This review summarises what we have learnt in the last two decades based on H
i
21 cm absorption observations about the cold interstellar medium (ISM) in the central regions of active galaxies and ...about the interplay between this gas and the active nucleus (AGN). H
i
absorption is a powerful tracer on all scales, from the parsec-scales close to the central black hole to structures of many tens of kpc tracing interactions and mergers of galaxies. Given the strong radio continuum emission often associated with the central activity, H
i
absorption observations can be used to study the H
i
near an active nucleus out to much higher redshifts than is possible using H
i
emission. In this way, H
i
absorption has been used to characterise in detail the general ISM in active galaxies, to trace the fuelling of radio-loud AGN, to study the feedback occurring between the energy released by the active nucleus and the ISM, and the impact of such interactions on the evolution of galaxies and of their AGN. In the last two decades, significant progress has been made in all these areas. It is now well established that many radio loud AGN are surrounded by small, regularly rotating gas disks that contain a significant fraction of H
i
. The structure of these disks has been traced down to parsec scales by very long baseline interferometry observations. Some groups of objects, and in particular young and recently restarted radio galaxies, appear to have a particularly high detection rate of H
i
. This is interesting in connection with the evolution of these AGN and their impact on the surrounding ISM. This is further confirmed by an important discovery, made thanks to technical upgrades of radio telescopes, namely the presence of fast, AGN-driven outflows of cold gas which give a direct view of the impact of the energy released by AGN on the evolution of galaxies (AGN feedback). In addition, evidence has been collected that clouds of cold gas can play a role in fuelling the nuclear activity. This review ends by briefly describing the upcoming large, blind H
i
absorption surveys planned for the new radio telescopes which will soon become operational. These surveys will allow to significantly expand existing work, but will also allow to explore new topics, in particular, the evolution of the cold ISM in AGN.
Galactic archaeology based on star counts is instrumental to reconstruct the past mass assembly of Local Group galaxies. The development of new observing techniques and data reduction, coupled with ...the use of sensitive large field of view cameras, now allows us to pursue this technique in more distant galaxies exploiting their diffuse low surface brightness (LSB) light. As part of the ATLAS3D project, we have obtained with the MegaCam camera at the Canada–France–Hawaii Telescope extremely deep, multiband images of nearby early-type galaxies (ETGs). We present here a catalogue of 92 galaxies from the ATLAS3D sample, which are located in low- to medium-density environments. The observing strategy and data reduction pipeline, which achieve a gain of several magnitudes in the limiting surface brightness with respect to classical imaging surveys, are presented. The size and depth of the survey are compared to other recent deep imaging projects. The paper highlights the capability of LSB-optimized surveys at detecting new prominent structures that change the apparent morphology of galaxies. The intrinsic limitations of deep imaging observations are also discussed, among those, the contamination of the stellar haloes of galaxies by extended ghost reflections, and the cirrus emission from Galactic dust. The detection and systematic census of fine structures that trace the present and past mass assembly of ETGs are one of the prime goals of the project. We provide specific examples of each type of observed structures – tidal tails, stellar streams and shells – and explain how they were identified and classified. We give an overview of the initial results. The detailed statistical analysis will be presented in future papers.
Leo T is the lowest mass gas-rich galaxy currently known and studies of its gas content help us understand how such marginal galaxies survive and form stars. We present deep neutral hydrogen (H I) ...observations from the Westerbork Synthesis Radio Telescope in order to understand its H I distribution and potential for star formation. We find a larger H I line flux than the previously accepted value, resulting in a 50% larger H I mass of 4.1 × 105 M⊙. The additional H I flux is from low surface brightness emission that was previously missed; with careful masking this emission can be recovered even in shallower data. We perform a Gaussian spectral decomposition to find a cool neutral medium component (CNM) with a mass of 3.7 × 104 M⊙, or almost 10% of the total H I mass. Leo T has no H I emission extending from the main H I body, but there is evidence of interaction with the Milky Way circumgalactic medium in both a potential truncation of the H I body and the offset of the peak H I distribution from the optical center. The CNM component of Leo T is large when compared to other dwarf galaxies, even though Leo T is not currently forming stars and has a lower star formation efficiency than other gas-rich dwarf galaxies. However, the H I column density associated with the CNM component in Leo T is low. One possible explanation is the large CNM component is not related to star formation potential but rather a recent, transient phenomenon related to the interaction of Leo T with the Milky Way circumgalactic medium.
We present the detection, performed with the Westerbork Synthesis Radio Telescope (WSRT) and the Karl Jansky Very Large Array (VLA), of a fast H I 21 cm outflow in the ultra-luminous infrared galaxy ...Mrk 231. The outflow is observed as shallow H I absorption blueshifted ~1300 km s-1 with respect to the systemic velocity and located against the inner kpc of the radio source. The outflowing gas has an estimated column density between 5 and 15 × 1018Tspin cm-2. We derive the Tspin to lie in the range 400−2000 K and the corresponding H I densities are nHI ~ 10−100 cm-3. Our results complement previous findings and confirm the multiphase nature of the outflow in Mrk 231. Although effects of the interaction between the radio plasma and the surrounding medium cannot be ruled out, the energetics and the lack of a clear kpc-scale jet suggest that the most likely origin of the H I outflow is a wide-angle nuclear wind, as earlier proposed to explain the neutral outflow traced by Na I and molecular gas in this source. Our results suggest that an H I component is present in fast outflows regardless of the acceleration mechanism (wind vs. jet driven) and that it must be connected with common properties of the pre-interaction gas involved. Considering the observed similarity of their column densities, the H I outflow likely represents the inner part of the broad wind identified on larger scales in atomic Na I. The mass outflow rate of the H I outflow (between 8 and 18 M⊙ yr-1) does not appear to be as large as that observed in molecular gas, partly owing to the smaller sizes of the outflowing region sampled by the H I absorption. These characteristics are commonly seen in other cases of outflows driven by the active galactic nucleus (AGN) suggesting that the H I may represent a short intermediate phase in the rapid cooling of the gas. The results further confirm H I as a good tracer for AGN-driven outflows not only in powerful radio sources. We also obtained deeper continuum images than previously available. They confirm the complex structure of the radio continuum originating both from the AGN and star formation. At the resolution obtained with the VLA (~1′′) we do not see a kpc-scale jet. Instead, we detect a plateau of emission, likely due to star formation, surrounding the bright nuclear region. We also detect a poorly collimated bridge which may represent the channel feeding the southern lobe. The unprecedented depth of the low-resolution WSRT image reveals radio emission extending 50′′ (43 kpc) to the south and 20′′ (17 kpc) to the north.
We use high-resolution (0.5 arcsec) CO(2−1) observations performed with the Atacama Large Millimetre/submillimetre Array to trace the kinematics of the molecular gas in the Seyfert 2 galaxy IC 5063. ...The data reveal that the kinematics of the gas is very complex. A fast outflow of molecular gas extends along the entire radio jet (~1 kpc), with the highest outflow velocities about 0.5 kpc from the nucleus, at the location of the brighter hot spot in the western lobe. The ALMA data show that a massive, fast outflow with velocities up to 650kms-1 of cold molecular gas is present, in addition to the outflow detected earlier in warm H2, H i and ionized gas. All phases of the gas outflow show similar kinematics. IC 5063 appears to be one of the best examples of the multi-phase nature of AGN-driven outflows. 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 an important result because IC 5063, although one of the most powerful Seyfert galaxies, is a relatively weak radio source (P1.4 GHz = 3 × 1023 W Hz-1). 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. A stronger, direct interaction between the jet and a gas cloud is present at the location of the brighter western lobe. This interaction may also be responsible for the asymmetry in the radio brightness of the two lobes. Even assuming the most conservative values for the conversion factor CO-to-H2, we find that the mass of the outflowing gas is between 1.9 and 4.8 × 107 M⊙, of which between 0.5 and 1.3 × 107 M⊙ is associated with the fast outflow at the location of the western lobe. These amounts are much larger than those of the outflow of warm gas (molecular and ionized) and somewhat larger than of the H i 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, although some of it could be the result of only partly dissociated clouds. Our CO observations demonstrate that fast outflows of substantial masses of molecular gas can be driven by relativistic jets, although in the case of IC 5063 the outflows are not fast enough to remove significant amounts of gas from the galaxy and the effects are limited to the central ~0.5 kpc from the centre.
We present high angular resolution (0.13–0.4 arcsec) ALMA CO(2–1) and 1.7 mm continuum observations of the far-infrared-bright galaxy PKS 0023−26 (
z
= 0.32), which hosts a young radio source as ...well as a luminous optical active galactic nucleus (AGN). Although young, the powerful radio source has already grown to a size of a few kiloparsec, making it potentially capable of affecting the interstellar medium (ISM) of the host galaxy. We detect a very extended distribution of molecular gas with a mass between 0.3 and 3 × 10
10
M
⊙
, depending on the
X
CO
conversion factor. The gas has a maximum radial extent of ∼5 arcsec (24 kpc) from the nucleus and is distributed in an asymmetric structure offset from the radio galaxy and with a fairly smooth velocity gradient. At large radii, tails of gas are observed in the direction of companion galaxies, suggesting that tidal interactions may be responsible for the origin of the gas. Overall, the observed properties are reminiscent of the molecular structures observed in some galaxy clusters. However, in the inner few kiloparsec, across the entire extent of the radio continuum, the kinematics of the gas appears to be affected by the radio source. In the central, sub-kiloparsec region, we observe the brightest emission from the molecular gas and the broadest velocity profiles with a full width at zero intensity (FWZI) of ∼500 km s
−1
, which indicate that in this region a direct interaction of the jet with dense clouds and outflowing molecular gas is happening. On larger, kiloparsec-scales, the molecular gas appears to avoid the radio lobes, while gas with a somewhat smaller velocity dispersion (FWZI of ∼350 km s
−1
) is observed around the radio lobes. Thus, in these regions, the gas appears to be affected by the expanding cocoon surrounding the radio source, likely dispersing and heating preexisting molecular clouds. The observations suggest that the mode of coupling between radio jets and the ISM changes from an outflowing phase limited to the sub-kiloparsec region to a maintenance phase, excavating cavities devoid of dense gas, at larger radii. This reveals that, already on galaxy scales, the impact of the AGN is not limited to outflows. This is in accordance with predictions from numerical simulations. With a star-formation rate of 25
M
⊙
yr
−1
, PKS 0023−26 is located on the SFR-
M
*
relation for star forming galaxies. Thus, the AGN does not appear to have, at present, a major impact on the host galaxy in terms of the overall level of star-formation activity. However, as the jet and lobes expand throughout the galaxy in the coming few ×10
7
yr, they will carry enough energy to be able to prevent further gas cooling and/or to inject turbulence and thus affect future star formation.
We present a detailed study of the properties of the molecular gas in the fast outflow driven by the active galactic nucleus (AGN) in the nearby radio-loud Seyfert galaxy IC 5063. By using ALMA ...observations of a number of tracers of the molecular gas (12CO(1–0), 12CO(2–1), 12CO(3–2), 13CO(2–1) and HCO+(4–3)), we map the differences in excitation, density and temperature of the gas as function of position and kinematics. The results show that in the immediate vicinity of the radio jet, a fast outflow, with velocities up to 800 km s-1, is occurring of which the gas has high excitation with excitation temperatures in the range 30–55 K, demonstrating the direct impact of the jet on the ISM. The relative brightness of the 12CO lines, as well as that of 13CO(2–1) vs. 12CO(2–1), show that the outflow is optically thin. We estimate the mass of the molecular outflow to be at least 1.2 × 106 M⊙ and likely to be a factor between two and three larger than this value. This is similar to that of the outflow of atomic gas, but much larger than that of the ionised outflow, showing that the outflow in IC 5063 is dominated by cold gas. The total mass outflow rate we estimated to be ~12 M⊙ yr-1. The mass of the outflow is much smaller than the total gas mass of the ISM of IC 5063. Therefore, although the influence of the AGN and its radio jet is very significant in the inner regions of IC 5063, globally speaking the impact will be very modest. We used RADEX non-LTE modelling to explore the physical conditions of the molecular gas in the outflow. Models with the outflowing gas being quite clumpy give the most consistent results and our preferred solutions have kinetic temperatures in the range 20–100 K and densities between 105 and 106 cm-3. The resulting pressures are 106–107.5 K cm-3, about two orders of magnitude higher than in the outer quiescent disk. The highest densities and temperatures are found in the regions with the fastest outflow. The results strongly suggest that the outflow in IC 5063 is driven by the radio plasma jet expanding into a clumpy gaseous medium and creating a cocoon of (shocked) gas which is pushed away from the jet axis resulting in a lateral outflow, very similar to what is predicted by numerical simulations.
We make use of interferometric CO and H i observations, and optical integral-field spectroscopy from the ATLAS3D survey, to probe the origin of the molecular and ionized interstellar medium (ISM) in ...local early-type galaxies. We find that 36 ± 5 per cent of our sample of fast-rotating early-type galaxies have their ionized gas kinematically misaligned with respect to the stars, setting a strong lower limit on the importance of externally acquired gas (e.g. from mergers and cold accretion). Slow rotators have a flat distribution of misalignments, indicating that the dominant source of gas is external. The molecular, ionized and atomic gas in all the detected galaxies are always kinematically aligned, even when they are misaligned from the stars, suggesting that all these three phases of the ISM share a common origin. In addition, we find that the origin of the cold and warm gas in fast-rotating early-type galaxies is strongly affected by environment, despite the molecular gas detection rate and mass fractions being fairly independent of group/cluster membership. Galaxies in dense groups and the Virgo cluster nearly always have their molecular gas kinematically aligned with the stellar kinematics, consistent with a purely internal origin (presumably stellar mass loss). In the field, however, kinematic misalignments between the stellar and gaseous components indicate that at least 42 ± 5 per cent of local fast-rotating early-type galaxies have their gas supplied from external sources. When one also considers evidence of accretion present in the galaxies' atomic gas distributions, ≳46 per cent of fast-rotating field ETGs are likely to have acquired a detectable amount of ISM from accretion and mergers. We discuss several scenarios which could explain the environmental dichotomy, including preprocessing in galaxy groups/cluster outskirts and the morphological transformation of spiral galaxies, but we find it difficult to simultaneously explain the kinematic misalignment difference and the constant detection rate. Furthermore, our results suggest that galaxy mass may be an important independent factor associated with the origin of the gas, with the most massive fast-rotating galaxies in our sample (M
K
≲−24 mag; stellar mass of ≈8 × 1010 M⊙) always having kinematically aligned gas. This mass dependence appears to be independent of environment, suggesting it is caused by a separate physical mechanism.
Fast radio bursts (FRBs) are extragalactic astrophysical transients
whose brightness requires emitters that are highly energetic yet compact enough to produce the short, millisecond-duration bursts. ...FRBs have thus far been detected at frequencies from 8 gigahertz (ref.
) down to 300 megahertz (ref.
), but lower-frequency emission has remained elusive. Some FRBs repeat
, and one of the most frequently detected, FRB 20180916B
, has a periodicity cycle of 16.35 days (ref.
). Using simultaneous radio data spanning a wide range of wavelengths (a factor of more than 10), here we show that FRB 20180916B emits down to 120 megahertz, and that its activity window is frequency dependent (that is, chromatic). The window is both narrower and earlier at higher frequencies. Binary wind interaction models predict a wider window at higher frequencies, the opposite of our observations. Our full-cycle coverage shows that the 16.3-day periodicity is not aliased. We establish that low-frequency FRB emission can escape the local medium. For bursts of the same fluence, FRB 20180916B is more active below 200 megahertz than at 1.4 gigahertz. Combining our results with previous upper limits on the all-sky FRB rate at 150 megahertz, we find there are 3-450 FRBs in the sky per day above 50 Jy ms. Our chromatic results strongly disfavour scenarios in which absorption from strong stellar winds causes FRB periodicity. We demonstrate that some FRBs are found in 'clean' environments that do not absorb or scatter low-frequency radiation.
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