Sasmirala Individual Information for NGC 6240S

Description

NGC 6240 is a very infrared-luminous, late-stage merger system consisting of two gas-rich massive spirals at a redshift of z = 0.025 (D ~ 114 Mpc) with a massive starburst and a binary AGN. Owing to this unique nature (discovered by [fosbury_unusual_1979, fried_ngc_1983]), NGC 6240 has been extensively studied at all wavelengths over the last decades. It belongs to the nine-month BAT AGN sample. Both active nuclei have been optically classified as LINERs, while the southern one, NGC 6240S, might also be classified as a Sy 2 or H II nucleus [rafanelli_subarcsec_1997]. We treat them as AGN/starburst composites (see also discussion in [yuan_role_2010]). The double AGN nature was discovered in X-rays [komossa_discovery_2003] and verified at radio [gallimore_parsec-scale_2004] and infrared wavelengths [risaliti_double_2006]. Both nuclei are separated by ~ 1.5arcsec ~ 0.8 kpc in the north-south direction (PA~ 10^∘; [max_locating_2007]) and are embedded within the kiloparsec-sized starburst [tecza_stellar_2000, engel_ngc_2010]. At radio wavelengths, both nuclei appear point-like at arcsecond resolution [colbert_radio_1994, beswick_merlin_2001], but the northern nucleus, NGC 6240N, shows jet-like east-west elongation at milliarcsecond resolution [gallimore_parsec-scale_2004, hagiwara_very_2011]. NGC 6240S emits strong water maser emission [hagiwara_search_2002, nakai_detection_2002, hagiwara_location_2003, hagiwara_two_2010]. The first attempt to detect NGC 6240 in the MIR failed [allen_near-infrared_1976]. Instead, it was first detected with IRAS [wright_ultraluminous_1984] and followed up with many ground-based MIR observations [rieke_10_1985, wright_recent_1988, smith_nature_1989, roche_atlas_1991, wynn-williams_luminous_1993, keto_infrared_1997, dudley_new_1999]. NGC 6240 has also been extensively studied ISO [klaas_infrared_1997, genzel_what_1998, rigopoulou_large_1999, charmandaris_mid-ir_1999, thornley_massive_2000, tran_isocam-cvf_2001, lutz_iso_2003, forster_schreiber_warm_2004]. The first arcsecond-resolution MIR images resolving both nuclei were obtained with ESO MPI 2.2 m/MANIAC in 1997 [krabbe_n-band_2001], followed by ESO 3.6 m/TIMMI2 images and spectroscopy [siebenmorgen_mid-infrared_2004]. [egami_subarcsecond_2006] reported the first subarcsecond-resolution MIR images using Keck/MIRLIN and an LR N-band spectrum using Keck/LWS. The images show both nuclei embedded within extended emission connecting them, while NGC 6240S completely dominates the total MIR emission of the system. They claim that the spectrum of NGC 6240S is consistent with pure star formation. No spectrum of NGC 6240N is presented. The nuclear structure is not resolved in the Spitzer/IRAC and MIPS images, which show a bright elongated nucleus embedded within diffuse host emission in the IRAC 5.8 and 8.0 μm bands (see also [bush_structure_2008]), and only a point source in the MIPS 24 μm band. Our MIPS 24 μm photometry agrees with the value published by [marshall_decomposing_2007]. The Spitzer/IRS LR staring-mode spectrum exhibits deep silicate 10 μm and weak silicate 18 μm absorption, strong PAH emission, and a red spectral slope in νF_ν-space (see also [armus_detection_2006, armus_observations_2007, farrah_high-resolution_2007]). The detection of AGN-associated [Ne V] emission indicates a significant AGN contribution to the arcsecond-scale MIR SED [armus_observations_2007]. Because both nuclei are blended in the Spitzer data, the resulting spectrophotometry is regarded as arcsecond-scale MIR SED for both. The nuclear region of NGC 6240 was observed with VISIR in two narrow N and one Q-band filter in 2005 (unpublished, to our knowledge). The VISIR images show a similar morphology as the MIRLIN images with both nuclei embedded within extended emission. NGC 6240S is elongated along PA~ 165^∘. We perform manual PSF-scaling to measure the unresolved nuclear fluxes in both nuclei. The resulting nuclear MIR SED of NGC 6240S is on average ~ 70% lower than the Spitzer spectrophotometry (~ 93% lower for NGC 6240N). Because these nuclear MIR SEDs might still be contaminated by star formation, this result is consistent with the ~ 20% AGN contribution estimate of [armus_observations_2007].

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Images

SEDs

MIR SED. The description of the symbols in all the SED plots (where present) is the following: Grey crosses and solid lines mark the Spitzer/IRAC, MIPS and IRS data. The colour coding of the other symbols is as follows: green for COMICS, magenta for Michelle, blue for T-ReCS and red for VISIR data. Darker-coloured solid lines mark spectra of the corresponding instrument. The black filled circles mark the nuclear 12 and 18 micron continuum emission estimate from the data (where present). The ticks on the top axis mark positions of common MIR emission lines, while the light grey horizontal bars mark wavelength ranges affected by the silicate 10 and 18 micron features.