What Powers the 12 μm Luminosities in AGNs: Spitzer/IRS Spectroscopic Study of the 12 μm Seyfert Sample

We present a mid-IR study of the 12 μm Seyfert sample, using 5-35 μm low-resolution spectroscopy from Spitzer/IRS. Sources in this sample display a wide variety of spectral shapes. We perform an analysis of the continuum emission, the strength of the Polycyclic Aromatic Hydrocarbon (PAH) emission, as well as fine-structure lines, in order to study the mid-IR properties of the local Seyfert galaxies. We find that the equivalent widths of PAHs decrease with increasing dust temperature. We also propose a method to estimate the AGN contribution to the integrated 12 μm emission of the galaxy

Intergalactic Star Formation

Star formation in interacting systems may take place in various locations, from the dust--enshrouded core of Ultraluminous Infrared Galaxies to more unusual places such as the debris of colliding galaxies expelled into the intergalactic medium. Determining whether star-formation proceeds in the latter environment, far from the parent galaxies, in a similar way as in spiral disks has motivated the multi--wavelength study presented here. We collected VLA/HI, UV/GALEX, optical Halpha and MIR/Spitzer images of a few nearby interacting systems chosen for their prominent "intergalactic" star formation activity. Preliminary results on the spectacular collisional HI ring around NGC 5291 are presented.Comment: 4 pages, 1 fig., tp appear in conference proceedings "Studying Galaxy Evolution with Spitzer and Herschel", eds. V. Charmandaris, D. Rigopoulou & N. Kylafi

Missing GRB host galaxies in deep mid-infrared observations: implications on the use of GRBs as star formation tracers

We report on the first mid-infrared observations of 16 GRB host galaxies performed with the Spitzer Space Telescope, and investigate the presence of evolved stellar populations and dust-enshrouded star-forming activity associated with GRBs. Only a very small fraction of our sample is detected by Spitzer, which is not consistent with recent works suggesting the presence of a GRB host population dominated by massive and strongly-starbursting galaxies (SFR >~ 100M[sun]yr^–1). Should the GRB hosts be representative of star-forming galaxies at high redshift, models of galaxy evolution indicate that >~ 50% of GRB hosts would be easily detected at the depth of our mid-infrared observations. Unless our sample suffers from a strong observational bias which remains to be understood, we infer in this context that the GRBs identified with the current techniques can not be directly used as unbiased probes of the global and integrated star formation history of the Universe

Mid-IR properties of Seyferts: Spitzer/IRS spectroscopy of the IRAS 12micron Seyfert sample

We performed an analysis of the mid-infared properties of the 12micron Seyfert sample, a complete unbiased 12micron flux limited sample of local Seyfert galaxies selected from the IRAS Faint Source Catalog, based on low resolution spectra obtained with the Infrared Spectrograph (IRS) on-board Spitzer Space Telescope. A detailed presentation of this analysis is dicussed in Wu et al. (2009). We find that on average, the 15-30micron slope of the continuum is -0.85+/-0.61 for Seyfert 1s and -1.53+/-0.84 for Seyfert 2s, and there is substantial scatter in each type. Moreover, nearly 32% of Seyfert 1s, and 9% of Seyfert 2s, display a peak in the mid-infrared spectrum at 20micron, which is attributed to an additional hot dust component. The Polycyclic Aromatic Hydrocarbon (PAH) equivalent width decreases with increasing dust temperature, as indicated by the global infrared color of the host galaxies. However, no statistical difference in PAH equivalent width is detected between the two Seyfert types, 1 and 2, of the same bolometric luminosity. Finally, we propose a new method to estimate the AGN contribution to the integrated 12micron galaxy emission, by subtracting the "star formation" component in the Seyfert galaxies, making use of the tight correlation between PAH 11.2micron luminosity and 12micron luminosity for star forming galaxies.Comment: Oral Contribution to Proceedings of IAU Symp. 267, "Co-evolution of Central Black Holes and Galaxies", 6 pages, 2 figure

The [Ne III]/[Ne II] line ratio in NGC 253

We present results of the mapping of the nucleus of the starburst galaxy NGC 253 and its immediate surroundings using the Infrared Spectrograph on board the Spitzer Space Telescope. The map is centered on the nucleus of the galaxy and spans the inner 800 × 688 pc^2. We perform a brief investigation of the implications of these measurement on the properties of the star formation in this region using theories developed to explain the deficiency of massive stars in starbursts

ALMA Maps of Dust and Warm Dense Gas Emission in the Starburst Galaxy IC 5179⋆^\star

We present our high-resolution ($0^{\prime\prime}.15\times0^{\prime\prime}.13$, $\sim$34 pc) observations of the CO(6-5) line emission, which probes the warm and dense molecular gas, and the 434 $\mu$m dust continuum emission in the nuclear region of the starburst galaxy IC 5179, conducted with the Atacama Large Millimeter Array (ALMA). The CO(6-5) emission is spatially distributed in filamentary structures with many dense cores and shows a velocity field that is characteristic of a circum-nuclear rotating gas disk, with 90% of the rotation speed arising within a radius of $\lesssim150$ pc. At the scale of our spatial resolution, the CO(6-5) and dust emission peaks do not always coincide, with their surface brightness ratio varying by a factor of $\sim$10. This result suggests that their excitation mechanisms are likely different, as further evidenced by the Southwest to Northeast spatial gradient of both CO-to-dust continuum ratio and Pa-$\alpha$ equivalent width. Within the nuclear region (radius$\sim$300 pc) and with a resolution of $\sim$34 pc, the CO line flux (dust flux density) detected in our ALMA observations is $180\pm18$ Jy km/s ($71\pm7$ mJy), which account for 22% (2.4%) of the total value measured by Herschel.Comment: Accepted for publication in Ap

Large-scale Star Formation Triggering in the Low-mass Arp 82 System: A Nearby Example of Galaxy Downsizing Based on UV/Optical/Mid-IR Imaging

As part of our Spitzer Spirals, Bridges, and Tails project to help understand the effects of galaxy interactions on star formation, we analyze GALEX ultraviolet, SARA optical, and Spitzer infrared images of the interacting galaxy pair Arp 82 (NGC 2535/6) and compare to a numerical simulation of the interaction. We investigate the multiwavelength properties of several individual star forming complexes (clumps). Using optical and UV colors, EW(Halpha), and population synthesis models we constrain the ages of the clumps and find that the median clump age is about 12 Myr. The clumps have masses ranging from a few times 10^6 to 10^9 solar masses. In general, the clumps in the tidal features have similar ages to those in the spiral region, but are less massive. The 8 micron and 24 micron luminosities are used to estimate the far-infrared luminosities and the star formation rates of the clumps. The total clump star formation rate is 2.0+/-0.8 solar masses per year, while the entire Arp 82 system is forming stars at a rate of 4.9+/-2.0 solar masses per year. We find, for the first time, stars in the HI arc to the southeast of the NGC 2535 disk. Population synthesis models indicate that all of the observed populations have young to intermediate ages. We conclude that although the gas disks and some old stars may have formed early-on, the progenitors are late-type or low surface brightness and the evolution of these galaxies was halted until the recent encounter.Comment: Accepted for publication in the AJ, 22 Figures, 5 Table

Probing the Evolution of the Galaxy Interaction/Merger Rate Using Collisional Ring Galaxies

We present the results from our program to determine the evolution of the galaxy interaction/merger rate with redshift using the unique star-forming characteristics of collisional ring galaxies. We have identified 25 distant collisional ring galaxy candidates (CRGCs) in a total of 162 deep Hubble Space Telescope Wide Field/Planetary Camera-2 images obtained from the HST Archives. Based on measured and estimated redshifts, these 25 CRGCs all lie in the redshift interval of 0.1 < z < 1. Using the local collisional ring galaxy volume density and the new ``standard'' cosmology, we find that in order to account for the number of identified CRGCs in our surveyed fields, the galaxy interaction/merger rate, parameterized as (1 + z)^m, must increase steeply with redshift.We determine a minimum value of m = 5.2 $\pm$ 0.7, though m could be as high as 7 or 8. We can rule out a non-evolving (m = 0) and weakly evolving (m = 1-2) galaxy interaction/merger rate at greater than the 4 sigma level of confidence.Comment: Accepted in the Astrophysical Journal (11 pages, 4 figures). Higher resolution version of the figures is available at http://www.astro.cornell.edu/~vassilis/papers