The emergence of metals and dust in galaxies at high redshifts

NUCLEAR PHYSICS A E~E~ER

Nuclear Physics A621 (1997) 566c-571c

The emergence of metals and dust in galaxies at high redshifts Max Pettini, a David L. King, a Linda J. Smith b and Richard W. Hunstead ¢ aRoyal Greenwich Observatory, Madingley Road, Cambridge CB3 0EZ, England bDepartment of Physics and Astronomy, University College London Gower Street, London W C I E 6BT, England CSchool of Physics, University of Sydney, NSW 2006, Australia

1. I N T R O D U C T I O N As highlighted by Jim Truran in his introductory review to this meeting, a new technique has recently been developed to complement studies of stellar populations in the Milky Way and of H II regions in nearby galaxies in our quest to unravel the history of cosmic chemical evolution. Observations of QSO absorption line systems give us the opportunity to measure directly element abundances in the interstellar media of galaxies observed when the universe was only a small fraction of its present age. For a Hubble constant H0 = 50 km s -1 Mpc -1 and a deceleration parameter q0 = 0.01, redshifts z = 2 - 3 correspond to look-back times of ~ 13 - 14 Gyr, comparable to the ages of globular clusters in our Galaxy. Although astronomers have been aware of the potential benefits of QSO absorption line spectroscopy for nearly thirty years, the turning point in the application of this technique to galactic chemical evolution was the identification in the mid-eighties by Artie Wolfe and his collaborators of the damped Lyman alpha systems (DLAs) as a population of high-redshift absorbers which are the most likely contenders for the title of 'primeval galaxies' (see Wolfe 1989, 1995 [1,2] for reviews of the properties of DLAs). The common occurrence of strong absorption lines in the spectra of z -~ 3 galaxies now being discovered in large numbers by Steidel and colleagues [3] supports this view of the damped systems. Characterised by high column densities of neutral hydrogen (N(H I)_> 2 × 102o cm-2), DLAs account for the bulk of the material available for star formation at z > 2; the mass-weighted average metallicity of this gas, irrespective of the morphological type and evolutionary state of the galaxies in which it resides, is a measure of the degree of metalenrichment of the universe as a whole at a given epoch. This fundamental quantity is in turn related to the evolution with redshift of the volume-averaged star-formation rate, the consumption of neutral gas, the integrated extragalactic background, and to the progressive emergence of dust in galaxies [4-6]. 0375-9474/97/$17.00 © 1997 - Elsevier Science B.V. All rights reserved. PII: S0375-9474(97)00304-7

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Figure 1. Portions of QSO spectra obtained in our survey of ZnII and C r I I lines in damped Ly~ systems. The x-axis is wavelength in/~; the y-axis is residual intensity. The vertical tick marks indicate the expected locations of the absorption lines, whether they are detected (as in Q0458-020 and Q1331+170) or not (as in Q0347-383). Line 1: Zn II 2025.484; line 2: C r I I 2055.596; line 3: C r I I 2061.575 + ZnII 2062.003 (blend); and line 4: Cr II 2065.501. The QSO spectra have been normalised to the underlying continuum; note the expanded vertical scale. The resolution of the spectra is between 0.7 and 1.5 FWHM and typical exposure times on 4-m class telscopes (WHT and AAT) are ~ 20 000 40 000 s; further details of the observations can be found in [7].

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2. O B S E R V A T I O N S

OF Q S O s W I T H D A M P E D

Lya SYSTEMS

Motivated by these considerations, my collaborators and I have been conducting over the last five years a large scale survey aimed at measuring the metallicity and dust content of damped Ly(~ galaxies. First results were published in 1994 [7]; the full sample [8,9] now stands at 33 DLAs, more than one third of the total number known. Our survey targets weak transitions of Zn II and Cr II which turn out to be of key diagnostic value and which are conveniently located at rest wavelengths between 2025 and 2065 ~. In metal-poor stars in our Galaxy and the Magellanic Clouds both Zn and Cr follow closely the abundance of Fe; [Zn/Fe] and [Cr/Fe] ~_ 0.0 for -2.0 < [Fe/H] < 0.0 [10]. Evidently, the nucleosynthetic productions of these three elements are linked, even though the tight correspondence between [Zn/H] and [Fe/H] is a challenge for current theoretical supernova yields [11]. In the Milky Way interstellar medium Zn is one of the few elements which are not readily incorporated into dust grains, while ~ 99% of Cr is in solid form (and therefore does not contribute to interstellar Cr II absorption lines) [12]. Thus from observations of a small wavelength interval in the spectra in QSOs with damped Lyc~ systems (see Fig. 1) it is possible to obtain a measure of the abundance of iron-peak elements in the interstellar gas via the N ( Z n + ) / N ( H °) column density ratio, and an indication of the amount of dust present from the N(Cr+)/N(Zn +) ratio.

3. C H E M I C A L

ENRICHMENT

IN THE EARLY UNIVERSE

All available measurements on the abundance of Zn in DLAs as a function of redshift are collected in Figure 2. Generally, the values of [Zn/H] are well below solar indicating that most damped Lya galaxies are chemically young. At z ~ 2--where most data are available--we deduce an average ([Zn/H]) of only 1/15 solar. Figure 2 also shows that there is a considerable range in the Zn abundance at z ~ 2. We know that values of [Zn/H] probably span more than 2 orders of magnitude because in several cases where only upper limits are available for N(Zn+), high resolution spectroscopy of other ions with intrinsically stronger lines has shown that the metallicities can be lower than 1/100 of solar [13,14]. This large range in the degree of metal enrichment at approximately the same redshift probably reflects the fact that the onset of star formation in galaxies did not occur a specific cosmic epoch, but more likely was a protracted process; furthermore, chemical evolution need not have proceeded at the same rate in different galaxies. The picture at z > 2.5 and z < 1.5 is more sketchy than between these redshifts, reflecting the observational difficulties in detecting the Zn II absorption lines in the near-infrared and ultraviolet respectively. Nevertheless, the available data do allow some tentative conclusions to be reached. In all but one case Zn is undetected at z > 2.5 and its abundance is generally very low, significantly less than 1/15 solar. On the other hand, at z -~ 2 - 2.5, some galaxies had apparently already attained near-solar metallicities. This suggests that the first major episodes of metal production in galaxies probably occurred between z _~ 3 and 2, and that in some cases this process may have proceeded rapidly, on a timescale of 1-2 Gyr. It is surprising that no damped Lya systems with near-solar metallicity have been

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Figure 2. The abundance of Zn relative to solar (on a log scale) in the 33 damped Lya galaxies in our survey plotted against redshift. Upper limits, corresponding to the nondetection of the Zn II lines, are indicated by downward pointing arrows. The dashed line corresponds to the solar abundance of Zn.

found at z < 1, as we approach the epoch when the Sun formed (at z - 0.32 in this cosmology). This may be simply due to the very small number of measurements available (see Fig. 2). Alternatively, the present sample of DLA systems, which is drawn from magnitude limited QSO surveys in the optical, may be biassed in favour of chemically unevolved--and therefore relatively unreddened--sightlines through the universe. In the models by Pei and Fall [15] such bias is most pronounced at z < 1. The suggestion of an increasing dust bias with decreasing redshift is supported by the fact that most DLAs imaged at z < 1 appear to be associated with relatively underluminous galaxies [16-18], calling into question the often assumed one-to-one correspondence between high-redshift damped Lya systems and today's disk galaxies. It seems more likely to us that galaxies of different masses and morphological types all contribute to the DLA population. In their study of the chemical evolution of the disk of the Milky Way, Edvardsson et al. 1993 [19] concluded that the stellar age-metallicity relation is relatively flat and shows a large scatter at all ages. If furthermore disk formation in the universe was not a coeval process and took place over a protracted epoch [20,21], we would not expect a tight trend in plots such as that in Fig. 2. Nevertheless even the broad characteristics of the chemical history of damped Lya galaxies appear to be significantly different from those of the disk of the Milky Way. In particular, as shown by Pettini et al. 1997 [9], the distribution of metallicities at a lookback time of ~ 13 Gyr resembles more closely that of stars in the halo, rather than the thick or thin disk. On the basis of the chemical evidence we would conclude that at z -~ 2 most galaxies had not yet collapsed to form disks and that the damped L y a systems trace an earlier stage in the formation of galaxies, possibly to be identified with the spheroidal component. It will be very interesting to assess whether other lines of evidence, particularly the kinematics and the morphology of high redshift galaxies (now accessible with the Hubble Space Telescope), support this conclusion or not.

M. Pettini et al./Nuclear Physics A621 (1997) 566c-571c

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Figure 3. Filled symbols: Cr abundance relative to Zn in the 18 damped Lya systems in our survey for which this ratio could be determined (in the other 15 DLAs Zn and Cr are both below the detection limit). Open circles: Typical [Cr/Zn] values in warm interstellar clouds in the disk and halo of the Milky Way [12]. The region within the dotted lines indicates how the [Cr/Fe] ratio varies in Galactic stars in this metallicity regime [22,23].

4. D U S T I N Y O U N G G A L A X I E S Turning now to Cr, we find its abundance to be generally lower than that of Zn, with values of [Cr/Zn] ranging from _~ 0 to < - 0.65 (see Fig. 3). The most straightforward interpretation is that in some DLAs Cr has been removed onto dust. On average Cr and other refractory elements are depleted by only a factor of ~ 2, significantly less than in local interstellar clouds. The lower dust-to-metals ratio, compared with the Milky Way, may be related to the lower metallicities, likely higher temperature of the ISM, and higher supernova rates in these young galaxies. Combining a metallicity ZDL A ~ 1/15Zo with a dust-to-metals ratio ~ 1/2 of that in local interstellar clouds, we deduce that the "typical" dust-to-gas ratio in damped Lya galaxies is ~ 1/30 of the Milky Way value. This amount of dust will introduce an extinction at 1500/~ of only A1500 ~ 0.1 in the spectra of background QSOs. Similarly, we expect little reddening of the broad spectral energy distribution of the high-z field galaxies now being found routinely by deep imaging surveys [3]. Even such trace amounts of dust, however, can explain the weakness of Lya emission from star-forming regions and may produce sufficient thermal emission for a L* galaxy to be detectable at sub-millimeter wavelengths if the dust temperature is greater than 60 K.

5. C O N C L U S I O N S In summary, it is clear that through the damped Lya absorption systems we have a direct view of the early stages in the evolution of galaxies. A substantial body of data is now being obtained on chemical enrichment, relative abundances of different elements,

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kinematics, and morphologies--and on how these properties evolve over cosmological timescales. Taken together, these observations provide an increasingly detailed picture to be explained by theories of the formation of structure in the universe. We are grateful to the AAT and WHT time assignment panels for their continuing support of this demanding observational programme. This work has benefited from several discussions with colleagues, particularly Chuck Steidel, Mike Fall, Bernard Pagel and Jim Truran. Max Pettini would like to express his sincere thanks to the organisers of the Nuclei in the Cosmos 1996 meeting for their kind hospitality. REFERENCES

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