Discovery of a new extragalactic circular radio source with ASKAP: ORC J0102–2450

We present the discovery of another Odd Radio Circle (ORC) with the Australian Square Kilometre Array Pathfinder (ASKAP) at 944 MHz. The observed radio ring, ORC J0102–2450, has a diameter of ∼70 arcsec or 300 kpc, if associated with the central elliptical galaxy DES J010224.33–245039.5 (z ∼ 0.27). Considering the overall radio morphology (circular ring and core) and lack of ring emission at non-radio wavelengths, we investigate if ORC J0102–2450 could be the relic lobe of a giant radio galaxy seen end-on or the result of a giant blast wave. We also explore possible interaction scenarios, for example, with the companion galaxy, DES J010226.15–245104.9, located in or projected onto the south-eastern part of the ring. We encourage the search for further ORCs in radio surveys to study their properties and origin.

X-raying the galaxy pair Arp 41: no collision in NGC 1232 and three ultraluminous sources in NGC 1232A

ABSTRACT We studied the apparent galaxy pair NGC 1232/NGC 1232A with Chandra, looking for evidence of interactions and collisions. We report that there is no cloud of diffuse emission in NGC 1232, contrary to previous claims in the literature. Instead, we find that the small ‘companion’ galaxy NGC 1232A contains three ultraluminous X-ray sources with peak 0.3–10 keV luminosities above 1040 erg s−1 (assuming a cosmological distance of ≈93 Mpc for this galaxy). For its mass, morphology, metal abundance, and bright ULX population, NGC 1232A is analogous to the more nearby late-type spiral NGC 1313.

Nature of the DLA towards Q 0528−250: High pressure and strong UV field revealed by excitation of C i, H2, and Si ii

ABSTRACT We present the detection of excited fine-structure energy levels of singly ionized silicon and neutral carbon associated with the proximate damped Lyman-α system at zabs = 2.811 towards Q 0528−250. This absorber has an apparent relative velocity that is inconsistent with the Hubble flow indicating motion along the line-of-sight towards the quasar, i.e. zabs > zem. We measure the metallicity of the system to be [Zn/H] = −0.68 ± 0.02. Using the relative populations of the fine-structure levels of Si ii and C i, as well as the populations of H2 rotational levels, we constrain the physical conditions of the gas. We derive hydrogen number densities of $n_{\rm H}=190^{+70}_{-50}$ cm−3 and $260^{+30}_{-20}$ cm−3 in two velocity components where both C i and H2 are detected. Taking into account the kinetic temperature in each component, ∼150 K, we infer high values of thermal pressure in the cold neutral medium probed by the observations. The strengths of the UV field in Draine’s unit are $I_{\rm UV} = 10^{+5}_{-3}$ and $14^{+3}_{-3}$ in each of these two components, respectively. Such enhanced UV fluxes and thermal pressure compared to intervening DLAs are likely due to the proximity of the quasar. The typical size of the absorber is ∼104 au. Assuming the UV flux is dominated by the quasar, we constrain the distance between the quasar and the absorber to be ∼150−200 kpc. This favours a scenario where the absorption occurs in a companion galaxy located in the group where the quasar-host galaxy resides. This is in line with studies in emission that revealed the presence of several galaxies around the quasar.

SMM J04135+10277: a distant QSO–starburst system caught by ALMA

ABSTRACT The gas content of galaxies is a key factor for their growth, starting from star formation and black hole accretion to galaxy mergers. Thus, characterizing its properties through observations of tracers like the CO emission line is of big importance in order to understand the bigger picture of galaxy evolution. We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of dust continuum, CO(5–4) and CO(8–7) line emission in the quasar–star-forming companion system SMM J04135+10277 (z = 2.84). Earlier low-J CO studies of this system found a huge molecular gas reservoir associated with the companion galaxy, while the quasar appeared gas-poor. Our CO observations revealed that the host galaxy of the quasar is also gas-rich, with an estimated molecular gas mass of $\sim (0.7{\!-\!}2.3)\times 10^{10}\, \rm M_{\odot}$. The CO line profiles of the companion galaxy are very broad ($\sim 1000\, \rm km\, s^{-1}$), and show signs of rotation of a compact, massive system. In contrast to previous far-infrared observations, we resolve the continuum emission and detect both sources, with the companion galaxy dominating the dust continuum and the quasar having a $\sim 25{{\ \rm per\ cent}}$ contribution to the total dust emission. By fitting the infrared spectral energy distribution of the sources with mr-moose and empirical templates, the infrared luminosities of the quasar and the companion are in the range of $L_{\rm IR, QSO}\sim (2.1{\!-\!}9.6)\times 10^{12}\, \rm L_{\odot}$ and $L_{\rm IR, Comp.}\sim (2.4{\!-\!}24)\times 10^{12}\, \rm L_{\odot}$, while the estimated star formation rates are $\sim 210{\!-\!}960$ and $\sim 240{\!-\!}2400\, \rm M_{\odot }\, yr^{-1}$, respectively. Our results demonstrate that non-detection of low-J CO transition lines in similar sources does not necessarily imply the absence of massive molecular gas reservoir but that the excitation conditions favour the excitation of high-J transitions.

Stellar and gas mass distributions for understanding the nature of spiral arms

Numerical simulations of disk galaxies with steady (long-lived) and dynamic (short-lived) spiral arms suggest that offsets between stellar and gas spiral arms depend on their nature or lifetime (Baba et al.2015). Based on this theoretical study, we investigated gas-star offsets in the nearby grand-design spiral galaxy M51, and found that its two spiral arms exhibit different offset dependences against radius. One arm is consistent with a steady arm, while the other is consistent with a dynamic arm. We deduce that this difference is likely due to a tidal interaction with the companion galaxy (Egusa et al.2017). For this study, a stellar mass distribution with a high accuracy at a high spatial resolution is essential, which has come to be available by applying recent SED fitting techniques to multi-wavelength images. We are now working to extend this study to other nearby spiral galaxies.

Discovery of the first heavily obscured QSO candidate at z > 6 in a close galaxy pair

While theoretical arguments predict that most of the early growth of supermassive black holes (SMBHs) happened during heavily obscured phases of accretion, current methods used for selecting z > 6 quasars (QSOs) are strongly biased against obscured QSOs, thus considerably limiting our understanding of accreting SMBHs during the first gigayear of the Universe from an observational point of view. We report the Chandra discovery of the first heavily obscured QSO candidate in the early universe, hosted by a close (≈5 kpc) galaxy pair at z = 6.515. One of the members is an optically classified type-1 QSO, PSO167–13. The companion galaxy was first detected as a [C II] emitter by Atacama large millimeter array (ALMA). An X-ray source is significantly (P = 0.9996) detected by Chandra in the 2–5 keV band, with < 1.14 net counts in the 0.5–2 keV band, although the current positional uncertainty does not allow a conclusive association with either PSO167–13 or its companion galaxy. From X-ray photometry and hardness-ratio arguments, we estimated an obscuring column density of NH > 2 × 1024 cm−2 and NH > 6 × 1023 cm−2 at 68% and 90% confidence levels, respectively. Thus, regardless of which of the two galaxies is associated with the X-ray emission, this source is the first heavily obscured QSO candidate at z > 6.

The host galaxy of GRB 980425/SN1998bw: a collisional ring galaxy

ABSTRACT We report Giant Metrewave Radio Telescope (GMRT), Very Large Telescope (VLT), and Spitzer Space Telescope observations of ESO 184−G82, the host galaxy of GRB 980425/SN 1998bw, that yield evidence of a companion dwarf galaxy at a projected distance of 13 kpc. The companion, hereafter GALJ193510-524947, is a gas-rich, star-forming galaxy with a star formation rate of $\rm 0.004\, M_{\odot }\, yr^{-1}$, a gas mass of $10^{7.1\pm 0.1} \, \mathrm{M}_{\odot}$, and a stellar mass of $10^{7.0\pm 0.3} \, \mathrm{M}_{\odot}$. The interaction between ESO 184−G82 and GALJ193510-524947 is evident from the extended gaseous structure between the two galaxies in the GMRT H i 21 cm map. We find a ring of high column density H i gas, passing through the actively star-forming regions of ESO 184−G82 and the GRB location. This ring lends support to the picture in which ESO 184−G82 is interacting with GALJ193510-524947. The massive stars in GALJ193510-524947 have similar ages to those in star-forming regions in ESO 184−G82, also suggesting that the interaction may have triggered star formation in both galaxies. The gas and star formation properties of ESO 184−G82 favour a head-on collision with GALJ193510-524947 rather than a classical tidal interaction. We perform state-of-the-art simulations of dwarf–dwarf mergers and confirm that the observed properties of ESO 184−G82 can be reproduced by collision with a small companion galaxy. This is a very clear case of interaction in a gamma-ray burst host galaxy and of interaction-driven star formation giving rise to a gamma-ray burst in a dense environment.

Using radio jets of PKS J0334-3900 to probe the intra-cluster medium in A3135

We present a multi-wavelength study of the radio galaxy PKS J0334-3900, which resides at the centre of Abell 3135. Using Australia Telescope Compact Array (ATCA) observations at 1.4, 2.5, 4.6 & 8.6 GHz, we performed a detailed analysis of PKS J0334-3900. The morphology and spectral indices give physical parameters that constrain the dynamical history of the galaxy, which we use to produce a simulation of PKS J0334-3900. This simulation shows that the morphology can be generated by a wind in the intracluster medium (ICM), orbital motion caused by a companion galaxy, and precession of the black hole (BH).Additionally, ATCA polarisation data was used to obtain rotation measure values along the line of sight to PKS J0334-3900. Using our simulation we are able to infer the distance between the jets along the line of sight to be 154 ± 16 kpc, which when combined with the difference in rotation measure between the jets provides a novel new way to estimate the average magnetic field within the cluster. A lower limit to the cluster magnetic field was calculated to be 0.09±0.03 μG. We have shown that different techniques can be applied to observations of jets in galaxies to infer information on cluster environments.