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Ultra-Luminous Infrared Galaxies
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Jackpot! Cosmic Magnifying-Glass Effect Captures Universe's Brightest Galaxies

Astronomers were fascinated in the 1980s with the discovery of nearby dust-enshrouded galaxies that glowed thousands of times brighter than our Milky Way galaxy in infrared light. Dubbed ultra-luminous infrared galaxies, they were star-making factories, churning out a prodigious amount of stars every year. What wasn't initially clear was what powered these giant infrared light bulbs. Observations by the Hubble Space Telescope helped astronomers confirm the source of the galaxies' light output. Many of them reside within "nests" of galaxies engaged in multiple pile-ups of three, four or even five galaxies. The dust is produced by the firestorm of star birth, which glows fiercely in infrared light.
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RE: ULIRGs
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Title: AKARI Near-Infrared Spectroscopy of Luminous Infrared Galaxies
Authors: Jong Chul Lee (KASI), Ho Seong Hwang (CfA), Myung Gyoon Lee (SNU), Minjin Kim (KASI), Joon Hyeop Lee (KASI)

We present the AKARI near-infrared (NIR; 2.5-5 micron) spectroscopic study of 36 (ultra)luminous infrared galaxies [(U)LIRGs] at z=0.01-0.4. We measure the NIR spectral features including the strengths of 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission and hydrogen recombination lines (Br\alpha. and Br\beta), optical depths at 3.1 and 3.4 micron, and NIR continuum slope. These spectral features are used to identify optically elusive, buried AGN. We find that half of the (U)LIRGs optically classified as non-Seyferts show AGN signatures in their NIR spectra. Using a combined sample of (U)LIRGs with NIR spectra in the literature, we measure the contribution of buried AGN to the infrared luminosity from the SED-fitting to the IRAS photometry. The contribution of these buried AGN to the infrared luminosity is 5-10%, smaller than the typical AGN contribution of (U)LIRGs including Seyfert galaxies (10-40%). We show that NIR continuum slopes correlate well with WISE [3.4]-[4.6] colours, which would be useful for identifying a large number of buried AGN using the WISE data.

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Ultra-Luminous Infrared Galaxies
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Title: A Spitzer Study of Interacting Luminous and Ultra-Luminous Infrared Galaxies
Authors: Joćo Rodrigo S. Lećo, Claus Leitherer

We conducted a Spitzer Space Telescope survey of 28 Luminous (11 < log(LIR/L_odot) < 12, LIRGs) and Ultra-Luminous Infrared Galaxies (log(LIR/L_odot) > 12, ULIRGs). Many of these galaxies are found in pairs or associations and are powered by either nuclear activity or starformation (Sanders & Mirabel 1996). Our main goal is to understand the relative importance of starbursts and AGNs in interacting systems. Is the frequency of AGN and starbursts in these interacting galaxies related to their luminosities? What is the importance of the merger stage and the frequency of AGNs? We present our conclusions and diagnostic diagrams based in the observed near infrared lines and compare to studies based solely in optical data.

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RE: ULIRGs
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Title: Extranuclear Halpha-emitting complexes in low-z (U)LIRGs: Precursors of tidal dwarf galaxies?
Authors: D. Miralles-Caballero, L. Colina, S. Arribas

This paper characterises the physical and kinematic properties of external massive star-forming regions in a sample of (U)LIRGs. We use high angular resolution ACS images from the HST B and I bands, as well as Halpha-line emission maps obtained with IFS. We find 31 external Halpha-emitting (young star-forming) complexes in 11 (U)LIRGs. These complexes have in general similar sizes, luminosities, and metallicities to extragalactic giant HII regions and TDG candidates found in less luminous mergers and compact groups of galaxies. We assess the mass content and the likelihood of survival as TDGs of the 22 complexes with simple structures in the HST images based on their photometric, structural, and kinematic properties. The dynamical tracers used (radius-sigma and luminosity-sigma diagrams) indicate that most of the complexes might be self-gravitating entities. The resistance to forces from the parent galaxy is studied by considering the tidal mass of the candidate and its relative velocity with respect to the parent galaxy. After combining the results of previous studies of TDG searches in ULIRGs a total of 9 complexes satisfy most of the applied criteria and thus show a high-medium or high likelihood of survival, their total mass likely being compatible with that of dwarf galaxies. They are defined as TDG candidates. We propose that they probably formed more often during the early phases of the interaction. Combining all data for complexes with IFS data where a significant fraction of the system is covered, we infer a TDG production rate of 0.3 candidates with the highest probabilities of survival per system for the (U)LIRGs class. This rate, though, might decrease to 0.1 after the systems in (U)LIRGs have evolved for 10 Gyr, for long-lived TDGs, which would imply that no more than 5-10 % of the overall dwarf population could be of tidal origin.

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RE: Ultraluminous infrared galaxies
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Title: A Physical Model for z~2 Dust Obscured Galaxies
Authors: Desika Narayanan (CfA), Arjun Dey (NOAO), Christopher Hayward (CfA), Thomas J. Cox (CfA), R. Shane Bussmann (Arizona), Mark Brodwin (CfA), Patrik Jonsson (UCSC), Philip Hopkins (UC Berkeley), Brent Groves (Leiden), Joshua D. Younger (IAS), Lars Hernquist (CfA)
(13 Oct 2009, Version v2)

We present a physical model for the origin of z~2 Dust-Obscured Galaxies (DOGs), a class of high-redshift ULIRGs selected at 24 micron which are particularly optically faint (F24/FR>1000). By combining N-body/SPH simulations of high redshift galaxy evolution with 3D polychromatic dust radiative transfer models, we find that luminous DOGs (with F24 > 0.3 mJy at z~2) are well-modelled as extreme gas-rich mergers in massive (~5x10^12-10^13 Msun) halos, with elevated star formation rates (~500-1000 Msun/yr) and/or significant AGN growth (Mdot(BH) > 0.5 Msun/yr), whereas less luminous DOGs are more diverse in nature. Merger-driven DOGs are caught in a stage transitioning from being starburst dominated to AGN dominated, evolving from a "bump" to a power-law shaped mid-IR (IRAC) spectral energy distribution (SED). While canonically power-law galaxies are associated with being AGN-dominated, we find that the power-law mid-IR SED can owe both to direct AGN contribution, as well as to a heavily dust obscured stellar bump at times that the galaxy is starburst dominated. Thus power-law galaxies can be either starburst or AGN dominated. Less luminous DOGs (100 < F24 < 300 microJy at z=2) can be well-represented either by mergers, or by less extreme secularly evolving gas-rich disk galaxies (with SFR > 50 Msun/yr). We find that some merger-driven DOGs can be selected as Submillimeter Galaxies (SMGs), while both merger-driven and secularly evolving DOGs typically satisfy the BzK selection criteria. Our models provide testable predictions of the physical masses, dust temperatures, CO line widths and location on the M*-MBH relation of DOGs. Finally, we provide public SED templates derived from these simulations.

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Dense Gas in Ultraluminous Infrared Galaxies

Ultraluminous infrared galaxies have luminosities that exceed a trillion suns. (For comparison, the Milky Way's luminosity is only that of about ten billion suns.) Extreme infrared activity is known to be associated with interacting galaxies, and optical imaging indeed shows that many ultraluminous systems are in collision. The physical mechanism(s) that actually power the luminosity, however, are still not understood. Might the same process(es) be underway at a low level in our galaxy?
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The Energy Sources of Ultraluminous Galaxies
Ultraluminous infrared galaxies (ULIRGs) are galaxies whose luminosity exceeds that of a trillion suns; for comparison, the Milky Way galaxy has a typical (and much more modest) luminosity of only about ten billion suns. ULIRGs were discovered by an all-sky infrared survey satellite in the 1980's, and since then the origin(s) of their huge infrared emission has been widely debated. Extreme infrared activity is known to be associated with interacting galaxies, and optical imaging indeed shows that many ULIRGs are in collision, but this fact does not answer the question of what physical mechanism powers the luminosity. Might the same process be underway at a low level in our galaxy?

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ULIRGs
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The Spitzer Space Telescope has observed a rare population of colliding galaxies whose entangled hearts are wrapped in tiny crystals resembling crushed glass.

The crystals are essentially sand, or silicate, grains that were formed like glass, probably in the stellar equivalent of furnaces. This is the first time silicate crystals have been detected in a galaxy outside of our own.

"We were surprised to find such delicate, little crystals in the centres of some of the most violent places in the universe. Crystals like these are easily destroyed, but in this case, they are probably being churned out by massive, dying stars faster than they are disappearing" - Dr. Henrik Spoon, Cornell University, Ithaca, N.Y.

Dr. Henrik Spoon is first author of a paper on the research appearing in the Feb. 20 issue of the Astrophysical Journal.
The discovery will ultimately help astronomers better understand the evolution of galaxies, including our Milky Way, which will merge with the nearby Andromeda galaxy billions of years from now.

"It's as though there's a huge dust storm taking place at the centre of merging galaxies. The silicates get kicked up and wrap the galaxies' nuclei in giant, dusty glass blankets" - Dr. Lee Armus, a co-author of the paper from the Spitzer Science Centre at the California Institute of Technology in Pasadena.

Silicates, like glass, require heat to transform into crystals. The gem-like particles can be found in the Milky Way in limited quantities around certain types of stars, such as our sun. On Earth, they sparkle in sandy beaches, and at night, they can be seen smashing into our atmosphere with other dust particles as shooting stars. Recently, the crystals were also observed by Spitzer inside comet Tempel 1, which was hit by NASA's Deep Impact probe.
The crystal-coated galaxies observed by Spitzer are quite different from our Milky Way. These bright and dusty galaxies, called ultraluminous infrared galaxies, or "Ulirgs," are swimming in silicate crystals. While a small fraction of the Ulirgs cannot be seen clearly enough to characterize, most consist of two spiral-shaped galaxies in the process of merging into one. Their jumbled cores are hectic places, often bursting with massive, newborn stars. Some Ulirgs are dominated by central supermassive black holes.

So, where are all the crystals coming from? Astronomers believe the massive stars at the galaxies' centres are the main manufacturers. According to Spoon and his team, these stars probably shed the crystals both before and as they blow apart in fiery explosions called supernovae. But the delicate crystals won't be around for long. The scientists say that particles from supernova blasts will bombard and convert the crystals back to a shapeless form. This whole process is thought to be relatively short-lived.

"Imagine two flour trucks crashing into each other and kicking up a temporary white cloud. With Spitzer, we're seeing a temporary cloud of crystallized silicates created when two galaxies smashed together" - Dr. Henrik Spoon .


Spectral lines from distant ultra-luminous infrared galaxies, as recorded by the Spitzer Space Telescope's infrared spectrograph, show the telltale bumps (in green) indicating the presence of crystalline silicates.
Credit: NASA/JPL-Caltech/H. W. W. Spoon (Cornell University)


Spitzer's infrared spectrograph spotted the silicate crystals in 21 of 77 Ulirgs studied. The 21 galaxies range from 240 million to 5.9 billion light-years away and are scattered across the sky. Spoon said the galaxies were most likely caught at just the right time to see the crystals. The other 56 galaxies might be about to kick up the substance, or the substance could have already settled.

Position (2000): RA: 09h00m25.4s Dec: +39d03m54s

Others authors of this work include Drs. A.G.G.M. Tielens and J. Cami of NASA's Ames Research Centre, Moffett Field, California; Drs. G.C. Sloan and Jim R. Houck of Cornell; B. Sargent of the University of Rochester, N.Y.; Dr. V. Charmandaris of the University of Crete, Greece; and Dr. B.T. Soifer of the Spitzer Science Centre.

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