Dark Halos of Fossil Groups and Clusters

Observations and Simulations

Ali Dariush, Trevor Ponman Graham SmithUniversity of Birmingham, UK

Frazer Pearce et alUniversity of Nottingham, UK

Andrew Benson et alCaltech, US

Habib Khosroshahi

Dark 2007 - Sydney

Motivation

• How do galaxies form and evolve? • How is the galaxy evolution connected to

the environment?• What aspect of the environment matters?

( SDSS results, Blanton, 2006)

• Can DM help us characterise environment?

• Is the shape of the DM distribution a true indicator of the evolutionary stage of galaxy systems?

• How well the models and cosmological simulations reproduce the observations of galaxies, DM and baryons? Challenges posed by observations of groups and clusters

A lensing or X-ray map

Dark 2007 - Sydney

XMM-IMACS

Osmond et al 2004

Rasm

uss

en e

t al

20

06

Fossil group

L* galaxies absent

BGG is a giant Elliptical

“Relaxed” X-ray emission

Fossil Galaxy Groups

Galaxy group

X-ray contours

L* galaxies MR - 21.5

BGG could be late-type

Irregular X-ray emission

Non-fossil group

Dark 2007 - Sydney

Search fossils

Large area X-ray survey Optical photometry

Optical Spectroscopy

• WARPS fossils • Fossils in SDSS: RAS + SDSS• Fossils in XCS (in progress)• Fossils in LoCuSS Are fossils really old systems?

Are there any differences in their halo and galaxy properties? Observations vs. Simulations

Extragalactic astronomy can benefit from fossil studies: Halo formation, Baryon physics, Feedback, Galaxy formation

Optical: m12 ≥2 (R-band)

X-ray: LX ≥1042 ergs/sec

Dark 2007 - Sydney

X-ray properties from Chandra observations

M - TX relation

Fossils appear to be hotter than normal groups for a given mass of the system

Finoguenov et al 2001

LX - Lopt relationExcess X-ray luminosity for a given optical luminosity of the groups

Fossils

Non-fossils

Khosroshahi et al 2007

Dark 2007 - Sydney

Fossils show higher concentration in their mass profiles compared to non-fossils systems with similar masses. This is an indication of early formation epoch.

J14

16

.4+

23

15

, K

hosr

osh

ahi et

al.

20

06

Mass concentration in fossils

Khosroshahi, Ponman & Jones 2007

• Hydrostatic equilibrium

• Spherical symmetry

• NFW profile (c200=r200/rs)

Dark 2007 - Sydney

Fossil Clusters in LoCuSSLocal Cluster Substructure Survey

A survey to probe the relationship between the structure (assembly history) of galaxy clusters and the evolution of the hot gas and galaxies.

LoCuSS targets 100 low z (0.15-0.3) clusters from X-ray through optical/IR to radio (PI Smith).

For fossil study we use Ks imaging and HST/ACS (cycle 16) or WFPC2 observations .

Dark 2007 - Sydney

Halo – Galaxy connection10 clusters with lensing and X-ray analysis

Mass concentration (X-ray)Clusters with largest luminosity gap are found in more concentrated halos (Zhang et al 2007).

Mass concentration (lensing)Clusters dominated by BCGs show least sub-structures (Smith et al 2005).

Dark 2007 - Sydney

Dark 2007 - Sydney

• Formation of fossils• Halo shape and

evolution• Halo isolation• Space density

• Cooling / heating• Semi-analytic

galaxies

Dark halo + GALAXIES + Hot gas

Fossils in the Millennium SimulationsLargest cosmological simulation available

Springel et al 2005 Croton et al 2006 Pearce et al 2007

Dark 2007 - Sydney

Halo mass evolution in Fossils

Fossils accumulate most of their mass at high redshifts,they are therefore old (Dariush et al 2007)

Dark 2007 - Sydney

Space density of fossilsA remarkable agreement non-fossil fossil

Dark 2007 - Sydney

Optical vs. X-ray fossils

No significant change in the fraction of fossils with mass suggests that may be merger is equally efficient in low and high velocity dispersion systems!

Dark 2007 - Sydney

Semi-analytic galaxies

Compare the semi-analytic galaxies of Bower et al (2006) and Croton et al (2006).

Better agreement in the luminosity gap (m12) in massive halos.

Dark 2007 - Sydney

Merger and star formation history

Star formation in central galaxy (BGG, BCG) as a result of equal and non-equal mass mergers.

Most of the merger induced star formation in fossils is originated from equal mass mergers in the first 5 Gyr while unequal mass mergers are the main source of SF in non-fossils.

m12 2

m12 ~ 1.0

m12 ~ 0.2

m1/m2>0.9

m1/m2~0.3

Dark 2007 - Sydney

Majority of brightest cluster galaxies have “boxy” isophotes.

Boxy isophotes are produced in gas poor, equal mass galaxy mergers.

Disky isophotes are produced in gas rich unequal mass galaxy mergers.

Naab & Burkert (2003)

Brightest Cluster Galaxy

K-band, J1416.4+2315

K-band, NGC 6482

The giant elliptical dominating fossils show non-boxy isophotes. No recent dry merger?

Dark 2007 - Sydney

Brightest Cluster Galaxy

A similar trend can be seen in LoCuSS sample. Clusters with largest luminosity gap are dominated by non-boxy isophote giant elliptical galaxies.

Dark 2007 - Sydney

Summary

• Both the observations and simulations point at fossils’ early formation.

• Fossils appear to be the extension of galaxy clusters to lower mass systems.

• There is a strong connection between the DM halo shape and galaxy properties.

• Fossil central galaxies show non-boxy isophotes which is an indication of gas rich merger.

• The space density of fossils in the observations and simulations are consistent.

• It is not clear if the galaxy merger is more efficient in low mass halos.

• Equal mass galaxy mergers form more stars in fossil central galaxies than in non-fossils.