X-Ray Emission from the Nuclear Region of Arp 220

Abstract

We present an imaging and spectral analysis of the nuclear region of the ultraluminous infrared galaxy merger of Arp 220, using deep Chandra-ACIS observations summing up to ∼ 300 {{ks}}. Narrowband imaging with subpixel resolution of the innermost nuclear region reveals two distinct Fe-K emitting sources, coincident with the infrared and radio nuclear clusters. These sources are separated by 1‧ (∼380 pc). The X-ray emission is extended and elongated in the eastern (E) nucleus, like the disk emission observed in millimeter radio images, suggesting a starburst dominance in this region. We estimate an Fe-K equivalent width of ≳ 1 {keV} for both sources and observe 2-10 keV luminosities of ∼ 2× {10}⁴⁰ {{erg}} {{{s}}}^-1 (western, W) and ∼ 3× {10}⁴⁰ {{erg}} {{{s}}}^-1 (E). In the 6-7 keV band the emission from these regions is dominated by the 6.7 keV Fe xxv line, suggesting a contribution from collisionally ionized gas. The thermal energy content of this gas is consistent with the kinetic energy injection in the interstellar medium by SNe II. However, nuclear winds from a hidden active galactic nucleus (AGN) (\upsilon ∼ 2000 {{km}} {{{s}}}^-1) cannot be excluded. The 3σ upper limits on the neutral Fe-Kα flux of the nuclear regions correspond to the intrinsic AGN 2-10 keV luminosities of < 1× {10}⁴² {{erg}} {{{s}}}^-1 (W) and < 0.4× {10}⁴² {{erg}} {{{s}}}^-1 (E). For typical AGN spectral energy distributions the bolometric luminosities are < 3× {10}⁴³ {{erg}} {{{s}}}^-1 (W) and < 8× {10}⁴³ {{erg}} {{{s}}}^-1 (E), and black hole masses of < 1× {10}⁵ {M}_⊙ (W) and < 5× {10}⁵ {M}_⊙ (E) are evaluated for Eddington limited AGNs with a standard 10% efficiency.