Molecular Gas Filaments and Fallback in the Ram Pressure Stripped Coma Spiral NGC 4921

Abstract

We investigate the effects of ram pressure on the molecular interstellar medium (ISM) in the disk of the Coma cluster galaxy NGC 4921 via high-resolution CO observations. We present 6″ resolution CARMA CO(1−0) observations of the full disk, and 0.″4 resolution Atacama Large Millimeter/submillimeter Array CO(2−1) observations of the leading quadrant, where ram pressure is strongest. We find evidence for compression of the dense ISM on the leading side, spatially correlated with intense star formation activity in this zone. We also detect molecular gas along kiloparsec-scale filaments of dust extending into the otherwise gas stripped zone of the galaxy, seen in Hubble Space Telescope images. We find the filaments are connected kinematically as well as spatially to the main gas ridge located downstream, consistent with cloud decoupling inhibited by magnetic binding, and inconsistent with a simulated filament formed via simple ablation. Furthermore, we find several clouds of molecular gas ∼1–3 kpc beyond the main ring of CO that have velocities that are blueshifted by up to 50 km s−1 with respect to the rotation curve of the galaxy. These are some of the only clouds we detect that do not have any visible dust extinction associated with them, suggesting that they are located behind the galaxy disk midplane and are falling back toward the galaxy. Simulations have long predicted that some gas removed from the galaxy disk will fall back during ram pressure stripping. This may be the first clear observational evidence of gas re-accretion in a ram pressure stripped galaxy.

LoTSS jellyfish galaxies

“…Close inspection of the HSC image shows small streams and clumps of dust that extend asymmetrically toward the western edge of UGC 2654 toward the direction of the radio continuum tail. This is is evidence for the stripping of dust in UGC 2654, consistent with previous works that have reported evidence for dust stripping in other nearby cluster galaxies (Kenney et al 2015;Cramer et al 2021). The Subaru g-band and INT Hα + [Nii] imaging for UGC 2654 is clearly asymmetric about the north-south axis.…”

“…These galaxy spectral indices are consistent with flattening due to ionization losses (Basu et al 2015;Chyży et al 2018) in a high density ISM. This flattening is expected at the low frequencies that we probe in this work and becomes increasingly important in the highest density regions of the ISM (Basu et al 2015), which is also consistent with enhanced ISM densities in jellyfish galaxies due to compression from ram pressure (e.g., Schulz & Struck 2001;Bekki 2014;Troncoso-Iribarren et al 2020;Cramer et al 2021).…”

“…Increasingly, RPS has been suggested to play an important role in this environmental quenching (e.g., Gavazzi et al 2001;Boselli & Gavazzi 2006;Chung et al 2007;Yagi et al 2010;Jáchym et al 2014;Muzzin et al 2014;Kenney et al 2015;Boselli et al 2018;Brown et al 2017;Poggianti et al 2017;Maier et al 2019;Roberts et al 2019;Ciocan et al 2020). Ram pressure can influence galaxy star formation both by removing cold gas (leading to rapid quenching) and by driving increased densities in the interstellar medium (ISM), potentially leading to starburst behavior (e.g., Schulz & Struck 2001;Bekki 2014;Vulcani et al 2018;Roberts & Parker 2020;Cramer et al 2021). The latter is predicted to occur on the "leading-side" of galaxies undergoing RPS, in other words, the side of the galaxy directly encountering the intracluster medium (ICM), opposite to the stripped tail (e.g., Gavazzi et al 2001;Boselli et al 2021).…”

VERTICO II: How H i-identified Environmental Mechanisms Affect the Molecular Gas in Cluster Galaxies

“…Cramer et al (2020) also report a region with increased CO surface brightness as a result of RPS in NGC4402, resulting in enhanced star formation (which aids the removal of diffuse gas through stellar feedback). In a similar study of the CO in Coma cluster galaxy NGC 4921, Cramer et al (2021) also find evidence for compression of the dense ISM on the leading side of this galaxy, accompanied by enhanced star-formation activity. As discussed above, similar results were found by Mok et al (2017).…”

“…More recently, it has been shown that, despite its more tightly bound and centrally located nature (e.g., Davis et al 2013), molecular gas can also be directly affected by environmental processes. For example, molecular gas disks in cluster galaxies have been found to be truncated and/or morphologically and kinematically disturbed, while molecular gas fractions have been found to be deficient compared to field galaxies at fixed stellar mass (e.g., Vollmer et al 2008;Fumagalli et al 2009;Boselli et al 2014a;Lee et al 2017;Lee & Chung 2018;Zabel et al 2019;Cramer et al 2021;Brown et al 2021). Additionally, molecular gas fractions can also be enhanced in cluster galaxies, as a result of ram pressure, which can facilitate the conversion of atomic into molecular gas (e.g., Moretti et al 2020).…”