The nearby system 4C12.50, also known as IRAS 13451+1217 and PKS 1345+12, is a merger of gas-rich galaxies with infrared and radio activity. It has a perturbed interstellar medium (ISM) and a dense configuration of gas and dust around the nucleus. The radio emission at small (∼100 pc) and large (∼100 kpc) scales, as well as the large X-ray cavity in which the system is embedded, are indicative of a jet that could have affected the ISM. We carried out observations of the CO(1-0), (3-2), and (4-3) lines with the Atacama Large Millimeter Array (ALMA) to determine basic properties (i.e., extent, mass, and excitation) of the cold molecular gas in this system, including its already-known wind. The CO emission reveals the presence of gaseous streams related to the merger, which result in a small (∼4kpc-wide) disk around the western nucleus. The disk reaches a rotational velocity of 200 km s −1 , and has a mass of 3.8(±0.4)×10 9 M . It is truncated at a gaseous ridge north of the nucleus that is bright in [O iii]. Regions with high-velocity CO emission are seen at signal-to-noise ratios of between 3 and 5 along filaments that radially extend from the nucleus to the ridge and that are bright in [O iii] and stellar emission. A tentative wind detection is also reported in the nucleus and in the disk. The molecular gas speed could be as high as 2200 km s −1 and the total wind mass could be as high as 1.5(±0.1)×10 9 M . Energetically, it is possible that the jet, assisted by the radiation pressure of the active nucleus or the stars, accelerated clouds inside an expanding bubble.
We present [C ii] synthetic observations of smoothed particle hydrodynamics (SPH) simulations of a dwarf galaxy merger. The merging process varies the star formation rate (SFR) by more than three orders of magnitude. Several star clusters are formed, the feedback of which disperses and unbinds the dense gas through expanding H ii regions and supernova (SN) explosions. For galaxies with properties similar to the modeled ones, we find that the [C ii] emission remains optically thin throughout the merging process. We identify the warm neutral medium ( 3 < log T gas < 4 with χ HI > 2χ H2) to be the primary source of [C ii] emission (∼58% contribution), although at stages when the H ii regions are young and dense (during star cluster formation or SNe in the form of ionized bubbles), they can contribute ≳50% to the total [C ii] emission. We find that the [C ii]/far-IR (FIR) ratio decreases owing to thermal saturation of the [C ii] emission caused by strong far-UV radiation fields emitted by the massive star clusters, leading to a [C ii] deficit medium. We investigate the [C ii]−SFR relation and find an approximately linear correlation that agrees well with observations, particularly those from the Dwarf Galaxy Survey. Our simulation reproduces the observed trends of [C ii]/FIR versus ΣSFR and ΣFIR, and it agrees well with the Kennicutt relation of SFR−FIR luminosity. We propose that local peaks of [C ii] in resolved observations may provide evidence for ongoing massive cluster formation.
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