Subaru Wide-Field Surveyof M87 Globular Cluster

Populations

N.Arimoto (NAOJ)

N.Tamura, R.Sharples (Durham) M.Onodera (Tokyo, NAOJ), K.Ohta(Kyoto)

J.-C.Cuillandre (CFHT)

Origin of GCs in Elliptical Galaxies

• Many luminous elliptical galaxies have bimodal or multimodal colour distributions of globular clusters (GCs) (Gebhardt & Kissler-Patig 1999, Larsen et al. 2001, Brodie et al. 2005).

• A model of elliptical galaxy formation with a simple monolithic collapse and subsequent star formation cannot produce the bimodal colour distribution of GCs.

• Multiple Collapse (Forbes et al. 1997) ― all GCs formed coevally with the host galaxy in massive star formation within a short time scale at high redshift but with discrete starburst phases, some of the red GCs formed in gas clouds more polluted.

• Merger (Ashman & Zepf 1992) ー luminous ellipticals formed via gas-rich mergers which induce formation of additional red GCs, while blue ones originate in the progenitor galaxies.

• Accretion (Cote et al. 1998) ー blue GCs were captured from other less luminous galaxies through tidal stripping and/or accretion. => intergalactic globular clusters (i-GCs)

search for intergalactic GCs

By surveying directly for i-GCs and characterizing this population,

it is possible to test the validity of the accretion scenario.

HST

Subaru

2 x 0.5 (640 kpc x 130 kpc)

A large area needs to be surveyed to see whether there are additionali-GCs and to discriminate them from GCs associated with the galaxy.

Subaru Suprime-Cam Observation of M87 GCs

Data for HDF-N and Lockman Hole are retrieved from SMOKA.

17 & 18 March 2004

Typical seeing sizes during the observations were 1.”5 in B and 1.”0 in V and I.

Selection of GCs

The colour criterion to select GC candidates is indicated by green.Open squares show z=0, 0.5, 1.0, 1.5, and 2.0 on each evolutionary track.

median smoothing & subtraction

before after

GC ColoursBimodal Distribution

M87 NGC4552

The colour distribution ofGCs in the inner most regionshows bimodality both in M87and NGC4552. This bimodalitybecomes less clear in the outerregions due to a decreasing contribution of the red GCpopulation.

Red GCs (V-I>1.1)Blue GCs (V-I<1.1)[Fe/H]= -0.13 & -1.0

Colour Gradients[Fe/H]=4.3(V-I)-5.3 , Vazdekis et al. (1996) SSP, Salpeter IMF, 12.6Gyr

Colour gradients are probably due to a combination of decreasing fractionof red GCs and colour gradients in each subpopulation. Blue GCs have nogradient in the outer region of luminous elliptical galaxies ([Fe/H]=-2.3).

Whilst red GCs has a similar distribution to the host galaxy halo light,

the blue GC distribution tends to be more extended.

Radial profiles of GC surface densities

M87 NGC4552

Halo light Halo light

1’ = 4.67 kpc

1’ = 4.47 kpc

RGC

BGC

TOTAL

Distribution of SN Frequency

BGCs BGCs

RGCs RGCs

M87 (SN=13.2)

NGC4552 (SN=4.8)

The increasing trend of local SN with distance is due to the fact that blue GCstend to be more extended that the host galaxy halo light distribution.

total

total

(luminosity of host galaxy halo calculated with de Vaucoulerurs law)

Surface Density of BGCs

M87 NGC4552

Virgo mass profile(McLaughlin 1999)

The blue GC population around M87 is not extended as the Virgo cluster mass density profile.

Intergalactic GCs (i-GCs)?

There is an excess of GCs by ～ 0.2 arcmin-2 in the surface number density atr ～ 100arcmin (450kpc) from M87, suggesting the existence of independent GC

populations; ie., intergalactic GCs of inhomogeneous distribution (cf i-PNe).

specific frequency SN=2.9

M87 NGC4552

GC Luminosity Functions (GCLFs)

A Gaussian is fit to the LF at magnitudes with ≥ 50% completenessVpeak = 23.61±0.08 mag (M87), 23.49±0.16 mag (NGC 4552)

(Consistent with the results from HST studies; Kundu et al. 1999; Kundu & Whitmore 2001)

GCLFs of Blue & Red GCs

Assuming that GCs are uniformly old, this offset

can be explained by a metallicity difference

between the two GC subpopulations;

[Fe/H] = -0.3 (red) and -1.6 (blue)

(Jordan et al. 2002).

In M87, the turnover mag for the red GC subpopulation appears to be fainter by ~ 0.5 mag than that for the blue GC subpopulation.

An unprecedented wide-field survey ( ～ 0.6 Mpc from the M87 centre) of GCs around M87 with Subaru/Suprime-CamSecure selection of GC candidates with an extended

source cut and a colour selection on the B-V vs. V-I diagram.Analyzed the Suprime-Cam data of the HDF-N and the Lockman Hole, which are compatible with the M87 data set in terms of filter set, seeing sizes and limiting mags and enable to select contaminating objects with the identical criteria to those for GC candidates in the M87 fields.

(1) Most of the blue GCs must be associated with the host galaxy, not the cluster, even in central cluster galaxies like M87.(2) Blue GCs are linked with the dark matter halo of a massive E galaxy, whilst red GCs are associated with the stellar body.(3) Formation of Blue GCs was accompanied by the collapse of the dark matter halo, whilst red GCs and field stars coevally formed in subsequent starbursts.

The blue GC population may have formed in situ at the very early stage of elliptical galaxy formation. If this is the case, the high SN values of luminous ellipticals at the cluster centre

could be explained with biased GC formation in denser environments.

Alternatively, the distinct spatial distribution between red and blue GCs is predicted by the merger scenario, although galaxymergers accompanied by starbursts and red GC formation wouldneed to be complete at very high redshift.

Simulation of Galaxy Formation

Abadi et al. (2003) ApJ 591, 499D

Mstar

The BGC populations seemto be associated with thedark matter distribution,which suggest that BGCswere formed even before

sub-galactic clumpshad coagulated into theprogenitor of a massive elliptical galaxy (z>4?).