DLA Surveys and Stats
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Transcript of DLA Surveys and Stats
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.