Sasmirala Individual Information for NGC 1068

Description

NGC 1068 is a face-on spiral-like luminous infrared galaxy at a distance 14.4 ± 3.0 Mpc (NED redshift-independent median) with an AGN optically classified either as Sy 1.8-2 [ho_search_1997-1, ho_search_1997, osterbrock_spectroscopic_1993] and has polarized broad emission lines [antonucci_spectropolarimetry_1985]. NGC 1068 is one of the original six Seyfert galaxies [seyfert_nuclear_1943] and thus its AGN is one of the most-studied at all wavelengths (see [honig_high-spatial_2008] for a recent detailed multiwavelength study), and the discovery of polarized broad emission lines in NGC 1068 led to the formulation of the unification scheme for AGN [antonucci_spectropolarimetry_1985]. It possesses a cone-like NLR in northern direction (length~ 10arcsec ~ 700 pc; PA~ 35^∘; [pogge_extended_1988, evans_hst_1991]) roughly coinciding with a bended radio jet [wilson_radio_1982, gallimore_subarcsecond_1996-1], and nuclear H₂O and OH maser emission perpendicularly orientated to the jet (PA~ 94^∘; [claussen_water-vapor_1984, greenhill_vlbi_1996, gallimore_h_1996]). Intense circum-nuclear star formation out to radii of ~ 1 kpc was detected as well [telesco_luminous_1984, balick_footprints_1985].

NGC 1068 possesses the MIR-brightest active nucleus in terms of flux. The first MIR photometry of NGC 1068 was obtained by [kleinmann_infrared_1970]. Since then a wealth of studies employing MIR photometry and spectroscopy with bolometers, including mapping observations, were performed [kleinmann_infrared_1970, neugebauer_infrared_1971, rieke_variability_1972, rieke_infrared_1972, becklin_size_1973, jameson_infrared_1974, jameson_infrared_1974-1, stein_observations_1974, rieke_infrared_1975, kleinmann_8-13_1976, lebofsky_infrared_1978, rieke_10_1978, lebofsky_extinction_1979, telesco_extended_1980, houck_medium-resolution_1980, malkan_stellar_1983, telesco_luminous_1984, roche_8-13_1984]. NGC 1068 was also observed by all space-based MIR facilities: e.g., ISO (e.g., [rigopoulou_large_1999, lutz_iso-sws_2000, alexander_infrared_2000, le_floch_mid-infrared_2001, siebenmorgen_isocam_2004]) and Spitzer (e.g., [goulding_towards_2009]). The first ground-based N-band images were obtained by [tresch-fienberg_structure_1987] with the Goddard IR array camera mounted on IRTF, revealing an extended complex structure, which was later resolved further with the first subarcsecond resolution image obtained with an upgraded Goddard Infrared Array Camera ([braatz_high-resolution_1993]; see also [cameron_subarcsecond_1993]). Further improved MIR images with better instruments and larger telescopes followed [bock_high-resolution_1998, bock_high_2000, alloin_0.6_2000, siebenmorgen_mid-infrared_2004, gorjian_10_2004, galliano_mid-infrared_2005, poncelet_original_2007]. In addition, spectroscopic [rhee_first_2006, mason_spatially_2006, poncelet_dynamics_2008] and polarimetric [knacke_infrared_1974, aitken_infrared_1984, lumsden_near-_1999, packham_gemini_2007] studies were dedicated to the nuclear region of NGC 1068. Finally, MIR interferometric observations with MIDI were performed as well [jaffe_central_2004, poncelet_new_2006, raban_resolving_2009].

We reanalysed all the eleven conventional VISIR N- and Q-band filter images, which were obtained in 2004 [galliano_mid-infrared_2005-1], in 2005 [poncelet_dynamics_2008], and in 2008 (this work). Some of the major results of all the previous MIR studies and our own can be summarized as follows. A compact north-south elongated MIR nucleus embedded within extended emission was detected in all subarcsecond-resolution images. From the core, a thin structure extends ~ 0.5arcsec (35 pc) to the north with a PA~-7^∘ before it bends to the east with a PA~ 20^∘ extending another ~ 1.3arcsec (90 pc). This structure follows the radio jet along the western side and coincides with the western part of the bright ionization cone emission. In addition an emission clump is located east of this structure at a distance of ~ 0.6arcsec (42 pc; PA~ 30^∘) coinciding with an eastern bright [O III] emission region. To the south, another thin structure extends with a PA~ 190^∘ for ~ 1.3arcsec (90 pc). The southern structure coincides as well with [O III] emission while there is no radio counterpart. In particular in the deep VISIR 12 μm images, additional but weak emission knots can be seen at larger distances (~ 3.3arcsec ~ 230 pc) towards the north-east and south-west along the same position angles and also aligned with [O III] emission. Therefore, it seems that the non-nuclear MIR emission in general coincides with bright [O III] clouds while bracketing the radio emission. The presence of the silicate feature and MIR polarization due to dichroic emission indicates that the MIR emission is caused by dust [packham_gemini_2007]. The MIR emission line kinematics indicate that the material is outflowing along the edges of the ionization cone [poncelet_dynamics_2008]. It remains uncertain which parts of the MIR emission structure are actually associated with the obscuring torus, but most works favour that it is unresolved in the subarcsecond-resolution images as in other local AGN. Indeed, the extensive MIR interferometric observations with MIDI have found a parsec-sized structure elongated along the system axis [raban_resolving_2009]. We isolate only the unresolved nuclear component in NGC 1068 by manual PSF scaling in order to leave a roughly smooth residual. This provides fluxes much lower than the total MIR emission on arcsecond scales, e.g., on average ~ 80 lower than the Spitzer/IRS HR staring-mode spectrum. As expected our fluxes are in between the nuclear Michelle N-band spectrum of [mason_spatially_2006] (2006; dark-magenta in the figure) and the MIDI correlated flux spectrum of [raban_resolving_2009]. The nuclear photometry also indicates a deep silicate 10 μm absorption feature and a declining flux at wavelengths > 18 μm in νF_ν-space. This agrees with the spectroscopic studies, which found that the silicate absorption becomes stronger towards the nucleus. The complex source morphology complicates any analysis of flux variations. which is therefore omitted. In summary, we note that the nuclear MIR emission of NGC 1068 is both the brightest and most complex compared to the other local AGN on subarcsecond scales, and it is unclear, whether the results from NGC 1068 can be applied to other AGN in general.

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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.