[Paper Review] Cold gas properties of the Herschel Reference Survey. III. Molecular gas stripping in cluster galaxies

A. Boselli, L. Cortese+6 more|arXiv (Cornell University)|Feb 3, 2014

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy98 references80 citations

TL;DR

This study investigates molecular gas deficiency in spiral galaxies within the Virgo cluster using the Herschel Reference Survey. It finds that ram pressure stripping removes molecular gas less efficiently than atomic gas due to its more centrally concentrated distribution, leading to H₂-deficient galaxies that evolve from the blue sequence to the red sequence via quenching of star formation.

ABSTRACT

The HRS is a complete volume-limited sample of nearby objects including Virgo cluster and isolated objects. Using a recent compilation of HI and CO data we study the effects of the cluster on the molecular gas content of spiral galaxies. We first identify M* as the scaling variable that traces the total H2 mass of galaxies better. We show that, on average, HI-deficient galaxies are significantly offset from the M(H2) vs. M* relation for HI-normal galaxies. We use the M(H2) vs. M* scaling relation to define the H2-deficiency parameter. This parameter shows a weak and scattered relation with the HI-def, here taken as a proxy for galaxy interactions with the cluster environment. We also show that, as for the HI, the extent of the H2 disc decreases with increasing HI-deficiency. These results show that cluster galaxies have, on average, a lower H2 content than similar objects in the field. The slope of the H2-def vs. HI-def relation is less than 1, while the D(HI)/D(i) vs. HI-def relation is steeper than the D(CO)/D(i) vs. HI-def relation, thereby indicating that the H2 gas is removed less efficiently than the HI. This result can be understood if the HI is distributed on a flat disc more extended than the stellar disc, thus less anchored to the gravitational potential well of the galaxy than the H2. There is a clear trend between the NUV-i colour and H2-def, which suggests that H2 removal quenches the activity of star formation. This causes galaxies migrate from the blue cloud to the green valley and, eventually, to the red sequence. The total gas-consumption timescale of gas deficient cluster galaxies is comparable to that of isolated systems, and is significantly larger than the typical timescale for total gas removal in a ram pressure stripping process, thus suggesting that ram pressure, rather than starvation, is the dominant process driving the evolution of these cluster galaxies.

Motivation & Objective

• Investigate the impact of the cluster environment on molecular gas content in spiral galaxies.
• Quantify molecular gas deficiency in cluster galaxies relative to field galaxies using a homogeneous sample.
• Determine whether ram pressure stripping or reduced molecular gas formation drives gas deficiency in cluster galaxies.
• Assess the role of molecular gas removal in quenching star formation and transforming galaxies from the blue sequence to the red sequence.

Proposed method

• Constructed a K-band-selected, volume-limited sample of nearby galaxies from the Herschel Reference Survey, including both Virgo cluster and isolated field galaxies.
• Used a stellar mass–molecular gas mass scaling relation to define an H₂-deficiency parameter as the logarithmic difference between observed and expected H₂ mass for a given stellar mass.
• Correlated H₂-deficiency with H i-deficiency (as a proxy for environmental perturbation) and with structural parameters like disc scalelengths.
• Analyzed CO and H i data from a subsample of 17 CO-mapped galaxies to assess the radial extent of molecular and atomic gas discs.
• Estimated gas consumption timescales (τ_gas, τ_gas,R) and compared them with ram pressure stripping timescales (τ_RP) from hydrodynamical simulations.
• Used the NUV-i colour index as a proxy for specific star formation rate to link H₂-deficiency to quenching.

Experimental results

Research questions

• RQ1 Is molecular gas deficiency in cluster galaxies driven by ram pressure stripping or by reduced molecular gas formation?
• RQ2 How does the efficiency of molecular gas removal compare to that of atomic gas removal in cluster environments?
• RQ3 What is the relationship between H₂-deficiency and the structural truncation of gaseous discs in cluster galaxies?
• RQ4 How do gas consumption timescales compare to ram pressure stripping timescales in shaping the evolution of cluster galaxies?
• RQ5 To what extent does molecular gas deficiency drive the migration of galaxies from the blue sequence to the red sequence?

Key findings

• H₂-deficient galaxies in the Virgo cluster are significantly offset (4σ) from the M(H₂) vs. M_star relation, indicating a systematic deficiency in molecular gas content.
• The H₂-deficiency parameter shows a weak, scattered correlation with H i-deficiency, suggesting that while environment affects both phases, the response differs.
• The radial extent of the molecular gas disc (D(CO)/D(i)) decreases with increasing H i-deficiency, with a slope of -0.36, indicating outside-in stripping.
• The radial extent of the atomic gas disc (D(H i)/D(i)) decreases more steeply (slope = -0.50) than the molecular disc, indicating atomic gas is stripped more efficiently.
• The total gas depletion timescale (τ_gas,R ≈ 3.0–3.3 Gyr) is significantly longer than the ram pressure stripping timescale (τ_RP ≈ 1.5 Gyr), indicating ram pressure is the dominant driver of gas removal.
• H₂-deficient cluster galaxies have a molecular gas consumption timescale of τ_H₂ ≈ 1.2 Gyr and a total gas depletion timescale of τ_gas ≈ 2.6 Gyr, consistent with quenching via gas stripping rather than internal consumption.