exploring the high-z frontier — galaxies at z 6 and beyond haojing yan (carnegie observatories)...
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Exploring the High-z Frontier
— Galaxies at z 6 and beyond
Haojing Yan (Carnegie Observatories)
CCAPP/OSU SeminarApril 8, 2008
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Outline• UV Luminosity Function of Galaxies at z 6
— a very steep faint-end slope
• Stellar Masses of Galaxies at z 6 — some high-mass, “old” galaxies already in place
Implications for (HI) Reionization
— dwarf galaxies did it!
• Unanswered Questions at z 6 — evolution of LF at the bright-end?
• Searching for Galaxies at z > 7-8
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Part I
LF of Galaxies at z 6 (5.5 z 6.5)
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Reionization might have ended at z 6
(Fan et al. 2006, AJ, 132,117)
• <XHI> ~ 1% is sufficient to create a complete GP-trough
• Practically, H still nearly fully ionized at z 6
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Searching Techniques
Lyman-Break Galaxy (LGB)
“Dropout”
Ly Emitter(LAE)
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Surface Density Expectation
• Assuming non-evolving M* (-21.23) & faint-end slope (-1.6) from z=
• Using the z=5.60 galaxy in the HDF-N (Weymann et al. 1999) to fix the normalization
Comparing to Simulation ofWeinberg et al. (2002)
Yan et al. 2002, ApJ, 580, 725
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Simple Prediction Seems to Work Well
• Consistent with all observations up to 2003, including new results from the HST/ACS
• Different groups emphasized different aspects:
Yan et al. (2003) Bouwens et al. (2003) Bunker et al. (2004) Dickinson et al. (2004)
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Source(s) of ReionizationYan & Windhorst 2004, ApJ, 600, L1
Critical value fromMadau, Haardt & Rees 1999
Contribution from reionizing sources
• Galaxies can account for the necessary reionizing photons, if the LF has a Galaxies can account for the necessary reionizing photons, if the LF has a steep faint-end slope; dwarf galaxies are important contributors.steep faint-end slope; dwarf galaxies are important contributors.
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To z<30 mag, 108 i-dropouts found in the HUDF
(Yan & Windhorst 2004, ApJ, 612, L93; YW04)
Note: ~ 1.5 mag deeper than Bunker et al. (2004; MNRAS, 355, 374)
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•Our HUDF z 6 candidate sample supports a very steep UV LF faint-end slope:
α = -1.8 to -1.9
• Dwarf galaxies can provide sufficient (re)ionizing photons at z 6
YW04 Constrain to the UV LF at z 6
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Recent Result Confirms the Steep Faint-end Slope (Bouwens et al.
2006)
506 i-drops: UDF, UDF-Pars, GOODS
But compare to YW04: M* = -21.03, * = 4.6x10-4/Mpc3
4.6x10-3
Msun/yr/Mpc3
1.1x10-2
Msun/yr/Mpc3
SFR is still uncertain by 2x
“Lilly-Madau Diagram”
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Luminosity Function of z 6 LAE• LAE : ~ 1/4 of the entire galaxy population (based on
results at z~3), but still very important — easier to identify; current redshift record holder is the LAE at z=6.96 (Iye et al. 2006)
• LAE as probe of the reionization epoch : neutral IGM — Lya line suppressed — LAE number drop (e.g., Marilada-Escude 1998; Malhotra & Rhoads 2001)
• LAE at z 6 are usually selected at two narrow windows at z=5.7 & 6.5 in order to avoid strong night-sky lines
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Evolution of LAE LF from z=5.7 t0 6.5
• Malhotra & Rhoads (2004): no evolution seen; IGM ionized up to z=6.5
• Haiman & Cen (2005): not necessarily; local HII bubble permits escape of Lya photons and the suppression is not as large; <XHI> up to 25%
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Better Statistics from Subaru Deep Field
Shimasaku et al. (2006) Kashikawa et al. (2006)
• Kashikawa et al. (2006): strong evolution from z=5.7 to z=6.5 !
• Significant fraction of HI at z=6.5 ?? WMAP zreion ~ 11.4?
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Part II
Stellar Masses of Galaxies at z 6
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Stellar Mass Assembly History in Early Universe
• Stellar mass density & SFR density: =
∫SFR dt
• Need measurements at rest-frame optical (and beyond) to reduce biases caused by dust extinction and short-lived stars when converting light to mass
• Study at high-z made possible by Spitzer IRAC
• GOODS Spitzer Legacy Program has played an important role
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3.6μm 4.5μm
5.6μm 8.0μm
z =5.83 galaxy
IRAC Sees z ~ 6 Galaxies in HUDF
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z=5.83
z=5.9
zp~5.9
Three i-drops in HUDF securely detected by IRAC
Yan et al. 2005, ApJ, 634, 109
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• Some high-mass (a few x 1010Msun) galaxies were already in place by z6 (age of Universe < 1.0 Gyr)
• A few hundred Myr old (formed at z>>6)
• Number density consistent with CDM simulation from Nagamine et al. (2004)
Some Major Conclusions from SED Fitting
See also Eyles et al. (2005)
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CDFS, 3.6μm HDFN, 3.6μm
Extending to Entire GOODS(Yan et al. 2006, ApJ, 651, 24)
IRAC-detected i-dropouts
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CDFS, 3.6μm HDFN, 3.6μm
IRAC-invisible i-dropouts
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Difficulty: no photometric info between z’ and IRAC 3.6μm
Have to take a different, simplified approach
(z’-3.6μm) color age for a given SFH M/L for a given SFH at this age stellar mass; repeat for all SFH in the set, and take min, max, median
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Stellar Mass Estimates Summarized
• IRAC-detected Sample
Mrep: 0.09 ~ 7.0x1010Msun (median 9.5x109Msun)
Trep: 50 ~ 400 Myr (median 290 Myr)
• IRAC-invisible Sample, using 3.6m upper limit
Upper-limit of Mmax (median 4.9x109Msun)
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IRAC-invisible sample stack Random stack
3.6μm
3.6μm mag = 27.44median z’ mag = 27.00
Mmin = 1.5x108
Mrep = 2.0x108 Msun
Mmax = 5.9x109
Stacking of IRAC-invisible i-dropouts
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Models courtesy of K. Nagamine;based on simulationsof Nagamine et al. (2004) and Night et al. (2006)
Implications (I): compare to simulation•ΛCDM models seem to be capable of
producing such high-mass galaxies by z 6
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Implications (II): Global Stellar Mass Density•Lower limit at z ~ 6: (1.0, 1.6, 6.5) x 106MsunMpc-3
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Implications (III): Source of Reionization• Critical SFR based on
Madau et al. (1999)
• Progenitors of all IRAC-detected z6 galaxies formed simultaneously with the same e-SFH: SFR e-t/
• The progenitors of high-mass galaxies alone CANNOT provide sufficient ionizing photons to sustain the reionization
• Dwarf (low-mass, low-luminosity) galaxies, which could be more numerous, must have played an important role
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Part III
Bright-end of LF at z 6
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L* & Bright-end of LBG LF
• Bouwens et al. (2006): L*(z=6) = 0.6L*(z=3)
•Effect of large-scale structure ( “cosmic variance”)??
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Need Degree-sized Surveys to Minimize Impact of “Cosmic
Variance” at Bright-end
(Millennium Simulation slice at z=5.7)
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D1(2h-4d)(overlap SWIRE)
D2 (10h+2d)(w/COSMOS)
D3 D4
16.5’x10’GOODS-Size Area
Bright i-drops in 4-deg2 CFHTLS
Yan et al. (in prep)
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Magellan High-z LAE Survey
Yan, McCarthy & Windhorst
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Survey Highlights
•Narrow-band imaging in 917nm & 971nm OH-free windows to search for LAE at z ≈ 6.5 & 7.0
•Four IMACS f/2 fields (~ 0.9 deg2); reducing cosmic variance with limited telescope time
•Survey depth (5-) AB=25.0 mag (2.4510-17 erg/s/cm2 for pure-line sources; 7-810-18 erg/s/cm2 for continuum-detected sources)
•Aiming at bright-end of the luminosity function
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6.46 — 6.62
6.91 — 7.07
~ 400 Mpc3/arcmin2
(Before upgrading, SITe CCDs)
o(917nm) p(971nm)
Survey Design: Filters
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Survey Design: Fields
•Use fields that have public, deep continuum images in multi-bands (especially in z’-band)
•Accessibility from Las Campanas
•CFHTLS Deep D1, D2 & D4 spreading out in RA
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Survey Status
• 1-night in Feb. 2007 + 2-night in Mar. 2008, 1 IMACS pointing in COSMOS field (CFHTLS-D2), 20hr in o(917nm)
• 3-night in Jul. 2007, 1 IMACS pointing in CFHTLS-D4, 20 hr in o(917nm)
• Achieved desired depth
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COSMOSCFHTLS-D4
1.48o
1.48o
1o
1o
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5- source counts
CFHTLSD4NW, 20hr in o
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3 candidates invisible in continuum
o=23.88
o=24.39
o=25.49?
(Now seeking time do spectroscopic identification)
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Kashikawaet al. 2006(in SubaruDeep Field)
Rapid Evolution from z=5.7 to 6.6 or not?
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Part IV
Searching for Galaxies at z > 7-8 and beyond
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Deep Space-based IR Imaging for LBG
Bouwens & Illingworth (2006); Bouwens et al. (2008)
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Space-based IR Imaging around Lensing Clusters for LBG
Bradley et al. (2008)Abell 1689
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Direct Slit-Spectroscopy around Lensing Clusters for LAE
Stark et al. (2007)
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Another Line of Thought
• There might be a much more luminous population at z>7; surface density as high as 0.01-0.05/arcmin2
From Yan et al. (2006)
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Wide-field near-IR Surveywith WIRCam at CFHT
PIs. Lihwai Lin & Luc Simard
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CFHT WIRCam J, 26 hrs Candidates to be observed by NICMOS in Cy-16 soon (PI. Yan)
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LBT Can Play an Important Role in the Study of the High-z Universe
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• MODS: efficient identification & detailed study of large samples of LBG candidates from z = 3 to 6
• LBC: large-field surveys along many sight-lines to overcome the bias caused by “cosmic variance”; deep Y-band imaging to look for z7 galaxies
• LUCIFER: identification of the luminous z>7 candidates found in the on-going wide-field IR surveys
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Summary• UV Luminosity Function of Galaxies at z 6
— a very steep faint-end slope
• Stellar Masses of Galaxies at z 6 — some high-mass, “old” galaxies already in place
Implications for (HI) Reionization
— dwarf galaxies did it!
• Unanswered Questions at z 6: Bright-end of LF (LBG/LAE)
— degree-sized surveys needed to reduce “cosmic variance”
• Searching for Galaxies at z > 7-8 — there might be a luminous population, detectable at ~24.5mag