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Lyman Break Galaxies in Large Quasar Groups at z~1
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  1. 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)

  2. 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

  3. 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

  4. 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)

  5. 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

  6. 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

  7. 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)

  8. 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

  9. Clowes-Campusano (CC) LQG fieldSmall box: CTIO 4m BTC field (VI) - - - MgII survey GALEX, CFHT imaging fields z~1.3 quasars O MgII absorbers z~0.8 quasars O MgII absorbers

  10. MgII overdensity Shaded regions: 65, 95, 99% confidence limits based on uniform distribution of MgII absorbers and selection function CC LQG z~0.8 LQG

  11. Red Galaxy Overdensity Contours: red galaxy density, V-I consistent with 0.8<z<1.4 Boxes: subfields observed in JK with ESO NTT+SOFI

  12. LQG: BRIGHT Quasar Overdensity • Compare region to DEEP2 (4 fields, 3 deg2, Coil et al. 2007) • No significant overdensity in CC LQG for moderateluminosity 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

  13. Overdensity in bright quasars ~2 deg2 11 bright, 34 faint quasars 3 deg2,4 fields on sky 6 bright, 35 faint quasars

  14. 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

  15. NASA mission, launched 2003 • 1.2° circular field of view, imaging + grism • 50cm mirror, 6 arcsec resolution • FUV channel: ~1500Å, NUV: ~2300Å

  16. 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

  17. 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

  18. Completeness limits

  19. GALEX NUV luminosity function and M* (Arnouts et al. 2005)

  20. 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

  21. 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)

  22. 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)

  23. GALEX, CTIO BTC, HST ACS close-up  28"   230 kpc  NUV FUV CTIO I CTIO V • ~80 kpc separation implies merger activity Possible merger in a z~1 LBG ACS F814W

  24. 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

  25. Preliminary LBG auto-correlation Correlation 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

  26. 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

  27. 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

  28. 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

  29. 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?

  30. 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

  31. 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

  32. 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