Unveiling the nucleus of NGC 7172

Abstract

Aims. We present the results of near-infrared (NIR) H + K European Southern Observatory SINFONI integral field spectroscopy (IFS) of the Seyfert 2 galaxy NGC 7172. We investigate the central 800 pc, concentrating on excitation conditions, morphology, and stellar content. NGC 7172 was selected from a sample of the ten nearest Seyfert 2 galaxies from the Veron-Cetty & Veron catalogue. All objects were chosen as test cases for adaptive optics (AO) assisted observations that allow a detailed study (at high spatial and spectral resolution) of the nuclear and host environments. NGC 7172 has a prominent dustlane crossing the central galaxy region from east to west, which makes it an ideal candidate to investigate the effect of obscuration by strong galactic extinction on (active) galaxies and their classification. Methods. The NIR is less influenced by dust extinction than optical light and is sensitive to the mass-dominating stellar populations. SINFONI integral field spectroscopy combines NIR imaging and spectroscopy and provides us with the opportunity to analyze several emission and absorption lines to investigate the stellar populations and ionization mechanisms over the 4 × 4 field of view (FOV). is predominantly inhabited by gas, dust, and an old K-M type giant stellar population. The gaseous, molecular, and stellar velocity maps show a related disturbed disk structure and similar velocities. Conclusions. We find evidence of nuclear activity located behind the prominent dustlane crossing the central region of the galaxy. The nucleus of NGC 7172, which is optically classified as a Seyfert 2 nucleus without any trace of broad emission lines, is a Seyfert 1 nucleus either surrounded by a molecular dust torus or hidden behind the strong galactic extinction. Our observation provides support for the unified model scheme. However, an evolutionary scenario cannot be ruled out by our observation.

LLAMA: nuclear stellar properties of Swift-BAT AGN and matched inactive galaxies

“…Overall, most galaxies (11/13) in our sample show nearly symmetric stellar continuum maps with regular elliptical isophotes. The two exceptions are NGC 7172 and NGC 7582, for both of which the maps exhibit extreme irregularities coinciding with known dust lanes (Bianchi et al 2007;Smajić et al 2012). These pass across their nuclei and are clearly visible in optical HST F606W images (Malkan, Gorjian & Tam 1998).…”

A low-luminosity type-1 QSO sample

“…Zuther et al 2007;Smajić et al 2012;Valencia-S. et al 2012a;Smajić et al 2014;Busch et al 2015;Fazeli et al, in prep.). The NIR H + K-band contains several diagnostic lines: Hydrogen recombination lines (Paα and Brγ) can be excited by the AGN (in the broad-and narrow-line region), but are also tracers of young star formation.…”

The central kpc of edge-on AGN

“…We show that the NIR is far more potent in discovering the true nature of AGN than the optical, even the polarized, light. For more information on NGC 7172 see [1].…”

“…We can compare the upper limit of M BH↓ =4×10 8 M to our prior calculated BH mass of M BH =(4.5±1.0)×10 8 M and we conclude that these are in good agreement. Subfigures taken from [1]. The LOSVD is not corrected for instrumental broadening.…”

“…Observations of polarized light can help to look "behind" the torus by detecting the scattered and reflected broad emission lines. This will usually not work with dust obscuration by the host because this dust obscuration often occurs on large galactic scales and far away from the nucleus where the broad emission lines are excited [1,7]. The last problem is supported by the finding that Seyfert 2 galaxies tend to be edge-on oriented [8,9].…”