Galaxy evolution in dense environments: a concise HI perspective

Abstract

Abstract. Observing the neutral hydrogen in galaxy clusters provides crucial insights in the physical processes that influence the evolution of gas-rich galaxies as they migrate from the lower-density filaments through the cluster outskirts into to the higher-density central regions. The morphology-density relation, the Butcher-Oemler effect, and the observed HI deficiencies in the central regions of galaxy clusters suggest that infalling galaxies experience a strong transformation of their morphologies, star formation rates, and gas content. The physical mechanisms that trigger and govern these transformations may depend strongly on environment. This contribution aims to illustrate that the morphological and kinematic characteristics of the cold gas provide a sensitive tool to determine which mechanisms dominate in which environments. Motivation for observing the cold gasHydrogen is the most abundant primordial baryonic matter in the Universe out of which the visible galaxies and galaxy clusters have formed. Observations of the distribution and kinematics of cold neutral hydrogen at the epoch of galaxy formation, and during their subsequent evolution, provide one of the most direct windows on the physical processes involved. Numerous studies indicate that the pace of a galaxy's evolution, like its growth by mass accretion, the rate at which it forms stars, the frequency of tidal interactions and the occurrence of minor or major mergers, depends significantly on its local and global environment. In particular the environment encountered in the outskirts of galaxy clusters is very favorable to rapid evolution and transformation of galaxies as they fall in along the lower-density filaments into the higher-density cluster cores. Such infalling galaxies encounter an increased local galaxy density, which results in more frequent tidal interactions, and they may be confronted with a hot Intra-Cluster Medium (ICM) as they approach the cluster core. This change of environment can have a dramatic impact on a galaxy's morphology, its gas content and its star formation rate. This is also implied by the well-known morphology-density relation combined with the notion that clusters continue to accrete galaxies from the surrounding large scale structure in which they are embedded. Consequently, infalling galaxies must experience a significant transformation on a relatively short time scale, which is also suggested by the ButcherOemler effect and the interpretation that the blue galaxies in the outskirts of galaxy clusters experience a period of vigorous star formation which can end abruptly once all the cold gas is consumed or removed. These galaxies eventually find themselves in the cluster core as gas-poor systems with a passively evolving stellar population.The presence of cold gas in galaxies is a prerequisite for star formation, and the amount and distribution of cold gas regulates the duration and intensity of star formation periods. The removal of this cold gas reservoir from infalling galaxies is a crucial element in t...

Finding faint HI structure in and around galaxies: Scraping the barrel

“…The main characteristic of the H I emission of NGC4111 is an extended faint filament between the three sources of the datacube. The orientation and kinematics of this filament suggest that the galaxies were tidally stripped from the outer disks by their nearby companions (Verheijen, 2004).…”

The Ursa Major Cluster Redefined as a ‘Supergroup’

“…Similar to Figure 9. The NGC3998 galaxy, which shows a disturbed HI content (Verheijen & Zwaan 2001) is highlighted with a black dot (skymap) and a green circle (distance–velocity diagram and velocity histogram). The BGG in the NGC3683 group is located at the kinematic centre (the velocity offset is only 3 km s −1 ). However, the projected spatial offset is 552 kpc, i.e.…”

“…Similar to Figure 9. The NGC3998 galaxy, which shows a disturbed HI content (Verheijen & Zwaan 2001) is highlighted with a black dot (skymap) and a green circle (distance–velocity diagram and velocity histogram). …”

MIGHTEE-HI: discovery of an H i-rich galaxy group atz = 0.044 with MeerKAT

“…This implies a relative deficit of low H -mass galaxies and indicates that denser environments, such as groups, are responsible for removing the H from low mass galaxies (e.g. Verheijen 2004;Freeland et al 2009;Busekool et al 2020). Using an Arecibo Legacy Fast ALFA (ALFALFA; Giovanelli et al 2005) sample, consisting of 1613 H galaxies in 620 groups in the redshift range of 0.020 < < 0.042, sensitivity limited to an H mass of HI ≥ 10 8.9 M , Hess & Wilcots (2013) conclude that as the optically-detected membership of a group increases, the H -detected galaxies reside increasingly towards the outskirts of the group.…”