1. IAP XVII1 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. IAP XVII2 Motivations and Astrophysical Context Mg II arises in environments ranging over five decades of N(H I ) Damped Lyman-  Absorbers (DLAs): N(HI) > 2 x 10 20 cm -2 Lyman Limit Systems (LLSs): N(HI) > 2 x 10 17 cm -2 sub-LLSs: (low redshift forest!) N(HI) < 6 x 10 16 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)

3. IAP XVII3 Mg II (2796) Transitions with HIRES/Keck

4. IAP XVII4

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

6. IAP XVII6 Weak Systems are Optically Thin, i.e. N(HI)<10 16.8 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 density dN/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)

7. IAP XVII7 W r (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)<10 16.8 cm -2

8. IAP XVII8 Variations in Fe II and in C IV indicate wide range of ionization parameters/densities When both Fe II and C IV are strong, multiphase conditions are suggested, inferred to be due to different densities. Ionization Conditions Constrained by Fe II and C IV Rigby, Charlton, & CWC (2001, submitted)

9. IAP XVII9 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

10. IAP XVII10 5. Densities -3.5 < log n H < 0 cm -2 low ionization, iron-rich clouds are most dense (degeneracy between UV background normalization and n H ) 7. Lower-iron clouds: N(Fe II )/N(Mg II ) < 0.3 6. Iron-rich clouds: N(Fe II )~N(Mg II ) best constrained clouds; log U ~ -4.5 & log n H ~ -1 cm -3 [  /Fe]~0; dust depletion not significant not as well constrained; -4 < log U < -2 & -3.5 < log n H < -1.5 cm -3 can have [  /Fe]~+0.5 or dust depletion effects (even with Z>0.1) those with best Fe II limits are significantly different than iron-rich Cloud Properties II

11. IAP XVII11 8. Cloud Sizes N H /n H 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 C IV or both strong Fe II and C IV in 4 systems Cloud Properties III

12. IAP XVII12 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(N HI ) for Ly  forest, If m 16 =-0.6 broken power law, then dN/dz~1; SCWS comprise 100% of log N(HI) ~ 16 cm -2 forest.

13. IAP XVII13 Space Density of Single Cloud Weak Systems Space density given by, n Fe /n gal = few x 10 6 h -2 Mpc -3 (dN/dz) (H 0 /c) (1+2q 0 z) 0.5  R 2 C f (1+z) n = Consider iron-rich SCWS; dN/dz=0.2, z=1, C f =1 n = 10 7 (1 pc / R ) 2 h Mpc -3 R = 10 pc gives n = 10 5 h Mpc -3 Galaxies have n = 0.04 h 3 Mpc -3 ; comparing gives …

14. IAP XVII14

15. IAP XVII15.W r =0.29A.W r =0.05A.W r =1.09A.W r =0.02A No galaxy ID

16. IAP XVII16 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 enriched 2.number density ratio to L * galaxies is 10 6 :1 3.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 fragments 7.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.