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Millions of Tiny, Weak Mg II Absorbers: What are They?

Chris Churchill(Penn State)

Jane Rigby (Steward); Jane Charlton (PSU)

Churchill, Rigby, Charlton, & Vogt (1999, ApJS, 120, 51)Rigby, Charlton, & Churchill (2001, ApJ, ready to submit)

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Motivations and Astrophysical Context

Mg II arises in environments ranging over five decades of N(H I)

• Damped Lyman-Absorbers (DLAs): N(HI) > 2x1020 cm-2

• Lyman Limit Systems (LLSs): N(HI) > 2x1017 cm-2

• sub-LLSs: (low redshift forest!) N(HI) < 6x1016 cm-2

eg. Biosse’ etal (1998); Rao & Turnshek (2000); Churchill etal (2000b)

eg. Steidel & Sargent (1992); Churchill etal (2000a)

eg. Churchill & Le Brun (1998); Churchill etal (1999); Rigby etal (2001)

Mg II selection probes a wide range of astrophysical sites where star formation has enriched gas; these sites can be traced from redshift 0 to 5

MgII -process ion – Type II SNe – enrichment from first stars (<1 Myr)

FeII iron-group ion – Type Ia SNe – late stellar evolution (>few Gyr)

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Mg II (2796) Transitions with HIRES/Keck

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n(W) ~ W-1

“Weak Mg II Systems”

• Mostly Single Clouds• Wr(MgII)<0.3by definition)• Isolated in redshift• Unresolved at 6 km s-1

• Power law equivalent width distribution down to Wr(MgII)=0.02

CWC etal. (1999, ApJS, 120, 51)Steidel & Sargent (1992, ApJS, 80, 1)

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Weak Systems are Optically Thin, i.e. N(HI)<1016.8 cm-2

The redshift path density of Lyman limit systems is equal to that of “strong” Mg II systems. If you find/count any more, they cannot be Lyman limit systems!

dN/dz = the redshift path densitydN/dz = the number of absorbers per z=1 interval

CWC etal. (1999, ApJS, 120, 51)

A Statistical Accounting

There is a factor of 3 greater MgII systems when weak absorbers are included!

~65% must be optically thin to N(HI)

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Wr(MgII)=0.3

CWC etal. (2000, ApJS, 130, 190)

10 of 10 strong systems have measured LL break

Only 1 of 7 weak systems has measured LL break;(has multiple clouds)

Confirmed by Direct Measurement of Lyman Limit Breaks in FOS/HST Spectra

Weak Systems are Optically Thin, i.e. N(HI)<1016.8 cm-2

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Variations in Fe II and in C IVindicate wide range of ionizationparameters/densities

When both Fe II and C IV arestrong, multiphase conditions aresuggested, inferred to be due todifferent densities.

Ionization Conditions Constrained by Fe II and C IV

Rigby, Charlton, & CWC (2001, submitted)

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1. 2/3 of weak systems are single clouds• comprise physically distinct population• preferred geometry (sheets) or small covering factor

4. Ionization Parameters: -5 < log U < -2

3. Metalicities are 0.1 solar or greater

2. Unresolved line widths at resolution 6 km s-1

• range of Doppler b parameters is 2-7 km s-1

• few systems- the profile is slightly asymmetric: non-thermal?

• no system had Mg II phase with less than 0.1 solar metalicity• no Lyman limit breaks; log N(HI)<16.8 cm-2

• When large, W(Ly) arises in broader, higher ionization phase

• often, [/Fe]>0 ruled out; Type Ia enrichment- advanced SFH• (degeneracy between [/Fe] and ionization parameter)

Cloud Properties I

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5. Densities -3.5 < log nH < 0 cm-2

• low ionization, iron-rich clouds are most dense• (degeneracy between UV background normalization and nH)

7. Lower-iron clouds: N(FeII)/N(MgII) < 0.3

6. Iron-rich clouds: N(FeII)~N(MgII)• best constrained clouds; log U ~ -4.5 & log nH ~ -1 cm-3

• [/Fe]~0; dust depletion not significant

• not as well constrained; -4 < log U < -2 & -3.5 < log nH < -1.5 cm-3

• can have [/Fe]~+0.5 or dust depletion effects (even with Z>0.1) • those with best FeII limits are significantly different than iron-rich

Cloud Properties II

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8. Cloud Sizes• NH/nH provides size estimate (factor of 2)• Iron-rich clouds well constrained to have D = 10 pc• Lower-iron clouds have 10 pc < D < few kpc

9. Multiple Ionization Phases• 7/15 systems require multi-phase ionization conditions• required by large W(Ly), yet no Lyman limit break in 3 systems• required by strong CIV or both strong FeII and CIV in 4 systems

Cloud Properties III

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Metal Enriched z=1 Lyman- Forest Clouds

Single-cloud, weak systems (SCWS) have: dN/dz=1.1 at z~1 log N(HI) > 15.8 cm-2 (Z>0.1 solar) log N(HI) < 16.8 cm-2 (no Lyman limit breaks)

If m=-1.3 single power law, then dN/dz~4 for Ly clouds; SCWS comprise 25% of log N(HI) ~ 16 cm-2 forest.

25-100% of log N(HI) > 15.8 cm-2 of z=1 Ly forest is significantly metal enriched

Using m=dlog(N)/dlog(NHI) for Ly forest,

If m<16=-1.8 and m>16=-0.6 broken power law, then dN/dz~1; SCWS comprise 100% of log N(HI) ~ 16 cm-2 forest.

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Space Density of Single Cloud Weak Systems

Space density given by,

nFe/ngal = few x 106 h-2 Mpc-3

(dN/dz) (H0/c) (1+2q0z)0.5

R2Cf (1+z)n =

Consider iron-rich SCWS; dN/dz=0.2, z=1, Cf=1

n = 107 (1 pc / R )2 h Mpc-3

R = 10 pc gives n = 105 h Mpc-3

Galaxies have n = 0.04 h3 Mpc-3; comparing gives …

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.Wr=0.29A

.Wr=0.05A

.Wr=1.09A

.Wr=0.02A

No galaxy ID

No galaxy ID

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• Churchill etal. (1999, ApJS, 120, 51)• Churchill etal. (2000, ApJS, 130, 91)• Churchill etal. (2000, ApJ, 547, 577)• Churchill & Vogt (2001, ApJ, 560, in press; astro-ph/0105044)• Charlton, Ding, Zonak, & Churchill (2001, ApJ, submitted)• Rigby, Charlton, & Churchill (2001, ApJ, submitted)

Thank you!Additional References:

Finally

1. high metallicity (Z>0.1); small (d~10 pc); Fe-group enriched2. number density ratio to L* galaxies is 106:13. comprise 25-100% of the Lyman- forest with N(HI)~16 cm-2

4. require in-situ star formation and Type Ia enrichment (>1Gyr)

5. multiphase ionization conditions (dwarf galaxy potential wells)6. velocity widths suggestive of star clusters / SNe shell fragments7. not closely associated with bright galaxies (D>50 kpc)

Arise in Population III star clusters or shell fragments of SNe in dwarf galaxies?

Could trace elusive small-mass, dark-matter halos predicted by simulations.