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Register a new userCold gas and the disruptive effect of a young radio jet
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Newly born and young radio sources are in a delicate phase of their life. Their jets are fighting their way through the surrounding gaseous medium, strongly experiencing this interaction while, at the same time, impacting and affecting the interstellar medium (ISM). Here we present the results from two studies of HI (in absorption) and molecular gas illustrating what can be learned from these phases of the gas. We first describe a statistical study with the WSRT. The study shows that the young radio sources not only have an higher detection rate of HI, but also systematically broader and more asymmetric HI profiles, most of them blueshifted. This supports the idea that we are looking at young radio jets making their way through the surrounding ISM, which also appears to be, on average, richer in gas than in evolved radio sources. Signatures of the impact of the jet are seen in the kinematics of the gas. However, even among the young sources, we identify a population that remains undetected in HI even after stacking their profiles. Orientation effects can only partly explain the result. These objects either are genuinely gas-poor or have different conditions of the medium, e.g. higher spin temperature. We further present the ALMA study of molecular gas in IC5063 to trace in detail the jet impacting the ISM. The kinematics of the cold, molecular gas co-spatial with the radio plasma shows this process in action. The ALMA data reveal a fast outflow of molecular gas extending along the entire radio jet (~1 kpc), with the highest outflow velocities at the location of the brighter hot-spot. We propose a scenario where the radio jet is expanding into a clumpy medium, interacting directly with the clouds and inflating a cocoon that drives a lateral outflow into the ISM.
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Cold neutral gas is a key ingredient for growing the stellar and central black hole mass in galaxies throughout cosmic history. We have used the Atacama Large Millimetre Array (ALMA) to detect a rare example of redshifted $^{12}$CO(2-1) absorption in PKS B1740-517, a young ($t sim 1.6 times 10^{3}$ yr) and luminous ($L_{rm 5 GHz} sim 6.6 times 10^{43}$ erg s$^{-1}$ ) radio galaxy at $z = 0.44$ that is undergoing a tidal interaction with at least one lower-mass companion. The coincident HI 21-cm and molecular absorption have very similar line profiles and reveal a reservoir of cold gas ($M_{rm gas} sim 10^{7} - 10^{8}$ M$_{odot}$), likely distributed in a disc or ring within a few kiloparsecs of the nucleus. A separate HI component is kinematically distinct and has a very narrow line width ($Delta{v}_{rm FWHM} lesssim 5$ km s$^{-1}$), consistent with a single diffuse cloud of cold ($T_{rm k} sim 100$ K) atomic gas. The $^{12}$CO(2-1) absorption is not associated with this component, which suggests that the cloud is either much smaller than 100 pc along our sight-line and/or located in low-metallicity gas that was possibly tidally stripped from the companion. We argue that the gas reservoir in PKS B1740-517 may have accreted onto the host galaxy $sim$50 Myr before the young radio AGN was triggered, but has only recently reached the nucleus. This is consistent with the paradigm that powerful luminous radio galaxies are triggered by minor mergers and interactions with low-mass satellites and represent a brief, possibly recurrent, active phase in the life cycle of massive early type galaxies.
At present neutral atomic hydrogen (HI) gas in galaxies at redshifts above $z sim 0.3$ (the extent of 21-cm emission surveys in individual galaxies) and below $z sim 1.7$ (where the Lyman-$alpha$ line is not observable with ground-based telescopes) has remained largely unexplored. The advent of precursor telescopes to the Square Kilometre Array will allow us to conduct the first systematic radio-selected 21-cm absorption surveys for HI over these redshifts. While HI absorption is a tracer of the reservoir of cold neutral gas in galaxies available for star formation, it can also be used to reveal the extreme kinematics associated with jet-driven neutral outflows in radio-loud active galactic nuclei. Using the six-antenna Boolardy Engineering Test Array of the Australian Square Kilometre Array Pathfinder, we have demonstrated that in a single frequency tuning we can detect HI absorption over a broad range of redshifts between $z = 0.4$ and $1.0$. As part of our early science and commissioning program, we are now carrying out a search for absorption towards a sample of the brightest GPS and CSS sources in the southern sky. These intrinsically compact sources present us with an opportunity to study the circumunuclear region of recently re-started radio galaxies, in some cases showing direct evidence of mechanical feedback through jet-driven outflows. With the sensitivity of the full ASKAP array we will be able to study the kinematics of atomic gas in a few thousand radio galaxies, testing models of radio jet feedback well beyond the nearby Universe
We use ALMA to detect and image CO (1-0) emission from Minkowskis Object, a dwarf galaxy that is interacting with a radio jet from a nearby elliptical galaxy. These observations are the first to detect molecular gas in Minkowskis Object. We estimate the range in the mass of molecular gas in Minkowskis Object assuming two different values of the ratio of the molecular gas mass to the CO luminosity, $alpha_{rm CO}$. For the Milky Way value of $alpha_{rm CO}=4.6~M_{odot}{rm (K~km~s^{-1}~pc^2)^{-1}}$ we obtain a molecular gas mass of $M_{rm H_2} =3.0 times 10^7~M_{odot}$, 6% of the HI gas mass. We also use the prescription of Narayanan et al. (2012) to estimate an $alpha_{rm CO}=27~M_{odot}{rm (K~km~s^{-1}~pc^2)^{-1}}$, in which case we obtain $M_{rm H_2} =1.8 times 10^8~M_{odot}$, 36% of the HI mass. The observations are consistent with previous claims of star formation being induced in Minkowskis Object via the passage of the radio jet, and it therefore being a rare local example of positive feedback from an AGN. In particular, we find highly efficient star formation, with gas depletion timescales $sim 5times 10^7 - 3times 10^8$yr (for assumed values of $alpha_{rm CO}=4.6$ and $27~M_{odot}{rm (K~km~s^{-1}~pc^2)^{-1}}$, respectively) in the upstream regions of Minkowskis Object that were struck first by the jet, and less efficient star formation downstream. We discuss the implications of this observation for models of jet induced star formation and radio mode feedback in massive galaxies.
Fast outflows of gas, driven by the interaction between the radio-jets and ISM of the host galaxy, are being observed in an increasing number of galaxies. One such example is the nearby radio galaxy 3C293. In this paper we present Integral Field Unit (IFU) observations taken with OASIS on the William Herschel Telescope (WHT), enabling us to map the spatial extent of the ionised gas outflows across the central regions of the galaxy. The jet-driven outflow in 3C293 is detected along the inner radio lobes with a mass outflow rate ranging from $sim 0.05-0.17$ solar masses/yr (in ionised gas) and corresponding kinetic power of $sim 0.5-3.5times 10^{40}$ erg/s. Investigating the kinematics of the gas surrounding the radio jets (i.e. not directly associated with the outflow), we find line-widths broader than $300$ km/s up to 5 kpc in the radial direction from the nucleus (corresponding to 3.5 kpc in the direction perpendicular to the radio axis at maximum extent). Along the axis of the radio jet line-widths $>400$ km/s are detected out to 7 kpc from the nucleus and line-widths of $>500$ km/s at a distance of 12 kpc from the nucleus, indicating that the disturbed kinematics clearly extend well beyond the high surface brightness radio structures of the jets. This is suggestive of the cocoon structure seen in simulations of jet-ISM interaction and implies that the radio jets are capable of disturbing the gas throughout the central regions of the host galaxy in all directions.
It has recently been shown that the abundance of cold neutral gas may follow a similar evolution as the star formation history. This is physically motivated, since stars form out of this component of the neutral gas and if the case, would resolve the longstanding issue that there is a clear disparity between the total abundance of neutral gas and star forming activity over the history of the Universe. Radio-band 21-cm absorption traces the cold gas and comparison with the Lyman-alpha absorption, which traces all of the gas, provides a measure of the cold gas fraction or the spin temperature. The recent study has shown that the spin temperature (degenerate with the ratio of the absorber/emitter extent) appears to be anti-correlated with the star formation density, undergoing a similar steep evolution as the star formation rate over redshifts of 0 < z < 3, whereas the total neutral hydrogen exhibits little evolution. Above z > 3, where the SFR shows a steep decline with redshift, there is insufficient 21-cm data to determine whether the spin temperature continues to follow the SFR. Knowing this is paramount in ascertaining whether the cold neutral gas does trace the star formation over the Universes history. We explore the feasibility of resolving this with 21-cm observations of the largest contemporary sample of reliable damped Lyman-alpha absorption systems and conclude that, while todays largest radio interferometers can reach the required sensitivity at z < 3.5, the Square Kilometre Array is required to probe to higher redshifts.
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