Effects of environmental gas compression on the multiphase ISM and star formation

Abstract

The cluster environment can affect galaxy evolution in different ways: via ram pressure stripping or by gravitational perturbations caused by galactic encounters. Both kinds of interactions can lead to the compression of the interstellar medium (ISM) and its associated magnetic fields, causing an increase in the gas surface density and the appearance of asymmetric ridges of polarized radio continuum emission. New IRAM 30m HERA CO(2−1) data of NGC 4501, a Virgo spiral galaxy currently experiencing ram pressure stripping, and NGC 4567/68, an interacting pair of galaxies in the Virgo cluster, are presented. We find an increase in the molecular fraction where the ISM is compressed. The gas is close to self-gravitation in compressed regions. This leads to an increase in gas pressure and a decrease in the ratio between the molecular fraction and total ISM pressure. The overall Kennicutt Schmidt relation based on a pixel-by-pixel analysis at ∼1.5 kpc resolution is not significantly modified by compression. However, we detected continuous regions of low molecular star formation efficiencies in the compressed parts of the galactic gas disks. The data suggest that a relation between the molecular star formation efficiency SFE H 2 = S FR/M(H 2 ) and gas self-gravitation (R mol /P tot and Toomre Q parameter) exists. Both systems show spatial variations in the star formation efficiency with respect to the molecular gas that can be related to environmental compression of the ISM. An analytical model was used to investigate the dependence of SFE H 2 on selfgravitation. The model correctly reproduces the correlations between R mol /P tot , SFE H 2 , and Q if different global turbulent velocity dispersions are assumed for the three galaxies. We found that variations in the N H 2 /I CO conversion factor can mask most of the correlation between SFE H 2 and the Toomre Q parameter. Dynamical simulations were used to compare the effects of ram pressure and tidal ISM compression. These models give direct access to the volume density. We conclude that a gravitationally induced ISM compression has the same consequences as ram pressure compression: (i) an increasing gas surface density; (ii) an increasing molecular fraction; and (iii) a decreasing R mol /P tot in the compressed region due to the presence of nearly self-gravitating gas. The response of SFE H 2 to compression is more complex. While in the violent ISM-ISM collisions (e.g., Taffy galaxies and NGC 4438) the interaction makes star formation drop by an order of magnitude, we only detect an SFE H 2 variation of ∼50% in the compressed regions of the three galaxies. We suggest that the decrease in star formation depends on the ratio between the compression timescale and the turbulent dissipation timescale. In NGC 4501 and NGC 4567/68 the compression timescale is comparable to the turbulent dissipation timescale and only leads to minor changes in the molecular star formation efficiency.

A diversity of starburst-triggering mechanisms in interacting galaxies and their signatures in CO emission

“…However, a decrease of the depletion time as observed, e.g. in LIRGs (Klaas et al 2010;Martinez-Badenes et al 2012;Nehlig et al 2016) translates into a deviation from this relation. A duality of star formation regimes has been proposed by Daddi et al (2010b) to distinguish galaxies with long and short depletion times, respectively disks and mergers, but independently of their SFR.…”

Two Orders of Magnitude Variation in the Star Formation Efficiency across the Premerger Galaxy NGC 2276

“…Surveys of nearby galaxies show a scatter in τ depl of ∼ 0.3 dex at galactic and sub-galactic scales (Saintonge et al 2011;Leroy et al 2013). However, interacting starburst galaxies (Klaas et al 2010;Nehlig et al 2016;Saito et al 2016) and ultra-luminous infrared galaxies (LIRGs, ULIRGS; Saintonge et al 2011;Martinez-Badenes et al 2012) exhibit a lower systematic τ depl of 0.05-0.8 Gyr.…”

“…Ram pressure, as another external force, is known for quenching star formation, particularly in dwarf galaxies (Steinhauser et al 2016), but can also locally compress gas and have the opposite effect (Ebeling et al 2014), especially in more massive systems where the background potential helps slowing down gas stripping. Studies of τ depl in galaxies at various stages of interaction indicate that the tidal gravitational forces change τ depl up to 0.4 dex (Martinez-Badenes et al 2012;Nehlig et al 2016;Lee et al 2017). Nehlig et al (2016) observed that ram pressure can decrease τ depl , but not as effectively as the tidal effects.…”

The effect of ram pressure on the molecular gas of galaxies: three case studies in the Virgo cluster

“…This leads to ISM compression and increasing H 2 formation; hence, it locally triggers intensive star formation (Fujita & Nagashima 1999;Kronberger et al 2008;Merluzzi et al 2013;Henderson & Bekki 2016). Indeed, ISM compression with high molecular fraction in the upstream side is seen in a number of galaxies experiencing ICM pressure (Vollmer et al 2012b;Nehlig, Vollmer & Braine 2016), which are often accompanied by an enhancement of star formation. Molecular gas enhancement among the Virgo cluster members has been also recently reported by Mok et al (2016).…”