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The Evolution of Damped Ly- a Absorbers: Element Abundances and Star Formation Rates

The Evolution of Damped Ly- a Absorbers: Element Abundances and Star Formation Rates. Varsha P. Kulkarni (Univ. of South Carolina, U.S.A.) Collaborators Donald G. York (Univ. of Chicago, U.S.A.) James T. Lauroesch (Northwestern Univ.,U.S.A. )

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The Evolution of Damped Ly- a Absorbers: Element Abundances and Star Formation Rates

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  1. The Evolution of Damped Ly-a Absorbers: Element Abundances and Star Formation Rates Varsha P. Kulkarni (Univ. of South Carolina, U.S.A.) Collaborators Donald G. York (Univ. of Chicago, U.S.A.) James T. Lauroesch (Northwestern Univ.,U.S.A. ) S. Michael Fall (Space Telescope Science Inst., U.S.A.) Pushpa Khare (Utkal Univ., India) Bruce E. Woodgate (NASA/GSFC, U.S.A.) Povilas Palunas (Univ. of Texas, U.S.A.) Joseph Meiring & Soheila Gharanfoli (Univ. of South Carolina, U.S.A.) Daniel E. Welty & James W. Truran (Univ. of Chicago, U.S.A.) ACKNOWLEDGMENTS NASA-STScI, National Science Foundation, Univ. of South Carolina Research Foundation

  2. DLAs as Probes of Metal Enrichment and Star Formation in Galaxies •Quasar absorbers probe galaxies at various cosmic epochs, selected independent of galaxy luminosities. •Damped Ly-alpha absorbers (DLAs) and sub-DLAs contribute a large fraction of H I in galaxies, and are best existing probes of element abundances in distant galaxies over ~90% of cosmic history. • So expected to shed light on the history of metal production and star formation in galaxies.

  3. Evolution of Metallicity • Most cosmic chemical evolution models predict rise in global mean interstellar metallicity of galaxies with time, from low values at high z to near-solar values at z=0 (e.g. Pei & Fall 1995; Malaney & Chaboyer 1996; Pei, Fall, & Hauser 1999; Somerville et al. 2001). • Mass-weighted mean Z of nearby galaxies is indeed near-solar (e.g. Kulkarni & Fall 2002). • Do DLA data show rise of global metallicity with time progressing to ~solar value at z~0?• Global Metallicity: N(HI)-weighted mean metallicity Z/Z = [S N(ZnII)i /SN(HI)i] / (Zn/H) • We take Zn as our primary metallicity indicator· Undepleted in Galactic ISM · [Zn/Fe] ~0 for –3 < [Fe/H] < 0 in Galactic stars· Lines often unsaturated, so reliable column density· Lines often outside the Lyman-alpha forest

  4. Low-z End of Metallicity-Redshift Relation (Kulkarni, Fall, Lauroesch, York, Welty, Khare, & Truran 2005) · Low-z end important to clarify shape of Z(z) relation and to understand connection of DLAs to present-day galaxies. · Zn II 2026, 2062 lie in UV for z < 0.6. So need space instruments · HST cycle 11 STIS survey: 4 DLAs, 0.09 < z < 0.52

  5. HST STIS Observations • STIS spectra with near-UV MAMA or CCD • 5 to 10 orbits per DLA to study Zn II, Cr II

  6. Abundances in Low-z DLAs

  7. Abundances at Intermediate Redshifts (Khare, Kulkarni, Lauroesch, York, Crotts, & Nakamura 2004) • MMT Blue Channel spectra for 8 DLAs or strong DLA candidates at 0.6 < z < 1.5 discovered with HST or SDSS • Spectral resolution ~75 km/s, but high S/N enables detection of Zn, Cr, Fe, Mn, Ni, Si, etc. •Together with HST data, doubled the z < 1.5 DLA Zn sample, tripled the z < 1 sample.

  8. DLA Abundances at 0.6 < z < 1.5 a: Rao & Turnshek 2000; b: Rao et al. 2005, in preparation c: Turnshek et al. 2004.

  9. Abundances at 0.6 < z < 1.5 a: Rao & Turnshek 2000; b: Rao et al. 2005, in preparation c: HST GO project 9382, PI Rao; d: Turnshek et al. 2004; e: Estimate from reddening

  10. Global Metallicity Evolution of DLAs 87 DLAs at 0.1 < z < 3.9 including our HST+MMT data, and the literature. Max. Limits: Zn limits treated as detections. Min. Limits: Zn limits treated as zeros or replaced with values inferred from other elements.

  11. Most DLAs at z < 1.5 appear to have low metallicities. • Global mean metallicity of DLAs seems to evolve slowly at best, at a rate of <~ 0.2 dex per unit redshift.

  12. Star Formation Rates of Quasar Absorbers(Kulkarni, Woodgate, York, Meiring, Thatte, Palunas, & Wassell 2005) • How does star formation history of absorbers compare with the global star formation history inferred from galaxy imaging studies? • Emission-line searches (e.g., Ly-a, H-a etc.) offer direct probes of star formation, if dust extinction is not significant. • Most attempts to detect continuum or line emission from intervening (zabs << zem) galaxies at z >~1.5 have failed. Our Goal: To increase the sample of star formation rate (SFR) measurements in heavy-element quasar absorbers. Our Technique: Ly-a search with narrow-band Fabry-Perot (FP) imaging. Tuning the FP to different wavelengths allows sampling of various redshifts. Sample Selection: Targets with well-detected systems at 2.3 < zabs < zem-0.6 from York et al. (1991) quasar absorber catalog, with mixed ionization (Si II, Al II, or O I with CIV and/or Si IV)

  13. Observations • NASA/GSFC Fabry Perot Imager at APO 3.5 m telescope. • Blue and “Vis-broad” etalons with various blocking filters to cover ~4000-4350 A. FWHM ~6-15 A. Automatic temperature control program kept wavelength settings fixed. Calibrated with emission lines from Ar/Kr lamps. • Nine observing runs during 10/2000-5/2004. • Total NB integrations of 24,000-43,200 s per field. (Among deepest existing NB images of quasar absorbers.) • Broad-band B images to sample rest-frame UV continuum emission near Ly-a. • Field of view ~ 3.5’ diameter around the quasar (~2.3 Mpc2 at z=2.4 for H0 = 70, Wm=0.27, WL=0.73)

  14. Data Reduction • Data reduced with standard IRAF packages. • Empirical extinction corrections made using relative photometry of stars in a given field in images taken on different nights. • Multiple images of same field registered and combined. • Effects of seeing differences between continuum (B) or NB images reduced by convolving one with a Gaussian of same FWHM as the other. • B images subtracted from NB images after alignment and appropriate scaling using interactive IDL program IDP3. • Photometry done with IDP3 and IRAF.

  15. Some Example Images Q0216: B Q0216: NB Q2233: B Q2233: NB

  16. Results of Ly-a Imaging Search • No significant Ly-a emission detected in any of our fields! • Not inconsistent with Ly-a emitter (LAE) density seen in other LAE searches of much larger field of view and redshift depth (e.g., Stiavelli et al. 2001; Palunas et al. 2004). • 3 s Ly-a flux limits of 4x10-18 to 1x10-16 erg s-1 cm-2  3 s SFR limits of 0.2-4.3 M yr-1

  17. The Star Formation History of Quasar Absorbers SFR estimates from emission-line searches for absorbers from our study and literature. LD5: Expected mean SFR for DLAs in large-disk scenario for closed-box model (Bunker et al. 1999) H5: Expected mean SFR for DLAs in hierarchical scenario (Bunker et al. 1999) Our limits are among the tightest existing limits for quasar absorbers. SFRs of most heavy-element absorbers lie below the large-disk prediction; many lie even below the hierarchical prediction.

  18. CONCLUSIONS & FUTURE WORK • Most DLAs at z < 1.5 appear to be metal-poor (~10-20% solar). Global mean metallicity of DLAs seems to evolve slowly at best, in contrast with predictions based on cosmic chemical evolution models and global star formation history from galaxy imaging surveys. • SFRs in majority of heavy-element quasar absorbers may be far below the global SFR inferred from emission-based galaxy surveys! • Overall, DLAs may have had a very different enrichment and star formation history than the general galaxy population. ---Small number statistics? ---Or is dust selection bias important? • ---Or are star formation regions compact (and lost in quasar PSF)? • Need abundance studies of larger z < 1.5 samples—In progress with MMT, VLT. Preliminary new MMT results agree with results so far. • Also need high-resolution imaging studies with redshift verification for more quasar absorbers.

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