GZ1 used for the fractions of early-types in clusters

We’re happy to report that we have once again used your (now public) GZ1 classifications to find an interesting result.

We use the classifications in a study we just submitted to MNRAS (or see the arXiv entry for a copy) looking at the observed fractions of early-type galaxies (and spiral galaxies), in groups and clusters of galaxies.

Recent work (De Lucia et al. (2011), which posted to the arxiv in September), used sophisticated semi
analytic models to determine the properties of galaxies found in massive
clusters in the Millennium Simulation. They identified elliptical galaxies
(or more accurately early-type galaxies) in the simulation, and found that the fraction these
galaxies, remained constant with cluster halo mass, over the range 10^14 to
10^14.8 solar masses. They compared their results with previous
observational studies which each contained less than 100 clusters.

With GZ1 we realised we could put together a much larger sample. We
used galaxies with GZ1 classifications, cross matched with cluster and
group catalogues, to compare the above results with almost 10 thousand
clusters. We found that the fraction of early-type galaxies is indeed
constant with cluster mass (see the included figure), and over a much larger range of 10^13 to 10^15
solar masses (with covers small groups of galaxies to rich clusters), than previously studied. We also found the well known result (to astronomers) that outside of groups and clusters, the fraction of early-type galaxies is
lower than inside of groups and clusters.

Plot showing the fraction of early-type galaxies (red lines) as a function of halo mass. We used two different halo mass catalogues, and the agreement between them is excellent. We also examine the fraction of spiral galaxies with halo mass (blue lines)

This work suggests that galaxies change from spiral to early-type when individual
galaxies join together to form small groups of galaxies, but that going from groups to rich clusters does not significantly change the morphologies of galaxies.

Without the GZ1 results at our finger-tips, this work, which was devised,
implemented, and written up in less than 2 months, would have taken much
longer to complete.

Thanks again for making the Zoo such a wealth of information,

Ben Hoyle (on behalf of Karen Masters, Bob Nichol, Steven Bamford, and Raul Jimenez)