We present the first ever detection of Cobalt in a Damped Lyman Alpha system
(DLA) at z = 1.92. In addition to providing important clues to the star
formation history of these high redshift galaxies, we discuss how studying the
Co abundance in DLAs may also help to constrain models of stellar
nucleosynthesis in a regime not probed by Galactic stars.Comment: 4 pages, to appear in the proceedings of `New Quests in Stellar
  Astrophysics: The Link Between Stars and Cosmology', eds. M. Chavez, A.
  Bressan, A. Buzzoni, D. Mayy

Ballester

Cardon

Centurión

Chen

Ellison

Fedchak

Federman

Gratton

Howk

Izotov

J.X. Prochaska

Kobulnicky

Lopez

McWilliam

Mullman

Nakamura

Nitz

Norris

Pettini

Pickering

Prantzos

Prochaska

Rich

Ryan

S.G. Ryan

S.L. Ellison

Savage

Savaglio

Tinsley

Viegas

Vladilo

English

arXiv

Ellison, S.L.

Ryan, Sean G.

Prochaska, J.X.

University of Hertfordshire Research Archive

arXiv:astro-ph/0104301v1  18 Apr 2001The First Detection of Cobalt in a Damped LymanAlpha System.Sara L. EllisonEuropean Southern Observatory, ChileSean G. RyanOpen University, UKJason X. ProchaskaCarnegie Observatories, USAApril 12, 2001Abstract. We present the first ever detection of Cobalt in a Damped LymanAlpha system (DLA) at z ∼ 2. In addition to providing important clues to the starformation history of these high redshift galaxies, we discuss how studying the Coabundance in DLAs may also help to constrain models of stellar nucleosynthesis ina regime not probed by Galactic stars.Keywords: Quasar absorption lines, ISM, abundances1. IntroductionThe study of elemental abundances in Damped Lyman Alpha systems(DLAs) at high redshift represents one of our best opportunities toprobe galaxy formation and chemical evolution at early times. Bycoupling measurements made in high z DLAs with our knowledge ofabundances determined locally and with nucleosynthetic models, wecan start to piece together the star formation histories of these galaxies.To this end, certain key elements are generally targeted, for exampleZn and Cr are traditionally used as a measure of the overall metallicityand gas-to-dust ratio in DLAs, whereas Si and S are used to determinewhether there is any evidence for α element overabundances, indicativeof SN Type II enrichment. Here, we propose that Co, which has neverbefore been featured in DLA studies, yet is well-studied in Galacticstars, may be an additional element that can provide useful clues tothe nature of star formation at high redshift.2. Why Study Cobalt?The intriguing trends of [Co/Fe] as a function of [Fe/H] in the variousGalactic stellar populations indicate that cobalt may be an elementsellison.tex; 1/08/2007; 13:21; p.12 Sara L. Ellison et al.that can provide important clues to chemical evolution, see Figure1. In sub-solar metallicity thin disk stars, Co is found to be slightlyunderabundant with respect to Fe ([Co/Fe] ∼ −0.1, Gratton and Sne-den 1991). This trend continues in moderately metal-poor halo starsdown to metallicities of [Fe/H] ∼ −2.5. For more metal-poor halo stars,a considerable overabundance of Co with respect to Fe is observed,0 < [Co/Fe] < 0.8, (McWilliam et al. 1995; Ryan, Norris and Beers1996). Interestingly, this coincides with a downturn in relative Cr toFe abundances, such that Co and Cr are closely anti-correlated in themetal poor population. Similar [Co/Fe] overabundances are observed inthe bulge ([Co/Fe] ∼ +0.3, McWilliam & Rich 1994), which, we recall,is as old as the halo, and to a lesser degree in the thick disk ([Co/Fe]∼ +0.1, Prochaska et al. 2000).The detection of Co II transitions is a challenging possibility for ISMabsorption line spectroscopy, both locally and at high z. Although thereare numerous transitions that lie at convenient rest wavelengths forboth local and high redshift studies, only two interstellar measurementscurrently exist (e.g. Mullman et al 1998). One of the advantages ofmoving to higher redshifts is that the relatively strong UV lines becomeshifted into the optical where one has access to more efficient detectorsand larger telescopes. In addition, the observed equivalent width isincreased by a factor of (z + 1). However, the fundamental problemremains that the Co II lines are all intrinsically weak and that thesolar abundance of Co is 8× 10−8 that of hydrogen.3. Cobalt in DLAsIn order to maximise the likelihood of detecting Co, we have selectedtwo DLAs with high N(Fe) that lie in front of relatively bright back-ground QSOs. The former criterion is to enable us to more easily reachour target detection limit of [Co/Fe] ≤ 0, which will be sufficient todistinguish between most of the populations plotted in Figure 1. Se-lecting bright QSOs obviously allows us to reach the requisite S/N infeasible amounts of time. The two systems chosen for this pilot studyare Q2206−199 (B = 17.8, zem = 2.559, zabs = 1.92) and Q1223+17(B = 18.5, zem = 2.936, zabs = 2.466). Observations were obtainedwith UVES (on the VLT) and HIRES (on Keck) the details of which(along with data reduction steps and complete abundance analysis) aredescribed in full by Ellison, Ryan and Prochaska (2001). Here, we limitourselves to reporting the final determined Co abundances which are[Co/H] = −0.51 for Q2206−199 and [Co/H] < −1.76 for Q1223+17.sellison.tex; 1/08/2007; 13:21; p.2Cobalt in DLAs. 3Figure 1. Relative cobalt abundances as a function of metallicity for different Galac-tic populations and DLAs: open stars — metal-poor halo stars from McWilliam etal (1995) and Ryan, Norris, & Beers (1996); open circles — moderately metal-poorthin disk and halo stars from Sneden & Gratton (1991); open triangles — bulge starsfrom McWilliam & Rich (1994); open squares — thick-disk stars from Prochaska etal. (2000). The solid circle represents Q2206-199 and the down-turned filled trianglesare upper limits for three other DLAs (Q1223+17 [this work], HE1104−1805 [Lopezet al 1999] and Q302−223 [Pettini et al 2000]). The cross in the lower left corner ofthe figure shows the typical error bar for stellar points.These points are plotted in Figure 1 along with Galactic stellar valuesand additional DLA limits derived from the literature.4. ResultsWe have shown that the detection of Co in DLAs is feasible with echellespectrographs on 8-m class telescopes. The first detection of Co in aDLA appears to be consistent with the abundances measured in theGalactic bulge and although this doesn’t necessarily equate DLAs withhigh redshift bulges, it may indicate that the two share some similaritiesin their star formation histories. However, the interpretation of theseresults is hindered by our lack of understanding of the nucleosyntheticprocesses that produce Co. Only recently have models been formulatedthat can successfully reproduce Galactic [Co/Fe] trends by forcing adeeper mass cut or higher explosion energies (Nakamura et al 1999,2000). However, matching the [Co/Fe] has come at the expense of highNi abundances which are not seen in stars. In the case of Q2206−199,sellison.tex; 1/08/2007; 13:21; p.34 Sara L. Ellison et al.however, we do find that high [Co/Fe] is accompanied by high [Ni/Fe].Therefore, although the results here represent a somewhat preliminarystudy of Co with a clear need for more data points, we have demon-strated the interest of studying this element both in the context ofunderstanding better the nature of DLAs and for providing a regimefor the testing of nucleosynthetic models.AcknowledgementsThe authors would like to express their gratitude to their colleaguesJacqueline Bergeron, Patrick Petitjean and Max Pettini for their con-sent to use some of the data presented here.ReferencesEllison, S. L., Ryan, S. G., Prochaska, J. X., 2001, MNRAS, submitted.Gratton, R., & Sneden, C., 1991, A&A, 241, 541Lopez, S., Reimers, D., Rauch, M., Sargent, W., Smette, A., 1999, ApJ, 513, 598McWilliam, A., & Rich, M., 1994, ApJS, 91, 749McWilliam, A., Preston, G. W., Sneden, C., Searle, L., 1995, AJ, 109, 2757Mullman, K., Lawler, J., Zsargo, J., Federman, S., 1998, ApJ, 500, 1064Nakamura, T., Umeda, H., Iwamoto, K., Nomoto, K., Hashimoto, M., Hix, R.,Thielemann, F., 2000, ApJ, submitted, astro-ph/0011184Nakamura, T., Umeda, H., Nomoto, K., Thielemann, F., Burrows, A., 1999, ApJ,517, 193Pettini, M., Ellison, S. L., Steidel, C. C., Shapley, A. E., & Bowen, D. V. 2000, ApJ,532, 65Prochaska, J. X., Naumov, S.O., Carney, B.W., McWilliam, A., & Wolfe, A. M.2000, ApJ, acceptedRyan, S. G., Norris, J. E., & Beers, T. C., 1996, ApJ, 471, 254sellison.tex; 1/08/2007; 13:21; p.4

Monthly Notices of the Royal Astronomical Society

ApJ, accepted

The first detection of cobalt in a damped Lyman alpha system

Crossref

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.257.7426

The First Detection of Cobalt in a Damped Lyman Alpha System

The First Detection of Cobalt in a Damped Lyman Alpha System

Abstract

Similar works

Full text

Available Versions

University of Hertfordshire Research Archive

Crossref