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Understanding Lyman Break Galaxies in Large Quasar Groups at z~1: Observations and Interpretation

Discover the background, structure, and significance of Large Quasar Groups (LQGs) at redshift z~1. Explore the Clowes-Campusano LQG and its implications for galaxy evolution and star formation. Analyze GALEX and SDSS data to gain insights into the structure and star formation within LQGs. Outline conclusions and areas for further research.

robin-hooper
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Understanding Lyman Break Galaxies in Large Quasar Groups at z~1: Observations and Interpretation

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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

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