Sasmirala Individual Information for NGC 1566

Description

NGC 1566 is a face-on grand-design spiral galaxy at a distance of D = 14.3 ± 5.9 Mpc (average of [tully_nearby_1988], [willick_homogeneous_1997] and D_L from NED) with a Sy 1.5 nucleus, which in the past showed spectral variation ranging between Sy 1.2 and Sy 1.9 [alloin_recent_1985, alloin_recurrent_1986]. It appears point-like at radio wavelengths (e.g., [morganti_radio_1999]). The NLR forms in projection a short but wide cone extending ~ 0.5arcsec ~ 35 pc to the south-east (PA~ 135^∘; e.g., [schmitt_comparison_1996]). After its detection in IRAS, NGC 1566 was observed in the MIR with ISO [clavel_2.5-11_2000, ramos_almeida_mid-infrared_2007] and ESO MPI 2.2 m/MANIAC [krabbe_n-band_2001], where the nucleus appeared unresolved. In the IRAC and MIPS images, a compact MIR nucleus embedded within diffuse extended emission and on larger scale surrounded by spiral-like emission was detected. Me measure the nuclear component only, which provides IRAC 5.8 and 8.0 μm and MIPS 24 μm fluxes significantly lower than the values by e.g., [dale_infrared_2005, munoz-mateos_radial_2009, gallimore_infrared_2010]. The IRS LR mapping-mode spectrum shows strong PAH emission, possible weak silicate 10 μm absorption and a rather flat spectral slope in νF_ν-space (see [smith_mid-infrared_2007, wu_spitzer/irs_2009, gallimore_infrared_2010] for more accurate versions). Thus, the MIR SED at arcsecond scales appears star-formation dominated. The nuclear region of NGC 1566 was observed with T-ReCS in the Si2 and Qa filters in 2005 [ramos_almeida_infrared_2009], and with VISIR in total in five narrow N-band and one Q-band filters spread over 2006 [reunanen_vlt_2010], 2009 [asmus_mid-infrared_2011] and 2010 (this work). In all cases, a compact MIR nucleus without any other host emission was detected. Note that for the T-ReCS images no corresponding standard star could be retrieved, and thus median flux conversion factors are used. The nucleus appears marginally resolved in all VISIR images except in the diffraction-limited PAH2_2 image from 2006 but with inconsistent position angles. Therefore, it remains uncertain, whether the MIR nucleus of NGC 1566 is in general extended at subarcsecond resolution. On the other hand, the N-band images with the largest FWHM values (PAH2 and NEII_1; ~ 0.5arcsec) also exhibit two times higher fluxes than in the SIV_2 and PAH2_2 filters with small FWHM values. It is thus possible, that the PAH2 and NEII_1 filters are significantly affected by extended emission, e.g., PAH. Deep subarcsecond-resolution MIR images covering the N-band obtained under good conditions are required to verify the nuclear MIR structure. The nuclear photometry in general is consistent with the previously published values and on average ~ 49% lower than the Spitzer spectrophotometry. We conclude that star formation dominates the MIR emission form the projected central ~ 250 pc in NGC 1566.

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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.