ISO spectroscopy of star formation and active nuclei in the luminous infrared galaxy NGC 6240

Abstract

Abstract. We present Infrared Space Observatory mid-and far-infrared spectroscopy of the merging galaxy NGC 6240, an object presenting many aspects of importance for the role of star formation and AGN activity in [ultra]luminous infrared galaxies. The mid-infrared spectrum shows starburst indicators in the form of low excitation fine-structure line emission and aromatic "PAH" features. A strong high excitation [O IV] line is observed which most likely originates in the Narrow Line Region of an optically obscured AGN. NGC 6240 shows extremely powerful emission in the pure rotational lines of molecular hydrogen. We argue that this emission is mainly due to shocks in its turbulent central gas component and its starburst superwind. The total shock cooling in infrared emission lines accounts for ∼0.6% of the bolometric luminosity, mainly through rotational H 2 emission and the [O I] 63 µm line. We analyse several ways of estimating the luminosities of the starburst and the AGN in NGC 6240 and suggest that the contributions to its bolometric luminosity are most likely in the range 50-75% starburst and 25-50% AGN.

NGC 6240: merger-induced star formation and gas dynamics

“…Based on silicate absorption at 9.7 μm, Armus et al (2006) find an extinction of A V ∼ 95 mag to the coronal line region. The more moderate estimates presented by Lutz et al (2003) are for a global extinction, and correspond to a dust screen A V ∼ 15−20 mag. This estimate is comparable to derivations at radio (N H ∼ (1.5−2) × 10 22 cm −2 , Beswick et al 2001) and X-ray (N H ∼ 10 22 cm −2 , Komossa et al 2003) wavelengths.…”

“…A second important issue is the impact of X-ray irradiation on the CO abundance around the nuclei, since the AGN are quite luminous. Lutz et al (2003) argue that as little as 25-50% of L bol is due to the AGN. If we take the lower end of this range, and distribute it equally between the two AGN, we find each radiates at L ∼ 3 × 10 44 erg s −1 .…”

The relation between AGN hard X-ray emission and mid-infrared continuum from ISO spectra: Scatter and unification aspects

“…also the lower estimate of Ikebe et al 2000), and the likely noticeable obscuration of the 6 µm continuum in this object. See Lutz et al (2003) for a full discussion of the mid-infrared properties. Mrk 463 has a very strong mid-IR AGN continuum but the relatively lowest absorption corrected hard X-ray emission compared to the average relation.…”

LIGHTING THE DARK MOLECULAR GAS: H2 AS A DIRECT TRACER

“…Hence, the H 2 gas that comprises the bulk of the molecular interstellar medium (ISM) is commonly believed to be too cold to be visible. And yet, despite these observational disadvantages, the advent of the Infrared Space Observatory and in particular Spitzer's Infrared Spectrograph (IRS, Houck et al 2004) revealed pure rotational H 2 emission from a rich variety of extragalactic sources, including normal star-forming galaxies (Roussel et al 2007), ultraluminous and luminous infrared galaxies (U/LIRGs Lutz et al 2003;Veilleux et al 2009;PereiraSantaella et al 2010;Stierwalt et al 2014), galaxy mergers (Appleton et al 2006), radio-loud active galactic nuclei (AGNs) (Ocaña Flaquer et al 2010), UV-selected galaxies (O'Dowd et al 2009), quasar hosts (Evans et al 2001), cooling-flow cluster systems (Egami et al 2006), sources with extreme shockdominated energetics (Ogle et al 2010), and even star-forming sources at redshifts 2 ).…”