Star-Forming Galaxies and the IGM in the z=1.5-2.5 “Redshift Desert”

C. Steidel, D. Erb, N. Reddy (Caltech)

A. Shapley (Berkeley)

M. Pettini (IoA, Cambridge)

K. Adelberger (OCIW)

Overview

• The “redshift desert” is largely a myth. The redshift range z~1.4-2.5 is actually quite easily observed and moreover there is (in principle) more information accessible on these objects than almost any other redshift

• Summary of some new results (mostly) from observed UV/optical and near-IR observations .

• Galaxies and the IGM at z~2: initial results

• Upshot: this is a very important epoch for the assembly and maturity of the massive galaxies of the present epoch

Why z~2 is interesting….• The peak of the QSO epoch, and (probably) of star formation in galaxies

• Allows for simultaneous study of diffuse IGM in the same cosmic volumes [z~2.5 QSOs are much more common than z~3.5 QSOs]; galaxy surface density within spectroscopic limit is much higher than at z>3

• Large numbers of galaxies are bright enough for detailed spectroscopic study with 8m-class telescopes

• Access to diagnostic spectroscopy in both the rest-frame far-UV and the rest-frame optical; well placed nebular lines in atmospheric windows!

The “Redshift Desert”

DEEP2 LBGs “Team Keck” in GOODS-N

Why the “redshift desert”?

• Familiar spectral features ([OII] 3727; 4000 Å break) used for redshift measurement at low redshift shift out of optical band for z>1.4.– Near-IR multi-object spectrographs planned or just coming on line– Sky is increasingly problematic in near-IR

• Difficult photometric selection because of absence of broad-band spectral features in optical window– Wide-field near-IR to adequate depth extremely expensive

• Most 8m-class optical spectrographs are optimized at visual and red wavelengths

• Common mis-conception: that emission lines are necessary to measure redshifts without heroic effort (Ly alpha, rest-optical nebular lines)

Far-UV Spectra of LBGs

Shapley et al 2003

(z~3)

Pushing into the “Spectroscopic

Desert” using optical (far-UV) color

selection

(poor man’s photo-z)

Adelberger et al 2004

LBG Analogs at lower redshifts

Optical Photometric Pre-Selection

Green/yellow (LBGs):

z=2.960.26

Cyan(“BX”):

Z=2.200.32

Magenta (“BM”):

Z=1.700.34

Total surface density is ~9

arcmin-2 to R=25.5

Why z~1.5-2.5 is now easy…•Sky is very dark (AB=22.5-23/arcsec2)

•LRIS-B is very efficient right where it counts: 3100-4500 Å

•Galaxies have lots of spectral features– mostly absorption lines

Z=1.41, 90 min

“Redshift Desert” Survey Statistics

• 7 fields, total ~0.5 sq. degrees– 5 are specially chosen to have several z~>2.5 QSOs for the

purposes of probing the IGM through the same volume (primary program is galaxy/IGM cross-correlation): Q1307+28, Q1623+27, Q1700+64, Q2343+12, Q2346+00

– 2 are fields chosen to have extensive existing or planned multi-wavelength data (GOODS-N, Groth/Westphal field)

• To date: total of 900 spectroscopic redshifts, z=1.4-2.6– Two primary selection criteria, targeting:

• Z=2.0-2.5 (“BX” ) 750 redshifts• Z=1.5-2.0 (“BM”) 150 redshifts [primarily in non-QSO fields]

– Contamination by low-z interlopers and stars is ~8% to R=25.5

z=1.5-2.5 galaxies:•Surface density to R=25.5 (K~22.5) is ~9.5/arcmin2

•~25% of the R-band surface density to R=25.5

Spectroscopy of Star-Forming Galaxies in the ``Redshift Desert’’

CS et al 2004

950750

150

All spectra 90 min, Keck/LRIS-B

CS et al 2004

Another Method for Exploring the “Desert”:Spectroscopic Failures from the DEEP Survey, observed with LRIS-B,

September 2003

z=1.657 z=1.958

DEEP Failure z-distribution

[OII] outside of DEIMOS coverage (z>1.4) 18/25 gals

[OII] falls within DEIMOS coverage (z<1.4) 7/25 gals, including 2 AGN

<z>=1.62+/-0.42

LRIS-B z’s for 25/26 DF targets

Deep Near-IR Photometry from

Palomar 5m/WIRC, 8.7’ x 8.7’ field of

view,

FWHM~0.6”

12 hours, reaches K~22.3 (KAB~24.2), 5

sigma~85% of galaxies with spectroscopic z’s are

detected in the K images (3 fields so far)

Near-IR Imaging of Spectroscopic z~2 Galaxies

•~10% have K<20

(note these are not all red in R-K)

•~30% have K<20.6 (cf. Gemini Deep Deep Survey)

•Distribution of K luminosities is not very different from z~3 sample

Near-IR Imaging of Spectroscopic z~2 Galaxies

Optical/IR colors are significantly redder at

z~2 than at z~3

(Galaxies with identical star formation histories would have identical R-K colors)

(283 gal)

(108 gal)

more extensive star formation histories than

z~3 analogs

Optical/IR Colors of z~2 Galaxies

(283) (108)

CS et al 2004; z~3 from Shapley et al 2001

R=25.5 limit

Galaxy Kinematics at z~2

Erb et al 2003

•7 of 15 show extended H emission (w/shear)

•Typical projected vc>150 km/sec

•Minimum dynamical masses >5x1010 Msun in several cases

•Intriguing differences compared to z~3 sample…

(Keck/NIRSPEC K- band Spectroscopy-May 2002)

H kinematics: rotation curves?

• Used ACS BViz images of GOODS-N field, in which we have 171 galaxies with z>1.4

• 10/13 galaxies targeted to have slit PA aligned with major axis. Only 2 show measurable shear.

•Puzzling result: elongated galaxies have smaller velocity dispersions than randomly selected objects

(Erb et al. 2004)

Galaxies at z=2.1-2.5

2 GOODS-N w/measured shear

Erb et al 2004

H kinematics: rotation curves?

• Seeing has significant effect on measured tilt

• In May 2002 seeing was 0.5”, and in May 2003 seeing was 0.9”

• Reobserved Q1700-BX691 to test this effect

•1-d is much more stable with respect to seeing

Q1700-BX691, z=2.1895

May 2002

vc~220 km/s

~170 km/s

May 2003

vc~120 km/s

~156 km/s Erb et al 2004

H-alpha Kinematics at z~2

• Z~2 galaxies have consistently larger 1-d velocity dispersions than z~3 sample

• 50% of sample has sigma > 100 km/sec (cf. 1/30 @z~3)

• H-alpha detections for ~80% of attempted targets in 1 hour with NIRSPEC

• ~25-30% have rotation-like kinematics

GOODS-N Results•171 spec redshifts between z=1.4-2.6

•4 BX/BMZ galaxies are also

SCUBA sources, 3 have spectroscopic

redshifts

Z=1.989

Z=1.865Z=2.098

94 z=2-2.5 Galaxies in the GOODS-N Field

50 GOODS-N Galaxies, z=1.5-2.0

X-Rays and Radio Emission from UV-selected Galaxies in CDF/GOODS-N Field

Reddy & CS 2004

Stacking of 171 spectroscopically confirmed z=1.4-2.5 S.F. galaxies (excluding all direct detections,

AGN)

•10 sigma detection <SFR(Xray)> = 42 Msun/yr

•5 sigma detection in radio<SFR(Radio)>=50Msun/yr

•Corresponding <SFR(1500)> = 8.5 Msun/yr

<A(1500)>= factor of 4.9, very similar to results at z~3, and to inference from UV colors

The average z~2 galaxy in this sample is a “LIRG”, w/Lbol~few x 1011

Chandra Stack

Blue: Starburst99 Solar Metallicity, Salpeter IMF

3250 4460

Far-UV Spectroscopy of “Desert” Galaxies

LRIS-B, June 2002

Far-UV Spectral Diagnostics: IMF constraints

At z~1.4-2.5, spectra of the quality necessary for detailed modeling

of the OB stellar population are

accessible for much more than lensed

galaxies

CS et al 2004

z=1.411

Best fit model: Salpeter IMF, solar metallicity

0

Far-UV Spectral Diagnostics: Metallicity Constraints

S. Rix, Pettini, CS, et al 2004

Q1307-BM1163 (z=1.410):

Data: Black, LRIS-B

Best fit model (blue):continuous star formation, Salpeter IMF,

solar metallicity

FeIII 1978 Index

cB58 (z=2.72):

Smoothed data (black)

Best-fit model: continuous star formation, Salpeter IMF,

0.4 solar metallicity

Rest UV + Rest Optical: What do you learn?

Keck/NIRSPEC

Keck/LRIS-B

H/[NII] ratio also implies [O/H]=0 (solar

abundance)

UV stellar P-Cygni and photospheric

absorption indicate solar abundances

•SFR(H)=SFR(UV)=30 Msun/year

•V(ISM)=270 km/sec with respect to both nebular lines and stellar photospheric lines

Direct Nebular Abundance Determination

• Use [OIII] 4363/(5007,4959) to get Te, [SII] to get ne

• Problem: 4363 weak, even in local low-Z gals; high z star-forming gals are not very metal-poor--NO HOPE at high redshift

(figure from van Zee 2000)

Z=1/25 Zsun

Indirect: Bright Lines (R23)

(Kobulnicky et al. 1999)

• High-Z branch: R23 decreases as Z increases

• Low-Z branch: R23 decreases as Z decreases

•Particularly ambiguous for near-solar abundances

• Systematic differences from direct method

•Practical issues for high-z galaxies

[NII]/H ratios: z~2 metallicities

• relationship between [NII]/Hand O/H

• after Denicolo et al 2002, but using only accurately determined values from literature

•[NII]/H saturates at high metallicity

(Pettini & Pagel 2004)

N2=log([NII] 6584/H)

12+log(O/H)=8.9+0.57xN2

~0.18, factor of 2.5 in O/H

[NII]/H ratios: z~2 metallicities

• at O/H greater than 0.25 solar O3N2 index is useful

• N2 increases as O3 decreases, so very sensitive to O/H

•Ratios are essentially reddening independent

(Pettini & Pagel 2004)

O3N2=log([NII] 6584/H)

12+log(O/H)=8.73-0.32xO3N2

~0.14

[NII]/H calibration from Denicolo et al 2002

•solar to super-solar metallicities are not uncommon among z~2 star-forming galaxies

•Initial indications are that choosing K-bright or R-K red objects is almost guaranteed to yield solar metallicities or greater…

Metallicities in z~2 Galaxies: Keck/NIRSPEC results

Rest-frame Optical Selection

• 90% of UV-selected “BX” objects have Ks>20

• Sample: 9 z=2.1-2.5 objects with Ks<20, 4 of which also have R-Ks>4

• Obtained NIRSPEC/H spectra

(Shapley et al. 2004)

Q1623+268 field, 121 with

spectroscopic z + K measurement

[NII]/H ratios: z~2 metallicities•Selected 8 K<20 galaxies (z=2.1-2.5) from among 12 with spectroscopic redshifts in a single (8’) field.

•All have abundances consistent with solar; half may have super-solar abundances.

•Population synthesis indicates stellar masses greater than 1011 Msun

•All are best-fit by ages >1Gyr (most >2Gyr) despite current star formation rates of ~50-100 Msun

Shapley et al 2004

Other redshift desert surveys

Some other surveys with galaxies at z~1.5-2.5

• K20 (Cimatti et al.) (K<20 selection) (~10)

• Gemini Deep Deep (Abraham et al.) (K<20.6, photo-z) (34)

• FIRES (Franx, van Dokkum et al.) (J-K selection) (4)

• Radio-detected SCUBA sources (Chapman et al.) (~50)

Reminder: K band is rest frame R @ z~2. Can easily be dominated by current star formation and not formed stellar mass

Optical vs. Near-IR Selection in the Desert: What’s the difference?

Shapley et al 2004

•Number for comparison are small, but:

•Of 9 z>1.7 K<20 galaxies in K20 survey (Daddi et al 2003), at least 6 would have been selected by UV BX/BM color criteria

•One of our fields (SSA22) is in common with GDDS; of 7 galaxies with z>1.6 (3 with spectr. Z and 4/photz), 6/7 satisfy BX/BM selection criteria.

•Space density of UV-selected objects w/K<20 is very similar to that of K-selected objects in the same redshift range.

Most massive galaxies at z~2 are still forming stars

•This might not be true at z=1.3, which is ~2 Gyr later

Other Interesting Points about z~2 IR-bright galaxies

• Change in R-K colors from z~3 to z~2.2, together with change in kinematics, suggests more than doubling of stellar mass of objects with the same range of star formation rates, on average, over that interval.

• Inferred star formation ages indicate long star formation histories for massive z~2 galaxies, consistent with their being galaxies which would have been easily observable at z~3.

Using optical (rest-UV) and near-IR (rest optical) to quantify physical properties of z~2 galaxies

Optical spectra: •IMF

• stellar photospheric abundances•ISM metallicity •ISM kinematics

Near-IR spectra:•Kinematics•Ionized gas metallicity•SFR estimate

Deep in the Desert: 10-20 hr Spectra w/LRIS-B (4-5 A resol’n)

*Quasar

H I

C IV

Probing The Intergalactic Medium Using Quasar Absorption Lines

Keck/HIRES Quasar Spectrum

Observer

•Idea is to:

•Observe the relative distribution of young galaxies and the diffuse IGM—(e.g., do they trace the same matter fluctuations, with different degrees of “bias”?)

• Place constraints on the evironmental influences of galaxy formation and the feedback of star formation and AGN .

•Directly measure the large-scale effects of “feedback”

Galaxies and the IGM

•The kinematics of outflowing ISM gas is similar in z~3 and z~2 star-forming galaxies

•Typical velocities are ~200-400 km/s with

respect to nebular line redshifts,

• Differences between Ly alpha emission and

interstellar absorption is ~500-1000 km/s

Z~2

Z~3

Galactic Scale Winds: Still Going Strong at z~2

Vwind=500-600 km s-1

dense shell of swept-up material

•Holes in ISM to allow escape of ionizing photons?

•Blow-out of dust and gas

•As for local “superwind” galaxies, mass loss rate in wind is comparable to SFR (5-1000 Msun yr-1 for bright LBGs) estimated from observations of the blue-shifted IS lines.

•Metals into IGM/ICM if Vwind > Vesc

•Expect gas heated to >> 106 K

•Expect metal mass ejected at ~SFR/100 ~ Msun yr-1

•Expected “sphere of influence” is Rwind~150 kpc (tSF/300 Myr) x (Vwind/600 km s-1)

Isotropic?

~20 Mpc (co-moving)

~10 Mpc (co-moving)

Mapping the Galaxy Distribution in the Same Volumes as Probed by the QSO Absorption Line Techniques

Map "On the Sky"

Along Our Line of Sight

~200 Mpc (co-moving)

Metals in the IGM and Galaxies at z~3

Even compressed into one dimension, galaxy

over-densities are closely related to metal

“over-densities”

Galaxy-Galaxy Clustering Galaxy-Metals Cross-Correlation

Adelberger, CS, Shapley, Pettini 2003

Galaxies and Metals at z~3

Metals and Galaxies on Scales of 0.75 Mpc (co-moving), ±600 km/sec (the expected sphere of influence of winds

from single LBGs)

Cross-correlationFraction of CIV systems that are

within this distance of a LBG

Adelberger, CS, Shapley, Pettini 2003

The “Galaxy Proximity Effect ”?Galaxies “clear out” a region in the neutral H within ~700 kpc (co-

moving)?

There is an excess of HI absorption within ~6 Mpc

of galaxies, on average

Average flux in the Lyman alpha

forest at z=3

Adelberger, CS, Shapley, Pettini 2003

Croft et al 2002

Metals in the Forest

• Metals, traced by CIV, are found in the same regions of space that contain detected LBGs

– Highest N(CIV) systems appear to be one and the same as LBGs (but with R ~150 kpc proper); extent consistent with expected sphere of influence of winds

– Even the weakest CIV systems are distributed like the observed LBGs

• Strongly suggests inhomogeneous enrichment of the IGM…and that most or all places that have observable metals have been recently “disturbed” at z~3

Other possible implications of winds from LBGs

• ICM metals in clusters of galaxies: these are after all the progenitors of rich environments today…

• “Entropy floor” in galaxy clusters?

• Feedback regulates star formation, suppresses subsequent star formation while wind-affected regions cool (~1-2 Gyr)

• Suppress formation of small galaxies within ~200 kpc of large ones?.

• The existence of winds is not surprising—what may be surprising is their strength and the suggestion that relatively massive galaxies (as opposed to dwarfs) may dominate the effects on the IGM, and not all of the effects are confined to very high redshifts (z>>3).

Galaxies and the IGM at z~2

• Much higher galaxy surface density (factor of 4 higher per unit solid angle)

– Further explore use of galaxy spectra to probe IGM

– Forest evolves extremely rapidly over the redshift range 3.51.8: how does the galaxy/forest relationship change?

• Many more QSO sightlines

– 17 lines of sight in 5 fields

• Simultaneously obtain first extensive information on z~2 galaxies.

– Epoch of peak star formation and black hole growth?

– Redshifts z=1.9-2.6 are ideal for near-IR spectroscopic follow-up

Multiple QSO Probes, z=1.8-2.5 IGM

7 QSO probes (HIRES/ESI/LRIS-B spectra) in 16’ by 12’ field

Goal: IGM “Tomography”+Galaxy influence/correlations with IGM metals, HI

Keck/LRIS-B 2002/3255 Galaxy Spectra

•∆(vneb-vism) = 460 km/s

•Dlos=115 kpc

•[O/H]~0 (i.e., solar metallicity)

•SFR~85 Msun/yr

•K~20.3, R~24

•M*~6 x 1010 Msun

HIRES Spectrum of QSO:

•∆v(CIV,NV)~570 km/s

•Weak associated Ly

Galaxy: Q1700-MD103

z=2.3148

D=115 kpc

Galaxy: Q1700-BX717

Z=2.4353

D=218 kpc

Other Examples: Individual Observations of Large-Scale Winds

QSO spectra from Simcoe

et al 2003

Galaxy Proximity Effect at z~2?

Adelberger et al 2004; sims by Kollmeier and Weinberg

Galaxies within 1h-1 co-moving Mpc of QSO sightline

A Special Case: The Pair at z=1.60/2.17

BX201 z=2.17

BM115 z=1.60

HST/WFPC2 F814W image=2” 11 h-1 kpc at z=1.6

Keck/LRIS spectra 3800-4100 AA

• Two UV-selected z~2 galaxies on one slit• Higher z galaxy probes outflow of lower-z gal at 11h-1 kpc• Outflowing gas: velocity & abs strength difference vs. radius

Other (Undigested) Results

• Far-UV Luminosity function, clustering in progress– z~3 spectroscopic LBGs: r0=4.0±0.3 h-1 Mpc (Adelberger et al

2003).• Spectral differences between z~3 and z~2: Lyman emission

becoming increasingly rare at the bright end of the L.F.• Impact of star forming galaxies on the z~2 IGM, cross correlation of

galaxies/metals/HI nearly ready.– Deep multi-wavelength fields are not particularly useful for this

sort of thing, which is a shame.• AGN content of z~2 sample: fraction of objects with obvious AGN

signatures is 3.2% (same as z~3). – Includes two narrow lined AGN in CDF-N that are X-ray

undetected.

Summary

• Galaxies at z=1.4-2.5 are easy to find and study; there is no redshift desert.

• There is a huge amount of accessible astrophysical information on these galaxies now, with much more to come in the next ~year from (e.g.) deep spectroscopy and from Spitzer.

• There are significant differences in the star forming galaxy population between z~3 and z~2, in the sense that there are many more “mature’’ galaxies by z~2 (that are still forming stars).

• Most massive galaxies at z~2 are still forming stars (?)– But perhaps not for much longer…

• The IGM represents a laboratory for studying important aspects of the galaxy formation process that are otherwise inaccessible observationally.