Sasmirala Individual Information for NGC 1808

Description

NGC 1808 is an inclined peculiar spiral galaxy at a distance of D = 12.3 ± 2.2 Mpc [tully_extragalactic_2009] with an active nucleus containing a prominent starburst an possibly an AGN (see e.g., [krabbe_near-infrared_1994]). Optically, the nucleus has been classified as a Sy 2 [veron-cetty_ngc_1985] or as an H II region [yuan_role_2010]. The detection of an obscured long-term variable hard X-ray source supports the AGN scenario [awaki_asca_1996, jimenez-bailon_x-ray_2005]. At radio wavelengths a number of compact non-thermal radio sources was detected in the centre of NGC 1808, most of which are associated with supernovae remnants (e.g., [collison_radio_1994]). The brightest central radio source is marginally extended with uncertain nature. [forbes_ngc_1992] conclude that the optical and radio properties can be explained with supernovae embedded within an H II region (see also [kotilainen_near-infrared_1996]). MIR forbidden emission line diagnostics neither support the presence of an obscured AGN [goulding_towards_2009]. The first MIR observations of NGC 1808 were performed by [frogel_8-13_1982], [phillips_8-13_1984], and [roche_atlas_1991]. After IRAS, the first ground-based N-band map of the nuclear region were made by [telesco_genesis_1993], showing a large extended MIR structure (PA~-40∘). ISO observations followed [laurent_mid-infrared_2000, siebenmorgen_infrared_2001, forster_schreiber_isocam_2003]. The nuclear region of NGC 1808 was later imaged in the MIR at arcsecond resolution with ESO 2.2 m/MANIAC [krabbe_n-band_2001], with ESO 3.6 m/TIMMI2 [galliano_mid-infrared_2005] and with CTIO 4 m/OSCIR [ramos_almeida_infrared_2009]. The deep TIMMI2 image further resolves the previously detected structure into several clumpy sources embedded within diffuse emission with a dominating elongated MIR nucleus (major axis~ 1.5arcsec ~  90pc; PA~-45∘). Spitzer lacks the angular resolution to resolve the complex nuclear structure and only the compact nucleus embedded within the extended emission is visible in the corresponding IRAC and MIPS images. Furthermore, the IRAC 8.0 μm PBCD image is saturated in the centre and thus not used. We measure the nuclear component in the IRAC 5.8 μm and MIPS 24 μm images. The IRS LR staring-mode spectrum displays typical star formation features with strong PAH emission, possible silicate 10 μm absorption, and a red spectral slope in νFν-space (see also [goulding_towards_2009, gonzalez-martin_dust_2013]). Here, we report VISIR imaging in the PAH2_2 filter performed in 2009 (unpublished, to our knowledge), which shows a morphology very similar to the previous TIMMI2 image despite higher angular resolution. In particular, the nucleus again appears elongated (FWHM(major axis)~ 0.64arcsec ~ 38 pc; PA~-50∘). The unresolved nuclear component flux is ~ 63% lower than the Spitzer spectrophotometry and consistent with the LR N-band T-ReCS spectrum by [gonzalez-martin_dust_2013]. The latter exhibits still PAH emission albeit weaker, which indicates that even the subarcsecond measurements are still star-formation contaminated. From the subarcsecond MIR point of view, we cannot exclude the existence of an AGN in NGC 1808 but agree with previous works that the MIR emission in the central ~ 200 pc is star formation dominated.

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.

Photometry details and reduced FITS files

ObjectRA
[deg]		Dec
[deg]		FilterImageInfoλ_c
[um]		HWHM
[um]		InstrPix. size
[arcsec/pix]		T_exp
[s]		ModeChop Throw
[arcsec]		Chop Angle
[deg]		Rot
[deg]		Prog IdCal. StarCal. DateDate Obs.Factor
[mJy/ct]		Err. Factor
[mJy/ct]		Cal. Flux
[mJy]		Upper Lim. Gauss?F_Gauss
[mJy]		E(F_Gauss)
[mJy]		Upper Lim. PSF?F_PSF
[mJy]		E(F_PSF)
[mJy]		Cal. Maj.
[arcsec]		Cal. Min.
[arcsec]		Cal. PA.
[deg]		Maj. Ax
[arcsec]		Min. Ax
[arcsec]		Pos. Ang.
[deg]
NGC 180876.926250-37.513056PAH2_2NGC1808_PAH2_2_2009-10-22T05-39.fits[Details]11.880.37VISIR0.075722.0PARA20.00.00.0384.B-0943(A)HD269672009-10-22T04:40:48Z2009-10-22T05:39:50Z0.020180.000398761.1False411.010.5False328.89.40.660.52123.00.640.54130.0