Early Science with the Large Millimeter Telescope: discovery of the¹²CO(1–0) emission line in the ring galaxy VIIZw466

Abstract

We report an early science discovery of the 12 CO(1-0) emission line in the collisional ring galaxy, VII Zw466, using the Redshift Search Receiver instrument on the Large Millimeter Telescope Alfonso Serrano.The apparent molecular-to-atomic gas ratio either places the ISM of VII Zw466 in the HI-dominated regime or implies a large quantity of CO-dark molecular gas, given its high star formation rate. The molecular gas densities and star formation rate densities of VII Zw466 are consistent with the standard Kennicutt-Schmidt star formation law even though we find this galaxy to be H 2 -deficient. The choice of CO-to-H 2 conversion factors cannot explain the apparent H 2 deficiency in its entirety. Hence, we find that the collisional ring galaxy, VII Zw466, is either largely deficient in both H 2 and HI or contains a large mass of CO-dark gas. A low molecular gas fraction could be due to the enhancement of feedback processes from previous episodes of star formation as a result of the star-forming ISM being confined to the ring. We conclude that collisional ring galaxy formation is an extreme form of galaxy interaction that triggers a strong galactic-wide burst of star formation that may provide immediate negative feedback towards subsequent episodes of star formation-resulting in a short-lived star formation history or, at least, the appearance of a molecular gas deficit.

Ring galaxies in the EAGLE hydrodynamical simulations

“…• Ring galaxies live in massive groups (Mhalo ∼ 10 13 M ) that are preferentially more concentrated than groups without ring galaxies at fixed halo mass. This is in agreement with the observations, in which ring galaxies are located within galaxy groups and have at least one companion galaxy (Higdon & Wallin 1997;Higdon 1995;Romano et al 2008;Conn et al 2016;Wong et al 2017;Elagali et al 2018).…”

“…This is different than the result presented in Figure 12, where the star formation rate surface density in ring galaxies is higher in the outer radii (r > 20 kpc) in comparison with the star-forming galaxy sample implying that the abundance of cold gas in the ring should be relatively normal. To examine the photodissociation hypothesis suggested in Higdon et al (2015); Wong et al (2017), we need to follow the formation and the evolution of the molecular hydrogen. This requires a detailed description of the ISM and the photoionisation effects of local sources on the H2 gas, which is not currently feasible in simulations with a boxsize as large as EAGLE and only prescribed in zoom-in simulations, see for example Hopkins et al (2014Hopkins et al ( , 2017.…”

“…Mihos & Hernquist (1994) and Mapelli et al (2008a) showed in their simulations that it takes around 100 Myr after the collision to develop the ring in the disk of these galaxies and that the ring morphology remains visible for ∼ 0.5 Gyr, which is very short compared to the Hubble timescale. Since the formation mechanisms of collisional ring galaxies are very well constrained via simulations, they can be considered as galaxy-scale perturbation experiments that facilitate the study of extreme modes of interaction-triggered star formation and feedback processes (Higdon et al 2012(Higdon et al , 2015Wong et al 2017;Renaud et al 2018).…”

“…Further, observations of collisional ring galaxies suggests that these systems contain on average high amounts of HI gas in comparison with galaxies that have the same stellar mass (Elagali et al 2018), yet are H2 deficient (Higdon et al 2015;Wong et al 2017) especially at the outer rings where the atomic hydrogen surface density is the highest (e.g., in the Cartwheel galaxy ΣHI = 19 − 65 M pc −2 ; Higdon et al 2015). The reason behind this deficiency is not yet well understood, however Wong et al (2017) and Higdon et al (2015) hypothesise that the ISM in ring galaxies behaves differently as a result of the extreme conditions in, e.g., pressure, temperature and metallicity, induced by the drop-through collision.…”

Early science with the Large Millimeter Telescope: a 1.1 mm AzTEC survey of red-Herschel dusty star-forming galaxies

“…The RSR data was reduced using the D package (Data REduction and Analysis Methods in PYthon, written by G. Narayanan) and following the standard procedure (e.g. Yun et al 2015;Cybulski et al 2016;Wong et al 2017). A careful visual inspection of individual scans was performed to identify and remove those with the noisiest spectral features.…”