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Intermediate Redshift Clusters Shed Light on Galaxy Evolution

February 1, 2013

Figure 1. The dynamical mass-to-light (M/L) ratio and the strength of the higher order Balmer lines Hδ and Hγ versus the velocity dispersion of the galaxy. The M/L versus velocity dispersion relation is a representation of the Fundamental Plane. The solid blue line shows the best fit to the low-redshift samples (Coma, Perseus and A194), while the dot-dashed lines show models for formation redshifts dependent on the velocity dispersions, color-coded for each of the clusters. The M/L ratios are modeled reasonably well by these models. However, the strength of the Balmer lines is significantly weaker than expected from the models, indicating that the M/L ratios and the Balmer line measurements cannot be reconciled with a simple passive evolution model with a formation redshift depending on the galaxy velocity dispersion.

Figure 2. Distributions of metallicity [M/H] and abundance ratios [α/Fe] for each of the clusters derived from the line indices. Under the assumption of passive evolution there should not be any significant differences from cluster to cluster. Instead we find that MS0451.6-0305 has [M/H] about 0.2 dex below that of the other clusters, while RXJ0152.7-1237 has [α/Fe] about 0.3 dex above that of the other clusters.

Figure 3. Sample grey scale images of cluster members of MS0451.6-0305 and RXJ1226.9+3332 made from Hubble Space Telescope imaging. The field size for each galaxy corresponds to approximately 75 kiloparsecs × 75 kiloparsecs at the distances of the two clusters. The analysis focuses on the elliptical and lenticular galaxies in the clusters, while spiral galaxies are excluded.

Using high signal-to-noise optical spectroscopy from Gemini, and Hubble Space Telescope imaging of three intermediate redshift galaxy clusters, Gemini astronomers Inger Jørgensen and Kristin Chiboucas have shed new light on galaxy evolution. The team’s paper, accepted for publication in The Astronomical Journal, shows that the mass to light ratios of the galaxies indicate that low-mass galaxies have experienced more recent star formation than high-mass galaxies. Both the mass-to-light ratios and the Balmer lines (hydrogen lines) can be used to estimate the ages of the galaxies. However, the Balmer lines indicate that all of the galaxies are very old, leading to a contradiction between the two methods of age determination.

Furthermore, the metal lines (or metallicity, an indication of the presence of elements heavier than hydrogen and helium which are formed in previous generations of stars) show that there are metallicity differences when comparing the clusters. This is in contradiction with the passive evolution of these galaxies and may indicate that either ongoing star formation or mergers may need to play a role in their future evolution to make them resemble nearby galaxies.

The results originate from the Gemini/HST Galaxy Cluster Project (GCP), and focus on early-type elliptical and lenticular galaxies in three of the richest clusters that are part of the GCP sample. The GCP aims to constrain models for galaxy formation and evolution by providing large data sets of galaxies in rich clusters between redshifts of one and the present. The full GCP covers 15 rich clusters selected based on their X-ray luminosity.

The accepted paper is available online on astro-ph.

For more information, contact the PI for the Gemini/HST Galaxy Cluster Project: Dr. Inger Jørgensen, inger@gemini.edu

 

In-Depth

The following discussion provides details for specialists.For further details see the paper at: http://arxiv.org/abs/1301.3177

In this work the team established scaling relations between the mass-to-light (M/L) ratios, absorption line indices, and velocity dispersions. They confirm that the relation between the M/L ratio and the velocity dispersion is steeper at z≈0.86 than at low redshifts. This indicates that, assuming passive evolution, the epoch of the last major star formation depends on the galaxy velocity dispersion (or alternatively mass), with the lower velocity dispersion (or lower mass) galaxies forming the majority of their stars more recently than is the case for the higher mass galaxies, see Figure 1a.

The three clusters follow similar scaling relations between absorption line indices and velocity dispersions as those found for low-redshift galaxies. The zero point offsets for the Balmer lines depend on cluster redshifts. However, the offsets indicate a slower evolution, and therefore higher formation redshift, than the zero point differences found from the M/L ratios, if interpreting the data using a passive evolution model (see Figure 1b).

Based on the absorption line indices and recent stellar population models, the team finds one cluster, MS0451.6-0305 (Figure 2a) with lower metallicity than the others, and one cluster, RXJ0152.7-1357 (Figure 2b), with higher alpha-element abundance ratio than the others. In order to evolve galaxies in these clusters into galaxies similar to low redshift galaxies, the MS0451.6-0305 galaxies would require on-going low-level star formation and the RXJ0152.7-1357 galaxies would need to experience mergers. Passive evolution alone in the time available is insufficient.

The analysis covers the clusters MS0451.6-0305 (z=0.54), RXJ0152.7-1357 (z=0.83), and RXJ1226.9+3332 (z=0.89) from the GCP sample. Figure 3 shows sample galaxies from two of the clusters.