Constraining Star Formation at z>7
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Constraining Star Formation at z>7 Dan Stark (Caltech) Collaborators: Richard Ellis, Johan Richard, Avi Loeb, Eiichi Egami, Graham Smith, Andy Bunker, Jean-Paul Kneib, Mike Santos A Century of Cosmology, San Servolo, Italy, 27 August 2007.

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Characterizing Star Formation at z>7: the Current Frontier

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Constraining Star Formation at z>7 Dan Stark (Caltech)Collaborators: Richard Ellis, Johan Richard, Avi Loeb, Eiichi Egami, Graham Smith, Andy Bunker, Jean-Paul Kneib, Mike SantosA Century of Cosmology, San Servolo, Italy, 27 August 2007

Characterizing Star Formation at z>7: the Current Frontier

  • Motivation:

  • Probing some of the earliest galaxies (t<750 Myr)

    • how does the global star formation rate density evolve?

    • what is the density and luminosity distribution of earliest star-forming systems?

    • what are physical properties (e.g. metallicity, stellar mass) of galaxies at the highest redshifts?

  • contribution of early star formation to reionization

    • what were the objects that reionized the IGM?

    • what is the redshift distribution of the reionizing systems?

    • if star-forming galaxies, were the galaxies primarily low luminosity?

  • note: current mode of observational study of z>7 universe is exploratory in nature

Decline in Star Formation Rate Density?

Bouwens et al., 2007


Does decline in SFRD extend to higher redshifts?

Old stars at z~5-6 Implies Star Formation at z>6

Measurement of stellar mass density allows constraints to be placed on required SF at z>5-6 to assemble galaxies (Stark & Ellis 2005, Stark et al. 2006, Yan et al. 2006, Eyles et al. 2006)

z = 5.554

1.1x1011 M

Stark et al. (2007a) ApJ, 659, 84

Assembled stellar mass density at z~5-6 requires greater star formation rate density at z>6 than has been observed thus far. How do we directly observe missing sources at z>6?

Evidence for Luminosity-Dependent Evolution

Bouwens et al. 2006, ApJ, 653, 53

Bouwens & Illingworth, 2006, Nature

Bouwens et al (2006, 2007) and Yoshida et al. (2006) propose L-dependent evolution - decline in abundance over 3<z<6 mostly for luminous sources

Observations suggest star formation increasingly dominated by low luminosity sources for z>6?

Beyond z~7 with Strong Gravitational Lensing


z = 6.8

Utilizing strong magnification (10-30) of clusters, probe much fainter than other methods in small areas (<0.1 arcmin2 /cluster)

Spectroscopic Surveys for Lensed Lyman-alpha Emitters

From arclet spectroscopy the location of the “critical lines” is known precisely for


and for


A Keck survey in the optical discovered 11 candidate LAEs between 2.2 < z < 5.6 -- Santos et al. ApJ 606, 683 (2004)

Low Luminosity z~10 Ly Emitters: Critical Line Mapping With Keck NIRSPEC

Wavelength sensitivity (1.5hr)

Cluster critical line for zS > 7


10-17 cgs

NIRSPEC slit positions

  • 9 clusters completed between 04-05 (Stark et al. 2007b, ApJ, 663,10)

  • Clusters have well-defined mass models & deep ACS imaging

  • Obs. sensitivity ~ 0.6-2 x10-17 cgs; magn. > 10-30 throughout

  • Sky area observed: 0.3 arcmin2; V(comoving)~30 Mpc3

  • 6 promising lensed emitter candidates (>5)

    • 8.6 < z < 10.2; L ~ 2 - 50. 1041 cgs; SFR ~ 0.2 -5 M yr-1

Candidate Ly Emitters

8.6 < z < 10.2; L ~ 2 - 50. 1041 cgs; SFR ~ 0.2 - 5 M yr-1

Recognize burden of proof that these are z~10 emitters is high! (see discussion in Stern et al. 2000ab, Bremer et al. 2004, G. Smith et al. 2006)

Each detection is > 5, seen in independent exposures/visits

Spectroscopic Elimination of Interlopers

If J-band emission is H…


Ly, [OII], and [OIII] observable with optical spectroscopy

Spectroscopic Elimination of Interlopers

  • Archival LRIS spectroscopy (Santos et al 2004) from 4000-9400Å available for all candidates

    • No emission lines detected: candidates are probably not H or [OII]

  • H-band spectroscopy obtained for 3 candidates with NIRSPEC

    • No emission lines detected: 2/6 candidates are probably not H, [O III]

  • J-band spectroscopy available for all candidates with NIRSPEC

    • No additional emission lines detected: candidates are most likely not[OIII] 4959

  • optical broadband? one candidate has marginal z-band detection

    • remove from high-z sample

At least 2 candidates are most easily explained as z~10 sources, and an additional 3 candidates have strong case to be at z~10.

Confirming Galaxies at z>8

1. Asymmetric Line Profile?

Long integrations needed.Only look for in best candidates.

Should be potentially feasible with MOSFIRE.

2. Lensed Counterimage?

Could confirm redshift, but difficult with longslit spectrograph.

Stern et al. 2005, ApJ

Contribution to Reionization?

# density required for reionizaton

Consider range:

fesc ~ 0.02-0.5 t ~250-575 Myr

None are real

All are real

If even 1-3 of the 6 candidates is at z~10, low luminosity galaxies may play a dominant role in cosmic reionization

Key uncertainty: if candidates are at z~10, are observed

densities characteristic given large cosmic variance?

A surprisingly large abundance of z~10 Lya Emitters?

Stark, Loeb, & Ellis, 2007, ApJ, in press

Observed densities of z~10 LAEs

model for z~10 LF assuming

increase in star formation efficiency

model for z~10 LF assuming

LLya-M mapping unchanged

from z~5.7/6.6

  • abundances of z~10 Lya emitters not consistent with extrapolation from z~6

  • require more Lya photons per unit halo mass, achievable through 1) increase in star formation efficiency or 2) change in IMF

  • possibilities tantalizing but more work needed to confirm candidates and additional survey volume needed to confirm densities.

Predictions for future LBG surveys at z>7

Stark, Loeb, & Ellis 2007, ApJ, in press


telescope regime

  • Upcoming surveys (e.g. VISTA / WFC3) should be able to detect large number of LBGs at z~7-8.

  • If increase in density suggested by z~9-10 lensed LAE candidates proves false, probing to z~10 may be much more difficult, especially from the ground


  • Strong lensing surveys are finding an abundant population of candidate faint Ly emitters at z~8-10 with SFR <1 M yr-1 - a population which may contribute significantly to reionization.

  • Exhaustive spectroscopic and imaging follow-up supports hypothesis that many of lensed Lya emitters are at z~10 but additional follow-up still required. Final confirmation is especially difficult at z>7!

  • Implied densities of z~9 LAEs still very uncertain, but if correct, would require increase in Lya photon output per unit halo mass from z~6.

  • Even with conservative assumptions, new instruments should result in reasonably large samples of galaxies at z~7-8 in the next few years. If increase in density implied in lensed LAE survey proves false, z~10 objects may be difficult to find before JWST, especially for (non-lensing) ground-based imaging surveys

Lensed z>7 Lyman-Break Galaxies

UDF flux


Bouwens & Illingworth, 2006

Only most luminous systems probed at z~7.5!

Predictions for Lya Emitters at z>7



  • Upcoming/ongoing surveys should be able to detect LAEs at z~7-8

  • Expected Poisson and cosmic variance will complicate conclusions about reionization.

  • Probing to z~10 may be much more difficult, unless there is upward evolution in LAE LF (as implied in lensed LAE searches).

HST+Spitzer Survey for Lensed z>7 LBGs

MS1358 - H

Critical line


> 2 mags

Richard, Stark, Ellis et al, in prep.

  • 11 NICMOS pointings in 6 lensing clusters

    (4 orbits J/F110W, 5 orbits H/F160W)

  • ACS/F850LP imaging of all clusters

  • K-band ground based imaging with Keck/NIRC + Subaru/MOIRCS

  • Deep IRAC imaging for all clusters

Candidate z~7.5 LBGs

Bouwens et al

Lensed candidates





  • 10 candidate z-drops in the 6 clusters surveyed with H ~ 26 - 26.8

  • Implied SFR ~ 0.1 - 2 M yr-1 (unlensed)

  • Time allocated to follow up these spectroscopically with NIRSPEC

Source Plane and Image Plane

Position of source relative to lens is crucial!

No rings: giant arcs with counter images

The Real Sky

What Observer Sees

In the general case (here elliptical lens) as background source moves closer in alignment, multiple images, some highly magnified appear. Lines where magnification is maximized are known as `caustics’ in the source plane, `critical lines’ in the image plane.

Bulk of candidates unlikely to be z~2 interlopers

z ~ 2 quiescent

Current limit

Possible limit in warm mission

z ~ 8 star-forming

Stacked IRAC limit for 8 unconfused candidates gives upper limit at 3.6 microns rejecting passive z~2 population as primary population

Implied Luminosity Function z~7.5

All 10 z-drops are real

None is real

Bouwens & Illingworth z~7.4


Lensed LBG survey probes faintward of UDF

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