IC 630: Piercing the Veil of the Nuclear Gas

Abstract

IC 630 is a nearby early-type galaxy with a mass of 6 × 10 10 M with an intense burst of recent (6 Myr) star formation. It shows strong nebular emission lines, with radio and X-ray emission, which classifies it as an AGN. With VLT-SINFONI and Gemini North-NIFS adaptive optics observations (plus supplementary ANU 2.3m WiFeS optical IFU observations), the excitation diagnostics of the nebular emission species show no sign of standard AGN engine excitation; the stellar velocity dispersion also indicate that a super-massive black hole (if one is present) is small (M • = 2.25 × 10 5 M ). The luminosity at all wavelengths is consistent with star formation at a rate of about 1-2 M /yr. We measure gas outflows driven by star formation at a rate of 0.18 M /yr in a face-on truncated cone geometry. We also observe a nuclear cluster or disk and other clusters. Photo-ionization from young, hot stars is the main excitation mechanism for [Fe II] and hydrogen, whereas shocks are responsible for the H 2 excitation. Our observations are broadly comparable with simulations where a Toomre-unstable, self-gravitating gas disk triggers a burst of star formation, peaking after about 30 Myr and possibly cycling with a period of about 200 Myr.

The AGN Ionization Cones of NGC 5728. I. Excitation and Nuclear Structure

“…This temperature is somewhat higher than given in standard references (e.g. 17000 K vs. 14300 K for B6 stars), as the hydrogen opacity is lower at infrared wavelengths; we observe a lower (and therefore hotter) layer in the stellar atmosphere (see Durré et al 2017). After telluric correction and flux calibration, the data cubes from the SINFONI H+K observations for separate dates were combined into single cubes.…”

“…The correlation between the two sets of ratios on a spaxel-by-spaxel basis is weak or non-existent (R 2 = 0.044), unlike that found for e.g. IC 630 (Durré et al 2017) and for IC 4687, but similar to NGC 7130 (both Colina et al 2015). This is due to the multi-component nature of this object; IC 630 is a starburst galaxy and IC 4687 a prototype star-forming luminous infrared galaxy (LIRG), whereas NGC 7130 has a mixture of star-forming regions and compact AGN excitation.…”

“…The hydrogen line ration are from Hummer & Storey (1987), assuming Case B recombination, an electron temperature T e = 10 4 K and a density n e = 10 3 cm −3 . The [Fe II] ratios in the literature are discrepant, we will use the commonly accepted value of 1.36 (see the discussion in Durré et al 2017). For this object, the [Fe II] 1257/1644 nm ratio will be somewhat uncertain, even with the corrected telluric feature in the 1257 nm emission line.…”

The life cycle of starbursting circumnuclear gas discs

“…IC 630 is a nearby early-type galaxy, classified as a radio galaxy (Brown et al 2011). With the help of VLT-SINFONI and Gemini North-NIFS adaptive optics observations, Durré et al (2017) show that the excitation of the circumnuclear gas within a few 100 pc of the centre can mostly be explained by star formation. The measured SFR (1-2 M yr −1 ), SNR (4 × 10 −3 yr −1 ), the gas outflow rate (0.18 M yr −1 ), total gas masses (a few times 10 7 M ) as well as the line of sight velocity channel maps of the gas all are in line (within a factor of a few) with our simulated system during the peak of the starbursting phase, seen face-on.…”

“…Such studies with integral field units at the largest available telescopes with resolutions of a few parsec find that gaseous and stellar structures are often cospatial and share similar kinematics, indicating that stars may have formed in-situ from the gas discs. Morphologies range from smooth, star forming discs and mini-spirals (Prieto et al 2005) over star formation concentrated in clumps (Durré & Mould 2014) to very disturbed filamentary outflowing structures (Durré et al 2017) and are readily visible in dust extinction maps as well (Prieto et al 2005(Prieto et al , 2014. Most of these sources do not show any signs of recent merging activity that could provide the necessary torques to transfer gas towards the central region.…”