Companion Galaxy Research Articles

On the Triggering of Extreme Starburst Events in Low-metallicity Galaxies: A Deep Search for Companions of Green Peas

Green pea galaxies are starbursting, low-mass galaxies that are good analogues to star-forming galaxies in the early universe. We perform a survey of 23 green peas using the Multi Unit Spectroscopic Explorer (MUSE) Integral Field Unit spectrograph on the Very Large Telescope to search for companion galaxies. The survey reaches an average point-source depth of ∼10−18 erg cm−2 s−1 for emission lines. The MUSE field of view allows us to probe a 1 × 1 arcmin2 field around these galaxies and to search their surroundings for faint companions that could have interacted with them and induced their starburst episodes. We search for companions using a variety of methods including template matching to emission- and absorption-line spectra. When restricting the search to the same physical area (R = 78 kpc) for all galaxies, we find that the fraction of green pea galaxies with companions is . We define a control sample of star-forming galaxies with the same stellar masses and redshifts as the green peas, but consistent with the star formation main sequence. We find that green pea galaxies are as likely to have companions as the control sample; for which the fraction of objects with companions is . Given that we do not find statistical evidence for an elevated companion fraction in the green peas in this study, we argue that the “companions” are likely unrelated to the bursts in these galaxies.

RAD@home citizen science discovery of an active galactic nucleus spewing a large unipolar radio bubble on to its merging companion galaxy

ABSTRACT Active galactic nucleus (AGN) feedback during galaxy merger has been the most favoured model to explain black hole–galaxy co-evolution. However, how the AGN-driven jet/wind/radiation is coupled with the gas of the merging galaxies, which leads to positive feedback, momentarily enhanced star formation, and subsequently negative feedback, a decline in star formation, is poorly understood. Only a few cases are known where the jet and companion galaxy interaction leads to minor off-axis distortions in the jets and enhanced star formation in the gas-rich minor companions. Here, we briefly report one extraordinary case, RAD12, discovered by RAD@home citizen science collaboratory, where for the first time a radio jet–driven bubble (∼ 137 kpc) is showing a symmetric reflection after hitting the incoming galaxy which is not a gas-rich minor but a gas-poor early-type galaxy in a major merger. Surprisingly, neither positive feedback nor any radio lobe on the counter jet side, if any, is detected. It is puzzling if RAD12 is a genuine one-sided jet or a case of radio lobe trapped, compressed and re-accelerated by shocks during the merger. This is the first imaging study of RAD12 presenting follow-up with the Giant Metrewave Radio Telescope, archival MeerKAT radio data and Canada-France-Hawaii Telescope optical data.

Diffuse non-thermal emission in the disks of the Magellanic Clouds

Context. The Magellanic Clouds, two dwarf galaxy companions to the Milky Way, are among the Fermi Large Area Telescope (LAT) brightest γ-ray sources. Aims. We present comprehensive modeling of the non-thermal electromagnetic and neutrino emission in both Clouds. Methods. We self-consistently model the radio and γ-ray spectral energy distribution from their disks based on recently published Murchison Widefield Array and Fermi/LAT data. All relevant radiative processes involving relativistic and thermal electrons (synchrotron, Compton scattering, and bremsstrahlung) and relativistic protons (neutral pion decay following interaction with thermal protons) are considered, using exact emission formulae. Results. Joint spectral analyses indicate that radio emission in the Clouds has both primary and secondary electron synchrotron and thermal bremsstrahlung origin, whereas γ rays originate mostly from π0 decay with some contributions from relativistic bremsstrahlung and Comptonized starlight. The proton spectra in both galaxies are modeled as power laws in energy with similar spectral indices, ~2.4, and energy densities, ~1 eV cm−3. The predicted 0.1–10 GeV neutrino flux is too low for detection by current and upcoming experiments. Conclusions. We confirm earlier suggestions of a largely hadronic origin of the γ-ray emission in both Magellanic Clouds.

Hot magnetic halo of NGC 628 (M 74)

Context. In several spiral galaxies that are observed face-on, large-scale ordered magnetic fields (the so-called magnetic arms) were found. One of the explanations was the action of the magnetic reconnection, which leads to a higher ordering of the magnetic fields. Because it simultaneously converts the energy of the magnetic fields into thermal energy of the surroundings, magnetic reconnection has been considered as a heating mechanism of the interstellar medium for many years. Until recently, no clear observational evidence for this phenomenon was found. Aims. We search for possible signatures of gas heating by magnetic reconnection effects in the radio and X-ray data for the face-on spiral galaxy NGC 628 (M 74), which presents pronounced magnetic arms and evidence for vertical magnetic fields. Methods. The strengths and energy densities of the magnetic field in the spiral and magnetic arms were derived, as were the temperatures and thermal energy densities of the hot gas, for the disk and halo emission. Results. In the regions of magnetic arms, higher order and lower energy density of the magnetic field is found than in the stellar spiral arms. The global temperature of the hot gas is roughly constant throughout the disk. Conclusions. The comparison of the findings with those obtained for the starburst galaxy M 83 suggests that magnetic reconnection heating may be present in the halo of NGC 628. The joint analysis of the properties of the magnetic fields and the hot gas in NGC 628 also provided clues for possible tidal interaction with the companion galaxy.

A tentative ∼1000 km s−1 offset between the [CII] 158 m and Lyα line emission in a star-forming galaxy at z = 7.2

GN-108036 is a star-forming galaxy at z = 7.21, and one of the most distant known sources in the Northern hemisphere. Based on observations from the NOrthern Extended Millimeter Array (NOEMA), here we report the tentative detection of the [C II] line at ≈4σ significance. The integrated [C II] line emission is spatially offset about ∼4 kpc from the rest-frame ultraviolet (UV) emission. The total [C II] luminosity (L[CII] = 2.7 × 108 L⊙) is consistent with the relation between [C II] luminosity and star formation rate (SFR) observed in nearby and high-z star forming galaxies. More interestingly, the [C II] line is blueshifted with respect to the Lyα line by 980 ± 10 km s−1. If confirmed, this corresponds to the largest velocity offset reported to date between the Lyα line and a non-resonant line at z ≳ 6. According to trends observed in other high redshift galaxies, the large Lyα velocity offset in GN-108036 is consistent with its low Lyα equivalent width and high UV absolute magnitude. Based on Lyα radiative transfer models of expanding shells, the large Lyα velocity offset in GN-108036 could be interpreted as the presence of a large column density of hydrogen gas, and/or an outflow with a velocity of vout ∼ ΔvLyα/2 ∼ 500 km s−1. We also report the 3σ detection of a potential galaxy companion located ∼30 kpc east of GN-108036, at a similar systemic velocity, and with no counterpart rest-frame UV emission.

Unusual Gas Structure in an Otherwise Normal Spiral Galaxy Hosting GRB 171205A/SN 2017iuk

We study the structure of atomic hydrogen (H i) in the host galaxy of GRB 171205A/SN 2017iuk at z = 0.037 through H i 21 cm emission line observations with the Karl G. Jansky Very Large Array. These observations reveal unusual morphology and kinematics of the H i in this otherwise apparently normal galaxy. High column density, cold H i is absent from an extended North–South region passing by the optical center of the galaxy, but instead is extended toward the South, on both sides of the galaxy. Moreover, the H i kinematics do not show a continuous change along the major axis of the galaxy as expected in a classical rotating disk. We explore several scenarios to explain the H i structure and kinematics in the galaxy: feedback from a central starburst and/or an active galactic nucleus, ram-pressure stripping, accretion, and tidal interaction from a companion galaxy. All of these options are ruled out. The most viable remaining explanation is the penetrating passage of a satellite through the disk only a few Myr ago, redistributing the H i in the GRB host without yet affecting its stellar distribution. It can also lead to the rapid formation of peculiar stars due to a violent induced shock. The location of GRB 171205A in the vicinity of the distorted area suggests that its progenitor star(s) originated in extreme conditions that share the same origin as the peculiarities in H i. This could explain the atypical location of GRB 171205A in its host galaxy.

The X-shaped Radio Galaxy J0725+5835 is Associated with an AGN Pair

X-shaped radio galaxies (XRGs) are those that exhibit two pairs of unaligned radio lobes (main radio lobes and wings). One of the promising models for the peculiar morphology is jet reorientation. To clarify this, we conducted a 5 GHz observation with the European VLBI Network (EVN) of XRG J0725+5835, which resembles the archetypal binary active galactic nuclei (AGNs) 0402+379 in radio morphology, but it is larger in angular size. In our observation, two milliarcsecond-scale radio components with nonthermal radio emission are detected. Each of them coincides with an optical counterpart with similar photometric redshift and (optical and infrared) magnitude, corresponding to dual active nuclei. Furthermore, with the improved Very Large Array (VLA) images, we find a bridge between the two radio cores and a jet bending in the region surrounding the companion galaxy. This further supports the interplay between the main and companion galaxies. In addition, we also report the discovery of an arcsecond-scale jet in the companion. Given the projected separation of ∼100 kpc between the main and companion galaxies, XRG J0725+5835 is likely associated with a dual jetted-AGN system. In both EVN and VLA observations, we find signatures that the jet is changing its direction, which is likely responsible for the X-shaped morphology. For the origin of jet reorientation, several scenarios are discussed.

A Dynamical Model of the M101/NGC 5474 Encounter

We present the first dynamical simulation that recreates the major properties of the archetypal nearby spiral galaxy M101. Our model describes a grazing but relatively close (∼14 kpc) passage of the companion galaxy NGC 5474 through M101's outer disk approximately 200 Myr ago. The passage is retrograde for both disks, yielding a relatively strong gravitational response while suppressing the formation of long tidal tails. The simulation reproduces M101's overall lopsidedness, as well as the extended NE Plume and the sharp western edge of the galaxy’s disk. The post-starburst populations observed in M101's NE Plume are likely a result of star formation triggered at the point of contact where the galaxies collided. Over time, this material will mix azimuthally, leaving behind diffuse, kinematically coherent stellar streams in M101's outer disk. At late times after the encounter, the density profile of M101's disk shows a broken “upbending” profile similar to those seen in spiral galaxies in denser environments, further demonstrating the connection between interactions and long-term structural changes in galaxy disks.

The halo of M 105 and its group environment as traced by planetary nebula populations

Context. M 105 (NGC 3379) is an early-type galaxy in the nearby Leo I group, the closest galaxy group to contain all galaxy types and therefore an excellent environment to explore the low-mass end of intra-group light (IGL) assembly. Aims. We present a new and extended kinematic survey of planetary nebulae (PNe) in M 105 and the surrounding 30′×30′ in the Leo I group with the Planetary Nebula Spectrograph (PN.S) to investigate kinematically distinct populations of PNe in the halo and the surrounding IGL. Methods. We use PNe as kinematic tracers of the diffuse stellar light in the halo and IGL, and employ photo-kinematic Gaussian mixture models to (i) separate contributions from the companion galaxy NGC 3384, and (ii) associate PNe with structurally defined halo and IGL components around M 105. Results. We present a catalogue of 314 PNe in the surveyed area and firmly associate 93 of these with the companion galaxy NGC 3384 and 169 with M 105. The PNe in M 105 are further associated with its halo (138) and the surrounding exponential envelope (31). We also construct smooth velocity and velocity dispersion fields and calculate projected rotation, velocity dispersion, and λR profiles for the different components. PNe associated with the halo exhibit declining velocity dispersion and rotation profiles as a function of radius, while the velocity dispersion and rotation of the exponential envelope increase notably at large radii. The rotation axes of these different components are strongly misaligned. Conclusions. Based on the kinematic profiles, we identify three regimes with distinct kinematics that are also linked to distinct stellar population properties: (i) the rotating core at the centre of the galaxy (within 1Reff) formed in situ and is dominated by metal-rich ([M/H] ≈ 0) stars that also likely formed in situ, (ii) the halo from 1 to 7.5Reff consisting of a mixture of intermediate-metallicity and metal-rich stars ([M/H] > −1), either formed in situ or was brought in via major mergers, and (iii) the exponential envelope reaching beyond our farthest data point at 16Reff, predominately composed of metal-poor ([M/H] < −1) stars. The high velocity dispersion and moderate rotation of the latter are consistent with those measured for the dwarf satellite galaxies in the Leo I group, indicating that this exponential envelope traces the transition to the IGL.

A Green Pea Starburst Arising from a Galaxy–Galaxy Merger

Green Pea galaxies are low-redshift starburst dwarf galaxies, with properties similar to those of the high-redshift galaxies that reionized the universe. We report the first mapping of the spatial distribution of atomic hydrogen (Hi) in and around a Green Pea, GP J0213+0056 at z = 0.0399, using the Giant Metrewave Radio Telescope (GMRT). Like many Green Peas, GP J0213+0056 shows strong Hi 21 cm emission in single-dish spectroscopy, strong Lyα emission, and a high [Oiii]λ5007 Å/[Oii]λ3727 Å luminosity ratio, O32 ≈ 8.8, consistent with a high leakage of Lyman-continuum radiation. Our GMRT Hi 21 cm images show that the Hi 21 cm emission in the field of GP J0213+0056 arises from an extended broken-ring structure around the Green Pea, with the strongest emission coming from a region between GP J0213+0056 and a companion galaxy lying ≈4.7 kpc away, and little Hi 21 cm emission coming from the Green Pea itself. We find that the merger between GP J0213+0056 and its companion is likely to have triggered the starburst, and led to a disturbed Hi spatial and velocity distribution, which in turn allowed Lyα (and, possibly, Lyman-continuum) emission to escape the Green Pea. Our results suggest that such mergers, and the resulting holes in the Hi distribution, are a natural way to explain the tension between the requirements of cold gas to fuel the starburst and the observed leakage of Lyα and Lyman-continuum emission in Green Pea galaxies and their high-redshift counterparts.

The ionizing properties of two bright Lyα emitters in the Bremer Deep Field reionized bubble at z = 7

Aims. We investigate the ionizing properties of the pair of bright Lyα emitting galaxies BDF521 and BDF2195 at z = 7.012 in order to constrain their contribution to the formation of the Bremer Deep Field (BDF) ‘reionized bubble’ in which they have been shown to reside. Methods. We obtain constraints on four UV emission lines (the CIVλ1548 doublet, HeIIλ1640, the OIII]λ1660 doublet, and the CIII]λ1909 doublet) from deep VLT X-shooter observations and compare them to those available for other high-redshift objects, and to models with mixed stellar and active galactic nucleus (AGN) emission. We use this spectroscopic information, together with the photometry available in the field, to constrain the physical properties of the two objects using the spectro-photometric fitting code BEAGLE. Results. We do not detect any significant emission at the expected position of the UV lines, with 3σ upper limits of equivalent width (EW) ≲2–7 Å rest-frame. We find that the two objects have a lower CIII] emission than expected on the basis of the correlation between the Lyα and CIII] EWs. The EW limits on CIV and HeII emission exclude pure AGN templates at ∼2 − 3σ significance, and only models with a ≲40% AGN contribution are compatible with the observations. The two objects are found to be relatively young (∼20–30 Myrs) and metal-poor (≲0.3 Z⊙), with stellar masses of a few 109 M⊙. Their production rate of hydrogen ionizing photons per intrinsic UV luminosity is log(ξion*/Hz erg−1) = 25.02–25.26, consistent with values typically found in high-redshift galaxies, but more than twice lower than values measured in z > 7 galaxies with strong CIII] and/or optical line emission (≃25.6–25.7). Conclusions. The two BDF emitters show no evidence of higher-than-average ionizing capabilities and are not capable of reionizing their surroundings by their own means, under realistic assumptions of the escape fraction of ionizing photons. Therefore, a dominant contribution to the formation of the reionized bubble must have been provided by fainter companion galaxies. The capabilities of the James Webb Space Telescope will be needed for spectroscopic confirmation of these objects.

Spatially Resolved Molecular Interstellar Medium in a z = 6.6 Quasar Host Galaxy

We present high spatial resolution (∼0.″4, 2.2 kpc) observations of the CO(6−5), CO(7−6), and [C i]369 μm lines and dust continuum emission from the interstellar medium (ISM) in the host galaxy of the quasar J0305−3150 at z = 6.6. These, together with archival [C ii]158 μm data at a comparable spatial resolution, enable studies of the spatial distribution and kinematics between the ISM in different phases. When comparing the radial profiles of CO, [C ii]158 μm, and the dust continuum, we find that the CO and dust continuum exhibit similar spatial distributions, both of which are less extended than the [C ii]158 μm, indicating that the CO and dust continuum are tracing the same gas component, while the [C ii]158 μm is tracing a more extended one. In addition, we derive the radial profiles of the [C ii]158 μm/CO, [C ii]158 μm/far-infrared (FIR), CO/FIR, and dust continuum S 98.7 GHz/S 258.1 GHz ratios. We find a decreasing S 98.7 GHz/S 258.1 GHz ratio with radius, possibly indicating a decrease of dust optical depth with increasing radius. We also detect some of the ISM lines and continuum emission in the companion galaxies previously discovered in the field around J0305−3150. Through comparing the line-to-line and line-to-FIR ratios, we find no significant differences between the quasar and its companion galaxies.

Enhanced star formation in z ∼ 6 quasar companions

ABSTRACT Quasars powered by supermassive black holes (>108 M⊙) at z ∼ 6 are predicted to reside in cosmic overdense regions. However, observations so far could not confirm this expectation due to limited statistics. The picture is further complicated by the possible effects of quasar outflows (i.e. feedback) that could either suppress or stimulate the star formation rate (SFR) of companion galaxies, thus modifying the expected bias. Here, we quantify feedback effects on the properties and detectability of companions by comparing cosmological zoom-in simulations of a quasar in which feedback is either included or turned-off. With respect to the no-feedback case, companions (a) directly impacted by the outflow have their SFR increased by a factor of 2−3, and (b) tend to be more massive. Both effects shift the [C ii] 158 μm and UV luminosity functions towards brighter magnitudes. This leads us to conclude that quasar feedback slightly increases the effective quasar bias, boosting the number density of observable quasar companions, in agreement with what has been found around the brightest quasars of recent Atacama Large Millimeter Array (ALMA) [C ii] surveys. Deeper observations performed with James Webb Space Telescope and/or ALMA will improve the statistical significance of this result by detecting a larger number of fainter quasar companions.

Multiwavelength and Multi-CO View of the Minor Merger Driven Star Formation in the Nearby LIRG NGC 3110

We present Atacama Large Millimeter/submillimeter Array observations of multiple 12CO, 13CO, and C18O lines and 2.9 mm and 1.3 mm continuum emission toward the nearby interacting luminous infrared galaxy NGC 3110, supplemented with similar spatial resolution Hα, 1.4 GHz continuum, and K-band data. We estimate the typical CO-to-H2 conversion factor of 1.7 M ⊙ (K km s−1 pc2)−1 within the disk using local thermal equilibrium-based and dust-based H2 column densities, and measure the 1 kpc scale surface densities of the star formation rate (ΣSFR), super star clusters (ΣSSC), molecular gas mass, and star formation efficiency (SFE) toward the entire gas disk. These parameters show a peak in the southern part of the southern spiral arm (SFE ∼ 10−8.2 yr−1, ΣSFR ∼ 10−0.6 M ⊙ kpc−2 yr−1, ΣSSC ∼ 6.0 kpc−2), which is likely attributable to the ongoing tidal interaction with the companion galaxy MCG-01-26-013, as well as toward the circumnuclear region. We also find that thermal free–free emission contributes to a significant fraction of the millimeter continuum emission at the position of the southern peak. These measurements imply that the peak of the southern arm is an active and young star-forming region, whereas the central part of NGC 3110 is a site of long-continued star formation. We suggest that during the early stage of the galaxy–galaxy interaction in which the mass ratio was high in NGC 3110, fragmentation along the main galaxy arms is an important driver of merger-induced star formation, and that massive gas inflow results in dusty nuclear starbursts.

CHANG-ES – XXVII. A radio/X-ray catalogue of compact sources in and around edge-on galaxies

ABSTRACT We present catalogues of discrete, compact radio sources in and around the discs of 35 edge-on galaxies in the Continuum Halos in Nearby Galaxies – an EVLA Survey. The sources were extracted using the pybdsf program at both 1.6 GHz (L band) and 6.0 GHz (C band) from matching resolution (≈3 arcsec) data. We also present catalogues of X-ray sources from Chandra data sets for 27 of the galaxies. The sources at the two radio frequency bands were positionally cross-correlated with each other, and the result cross-correlated with the X-ray sources. All catalogues are included for download with this paper. We detect a total of 2507 sources at L band and 1413 sources at C band. 75 sources have been successfully cross-correlated in both radio bands plus X-ray. Three new nuclear sources are candidates for low luminosity active galactic nuclei in NGC 3877, NGC 4192, and NGC 5792; the one in NGC 3877 also appears to be variable. We also find new nuclear sources in two companion galaxies: NGC 4435 (companion to NGC 4438) and NGC 4298 (companion to NGC 4302). We have also discovered what appears to be a foreground double star; each star has X-ray emission and there is radio emission at both L band and C band in between them. This could be a colliding wind binary system. Suggestions for follow-up studies are offered.

Toward a More Complete Optical Census of Active Galactic Nuclei via Spatially Resolved Spectroscopy

Abstract While emission line flux ratio diagnostics are the most common technique for identifying active galactic nuclei (AGNs) in optical spectra, applying this approach to single-fiber spectra of galaxies can omit entire subpopulations of AGNs. Here, we use spatially resolved spectroscopy from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to construct a sample of 10 galaxies where Baldwin–Philips–Terlevich line flux ratio diagnostics classify each galaxy’s central 3″ spectrum as LINER or star-forming, while >10% of the spaxels in the galaxy’s MaNGA footprint are classified as Seyfert. We obtain Chandra observations of these 10 galaxies with off-nuclear Seyfert regions to determine whether AGNs are actually present in them. Our main result is that 7–10 (depending on strictness of criteria) of the galaxies host one or more X-ray AGNs, even though none of them were classified as AGNs based on their single-fiber optical spectra. We find that these AGNs were not identified in the single-fiber spectra because they are AGNs in the nuclei of companion galaxies, low-luminosity AGNs, dust-obscured AGNs, and/or flickering AGNs. In summary, we find that off-nuclear AGN signatures may increase the number of known AGNs by a factor of two over what conventional single nuclear fiber spectra identify. Our results show that spatially resolved spectroscopy can be leveraged to reveal a more complete census of AGNs that are traditionally missed by single-fiber spectra.

Conditions for Direct Black Hole Seed Collapse near a Radio-loud Quasar 1 Gyr after the Big Bang

Abstract Observations of luminous quasars and their supermassive black holes at z ≳ 6 suggest that they formed at dense matter peaks in the early universe. However, few studies have found definitive evidence that the quasars lie at cosmic density peaks, in clear contrast with theory predictions. Here we present new evidence that the radio-loud quasar SDSS J0836+0054 at z = 5.8 could be part of a surprisingly rich structure of galaxies. This conclusion is reached by combining a number of findings previously reported in the literature. Bosman et al. obtained the redshifts of three companion galaxies, confirming an overdensity of i 775 dropouts found by Zheng et al. By comparing this structure with those found near other quasars and large overdense regions in the field at z ∼ 6–7, we show that the SDSS J0836+0054 field is among the densest structures known at these redshifts. One of the spectroscopic companions is a very massive star-forming galaxy ( log 10 (  ⋆ / M ⊙ ) = 10.3 − 0.2 + 0.3 ) based on its unambiguous detection in a Spitzer 3.6 μm image. This suggests that the quasar field hosts not one, but at least two rare, massive dark matter halos ( log 10 (  h / M ⊙ ) ≳ 12 ), corresponding to a galaxy overdensity of at least 20. We discuss the properties of the young radio source. We conclude that the environment of SDSS J0836+0054 resembles, at least qualitatively, the type of conditions that may have spurred the direct collapse of a massive black hole seed according to recent theory.

HSTWFC3/Grism observations of the candidate ultra-high-redshift radio galaxy GLEAM J0917–0012

AbstractWe presentHubble Space TelescopeWide Field Camera 3 photometric and grism observations of the candidate ultra-high-redshift ($z>7$) radio galaxy, GLEAM J0917–0012. This radio source was selected due to the curvature in its 70–230 MHz, low-frequency Murchison Widefield Array radio spectrum and its faintness inK-band. Follow-up spectroscopic observations of this source with the Jansky Very Large Array and Atacama Large Millimetre Array were inconclusive as to its redshift. Our F105W and F0986M imaging observations detect the host of GLEAM J0917–0012 and a companion galaxy,$\sim$one arcsec away. The G102 grism observations reveal a single weak line in each of the spectra of the host and the companion. To help identify these lines we utilised several photometric redshift techniques including template fitting to the grism spectra, fitting the ultraviolet (UV)-to-radio photometry with galaxy templates plus a synchrotron model, fitting of the UV-to-near-infrared photometry withEAZY, and fitting the radio data alone withRAiSERed. For the host of GLEAM J0917–0012 we find a line at$1.12\,\mu$m and the UV-to-radio spectral energy distribution (SED) fitting favours solutions at$z\sim 2$or$z\sim 8$. While this fitting shows a weak preference for the lower redshift solution, the models from the higher redshift solution are more consistent with the strength of the spectral line. The redshift constraint byRAiSERedof$>6.5$also supports the interpretation that this line could be Lyman$-\alpha$at$z=8.21$; howeverEAZYfavours the$z\sim 2$solution. We discuss the implications of both solutions. For the companion galaxy we find a line at$0.98\,\mu$m and the SED fitting favours solutions at$z<3$implying that the line could be the [OII]$\lambda3727$doublet at$z=1.63$(although theEAZYsolution is$z\sim 2.6\pm 0.5$). Further observations are still required to unambiguously determine the redshift of this intriguing candidate ultra-high-redshift radio galaxy.

Taking snapshots of the jet-ISM interplay: The case of PKS 0023–26

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 × 1010 M⊙, depending on the XCO 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 ×107 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.

RAD@home discovery of a one-sided radio jet hitting the companion galaxy

AbstractMinkowski’s Object and ‘Death Star galaxy’ are two of the famous cases of rare instances when a radio jet has been observed to directly hit a neighbouring galaxy. RAD12, the RAD@home citizen science discovery with GMRT being presented here, is not only a new system being added to nearly half a dozen rare cases known so far but also the first case where the neighbouring galaxy is not a minor/dwarf companion but a galaxy bigger than the host of the radio jet. Additionally, the jet appears to be one-sided and the jet after interaction completely stops and forms a bubble inflating laterally which is unlike previous cases of minor deviation or loss of collimation. Since the nature of radio jet-ISM coupling is poorly understood so far, more discovery of objects like RAD12 will be important to the understanding of galaxy evolution through merger and AGN feedback.