The jet-ISM interaction in the Outer Filament of Centaurus A

The interaction between the radio plasma ejected by the active nucleus of a galaxy and the surrounding medium is a key process that can have a strong impact on the interstellar medium of the galaxy and hence on galaxy evolution. The closest laboratory where we can observe and investigate this phenomenon is the radio galaxy Centaurus A. About 15 kpc north-east of this galaxy, a particularly complex region is found: the so-called Outer Filament where jet-cloud interactions have been proposed to occur. We investigate the presence of signatures of jet-ISM interaction by a detailed study of the kinematics of the ionized gas, expanding on previous results obtained from the HI. We observed two regions of the outer filament with VLT/VIMOS in the IFU observing mode. Emission from Hbeta and [OIII]4959,5007\AA\ is detected in both pointings. We found two distinct kinematical components of ionized gas that well match the kinematics of the nearby HI cloud. One component follows the regular kinematics of the rotating gas while the second shows similar velocities to those of the nearby HI component thought to be disturbed by an interaction with the radio jet. We suggest that the ionized and atomic gas are part of the same dynamical gas structure originating as result of the merger that shaped Centaurus A and which is regularly rotating around Centaurus A as proposed by other authors. The gas (ionized and HI) with anomalous velocities is tracing the interaction of the Large-Scale radio Jet with the ISM, suggesting that, although poorly collimated as structure, the jet is still active. However, we can exclude that a strong shock is driving the ionization of the gas. It is likely that a combination of jet entrainment and photoionization by the UV continuum from the central engine is needed in order to explain both the ionization and the kinematics of the gas in the Outer Filament.Comment: 6 pages, 6 figures, 1 table. Final version accepted for publication on A&

The outer filament of Centaurus A as seen by MUSE

We investigate signatures of a jet-interstellar medium (ISM) interaction using optical integral-field observations of the so-called outer filament near Centaurus A, expanding on previous results obtained on a more limited area. Using the Multi Unit Spectroscopic Explorer (MUSE) on the VLT during science verification, we observed a significant fraction of the brighter emitting gas across the outer filament. The ionized gas shows complex morphology with compact blobs, arc-like structures and diffuse emission. Based on the kinematics, we identified three main components. The more collimated component is oriented along the direction of the radio jet. The other two components exhibit diffuse morphology together with arc-like structures also oriented along the radio jet direction. Furthermore, the ionization level of the gas is found to decrease from the more collimated component to the more diffuse components. The morphology and velocities of the more collimated component confirm our earlier results that the outer filament and the nearby HI cloud are likely partially shaped by the lateral expansion of the jet. The arc-like structures embedded within the two remaining components are the clearest evidence of a smooth jet-ISM interaction along the jet direction. This suggests that, although poorly collimated, the radio jet is still active and has an impact on the surrounding gas. This result indicates that the effect on the ISM of even low-power radio jets should be considered when studying the influence Active Galactic Nuclei can have on their host galaxy.Comment: 5 pages, 3 figures, Accepted for publication by A&

The fast molecular outflow in the Seyfert galaxy IC5063 as seen by ALMA

We use high-resolution (0.5 arcsec) CO(2-1) observations performed with ALMA to trace the kinematics of the molecular gas in the Seyfert 2 galaxy IC5063. A fast outflow of molecular gas extends along the entire radio jet, with the highest outflow velocities about 0.5kpc from the nucleus, at the location of the brighter hot-spot in the W lobe. The data show that a massive, fast outflow with velocities up to 650 km/s of cold molecular gas is present, in addition to one detected earlier in warm H2, HI and ionised gas. Both the central AGN and the radio jet could energetically drive the outflow. However, the characteristics of the outflowing gas point to the radio jet being the main driver. This is important, because IC5063, although one of the most powerful Seyfert galaxies, is a relatively weak radio source (P = 3x10^23 W/Hz). All the observed characteristics can be described by a scenario of a radio plasma jet expanding into a clumpy medium, interacting directly with the clouds and inflating a cocoon that drives a lateral outflow into the interstellar medium. This model is consistent with results obtained by recent simulations such as those of Wagner et al.. A stronger, direct interaction between the jet and a gas cloud is present at the location of the brighter W lobe. Even assuming the most conservative values for the conversion factor CO-to-H2, the mass of the outflowing gas is between 1.9 and 4.8x10^7 Msun. These amounts are much larger than those of the outflow of warm gas (molecular and ionized) and somewhat larger than of the HI outflow. This suggests that most of the observed cold molecular outflow is due to fast cooling after being shocked. This gas is the end product of the cooling process. Our CO observations demonstrate that fast outflows of molecular gas can be driven by relativistic jets.Comment: Accepted for publication in A&A. 11 pages, 8 figure

LOFAR observations of 4C+19.44. On the discovery of low frequency spectral curvature in relativistic jet knots

We present the first LOFAR observations of the radio jet in the quasar 4C+19.44 (a.k.a. PKS 1354+19) obtained with the long baselines. The achieved resolution is very well matched to that of archival Jansky Very Large Array (JVLA) observations at higher radio frequencies as well as the archival X-ray images obtained with {\it Chandra}. We found that, for several knots along the jet, the radio flux densities measured at hundreds of MHz lie well below the values estimated by extrapolating the GHz spectra. This clearly indicates the presence of spectral curvature. Radio spectral curvature has been already observed in different source classes and/or extended radio structures and it has been often interpreted as due to intrinsic processes, as a curved particle energy distribution, rather than absorption mechanisms ({ Razin-Tsytovich} effect, free-free or synchrotron self absorption to name a few). Here we discuss our results according to the scenario where particles undergo stochastic acceleration mechanisms also in quasar jet knots.Comment: 13 pages, 4 tables, 4 figures, pre-proof version, published on the Astrophysical Journal (Harris, et al. 2019 ApJ, 873, 21

Spectroscopy with the Engineering Development Array: cold H+^{+} at 63 MHz towards the Galactic Centre

The Engineering Development Array (EDA) is a single test station for Square Kilometre Array (SKA) precursor technology. We have used the EDA to detect low-frequency radio recombination lines (RRLs) from the Galactic Centre region. Low-frequency RRLs are an area of interest for future low-frequency SKA work as these lines provide important information on the physical properties of the cold neutral medium. In this project we investigate the EDA, its bandpass and the radio frequency interference environment for low-frequency spectroscopy. We present line spectra from 30 to 325 MHz for the Galactic Centre region. The decrease in sensitivity for the EDA at the low end of the receiver prevents carbon and hydrogen RRLs to be detected below 40 and 60 MHz respectively. RFI strongly affects frequencies in the range 276-292, 234-270, 131-138, 95-102 and below 33 MHz. Cn$\alpha$ RRLs were detected in absorption for quantum levels n = 378 to 550 (39-121 MHz) and in emission for n = 272 to 306 (228-325 MHz). Cn$\beta$ lines were detected in absorption for n = 387 to 696 (39-225 MHz). Hn$\alpha$ RRLs were detected in emission for n = 272 to 480 (59-325 MHz). Hn$\beta$ lines were detected for n = 387 to 453 (141-225 MHz). The stacked Hn$\alpha$ detection at 63 MHz is the lowest frequency detection made for hydrogen RRLs and shows that a cold (partially) ionized medium exists along the line of sight to the Galactic Centre region. The size and velocity of this cold H$^{+}$ gas indicates that it is likely associated with the nearby Riegel-Crutcher cloud.Comment: 18 pages, 6 figures and 5 table

Extended, Dusty Star Formation Fueled by a Residual Cooling Flow in the Cluster of Galaxies Sersic 159-03

While the cooling of the hot intra-cluster medium (ICM) in the cores of galaxy clusters is mostly counteracted by heating from the central active galactic nucleus (AGN), the balance is not perfect. This can lead to residual cooling flows and low-level star formation, the physics of which is not well understood. Here we present a detailed study of the residual cooling flow in the center of the low mass galaxy cluster Sersic 159-03 (A S1101) using far-ultraviolet imaging from the Hubble Space Telescope and far-infrared (FIR) spectroscopy and photometry from the Herschel space observatory, along with a wealth of archival data. We detect extended emission at UV, FIR, and [CII], indicating a star formation rate of ~1-3 Msun/yr, depending on the indicator and assumptions made. The most recently formed stars appear spatially coincident with the lowest entropy ICM. We speculate that this low-entropy gas has been displaced by the central AGN ~7.5 kpc north of the cD galaxy. These data demonstrate that the displacement of the cooling core from the direct vicinity of the central AGN can temporarily break the feedback cycle and lead to cooling and star formation that is offset from the center of the galaxy. We find an abundance (~10^7 Msun) of cold (20K) dust in the center of the cluster and a second FIR peak ~30kpc to the north of the central galaxy. If confirmed to be associated with the cooling filaments, this would be the most extended complex of dust yet found in a cool core cluster.Comment: 10 pages, 8 figures. Submitted to ApJ - comments welcom

Carbon radio recombination lines from gigahertz to megahertz frequencies towards Orion A

Context. The combined use of carbon radio recombination lines (CRRLs) and the 158 $\mu$m-[CII] line is a powerful tool for the study of the energetics and physical conditions (e.g., temperature and density) of photodissociation regions (PDRs). However, there are few observational studies that exploit this synergy. Aims. Here we explore the relation between CRRLs and the 158 $\mu$m-[CII] line in light of new observations and models. Methods. We present new and existing observations of CRRLs in the frequency range 0.15--230 GHz with ALMA, VLA, the GBT, Effelsberg 100m, and LOFAR towards Orion~A (M42). We complement these observations with SOFIA observations of the 158 $\mu$m-[CII] line. We studied two PDRs: the foreground atomic gas, known as the Veil, and the dense PDR between the HII region and the background molecular cloud. Results. In the Veil we are able to determine the gas temperature and electron density, which we use to measure the ionization parameter and the photoelectric heating efficiency. In the dense PDR, we are able to identify a layered PDR structure at the surface of the molecular cloud to the south of the Trapezium cluster. There we find that the radio lines trace the colder portion of the ionized carbon layer, the C$^{+}$/C/CO interface. By modeling the emission of the $158$~$\mu$m-[CII] line and CRRLs as arising from a PDR we derive a thermal pressure $>5\times10^{7}$ K cm$^{-3}$ and a radiation field $G_{0}\approx10^{5}$ close to the Trapezium. Conclusions. This work provides additional observational support for the use of CRRLs and the 158 $\mu$m-[CII] line as complementary tools to study dense and diffuse PDRs, and highlights the usefulness of CRRLs as probes of the C$^{+}$/C/CO interface.Comment: 18 pages, 16 figures, accepted for publication in A&