Dust and gas in the central region of NGC 1316 (Fornax A)

Abstract

Context. The early-type galaxy NGC 1316, associated with the radio source Fornax A, hosts about 107 M⊙ of dust within a central radius of 5 kpc. These prominent dust structures are believed to have an external origin, which is also a popular interpretation for other dusty early-type galaxies. Moreover, it has been long known that ionised gas is present in NGC 1316, but to date there has been a lack of detailed investigation. Aims. Our aim is to understand the nature of ionised gas and dust in NGC 1316 and to offer an interpretation for the origin of the dust. Methods. We use archival Hubble Space Telescope/Advanced Camera for Surveys data to construct colour maps that delineate the dust pattern in detail, and we compare these data with maps constructed with data from the Multi Unit Spectroscopic Explorer (MUSE) instrument of the Very Large Telescope at the European Southern Observatory. Twelve MUSE pointings in wide field mode form a mosaic of the central 3.3 × 2.4 arcmin2. We use the tool PyParadise to fit the stellar population. We use the residual emission lines and the residual interstellar absorption NaI D-lines, and we measure line strengths, the velocity field, and the velocity dispersion field. Results. The emission lines resemble low-ionisation nuclear emission-line region lines, with [NII] being the strongest line everywhere. Ionising sources are plausibly the post-asymptotic giant branch stars of the old or intermediate-age stellar population. There is a striking match between the dust structures, ionised gas, and atomic gas distributions, the last of which is manifested by interstellar absorption residuals of the stellar NaI D-lines. In the dust-free regions, the interstellar NaI D-lines appear in emission, which is indicative of a galactic wind. The velocity field of the ionised gas (and thus of the dust) is characterised by small-scale turbulent movements that indicate short lifetimes. At the very centre, a bipolar velocity field of the ionised gas is observed, which we interpret as an outflow. The low-velocity part is associated with dust. We identify a strongly inclined gaseous dusty disc along the major axis of NGC 1316. A straight beam of ionised gas with a length of about 4 kpc emanates from the centre. Conclusions. The dust in NGC 1316 has different origins. Our findings are strongly suggestive of a dusty outflow that is curved along the line-of-sight. Nuclear outflows may be important dust-producing machines in galaxies. Another dusty gaseous component forms a disc that we identify as the predecessor of a central dust lane.

AGN feeding and feedback in Fornax A

“…Recently, Richtler et al (2020) presented the same observations focusing on the [N ii]λ6583 line and the NaI D absorption doublet. Our study presents both points of agreement and tension with the conclusions of these authors, as detailed in Sects.…”

“…A major merger occurred 1-3 Gyr ago (e.g., Schweizer 1980;Serra et al 2019). This event likely caused the first triggering of the radio source and brought large amounts of dust (Schweizer 1980;Lanz et al 2010;Galametz et al 2012;Duah Asabere et al 2016), molecular gas (Horellou et al 2001;Galametz et al 2014;Morokuma-Matsui et al 2019), and neutral hydrogen (Horellou et al 2001;Serra et al 2019;Richtler et al 2020;Kleiner et al 2021) into the centre and around the galaxy. Figure 1 shows that the dust seen in extinction in the innermost ≈8 kpc is distributed mainly in two structures: a stripe crossing the centre from the north-west to the south-east; and two outer arcs that seem to originate at the bend of the radio jets.…”

AGN feeding and feedback in Fornax A: kinematical analysis of the multi-phase ISM

“…The EW-stripe is aligned with the major axis of the stellar body and is perpendicular to the dust lane, which may lead to misinterpret the nature of this feature as an edge-on circumnuclear disk (e.g. Richtler et al 2020) as seen in several active early-type galaxies, (e.g. NGC 1813 and PKSB 1718-649; Krajnović et al 2015;Maccagni et al 2016Maccagni et al , 2018.…”

“…Schweizer 1980;Serra et al 2019). This event likely caused the first triggering of the radio source and brought large amounts of dust (Schweizer 1980;Lanz et al 2010;Galametz et al 2012;Duah Asabere et al 2016), molecular gas (Horellou et al 2001;Galametz et al 2014;Morokuma-Matsui et al 2019), and neutral hydrogen (Horellou et al 2001;Serra et al 2019;Richtler et al 2020;Kleiner et al 2021) into the centre and around the galaxy. Fig.…”

The Stellar Initial Mass Function and Population Properties of M89 from Optical and NIR Spectroscopy: Addressing Biases in Spectral Index Analysis

“…Xu et al 2010) and internal causes (e.g. Lauer et al 2005;Richtler et al 2020). Since M89 exhibits shells and stellar streams (Schweizer & Seitzer 1992;Bonfini et al 2018), the presence of dust (and gas, that is observed at larger radii) can be a signature of an accreted small stellar component that can have different characteristics (although diluted by the bulk of stars of M89).…”