Lyman Break Galaxies in Large Quasar Groups at z~1

G Williger (Louisville/JHU), R Clowes (Central Lancashire), L Campusano (U de Chile), L Haberzettl & J Lauroesch

(Louisville), C Haines (Naples,Birmingham), J Loveday

(Sussex), D Valls-Gabaud (Meudon), I Söchting (Oxford), R Davé (Arizona), M

Graham (Caltech)

Outline

• Background on large quasar groups (LQGs)

• Clowes-Campusano LQG• Observations:

– Galaxy Evolution Explorer (GALEX), Lyman Break Galaxies

– SDSS for Ground-based wide-field imaging

• Analysis, interpretation• Conclusions/further work

Background: LQGs

• Discovered: late 1980s

• Shapes: irregular, filamentary agglomerations

• Numbers: ~10-20 member quasars

• Sizes: 100-200 Mpc not virialised

• Frequency: ~10-20 catalogued, but probably more in sky

Why Study LQGs? Star Formation

• Quasars likely triggered by gas-rich mergers in local (~1 Mpc) high density environments (Ho et al. 2004; Hopkins et al. 2007)– Quasars avoid cluster centres at z~<0.4

(Söchting et al. 2004), analogous to star formation quenching

– Quasars at z~1 preferentially in blue (U-B<1) galaxy environments, presumably merger-rich (Coil et al. 2007, DEEP2)

LQGs: Structure Tracers

• Quasars + AGN delineate structure at z~0.3 (Söchting et al. 2002)

• Quasar-galaxy correlation similar to galaxy-galaxy correlation (Coil et al. 2007)

• Quasars are most luminous structure tracers

LQGs: Structure+Star Formation Probes

• At z~1– star formation much higher than present

quasars should mark regions of high star formation

– Galaxy surveys time-intensive more efficient to use quasars as structure markers

Clowes-Campusano LQG z~1.3

• Discovered via objective prism survey, ESO field 927 (1045+05) (Clowes et al. 1991, 94, 99; Graham et al. 1995)

• >=18 quasars Bj<20.2, 1.2<z<1.4, overdensity of 6 from SDSS DR3

• 2.5°x5° (120x240 h-2 Mpc-2, H0=70 km s-1 Mpc, Ωm=0.3, Λ=0.7)

• Overdensity of 3 in MgII absorbers (Williger et al. 2002)

• Overdensity of ~30% in red galaxies (Haines et al. 2004)

Bonus: Foreground LQG z~0.8

• >=14 quasars, 0.75~<z~<0.9, bright quasar overdensity ~2

• ~3°x3.5° (100x120 h-2 Mpc-2)

• Marginal overdensity of MgII absorbers

Clowes-Campusano

(CC) LQG field

Small box: CTIO 4m BTC field

(VI)

z~1.3 quasars

O MgII absorbersz~0.8 quasarsO MgII absorbers

- - - MgII survey

GALEX, CFHT imaging fields

MgII overdensityCC LQG

Shaded regions: 65, 95, 99% confidence limits based on uniform distribution of MgII absorbers and selection function

z~0.8 LQG

Red Galaxy OverdensityContours:

red galaxy

density, V-I

consistent

with

0.8<z<1.4

Boxes:

subfields

observed in

JK with ESO

NTT+SOFI

LQG: BRIGHT Quasar Overdensity

• Compare region to DEEP2 (4 fields, 3 deg2, Coil et al. 2007)

• No significant overdensity in CC LQG for moderate luminosity quasars to AGN -25.0<MI<-22.0 (Richardson et al. 2004 SDSS photometric quasar catalogue)

• ~3x overdensity for bright MI<-25.0 quasars lots of merging

Overdensity in bright quasars

~2 deg2

11 bright, 34 faint quasars

3 deg2, 4 fields on sky

6 bright, 35 faint quasars

CC LQG: Unique Laboratory

• Deep fields (DEEP2, Aegis etc.) NOT selected for quasar overdensity

• Clowes-Campusano LQG: UNIQUE opportunity to study galaxies and quasar-galaxy relation in DENSE quasar environment

• NASA mission, launched 2003

• 1.2° circular field of view, imaging + grism

• 50cm mirror, 6 arcsec resolution

• FUV channel: ~1500Å, NUV: ~2300Å

• Surveys:– All sky: 100 s exposure, AB~20.5– Medium imaging survey: 1500s exp, 1000

deg2, AB~23– Deep imaging survey: 30ks exp, 80 deg2,

AB~25 – OUR CONTROL (e.g. CDF-S, NOAO Wide Deep Survey, COSMOS, ELAIS, HDF-N)

– Ultra-deep imaging survey: 200ks, 4 deg2, AB~26

– NOTE: confusion starts at NUV(AB)~23 – deconvolution techniques with higher resolution optical data appear to work

UV Observations

• GALEX: 2 overlapping ~1.2° fields• Exp times ~21-39 ksec, 70-90%

completeness for AB mags ~24.5 in FUV, NUV– M* at z~1.0, M*+0.5 at z~1.4

• FUV-NUV reveals Lyman Break Galaxies (LBGs) at z~1 – key star-forming population

Completeness limits

GALEX NUV

luminosity

function and

M* (Arnouts

et al. 2005)

Lyman Break Galaxies (LBGs)

• Break at rest-frame Lyman Limit 912Å sign of intense star formation– Often associated with merger activity

• Easily revealed in multi-band imaging– First found at z~3.0, in u-g bands

• UV flux strongly quenched (scattered) by dust– LBGs only reveal fraction of star-forming

galaxies

Sloan Survey: optical photometry

• For initial optical colours, use Sloan Digital Sky Survey: 95% point source completeness u=22.0, g=22.2, r=22.2, i=21.3, z=20.5 (Adelman-McCarthy et al. 2006)

LBG sample in LQG

• FUV-NUV>=2.0 and NUV<=24.5 – 95% SDSS detections

• SDSS resolved as galaxies

• 7-band photo-z's of z>0.5 (Δz~0.1)

• 690 candidates (~50% of number density from Burgarella et al. 2007)

GALEX, CTIO BTC, HST ACS close-up

• ~80 kpc separation implies merger activity

Possible merger in a z~1 LBG

FUV NUV CTIO V

ACS F814W

CTIO I

28" 230 kpc

LBG Auto-correlation, LBG-quasar clustering

• Preliminary Limber inversion of LBG power law auto-correlation – Evidence for strong clustering

• No significant overdensity of LBGs around 13 brightest quasars

Preliminary LBG auto-correlationCorrelation

length

r0=13

Mpc: 3x

stronger

than NUV

sample of

Heinis et al.

(2007), L*

galaxies at

z~1 and

LBGs at z~4 –

Implies strong clustering

Mean Galaxy Ages

• Calculate mean, std dev of rest-frame LBG 7-band photometry

• Fit spectral energy distributions (SEDs; PEGASE, Fioc & Rocca-Volmerange 1997)– Closed-box models metallicity not free

parameter– Dust and dust-free models used

Mean LBG galaxy ages

• Most promising constraint for galaxy ages from highest z bin

• Best fit: 2.5 Gyr, exponentially decreasing SFR with decay time 5 Gyr (no dust)

• Youngest acceptable fit: 120 Myr burst model (with dust)

Only 64 galaxies in this z-bin

Interpretation

• Strong LBG auto-correlation– due to observing only brightest galaxies?

• Lack of quasar-galaxy clustering– small number statistics?

• Best fit age >> 250-500 Myr found by Burgarella et al. toward CDF-South– Due to our observing only brightest, most massive

galaxies?– Burgarella et al sample went 2x deeper in UV, has

COMBO-17, Spitzer, Chandra supporting data

Questions to address

• Does blue galaxy environmental preference of Coil et al. persist to same degree in LQG?

• Burgarella et al. (2007) found 15% of z~1 LBGs are red from Spitzer data. Is LQG population consistent?

Ground-based Supporting Data

• 2x1° imaging in rz (CFHT Mega-Cam)• ~1.5° imaging in gi (Bok 2.3m)• ~1° imaging in JK (KPNO 2.1m)• ~0.5° imaging VRIz (CTIO 4m) – away from GALEX

fields around group of 4 LQG members• ~600 redshifts from Magellan 6.5m• 5 subfields in JK with NTT+SOFI, additional MgII spectra

with VLT, 30' subfield in VI with CTIO 4m• Proposed Chandra images of bright quasars search

for hot gas in rich clusters

Further work

• Reduce, analyse deeper optical-IR images – Individual galaxy SEDs, better discrimination on red

end– Search for red-selected galaxies

• Use Magellan spectra, observed near-IR bands for better photo-z's

• Proposed deeper (2x) exposures for GALEX Cy4

• Will propose for Spitzer to get evolved stellar populations

SUMMARY

• Large quasar groups (LQGs): excellent tracers of star formation and large structures

• Largest, richest LQG at z~1 observed with GALEX (FUV+NUV) over 2 deg2

• 690 bright z~1 LBGs– Strong clustering: r0~13 Mpc– Mean ages best fit ~2.5Gyr, but 120Myr allowed

• Working with ground-based data, proposing deeper GALEX exposures to probe down luminosity function