Luminosity and Mass functions in spectroscopically-selected groups at z~0.5

Luminosity and Mass functions in spectroscopically-selected groups at z~0.5

George Hau, Durham University

Dave Wilman (MPE)

Mike Balogh (Waterloo)

Richard Bower (Durham)

John Mulchaey, Gus Oemler (Carnegie)

Ray Carlberg (Toronto)

Plan of talk

• Motivation• CNOC2 Groups at 0.3 < z < 0.55 survey• Preliminary LFs for individual groups• Mass function• Summary

Motivation

• Kodama & Bower 2004: Little evolution in stellar mass of cluster galaxies since z~1

• Decline of volume-averaged SFR by fact of ~5 from z~1-2 to present.

• Poorer environments such as groups must drive the evolution of field galaxy population since z~1.

Little evolution in clusters since z~1

Kodama & Bower 2004

Motivation

• > 50% of present day galaxies in groups. Groups = typical environment of galaxies.

• Understanding "cosmic downsizing”: Active star formation shifts to lower and lower mass galaxies as the Universe evolves.

But…• Groups have low density contrast against background-- hard to build large

samples (Eke review. See also works by Mulchaey, Zabludoff, Ponman etc)

Compact Groups (eg. Hickson et al 1989)

X-ray luminous (e.g. Mulchaey et al 2003)

Radio selected (e.g. Allington-Smith et al 1993)

Our approach:

Select groups based on redshifts and positions

CNOC2 Groups at 0.3≤z≤0.55:

• Based on CNOC2 redshift survey (Yee et al 2000): 4x104 galaxies with UBVRI photometry + 6000 redshifts

• Groups detection by

– Friends-of-friends algorithm

– Spatial (1.5*r200) + velocity (3σ) trimming + iteration.

• 200 Groups 0.12 < z < 0.55 identified (Carlberg et al 2001).

Magellan followup spectroscopy (Wilman et al 2004)

• Extended spectroscopic sample to RC=22.0

• Final sample contains (0.3 < z < 0.55):

– 282 members in 26 groups.– 334 serendipitous field galaxies.

• Selection function is well understood and sample is representative.

• [OII] emission infers presence of star formation (EW≥5Ǻ).

• SED fitting -> rest-frame luminosities.

• Groups range in σv from < 100 km/s to ~800 km/s (~poor cluster)

[OII] [OII]

[OII] [OII]

Wilman et al 2004 groupsQuickTime™ and a

TIFF (LZW) decompressorare needed to see this picture.

Wilman et al 2004

Galaxy properties and evolution in groups -- see

Dave Wilman’s talk

Additional data

• GALEX UV imaging (current)

– star formation

• NIR imaging: SOFI, Spitzer GTO

• Chandra observations scheduled

HST Morphologies

• Observations of 20 groups for 1 orbit each in F775W filter.

• Visual classification by Gus Oemler of all 0.3 ≤ z ≤ 0.55 galaxies in the 16 fields

• Classification possible to R~23.9, according to the MORPHS scheme.

• Preliminary results for 16 ACS fields.

• 158 classified group galaxies & 124 classified field galaxies.

New FORS2 spectroscopy: probing z~0.5 counterparts of local post-starburst population

Kauffmann et al 2003

stellar age indicator for SDSS galaxies

New VLT FORS spectroscopy

• FORS2 Followup spectroscopy:

• Targeting faint galaxies down to R~23 to improve completeness & probe star forming population

– June 2005: 3 nights, 6 groups -- preliminary results presented here

– ~240 new redshifts, >27 new group members

– Sep 2005: 10.4 hrs, 4 groups. Data not reduced yet

FORS2 mask

Galaxy selection function

Old LDSS2 data

+ new FORS2 data

After correction

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Survey limit

R

Fractio

n of gala

xies w

ith

z













Luminosity function of individual groups (preliminary)

z=0.359

z=0.536z=0.471z=0.361

z=0.510z=0.372

M*

New dataOld data

Morphological segregation

Group 37

dec

r’0

dv (km/s)

RA

Morphological segregation

Group 38

dec

r’0

dv (km/s)

RA

Mass function of CNOC2 groups (Balogh et al in prep)

• Spitzer GTO data in sub area of survey• Chose groups with >60% IR spatial coverage• Matched IR data (IRAC 3.6µm) to CNOC2 groups catalogue.• Mass from stellar M/L ratios calculated using population

synthesis models

• Total stellar mass Mstellar for all gals within R200.

Mass Functions as function of z and σ

σ

z

Combined Mass Function

Local MF, α=-0.6

Combined MF, z~0.3

More IR data will help--Valentine take note! :)

Summary

• New FORS2 spectroscopy down to R~23.2• Presented LF functions for 6 groups• Presented mass functions using SPITZER imaging • No evidence for trends in Mass Functions -- in concordance with idea that group

stellar mass in place by z~1. (cf. Conselice’s talk)

• Future Plan:• Combined LF & MS & comparison with local functions• Investigation of SFH with [OII] • Serendipitous groups & filaments.• Improving mass function using SOFI data

See Dave Wilman’s talk

• Galaxy properties and evolution in groups

Documents

Luminosity and Mass functions in spectroscopically-selected groups at z~0.5