Inefficient jet-induced star formation in Centaurus A

Abstract

NGC 5128 (Centaurus A) is one of the best targets to study AGN feedback in the local Universe. At 13.5 kpc from the galaxy, optical filaments with recent star formation lie along the radio jet direction. This region is a testbed for positive feedback, here through jet-induced star formation. Atacama Pathfinder EXperiment (APEX) observations have revealed strong CO emission in star-forming regions and in regions with no detected tracers of star formation activity. In cases where star formation is observed, this activity appears to be inefficient compared to the Kennicutt-Schmidt relation. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to map the 12 CO(1-0) emission all along the filaments of NGC 5128 at a resolution of 1.3 ∼ 23.8 pc. We find that the CO emission is clumpy and is distributed in two main structures: (i) the Horseshoe complex, located outside the HI cloud, where gas is mostly excited by shocks and where no star formation is observed, and (ii) the Vertical filament, located at the edge of the HI shell, which is a region of moderate star formation. We identified 140 molecular clouds using a clustering method applied to the CO data cube. A statistical study reveals that these clouds have very similar physical properties, such as size, velocity dispersion, and mass, as in the inner Milky Way. However, the range of radius available with the present ALMA observations does not enable us to investigate whether or not the clouds follow the Larson relation. The large virial parameter α vir of the clouds suggests that gravity is not dominant and clouds are not gravitationally unstable. Finally, the total energy injection in the northern filaments of Centaurus A is of the same order as in the inner part of the Milky Way. The strong CO emission detected in the northern filaments is an indication that the energy injected by the jet acts positively in the formation of dense molecular gas. The relatively high virial parameter of the molecular clouds suggests that the injected kinetic energy is too strong for star formation to be efficient. This is particularly the case in the horseshoe complex, where the virial parameter is the largest and where strong CO is detected with no associated star formation. This is the first evidence of AGN positive feedback in the sense of forming molecular gas through shocks, associated with low star formation efficiency due to turbulence injection by the interaction with the radio jet.

M 31 circum-nuclear region: A molecular survey with the IRAM interferometer

“…We have a mean M H 2 of (7.0 ± 0.3) × 10 3 M for the detected clouds. We find our values for the mass and size of clumps to be consistent with Larson's mass -size relation found in Salomé et al (2017), Gratier et al (2012) and Rosolowsky (2007), cf Figure 9. It is not the case for the velocity -size and velocity -mass relations though.…”

“…of 15.3 ± 2.6 M pc 2 . This is three times less than in NGC 5128, about two times less than in the Milky Way but close to the value found in the LMC, 21 ± 9 M pc 2 (Salomé et al 2017;Miville-Deschênes et al 2017;Hughes et al 2013).…”

“…In our case, clumps have α vir 2, their kinetic energy appears dominant and they are not virialized. Compared to the virial parameter in Salomé et al (2017); Miville-Deschênes et al (2017) and Hughes et al (2013) for giant molecular clouds, our mean value is 140 ± 72 which is one to two orders of magnitude higher than these since our clumps are smaller and less massive. Figure 10 reveals that the velocity dispersions of our measured clumps lie well above the relation for Milky Way clouds.…”

A 1D fluid model of the Centaurus A jet

“…We have no evidence for clouds in the vicinity of the Centaurus A X-ray jet; the presence of clouds is merely demonstrated for the 'middle regions' (e.g. Salomé et al 2017 and references therein), several kpc beyond the extent of the X-ray jet.…”

An Enormous Molecular Gas Flow in the RX J0821+0752 Galaxy Cluster

“…The molecular gas in M87 is located at the truncation of the radio lobe, appearing to be either excited or destroyed by AGN activity (Simionescu et al 2018). Jet-induced star formation is also observed in Centaurus A (Salomé et al 2017). In A1795 the radio jet is projected along the inner edge of a curved molecular filament, suggesting that the jet has either deflected off of the molecular gas or the gas is entrained in the expanding radio bubble (Russell et al 2017b).…”