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 νFν-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
Optical image (DSS, red filter). Displayed are the
central 4 arcmin with North being up and East to the left. The colour
scaling is linear with white corresponding to the median background (BG)
and black to the 0.01% pixels with the highest intensity.
Spitzer MIR images. Displayed are the inner 40 arcsec
with North being up and East to the left. The colour scaling is logarithmic
with white corresponding to median BG and black to the 0.1% pixels with the
highest intensity. The label in the bottom left states instrument and
central wavelength of the filter in micron (I: IRAC, M: MIPS).
Subarcsecond-resolution MIR images sorted by increasing
filter central wavelength. Displayed are the inner 4 arcsec with North
being up and East to the left. The colour scaling is logarithmic with white
corresponding to median BG and black to the 75% of the highest intensity of
all images in units of sig_bg. The inset image (where present; either
bottom or top right) shows the central arcsecond of the PSF from the
calibrator star, scaled to match the science target. The labels in the
bottom left state instrument and filter names (C: COMICS, M: Michelle, T:
T-ReCS, V: VISIR).
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.