[Paper Review] Molecular gas in the galaxy cluster Abell 262. CO observations of UGC 1347 and other galaxies of the cluster

Thomas Bertram, A. Eckart+3 more|arXiv (Cornell University)|Oct 18, 2005

Galaxies: Formation, Evolution, Phenomena36 references3 citations

TL;DR

This study investigates molecular gas deficiency in the Abell 262 galaxy cluster using CO(1-0) line observations of 12 galaxies and high-resolution interferometry of UGC 1347. It finds indirect evidence for a molecular gas and dust deficiency in the cluster core, linked to HI stripping, with a predicted 100 µm flux drop of ~1.1 Jy consistent with combined atomic and molecular gas deficiencies.

ABSTRACT

We present millimeter CO line emission observations of 12 galaxies within the Abell 262 cluster, together with L_FIR data, in the context of a possible molecular gas deficiency within the region of the cluster center. Several indications of the presence of such a deficiency are highlighted and connected to a model of cirrus-like cloud stripping. The model predicts a drop in the average 100 micron flux density of galaxies in the core of the cluster compared to the average 100 micron flux density in the outer regions, which is actually indicated in the IRAS data of the cluster members. This drop is explained by the decrease in the total hydrogen column density N(H) and, therefore, also includes a decrease in the molecular gas content. In addition to results for the global CO content of the galaxy sample, high-resolution interferometric CO(1-0) observations of one of the cluster members, UGC 1347, exemplify the spatial distribution of the molecular gas in a galaxy of the cluster. With these observations, it was possible to confirm the existence of a bright off-nuclear CO-emission source and to derive molecular masses and line ratios for this source and the nucleus.

Motivation & Objective

• To investigate the presence of molecular gas deficiency in galaxies within the Abell 262 cluster, particularly in the cluster core.
• To examine the relationship between molecular gas content (traced by CO emission) and distance from the cluster center.
• To analyze the spatial distribution of molecular gas in a representative cluster member, UGC 1347, using high-resolution interferometry.
• To test the hypothesis that HI deficiency in cluster galaxies may be accompanied by a deficiency in molecular gas and dust, as predicted by cirrus cloud stripping models.
• To quantify the contribution of molecular and atomic gas deficiencies to the observed drop in 100 µm flux density in the cluster core.

Proposed method

• Conducted millimeter-wave CO(1-0) line emission observations of 12 galaxies in Abell 262 using single-dish telescopes to measure global molecular gas content.
• Performed high-resolution interferometric CO(1-0) observations of UGC 1347 to map the spatial distribution of molecular gas and identify off-nuclear sources.
• Used IRAS 100 µm flux density data to assess dust emission trends and correlate them with cluster-centric distance.
• Applied the cirrus cloud model of de Vries et al. (1987) to predict a drop in 100 µm flux density due to reduced hydrogen column density in the cluster core.
• Calculated expected contributions to the 100 µm flux drop from atomic (HI) and molecular (H2) gas deficiencies using HIDEF comparison sample data.
• Analyzed color-color diagrams in IRAS bands to identify cirrus-like emission and assess the role of star formation versus diffuse dust in cluster galaxies.

Experimental results

Research questions

• RQ1 Is there a measurable deficiency in molecular gas content among galaxies in the central region of the Abell 262 cluster compared to outer regions?
• RQ2 Does the observed drop in 100 µm flux density in the cluster core correlate with predicted decreases due to gas stripping, as modeled by cirrus cloud stripping?
• RQ3 What is the spatial distribution of molecular gas in UGC 1347, and does it show signs of enhanced star formation or asymmetric outflows?
• RQ4 To what extent do atomic and molecular gas deficiencies contribute to the observed 100 µm flux deficit in Abell 262 galaxies?
• RQ5 Can the HI deficiency in Abell 262 galaxies be causally linked to a potential molecular gas deficiency, as suggested by environmental stripping mechanisms?

Key findings

• Galaxies in the core of Abell 262 (within 1 Abell radius) show a trend toward lower CO line emission and FIR luminosity, suggesting a molecular gas deficiency.
• The observed drop in median 100 µm flux density of ~1.1 Jy in the cluster core is consistent with predictions from the cirrus cloud stripping model.
• The contribution of molecular gas deficiency to the 100 µm flux drop is estimated at 0.26 ± 0.14 Jy, while atomic gas deficiency contributes 0.68 +1.58/-0.55 Jy.
• High-resolution CO(1-0) observations of UGC 1347 reveal a bright off-nuclear CO-emitting source south of the nucleus, indicating localized gas concentration.
• The velocity profile of the off-nuclear source deviates from the disk rotation curve, suggesting possible asymmetric gas expansion or outflow, consistent with enhanced star formation.
• The color-color diagram shows that most core galaxies have cirrus-like FIR colors, but a few with higher 60/100 µm ratios suggest additional star formation contributions.