Nucleosynthetic signatures of early stars in IGM and DLA absorption lines

1. Nucleosynthetic signatures of early stars in IGM and DLA absorption lines Bryan E. Penprase1, Wallace L. W. Sargent2, Jason X. Prochaska3 , and Catherine Wilka1 Abstract Using the Keck and ESI spectrograph we have observed a sample of 33 low‐metallicity DLA systems in which we detect low metallicity ([X/H] < ‐2.4) and for which patterns of abundances such as [C/O] enhancement at low metallicity and [Alpha/Fe‐peak] ratios are consistent with intermediate‐mass nucleosynthesis from Population III stars. The sample also indicates a "floor" of low metallicity at [X/H] > ‐3.2, suggesting a prompt enrichment of the IGM, and we estimate that systems with lower metallicity would be extremely rare. The low-Z DLA observations made with Keck/Echellete Spectrograph and Imager during observing runs in 2007 and 2008, and have been published in the recent paper entitled “Keck Echellette Spectrograph and Imager Observations of Metal-poor Damped Lyα Systems” (Astrophysical Journal, 721, 1 (2010)). Further low-Z DLA observations using the Keck HIRES spectrograph are scheduled for October 2011, and will help provide more accurate constraints on the metallicities obtained from the ESI spectrograph. New results from Cooke at al (2011a, 2011b) with the Keck/HIRES and VLT/UVES spectrographs have shown similar trends in abundance ratios for such low-metallicity DLA systems, and such observations help constrain nucleosynthesis and stellar populations at high redshift. A second probe of the enrichment and photoionization of the IGM can be found from detections of high ionization species in the Lyman- forest such as OVI, CIV, and SiIV. To this end, we have obtained very high S/N spectra of 12 quasars using the Keck telescope and HIRES spectrograph. For these spectra we have analyzed over 120 CIV absorption systems, for which we have obtained 83 detections of OVI absorption. With Voigt profile fitting to the components, we provide summaries of the ratios of CIV/OVI, OVI columns and b-values within this sample, and plots of these quantities with redshift. Our results will be interpreted using photoionization models to help constrain the density and ionizing radiation of the OVI, and whether these systems are part of a filamentary system of IGM or associated with galaxy halos. Discussion The lowest abundances of our DLA sample enable a comparison with stellar samples, and provide a complementary probe of important nucleosynthetic quantities such as C/O and Si/O in the intergalactic medium. Our enhanced abundances of [C/O] and enhanced ratios of α-peak elements over Fe-peak elements are consistent with production from massive stars with m ≥ 100 M which release generous amounts of C and O in the pre-supernova phase, which could explain the α-process overabundance with respect to Fe-peak elements. The upward trend of C/O can be explained with a top-heavy IMF of Population III stars, with Mlow > ~ 10 M, as tested to fit the C and O yields by Chieffi and Limongi (2002) for metallicities in the range 0 ≤ Z ≤ 10-5. Figure 4 compares our observed abundances of other elements with nucleosynthesis models from Heger and Woosley (2002), who used massive stars with helium cores of 65-130 M integrated over an initial mass function. Our results show an underabundance of O, Si, and Fe and an overabundance of Al, compared to the predicted yields of C. In the near future we hope to compare our results with other nucleosynthesis models, like those by Chieffi & Limongi (2002), and test different initial mass function parameters to achieve yields that more closely match our data. Results Figure 1: Two representative low-metallicity spectra from our sample. (left) Sections of our spectrum SDSS0955+4116 showing the lines of C II λ1334, Si II λ1304, and OI λ1302 for the DLA system at z = 3.28102, the HI damped Lyα profile at the same redshift, and close-ups of individual line profiles. (right) Same type of plot for the SDSS1001+0343 DLA system at z = 3.07867, showing the HI profile and sections of the quasar spectrum with the lines of CII, SiII OI indicated. Introduction Damped Lyman- systems (DLAs) are massive clouds of hydrogen-rich gas (log N (H I) > 2 x 1020 cm-2) that can be observed as absorption in the spectra of background quasars and can trace the chemical evolution of galaxies in the early epochs of the universe. DLAs comprise the majority of neutral hydrogen available for star formation in the universe up to z ≥ 4 (Storrie-Lombardi & Wolfe, 2000; Peroux et al, 2003) and are thought to be the progenitors of modern galaxies. Recent studies have revealed that the abundance patterns of Z ~ 1/10 – 1/300 Z DLAs resemble those of Milky Way metal-poor halo stars (e.g., Pettini et al, 1997; Prochaska & Wolfe, 1999), which supports the hypothesis that at least some DLAs have a similar evolution history as our own Galaxy (Lopez et al, 2002). While observations of the metal-poor halo stars have been useful for comparisons with nucleosynthesis models, it is unclear to what extent mixing and convection have altered the chemical compositions of the stars, and the exact effects of gas-dust separation and nearby companion stars are as yet unknown. Low-metallicity DLAs are therefore of particular importance because they provide a look at the relatively unpolluted reserve from which the short-lived and high-mass first stars, Population III stars, formed. Surveys (Sargent et al 1989; Bechtold et al 1984, 1994; Prochaska et al, 2005, 2007) have brought the number of DLAs up to the hundreds, and the larger among these samples have revealed a modest evolution in metallicity with time (Prochaska et al, 2003). Metal-poor DLAs are intriguing because they provide information on the conditions in which the first stars and galaxies formed. We present results from the thorough spectral analysis of several DLAs in the lines of sight of 33 optically bright quasars at redshifts 2 < z < 4 selected from the Sloan Digital Sky Survey for their low metallicity potential. The observations were made with Keck/Echellete Spectrograph and Imager during observing runs in 2007 and 2008, and have been published in the recent paper entitled “Keck Echellette Spectrograph and Imager Observations of Metal-poor Damped Lyα Systems” (Astrophysical Journal, 721, 1 (2010)). To gather a large sample of low-metallicity DLAs, the Sloan Digital Sky Survey's (SDSS) fifth data release (DR5), which contains 77,229 newly detected quasars (Schneider, et al, 2007) was examined by JXP in order to produce a list of quasars with strong damped Lyman- absorption and weak absorption from the CII line at 1334 Å and the SiII line at 1260 Å. These quasars were then observed with the Keck ESI echellete spectrograph, and a thorough analysis provided accurate measurements of the column densities of CII, OI, SiII and other species, and their abundances relative to solar. Higher spectral resolution follow-up observations with the HIRES spectrograph are ongoing. IGM observations of OVI, CIV, and SiIV The enrichment of the IGM by outflows and supernovae, and photoionization of the IGM by quasars and high mass stars can be detected from highly ionized IGM absorbers. A sample of 12 quasars were observed at high S/N with the Keck telescope and HIRES spectra, and over 120 CIV absorption systems were seen, for which 83 detections of OVI have been observed. These data are being compared with the survey of OVI from Fox et al (2011), and from low-redshift observations of OVI described by Prochaska et al (2011) and Tumlinson et al (2011). Our preliminary results show most b values in the range of 10 < b < 40 km/s and column densities of OVI ranging from 13 < log(N(OVI)) < 15 cm-2. We have fitted the CIV, OVI, SiIV, CII, SiII, and HI absorption systems using Voigt profile models, and will comparing the observed ratios to results from photoionization models. Table 2:For each element, we measured the column density of the species, and then adjusted for solar abundances using recently recalibrated values (Lodders, 2003), after combining with the measured HI column density. The results in Table 2 then are [X/H], or abundance of element relative to solar. Figure 5:Histograms of b-values and column densities for OVI, from our sample of CIV systems observed with HIRES. Our range of values agree well with those reported in Fox, et al (2011). Data analysis Observations of a sample of 10 quasars selected for low metallicity were taken at the Keck II telescope on 16 March 2007, and were supplemented with additional observations at the Keck II telescope during three nights of 7-9 May 2008. The ESI was used in echellette mode, which provides a free spectral range of 3900 Å to 10900 Å at a dispersion ranging from 0.16 Å pixel-1 to 0.30 Å pixel-1, corresponding to a constant velocity dispersion of 11.5 km s-1 pixel-1. The observed sample of quasars, their redshifts, apparent magnitudes, signal to noise ratios and other quantities are presented below in Table 1. For each of the quasars, the HI profile was fit using XIDL and IDL routines, and the damping wings of the HI profiles were visually compared to the observed continuum in the quasar spectrum. Column densities of a wide range of species derived using both the weak line limit and apparent optical depth (AOD) technique (Savage and Sembach, 1991). In some cases a local continuum fit was performed during the AOD column density measurement to improve accuracy. Where multiple transitions existed, the transitions were combined with a weighted average that created a single optical depth profile for which the column density can be derived. Spectral lines from transitions of the elements C, O, Si, Al, Fe, Mg, Mn, and S, were observed and averaged to provide a mean column density, which was converted to logarithmic abundances using the solar abundances of Lodders (2003). While the low resolution of the Keck ESI spectrograph limits the precision of these observations, and in particular is vulnerable to saturation effects, our results agree well the those from the Keck/HIRES and VLT/UVES spectrographs recently published by Cooke, Pettini, et al (2011a, 2011b). In particular the enhancement of [C/O] at low metallicity is observed in both our study and that of Cooke, et al, and the pattern of abundances [X/H] from these studies will be helpful in constraining nucleosynthesis at high redshifts. Table 1 – Quasars Observed with the Keck ESI Spectrograph Figure 6:Trends in ratios of CIV and OVI against CIV column density (left) and reshift (right). Additional plots of CII/CIV, and SiII/SiIV will be analyzed to help constrain the photoionization and enrichment processes of the IGM. References Adelman-McCarthy, J. et al. 2007, ApJS, 172, 634. Akerman, C.J., Carigi, L., Nissen, P.E., Pettini, M. & Asplund, M. 2004, A&A, 414, 931. Aoki, W., et al. 2006, Proceedings of Science, Chemical abundance patterns of extremely metal-poor stars. Bechtold, J. 1994, ApJS, 91, 1. Bechtold, J., et al. 1984, ApJ, 281, 76. Chieffi, A. & Limongi, M. 2002, ApJ, 577, 281 Cohen, J., et al, 2007, ApJ, 659, L161. Cooke, R., Pettini, M., Steidel, C.C., Rudie, G.C., & Jorgenson, R.A., 2011a, R.A., MNRAS, 412, 1047. Cooke, R., Pettini, M., Steidel, C.C., Rudie, G.C., & Nissen, P.E., 2011b, arXiv 1106-2805. Fenner, Y., Prochaska, J.X. & Gibson, B.K. 2004, ApJ, 606, 116. Frebel, A., et al. 2007, ApJ, 658, 534. Fox, A., 2011, ApJ, 730, 58. Gawiser, E., Wolfe, A.M., Prochaska, J.X., Lanzetta, K.M., Yahata, N. & Quirrenbach, A. 2001, ApJ, 562, 628. Heger, A. & Woosley, S.E. 2002, ApJ, 567, pp. 532. van der Hoek, L.B. & Groenewegen, M.A.T. 1997, A&A 123, 305. Lodders, K. 2003, ApJ, 591, 1220. Lopez, S., Reimers, D., D'Odorico, S. & Prochaska, J.X. 2002, A&A, 385, 778. Péroux, C., et al., 2003, Mem.Soc. Astro. Ital. Supp, 3,261. Pettini, M., Smith, L.J., King, D.L. & Hunstead, R.W. 1997, ApJ, 486, 665. Prochaska, J.X., Gawiser, E., Wolfe, A.M., Castro, S. & Djorgovski, S.G. 2003, ApJ, 595, L9. Prochaska, J.X., Herbert-Fort, S. & Wolfe, A.M. 2005, ApJ, 635, 23. Prochaska, J.X. & Wolfe, A.M. 1999, ApJS, 121, 369. Prochaska, J.X., Wolfe, A.M., Howk, J.C., Gawiser, E., Burles, S.M. & Cooke, J. 2007, ApJS, 171, 29. Prochaska, J.X., et al., 2011, arXiv 1103.1891. Rodríguez, E., Petitjean, P., Aracil, B., Ledoux, C. & Srianand, R. 2006, A&A, 446, pp. 791. Sargent, W.L.W., Steidel, C.C. & Boksenberg, A. 1989, ApJS, 69, 703. Savage, B.D. & Sembach, K.R. 1991, ApJ, 379, 245. Storrie-Lombardi, L.J. & Wolfe, A.M. 2000, ApJ, 543, 552. Tumlinson, J. et al, 2011, arXiv 1103.5252. Wolfe, A.M., Turnshek, D.A., Smith, H.E. & Cohen, R.D. 1986, ApJS, 61, 249. Woosley, S.E. & Heger, A. 2007, Physics Reports, 442, 269. Woosley, S.E., Heger, A. & Weaver, T. A. 2002, APS, Reviews of Modern Physics, 74, issue 4, pp. 1015-1071. Figure 3:[C/O] vs. [O/H] for lines with small equivalent widths with equivalent width less than 130 mÅ for both CII and OI. The upward trend in [C/O] is seen even after conservative estimates of saturation effects. Stellar measurements of [C/O] (squares and triangles) from Akerman et al. (2004) are also plotted for comparison. Figure 4: Plot of elemental abundances for the DLA systems against element number, with open squares representing individual DLA systems, and the mean and standard deviation of the sample of four DLAs shown by the filled squares and error bars. The nucleosynthetic yields of Heger & Woosley (2002) for the various elements are shown with triangles. We observe reduced abundances of O, Si, and Fe relative to C compared to the nucleosynthetic model, and a slight enhancement of Al relative to C. Acknowledgements: We would like to thank funding sources which include Pomona College, who funded travel to Keck via internal travel grants, and to thank Wallace Sargent, who allowed us to participate in the Keck Observatory runs of March and April 2007. Finally, this project would not have been possible without the support of Pomona College SURP grants to Irene Toro Martinez, Catherine Wilka, and additional support from Pomona College Research funds. Figure 2:Plot of DLA element abundance relative to solar ([α/H]) vs. redshift for our sample (symbols with error bars), compared with that of Prochaska et al. (2003b) (plus signs) for the α-process elements (top). The α-process elements presented include [O/H] (triangles) and [Si/H] (diamonds). The lower panel depicts the element abundance for Fe-peak elements. The Fe-peak elements presented include [Fe/H] (squares) and [Al/H] (×’s). 1Pomona College, 2California Institute of Technology, 3University of California, Santa Cruz