THE EXCEPTIONAL SOFT X-RAY HALO OF THE GALAXY MERGER NGC 6240

Journal Article · Sun Mar 10 04:00:00 EDT 2013 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/765/2/141· OSTI ID:22126954

Nardini, E.; Junfeng, Wang; Fabbiano, G.; Elvis, M.; Risaliti, G.; Karovska, M.; Zezas, A. ^[1]

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1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

We report on a recent {approx}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 {approx}110 Multiplication-Sign 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 {approx}7.5 million K, an estimated density of 2.5 Multiplication-Sign 10{sup -3} cm{sup -3}, and a total mass of {approx}10{sup 10} M{sub Sun }, resulting in an intrinsic 0.4-2.5 keV luminosity of 4 Multiplication-Sign 10{sup 41} erg s{sup -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 a steady rate over the entire dynamical timescale ({approx}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 {approx} 0.1 solar) and temperature (kT {approx} 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.

• OSTI ID: 22126954
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 765; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ASTRONOMY
ASTROPHYSICS
BINDING ENERGY
ENERGY LOSSES
IRON
KEV RANGE
LUMINOSITY
NONLUMINOUS MATTER
PHOTON EMISSION
SOFT X RADIATION
SUPERNOVAE
X-RAY GALAXIES
X-RAY SPECTRA