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
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