DLA Surveys and StatsDLA Surveys and Stats

Sandhya RaoUniversity of Pittsburgh

Outline

A bit of history DLAs at high redshift (z>1.65) HI stats at z = 0 DLAs at low z

New results at low z MgII, FeII, N(HI) correlations dn/dz, Ω, f(N) Star formation history of DLAs

a paradigm shift

The First DLA:Title: Absorption Lines in the Quasistellar Object PHL 957.Authors: Lowrance, J. L., Morton, D. C., Zucchino, P., Oke, J. B., & Schmidt, M.

Bibliographic Code: 1971BAAS....3..238L

Lowrance et al. 1972, ApJ, 171, 233

Beaver et al. 1972, ApJ, 178, 95

Hale telescopeImage-tube5Å resolution

Lick ObservatoryImage-tube8Å resolution

Black, Chaffee, & Foltz1987, ApJ, 317, 442

MMT spectrographCCD1Å resolution

Optical surveys for DLAs:

The Lick Survey for DLAs Wolfe, Turnshek, Smith, & Cohen 1986 - first systematic search for DLA candidates - z ≈ 2 - follow-up spectroscopy to confirm detections Turnshek et al. 1989, Wolfe et al. 1993 - 15 DLAs in 68 spectra, Δz=55 Lanzetta et al. 1991 - expanded sample, 1.6<z<4.1 - 38 DLAs in 156 spectra, Δz=155 - first determinations of dn/dz, Ω, f(N) vs. z

Wolfe et al. 1995 - LBQS: 62 DLAs in 228 spectra, Δz=324 - SF model to explain Ω, f(N)

Motivation: Milky Way column densities Low ions & narrow fwhm’s Low 21 cm spin temps search for high z disks

Optical surveys for DLAs (contd):

Storrie-Lombardi & Wolfe 2000 - extended survey to z=4.7 using LRIS on Keck - 85 DLAs along 646 sightlines, Δz=420

Peroux et al. 2001, 2003 - surveyed 66 QSOs at z>4 - 26 additional DLAs, 15 at z>3.5 - LLS include half the Ω at z>3.5

Prochaska & Herbert-Fort 2004 - used SDSS-DR1 spectra, 71 DLAs in 1252 QSOs - a total of 163 DLAs in high-z statistical sample - most sensitive at z=2 — 3.2 - LLS contribute <15%

HI at the present epoch

Need a statistical description of HI at z=0 to help interpret DLA stats.

The relevant questions are:

1. How much of it is there? Ω

2. What is its cross-section? dn/dz

3. What is the column density distribution? f(N)

4. Where does it reside, and have we found it all?

Rao and Briggs 1993 (pre- HI, pre-large-galaxy surveys era)

- used the optical luminosity function of gas-rich galaxies + HI maps of a ‘complete’ sample of 27 nearby galaxies

f(N) ~ ∫Φ(M) <A(M,N)> dM

∫f(N)dN ~ dn/dz

∫Nf(N)dN ~ Ω

Rao and Briggs 1993 (pre- HI, pre-large-galaxy surveys era)

- used the optical luminosity function of gas-rich galaxies + HI maps of a ‘complete’ sample of 27 nearby galaxies

f(N) ~ ∫Φ(M) <A(M,N)> dM

∫f(N)dN ~ dn/dz

∫Nf(N)dN ~ Ω

Rao and Briggs 1993 (pre- HI, pre-large-galaxy surveys era)

f(N) ~ ∫Φ(M) <A(M,N)> dM

∫f(N)dN ~ dn/dz

∫Nf(N)dN ~ Ω

HI 21cm surveys: HI Mass Function and f(N) distribution

AHISS: Arecibo HI Strip Survey — Zwaan et al. 1997

HI survey of the Ursa Major cluster — Zwaan, Verheijen, & Briggs 1999

ADBS: Arecibo Dual Beam Survey — Rosenberg & Schneider 2001

HIPASS: HI Parkes All Sky Survey — Zwaan et al. 2003, Zwaan et al. 2005 Ryan-Weber et al. 2003

HIDEEP: 20x deeper in 4° x 8° fld — Minchin et al. 2003, 2004

HI 21cm surveys: HI Mass Function and f(N) distribution Current status of results:

All gas rich galaxies are included in the optical luminosity function. (Until last month, that is.)

ΩHI (z=0) is still dominated by massive, HI rich galaxies (spirals) , but LSB contribution is now 30%. Larger than RB93 result by 40%.

dn/dz is larger than RB93 result by about a factor of 3.

The contribution of LSB galaxies to the HI cross-section, 40%, is larger than previously thought.

Thanks to large surveys, DLA stats at high z and HI statsat z=0 are much better understood now than they werea few years ago.

There is no all-sky UV spectroscopic survey of QSOs(one can only wish!), but we managed to get the best outof STIS before its untimely demise.

The need for UV surveys

DLAs at low redshift: UV surveys

• IUE Survey: Lanzetta, Wolfe, & Turnshek 1995

• HST Key Project: Jannuzi et al. 1998

• IUE+HST Archival Survey: Rao, Turnshek, & Briggs 1995

• HST-FOS Survey: Rao & Turnshek 2000

• HST-STIS Survey: Rao, Turnshek, & Nestor 2005

Our approach:

• High-z DLAs have MgII, SiII, CII, FeII absorption

• MgII (2796Å, 2803Å) can be seen in the optical at z > 0.1

• We targeted QSOs that had low-z MgII absorption

• MgII statistics (dn/dz and REW distribution) are known bootstrap from MgII stats to DLA stats

Rao & Turnshek (2000): MgII systems from literature (primarily Steidel & Sargent 1992)

HST-FOS Cycle 6 survey + HST Archival survey: 12 DLAs in 81 MgII systems with W0

2796 > 0.3Å + 4 more in a Cycle 9 survey

Rao, Turnshek, & Nestor (2005): MgII Systems from SDSS EDR (Dan Nestor 2004, PhD Thesis, U. Pittsburgh) 118 HST orbits – 1 of 7 Large Programs approved in Cycle 11 75 SDSS QSOs with 82 MgII systems 0.472 ≤ z ≤ 1.646 1.0Å ≤ W0

2796 ≤ 3.7 25 are DLAs

The MgII-DLA Surveys

We now have a sample of 197 MgII systems at z < 1.65 that have measurements of N(HI). 41 are DLAs.

MgII REW distribution

Shaded histogram: DLAs

Fraction of systems that are DLAs increases with W.

But the mean valueof N(HI) remainsconstant for W>0.6.

<N(HI)> = (3.4±0.7)E20 cm-2

There are no DLAs for W < 0.6 Å. <N(HI)> = (9.7±2.7)E18 cm-2 .

Fraction of systems that are DLAs increases with W.

But the mean valueof N(HI) remainsconstant for W>0.6.

<N(HI)> = (3.4±0.7)E20 cm-2

There are no DLAs for W < 0.6 Å. <N(HI)> = (9.7±2.7)E18 cm-2 .

W = 0.6 Å implies a spread in sightline velocity of Δv = 64 km/s.DLAs do not have kinematic spreads less than this.

Turnshek (tomorrow): kinematic spread metallicity halo mass, galaxy type

MgII-FeII selection

W02796 ≥ 0.3 Å : 21% DLAs

W02796 ≥ 0.6 Å : 27% DLAs

W02796 ≥ 0.5 Å

+ W0

2600 ≥ 0.5 Å : 36% DLAs

Red: all systems slope = 1.12 ± 0.06

Blue: DLAs only slope = 1.30 ± 0.11

All DLAs remain if the sample is restricted to W02796/ W0

2600 < 2.

38% DLAs

MgII 2796 vs. MgI2852

W02796/ W0

2600 < 2.

The DLAs occupy a regime where 1 < W02796/W0

2600 < 2 and W02852 > 0.2 Å.

43% DLAs

Upper limits not plotted.

Number of DLAs per unit redshift

nDLA(z) = dn/dz

High z: Prochaska & Herbert-Fort 2004

Low z: Rao, Turnshek, & Nestor 2005

z=0:Ryan-Weber et al. (2005)Zwaan et al. (2005)

No-evolution curve in the “737” cosmology.

h=0.7ΩM=0.3ΩΛ=0.7

n(z) = n0(1+z)

= 1.2

No-evolution curve and power-law fit.

Cosmological Neutral Gas Mass Density in DLAs: ΩDLA(z)

Ωlum(z=0)SDSS LFPanter et al. 2004

Ωg(z=0)HIPASSZwaan et al. 2005

Out with the old, in with the new. Much better.

ΩDLA is constant for 0.5 < z < 4.5.

ΩDLA = (9.7 ± 0.1) x 10-4

Ωgas (z=0) = (4.88 ± 0.56) x 10-4

The HI column density distribution function f(N)

Low z slope = -1.4 ± 0.2

High z slope = -1.8 ± 0.1

z=0 slopes: -1.4 ± 0.2, log N(HI) < 20.9 -2.1 ± 0.9, log N(HI) > 20.9

Now, all three results together:

Simple picture:

High z: higher comoving c.s./volume, lower <N(HI)>Low z: lower comoving c.s./volume, higher <N(HI)> constant mass density

Column densities increase as clouds condense and mergers proceed,and then decrease when star formation depletes gas?

The Star Formation History of Galaxies

Compilation of SFR measurements by A. Hopkins (2004) + parameterization: Hopkins, Rao, &Turnshek 2005 (submitted)

The Star Formation History of DLAs

global Schmidt Law – Kennicutt 1998

* = nDLA ΣSFR = 4.0 x 10-15 nDLA Σgas

dX/dz

.

dX/dz

1.4 Σgas = mH <N(HI)>

dX/dz = (c/H0)(1+z)2/E(z)

E(z)=(ΩM(1+z)3 + ΩΛ)0.5

Hopkins, Rao, & Turnshek 2005

(in units of Msun/yr/Mpc3)

Evolution of the mass density in metals.

63.7Z

Conti et al. 2003

Calura & Matteucci2004

Dunne et al. 2003 (submillimeter)

Rao, Prochaska,

Wolfe, Howk 2005

Mass density in metals derived from the SFR history. . .

(baryon)

Fukugita & Peebles 2004

(DLA)

evolution of stellar mass density derived from SFRs.

(gas) assuming that the total gas+stellar mass density at all epochs equals the z=0 value of (DLA)+(stars).

Stellar mass density and DLA gas mass density

DLAs do not trace the majority of the neutral gas at all epochs - particularly at high redshifts.

1. This can’t be attributed to missed QSOs due to dust obscuration: Ellison et al. radio loud QSOs DLA survey.

2. Contribution from subDLAs? Peroux et al. claim 50% of neutral gas mass at z>3.5 could be from subDLAs; but refuted by Prochaska & Herbert-Fort.

3. Explains disparity in (metals). Low average metallicity + low gas mass density = metal mass density much lower than in luminous galaxies.

4. Very likely that very high column density systems have very low gas cross-sections, and are missed in QSOAL surveys. Evidence for this is shown in the next few slides.

Global Schmidt Law from Kennicutt 1998.

1 Msun/pc2 = 1.2 x 1020 cm-2

Incidence of SFR surface densities from Lanzetta et al. 2002

From high-z DLA f(N) distribution

From galaxies in the HDFs

At high redshift:

DLAs and luminous galaxies are distinct populations.

Column densities up to 4 orders higher are observed in the luminous population. If the highest SFR density objects have gas radii 2 orders smaller than DLAs, they will contribute to dn/dz and Ω.

Integral SFR density in DLAs is higher than integral SFR density in luminous objects!

Luminous galaxy surveys do not include DLAs.DLA surveys do not include luminous galaxies.

However, at low z there is some overlap.

(baryon)Fukugita & Peebles 2004

(DLA)

evolution of stellar mass density derived from SFRs.

(gas) assuming that the total gas+stellar mass density at all epochs equals the z=0 value of (DLA)+(stars).

Stellar mass density and DLA gas mass density

stars gas

Summary

1. 43% of MgII systems with 1 Å <W2796/W2600 < 2 Å and MgI W2852 > 0.2 Å are DLAs. And DLAs are confined to these regimes.

2. dn/dz evolves from high redshift to z=1.5 or 2 and then does not.

3. ΩDLA stays flat from z≈5 to z≈0.5. DLA value of Ω is 2x larger than z=0 value.

4. f(N) changes with redshift: seems to show assembly of high density clouds at z>2 followed by depletion due to star formation.

5. By comparing star formation histories of luminous galaxies and the gas in DLAs, one has to conclude that DLAs do not trace all the neutral gas, particularly at high z, and luminous galaxies do not trace all the star formation at high z.