Environmental Dependence of Environmental Dependence of Brightest Cluster Galaxy EvolutionBrightest Cluster Galaxy Evolution

Sarah Brough, Liverpool John Moores UniversityChris Collins, Liverpool John Moores UniversityDoug Burke, Centre for Astrophysics, Harvard

Bob Mann, Institute for Astrophysics, EdinburghPaul Lynam, ESO

IntroductionIntroduction

Why study BCGs?

K-band photometry of BCGs

K-band structural properties of BCGs

Conclusions

Brightest Cluster GalaxiesBrightest Cluster Galaxies

Most luminous galaxies emitting purely photospheric light in the Universe.

Found at the centre of clusters and groups of galaxies.

Originally studied to determine cosmological parameters (e.g. Sandage et al 1972) and measure large-scale streaming (e.g. Lauer & Postman 1994)

Monolithic collapse models predict that these galaxies formed first at very high redshift and have passively evolved since.

Hierarchical models predict that these most massive galaxies should have assembled their stellar mass most recently.

Therefore, the differences in the predictions of these models should be most apparent in these, most massive galaxies.

Aim to examine the evolution of BCGs in host clusters with known X-ray luminosities by studying their physical properties over a wide span of cosmological time.

Cluster X-ray luminosity allows an objective, quantitative measure of galaxy environmental density.

X-ray selected cluster catalogues: EMSS (Gioia & Luppino 1994); SHARC (Burke et al. 2003); NORAS (Bohringer et al. 2000).

Present StudyPresent Study

Flux limits of X-ray cluster cataloguesFlux limits of X-ray cluster catalogues

Ebeling et al. 2001

X-ray luminosity distribution of cluster sampleX-ray luminosity distribution of cluster sample

Absolute MagnitudesAbsolute Magnitudes

K-Band Hubble Diagram

Brough et al. 2002

Inferred Mass of BCGsInferred Mass of BCGs

• Difference with environment also observed by Nelson et al. (2002) with Las Campanas Distant Cluster Survey and Ellis & Jones (2004) with 3 X-ray selected clusters.

• The colour evolution observed by Nelson et al. (2002) is consistent with accretion consisting of old stellar populations.

Structural PropertiesStructural Properties

•The accretion rates inferred assume that increases in luminosity mean increases in mass.

•Numerical simulations of dissipationless mergers predict that radii increase, and the surface brightnesses fade (e.g. Navarro 1990, Capelato et al. 1995)

•Therefore, expect a difference in structural properties with host cluster environment.

Petrosian Structural ParametersPetrosian Structural Parameters

Measure K-band Petrosian structural parameters.

Petrosian Eta(r) = SB(r) - <SB(r)>

Petrosian parameters are not model dependent, are less affected by zero-point or extinction errors and are more robust than model fits.

Correlation with EnvironmentCorrelation with Environment

> 99% Significance

95-98% Significance

Petrosian Eta ProfilePetrosian Eta Profile

Kjaergaard et al. 1993

Petrosian Eta ProfilePetrosian Eta Profile

Abell 644

ConclusionsConclusions• BCGs in high X-ray luminosity clusters

assembled their stellar mass at z>1 and have been passively evolving since.

• BCGs in low X-ray luminosity clusters have assembled more recently and have been undergoing significant mass evolution since z~1.

• BCG profiles not consistent with those of normal cluster elliptical galaxies.