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

1. 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)

2. Motivations and Astrophysical Context MgII arises in environments ranging over five decades of N(HI) • Damped Lyman-a Absorbers (DLAs): N(HI) > 2x1020 cm-2 eg. Biosse’ etal (1998); Rao & Turnshek (2000); Churchill etal (2000b) • Lyman Limit Systems (LLSs): N(HI) > 2x1017 cm-2 eg. Steidel & Sargent (1992); Churchill etal (2000a) • sub-LLSs: (low redshift forest!) N(HI) < 6x1016 cm-2 eg. Churchill & Le Brun (1998); Churchill etal (1999); Rigby etal (2001) MgII a-process ion – Type II SNe – enrichment from first stars (<1 Myr) FeII iron-group ion – Type Ia SNe – late stellar evolution (>few Gyr) 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

3. Mg II (2796) Transitions with HIRES/Keck

5. “Weak MgII Systems” • Mostly Single Clouds • Wr(MgII)<0.3A (by definition) • Isolated in redshift • Unresolved at 6 km s-1 • Power law equivalent width distribution • down to Wr(MgII)=0.02A n(W) ~ W-1 CWC etal. (1999, ApJS, 120, 51) Steidel & Sargent (1992, ApJS, 80, 1)

6. Weak Systems are Optically Thin, i.e. N(HI)<1016.8 cm-2 A Statistical Accounting dN/dz = the redshift path density dN/dz = the number of absorbers per Dz=1 interval 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! There is a factor of 3 greater MgII systems when weak absorbers are included! ~65% must be optically thin to N(HI) CWC etal. (1999, ApJS, 120, 51)

7. Weak Systems are Optically Thin, i.e. N(HI)<1016.8 cm-2 Confirmed by Direct Measurement of Lyman Limit Breaks in FOS/HST Spectra 10 of 10 strong systems have measured LL break Only 1 of 7 weak systems has measured LL break; (has multiple clouds) Wr(MgII)=0.3 A CWC etal. (2000, ApJS, 130, 190)

8. Ionization Conditions Constrained by Fe II and C IV Variations in Fe II and in C IV indicate wide range of ionization parameters/densities Rigby, Charlton, & CWC (2001, submitted) When both Fe II and C IV are strong, multiphase conditions are suggested, inferred to be due to different densities.

9. Cloud Properties I 1. 2/3 of weak systems are single clouds • comprise physically distinct population • preferred geometry (sheets) or small covering factor 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? 3. Metalicities are 0.1 solar or greater • 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(Lya) arises in broader, higher ionization phase 4. Ionization Parameters: -5 < log U < -2 • often, [a/Fe]>0 ruled out; Type Ia enrichment- advanced SFH • (degeneracy between [a/Fe] and ionization parameter)

10. Cloud Properties II 5. Densities -3.5 < log nH < 0 cm-2 • low ionization, iron-rich clouds are most dense • (degeneracy between UV background normalization and nH) 6. Iron-rich clouds: N(FeII)~N(MgII) • best constrained clouds; log U ~ -4.5 & log nH ~ -1 cm-3 • [a/Fe]~0; dust depletion not significant 7. Lower-iron clouds: N(FeII)/N(MgII) < 0.3 • not as well constrained; -4 < log U < -2 & -3.5 < log nH < -1.5 cm-3 • can have [a/Fe]~+0.5 or dust depletion effects (even with Z>0.1) • those with best FeII limits are significantly different than iron-rich

11. Cloud Properties III 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(Lya), yet no Lyman limit break in 3 systems • required by strong CIV or both strong FeII and CIV in 4 systems

12. Metal Enriched z=1 Lyman-a 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) Using m=dlog(N)/dlog(NHI) for Lya forest, If m=-1.3 single power law, then dN/dz~4 for Lya clouds; SCWS comprise 25% of log N(HI) ~ 16 cm-2forest. 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-2forest. 25-100% of log N(HI) > 15.8 cm-2 of z=1 Lya forest is significantly metal enriched

13. Space Density of Single Cloud Weak Systems Space density given by, (dN/dz) (H0/c) (1+2q0z)0.5 n = pR2Cf (1+z) Consider iron-rich SCWS; dN/dz=0.2, z=1, Cf=1 n = 107 (1 pc / R )2h Mpc-3 R = 10 pc gives n = 105h Mpc-3 Galaxies have n = 0.04 h3 Mpc-3; comparing gives … nFe/ngal = few x 106h-2 Mpc-3

15. .Wr=0.05A .Wr=1.09A No galaxy ID No galaxy ID .Wr=0.02A .Wr=0.29A

16. Finally • high metallicity (Z>0.1); small (d~10 pc); Fe-group enriched • number density ratio to L* galaxies is 106:1 • comprise 25-100% of the Lyman-a forest with N(HI)~16 cm-2 • require in-situ star formation and Type Ia enrichment (t>1Gyr) • multiphase ionization conditions (dwarf galaxy potential wells) • velocity widths suggestive of star clusters / SNe shell fragments • 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. Additional References: Thank you! • 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)