NGC 3801 caught in the act: a post-merger star-forming early-type galaxy with AGN—jet feedback

Abstract

In the current models of galaxy formation and evolution, AGN feedback is crucial to reproduce galaxy luminosity function, colour–magnitude relation and M•–σ relation. However, whether AGN feedback can indeed expel and heat up significant amount of cool molecular gas and consequently quench star formation is yet to be demonstrated observationally. Only in four cases so far (Cen A, NGC 3801, NGC 6764 and Mrk 6), X‐ray observations have found evidences of jet‐driven shocks heating the ISM. We chose the least explored galaxy NGC 3801, and present the first ultraviolet imaging and stellar population analysis of this galaxy from GALEX data. We find this merger‐remnant early‐type galaxy to have an intriguing spiral wisp of young star‐forming regions (age ranging from 100 to 500 Myr). Taking clues from dust/PAH, H i and CO emission images, we interpret NGC 3801 to have a kinematically decoupled core or an extremely warped gas disc. From the HST data, we also show evidence of ionized gas outflow similar to that observed in H i and molecular gas (CO) data, which may have caused the decline of star formation leading to the red optical colour of the galaxy. However, from these panchromatic data, we interpret that the expanding shock shells from the young (∼2.4 Myr) radio jets are yet to reach the outer gaseous regions of the galaxy. It seems we observe this galaxy at a rare stage of its evolutionary sequence where post‐merger star formation has already declined and new powerful jet feedback is about to affect the gaseous star‐forming outer disc within the next 10 Myr, to further transform it into a red‐and‐dead early‐type galaxy.

Detection of a High Brightness Temperature Radio Core in the Active-Galactic-Nucleus-Driven Molecular Outflow Candidate NGC 1266

“…An HST B-band (F435W) image is presented in Figure 1. 13 These images reveal filamentary structures in the central dust emission of NGC 1266, similar to those seen in other systems with outflows (e.g., NGC 3801; Das et al 2005;Hota et al 2012). A detailed analysis of the full set of HST data from this program will be presented in a future paper.…”

“…2012; Greene et al 2012;Maiolino et al 2012). It has been suggested that the less powerful AGNs that populate the nearby universe may also be capable of driving molecular outflows and shutting down late-time SF in some local galaxies (Schawinski et al 2009;Chung et al 2011), and the number of such candidates has been increasing (Das et al 2005;Matsushita et al 2007;Alatalo et al 2011;Dasyra & Combes 2012;Hota et al 2012;Aalto et al 2012b;Morganti et al 2013;Sakamoto et al 2013). Of these intriguing local candidates for AGN-driven SF quenching, NGC 1266 (Alatalo et al 2011(Alatalo et al , 2013Davis et al 2012) provides a rare example of a host galaxy that is both non-starbursting and shows no evidence of a recent major merger or interaction (although see Aalto et al 2012b andSakamoto et al 2013…”

Parent population of flat-spectrum radio-loud narrow-line Seyfert 1 galaxies

“…Its spectrum is extremely red, with weak emission lines. We cannot exclude that the AGN is in its last phase before reaching a quiescent state (Hota et al 2012). The source has symmetric jets, and it appears to be forming a disk.…”

Extended Catalog of Winged or X-shaped Radio Sources from the FIRST Survey

“…We have classified it as a probable XRG (or an intermediate between the X-and Z-shaped radio galaxies) based on its FIRST image shown in the top right panel of Figure 7. As it is known to be a post-merger star-forming galaxy (Hota et al 2012), its wings could even have formed owing to pre-merger gravitational interaction between the two galaxies. Coming to some other "probable" XRG candidates in the FR I domain (Figure 6), we find that a few of them (e.g., J1044+3540; see the bottom right panel of Figure 7) appear remarkably similar in radio morphology to the early lowresolution radio map of XRG NGC 326 (Fanti et al 1977), which is a classical example of an FR I XRG, on account of its prominent Figure 6.…”