the circumnuclear regions). One new area of emphasis is the mid-IR spectral region. We will use the IRS data to supplement the SED conclusions via our own Bayesian algorithm which also infers metallicity, interstellar medium (ISM) ambient pressure, and embedded young star fractions.
Finally, we will compare each merger to the simulated photometry/ morphology of a suite of simulations based on Hernquist et al. models. SEDs for the simulations will be calculated in each case using the SUNRISE radiative transfer code. We will identify the simulation and interacting stage/age that best reproduces each observed merger’s properties. The results will allow us to refine and test how physical parameters develop in mergers of various kinds, as well as help test the reliability of the simulation assumptions. We will in addition analyze 25 non-interacting galaxies as comparisons.
Our new, much larger sample will enable us to reach much firmer statistical conclusions, and to address the processes in intermediate interaction stages across a much wider range of galaxy mass-ratios and impact parameters. Our catalog of ~23-band UV-FIR photometry on these 180 (+25) galaxies will constitute a legacy contribution. The conclusions will have application for mergers in the more distant universe. Other members of the current team include: Lauranne Lanz, Chris Hayward, Andreas Zezas, Rafael Martinez, Matt Ashby, and Andres Guzman, Steve Willner, and Volker Tolls. We expect that 1-2 students will also participate.
Fitting a typical SED; points are phtometric data, and the black line is the model fit (the blue line shows the stellar contribution before extinction is included)
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