Resolving the innermost parsec of Centaurus A at mid-infrared wavelengths

Abstract

Context. To reveal the origin of mid-infrared radiation from the core of Centaurus A, we carried out interferometric observations with the MID-infrared Interferometer (MIDI) at ESO's VLTI telescope array. Aims. Observations were obtained with four baselines between unit telescopes of the VLTI, two of them roughly along the radio axis and two orthogonal to it. The interferometric measurements are spectrally resolved with λ/∆λ = 30 in the wavelength range 8 to 13 µm. Their resolution reaches 15 mas at the shortest wavelengths. Supplementary observations were obtained in the near-infrared with the adaptive optics instrument NACO, and at mm wavelengths with SEST and JCMT. Methods. The mid-infrared emission from the core of Centaurus A is dominated by an unresolved point source (<10 mas). Observations with baselines orientated perpendicular to the radio jet reveal an extended component which can be interpreted as a geometrically thin, dusty disk, the axis of which is aligned with the radio jet. Its diameter is about 0.6 pc. It contributes between 20% (at λ 8 µm) and 40% (at λ 13 µm) to the nuclear flux from Centaurus A and contains dust at about 240 K. We argue, that the unresolved emission is dominated by a synchrotron source. Its overall spectrum is characterized by an F ν ∼ ν −0.36 power-law which cuts off exponentially towards high frequencies at ν c = 8 × 10 13 Hz and becomes optically thick at ν < ν 1 45 GHz. Results. Based on a Synchrotron Self Compton (SSC) interpretation for the γ-ray emission, we find a magnetic field strength of 26 µT and a maximum energy of relativistic electrons of γ c = E c /m e c 2 = 8500. Near γ c , the acceleration time scale is τ acc = 4 days, in good agreement with the fastest flux variations, observed at X-ray frequencies. Our SSC model argues for a Doppler factor δ 1 whichtogether with the jet-counter jet ratio of the radio jets on parsec scale -results in an upper limit for the bulk Lorentz factor Γ jet < 2.5, at variance with the concept of a "mis-directed BL Lac object". Conclusions. We estimate a thermal luminosity of the core, P th 1.3 × 10 34 W = 1.5 × 10 −4 × L Edd , intermediate between the values for highly efficiently accreting AGN (e.g. Seyfert galaxies) and those of typical FR I radio galaxies. This luminosity, which is predominantly released in X-rays, is most likely generated in an Advection Dominated Accretion Flow (ADAF) and seems just sufficient to heat the dusty disk.

The dusty heart of nearby active galaxies

“…The most promising tool for this task is IR interferometry, which was already successfully used for a number of nearby objects (Swain et al 2003;Wittkowski et al 2004;Weigelt et al 2004;Jaffe et al 2004;Poncelet et al 2006;Meisenheimer et al 2007;Tristram et al 2007Tristram et al , 2009Beckert et al 2008;Raban et al 2009;Burtscher et al 2009;Kishimoto et al 2009b). However, except for reconstructed images of NGC 1068 obtained from Speckle interferometry (Weigelt et al 2004), long-baseline interferometry usually provides spatial information coded as Fourier space observables, i.e.…”

Resolving the Nucleus of Centaurus A at Mid-Infrared Wavelengths

“…This is possible since only the correlated spectrum depends on the projected baseline length and orientation and it is not expected that the N band source changes within one week. The 2005 observations that led to Meisenheimer et al (2007) were not included in this analysis since their total flux differs considerably from the 2008 observations. Finally, the calibrated correlated spectra were divided by the average single-dish spectrum to derive visibilities.…”

“…With limited (u,v) coverage, Meisenheimer et al (2007) found from earlier MIDI data that the mid-infrared (IR) emission from the central parsec of Centaurus A is dominated (60% at 13 mm) by an unresolved point source (diameter o10 mas). By comparison with multiwavelength data, this emission was classified as being most likely nonthermal (synchrotron) in origin.…”

The innermost dusty structure in active galactic nuclei as probed by the Keck interferometer

“…early results on Seyfert 2 galaxies by Jaffe et al 2004; radio galaxy, Meisenheimer et al 2007; type 1 AGNs, Beckert et al 2008;Kishimoto et al 2009b;Burtscher et al 2009; snapshot survey, Tristram et al 2009). In the near-IR, after the very early results for the brightest Seyfert 1 galaxy NGC 4151 (Swain et al 2003) and Seyfert 2 galaxy NGC 1068 (Wittkowski et al 2004), long-baseline interferometry has now started to explore the innermost dusty structure extensively at milli-arcsec (mas) resolution (Kishimoto et al 2009a, hereafter Paper I; Pott et al 2010).…”