Monday 01/28/2013

The Exceptional Soft X-ray Halo of the Galaxy Merger NGC 6240 Authors: Emanuele Nardini, Junfeng Wang, Giuseppina Fabbiano, Martin Elvis, Silvia Pellegrini, Guido Risaliti, Margarita Karovska, Andreas Zezas
(Submitted on 24 Jan 2013)

Abstract: We report on a recent ~150-ks long Chandra observation of the ultraluminous infrared galaxy merger NGC 6240, which allows a detailed investigation of the diffuse galactic halo. Extended soft X-ray emission is detected at the 3-sigma confidence level over a diamond-shaped region with projected physical size of ~110×80 kpc, and a single-component thermal model provides a reasonably good fit to the observed X-ray spectrum. The hot gas has a temperature of ~7.5 million K, an estimated density of 2.5×10^{-3} cm^{-3}, and a total mass of ~10^10 M_sun, resulting in an intrinsic 0.4-2.5 keV luminosity of 4×10^41 erg s^{-1}. The average temperature of 0.65 keV is quite high to be obviously related to either the binding energy in the dark-matter gravitational potential of the system or the energy dissipation and shocks following the galactic collision, yet the spatially-resolved spectral analysis reveals limited variations across the halo. The relative abundance of the main alpha-elements with respect to iron is several times the solar value, and nearly constant as well, implying a uniform enrichment by type II supernovae out to the largest scales. Taken as a whole, the observational evidence is not compatible with a superwind originated by a recent, nuclear starburst, but rather hints at widespread, enhanced star formation proceeding at steady rate over the entire dynamical timescale (~200 Myr). The preferred scenario is that of a starburst-processed gas component gently expanding into, and mixing with, a pre-existing halo medium of lower metallicity (Z ~ 0.1 solar) and temperature (kT ~ 0.25 keV). This picture cannot be probed more extensively with the present data, and the ultimate fate of the diffuse, hot gas remains uncertain. Under some favorable conditions, at least a fraction of it might be retained after the merger completion, and evolve into the hot halo of a young elliptical galaxy.

Comments: 56 pages, 16 figures, 7 tables. Accepted for publication in ApJ
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1301.5907 [astro-ph.CO]
(or arXiv:1301.5907v1 [astro-ph.CO] for this version)

Spectral Energy Distributions of HII regions in M33 (HerM33es) Authors: M. Relano, S. Verley, I. Perez, C. Kramer, D. Calzetti, E. M. Xilouris, M. Boquien, J. Abreu-Vicente, F. Combes, F. Israel, F. S. Tabatabaei, J. Braine, C. Buchbender, M. Gonzalez, P. Gratier, S. Lord, B. Mookerjea, G. Quintana-Lacaci, P. van der Werf
(Submitted on 24 Jan 2013)

Abstract: Within the framework of the Herschel M 33 extended survey HerM33es we study the Spectral Energy Distribution (SED) of a set of HII regions in M 33 as a function of the morphology. We present a catalogue of 119 HII regions morphologically classified: 9 filled, 47 mixed, 36 shell, and 27 clear shell HII regions. For each object we extract the photometry at twelve available wavelength bands (from FUV-1516A to IR-250mi) and obtain the SED. We also obtain emission line profiles across the regions to study the location of the stellar, ionised gas, and dust components. We find trends for the SEDs related to the morphology, showing that the star and gas-dust configuration affects the ratios of the emission in different bands. The mixed and filled regions show higher emission at 24mi than the shells and clear shells, which could be due to the proximity of the dust to the stellar clusters in the case of mixed and filled regions. The FIR peak for shells and clear shells seems to be located towards longer wavelengths, indicating that the dust is colder for this type of objects.The logarithmic 100/70mi ratio for filled and mixed regions remains constant over one order of magnitude in Halpha and FUV surface brightness, while the shells and clear shells exhibit a wider range of values of almost two orders of magnitude. We derive dust masses and temperatures fitting the individual SEDs with dust models proposed in the literature. The derived dust mass range is between 10^2-10^4 Msun and the cold dust temperature spans T(cold)~12-27 K. The spherical geometrical model proposed for the Halpha clear shells is confirmed by the emission profile obtained from the observations and is used to infer the electron density within the envelope: the typical electron density is 0.7+-0.3 cm^-3, while filled regions can reach values two to five times higher.

Comments: 21 pages, 19 figures. Accepted to A&A
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); Galaxy Astrophysics (astro-ph.GA)
Cite as: arXiv:1301.5917 [astro-ph.CO]
(or arXiv:1301.5917v1 [astro-ph.CO] for this version)
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