Annu. Rev. Astron. Astrophys. 2005. 43: 861-918
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8. STAR FORMATION IN DAMPED Lyalpha SYSTEMS

Several lines of evidence imply that damped Lyalpha systems experience ongoing star formation. At z < 1.6 the evidence is unambiguous, since more than half of the sample is associated with galaxies of stars. At z geq 1.6 the evidence is indirect because only one damped Lyalpha system is identified with a resolved object of galactic dimensions that emits starlight. While starlight likely ionizes the gas that gives rise to Lyalpha emission in two other high-z damped Lyalpha systems, the starlight itself has not been directly detected. The expectation is that most of the metals seen in damped Lyalpha systems were generated through star formation in their host systems, but the instantaneous (SFRs) characterizing this important population of objects remained unknown until recently.

8.1. Direct Emission Measurements of Star-Formation Rates in Damped Lyalpha Systems with z > 1.6

Table 1 summarizes results including SFRs for the three positive detections and for objects with upper limits on SFR at z geq 1.6. Observed lower limits on L(Halpha) were coverted to upper limits on SFR using the Kennicutt (1998) calibration, SFR (Modot year-1) = 7.9 × 10-42 L (H alpha)(ergs s-1). Detections of Lyalpha emission 7 were combined with the expression for L (Halpha) and case B radiative recombination to find lower limits of SFR (Modot year-1) = 1.1 × 10-42 L(Lyalpha) (ergs s-1). The total magnitude integrated over DLA2206 - 19A of V = 23 and the very sensitive B > 27 limit on DLA0953 + 47A (A. Bunker 2004, priv. comm.) were combined with the rest-frame UV Kennicutt (1998) calibration, SFR (Modot year-1) = 1.4 × 10-28 Lnu (ergs s-1 Hz-1), to set a lower limit on the SFR for DLA2206 - 19A and a rough upper limit for DLA0953 + 47A assuming no dust extinction for the latter object. An estimated dust extinction correction for DLA2206 - 19A (Wolfe et al. 2004) suggests an upper limit on SFR of 50 Modot year-1, which is comparable to the SFRs of the more luminous Lyman Break Galaxies (Shapley et al. 2003).

8.2. Star-Formation Rates from the C II* Technique

A method recently developed by Wolfe, Prochaska & Gawiser (2003) makes it possible to infer the SFR per unit area for individual damped Lyalpha systems. The basic method is to use measurements of the C II* 1335.7 column density to measure the [C II] 158 µm cooling rate in the neutral gas producing the damped Lyalpha absorption. This is possible because the C II* lambda 1335.7 transition arises from the excited 2P3/2 state in C+, and spontaneous photon decay of the 2P3/2 state to the 2P1/2 state results in [C II] 158 µm emission, which is the principal coolant of neutral gas in the Galactic ISM (Wright et al. 1991). Under the presumed condition of thermal balance, the cooling rate equals the heating rate and it is possible to calculate the SFR per unit area that generates the implied heating rate.

By analogy with the Galactic ISM, Wolfe, Prochaska & Gawiser (2003) adopt the grain photoelectric effect as the principal heating mechanism for damped Lyalpha systems. In that case FUV radiation (hnu approx 6-13.6 eV) ejects photoelectrons from grain surfaces, which heat ambient electrons through Coulomb interactions (Bakes & Tielens 1994, Weingartner & Draine 2003). The heating rate from the grain photoelectric effect is proportional to the FUV radiation intensity Jnu, which consists of a contribution from the FUV background radiation plus a local contribution from hot stars located inside the galaxy that is proportional to the instantaneous SFR per unit area, dot{psi}*.

To infer dot{psi}* and other properties from observations, Wolfe, Prochaska & Gawiser (2003) deduce the [C II] 158 µm spontaneous emission rate per H atom from the observational quantity:

Equation 8 (8)

where Aul is the Einstein coefficient and hnuul is the energy of the 158 µm transition. These are known quantities, and N(C II*) and N(HI) are measured from the absorption line spectra. The total heating rate includes inputs due to the grain photoelectric effect, X-ray photoionization, cosmic ray ionization, C I photoionization, and collisional heating. The total cooling rate includes cooling due to Lyalpha, grain radiative recombination, and emission by fine-structure states of O0, Si+, Fe+, etc., in addition to the [C II] 158 µm fine-structure emission. Radiative excitation of the C+ fine-structure states by CMB radiation is included because it can be significant at high redshift. Wolfe et al. (2004) showed that the heating rates predicted for the Haardt & Madau (2003) backgrounds were significantly lower than the 158 µm cooling rates implied for damped Lyalpha systems with detected C II*lambda 1335.7 absorption, requiring active star formation to explain the observed cooling rates. On the other hand, Wolfe et al. (2004) also showed that heating by background radiation alone cannot be ruled out for systems in which C II*lambda 1335.7 absorption was not detected. A summary of the ellc data for damped Lyalpha systems (and the Galaxy) is given in Figure 12. Wolfe, Prochaska & Gawiser (2003) argued that the heating rate in the Galaxy is much larger than that in damped Lyalpha systems because the dust content in the Galaxy is at least a factor of 30 higher than that in damped Lyalpha systems, whereas dot{psi}* in the Galaxy is only a factor of 2 to 3 lower than that in damped Lyalpha systems (see below).

Figure 12

Figure 12. ellc versus N(H I) for sample of 52 damped Lyalpha systems. Red data points are positive detections, blue are 2sigma upper limits, and green are 2sigma lower limits. Small stars depict positive detections from sightlines in the Galaxy ISM. Large star depicts [C II] 158 µm emission rate per H atom averaged over the disk of the Galaxy. The latter is about 30 times higher than the average of the DLA detections.

In the case of systems with detected C II*lambda 1335.7 absorption, one compares 158 µm emission rates computed for a range of SFRs per unit area with the empirical quantity, ellc. This results in two solutions for dot{psi}* corresponding to thermally stable states of a two-phase medium in which warm neutral medium (WNM) gas is in pressure equilibrium with cold neutral medium (CNM) gas: the WNM solution corresponds to the gas with T ~ 8000 K and n ~ 0.02 cm-3, and the CNM solution corresponds to the gas with T ~ 100 K and n ~ 10 cm-3. In every case the WNM solution for dot{psi}* is a factor of 10 or more higher than the CNM solution: since C II emission in the WNM is a small fraction of the total cooling rate, the total heating rate implied for an observed value of ellc must be higher in the WNM than in the CNM. Owing to the low dust optical depths of damped Lyalpha systems the resulting dot{psi}* are system-wide average SFRs rather than local values as is the case in the dusty Galactic ISM.

This method was used by Wolfe et al. (2004) to analyze a sample of 45 damped Lyalpha systems, 23 with measured C II* column densities and 22 for which C II* was not detected. In the redshift interval z = [1.6, 4.5] the average SFR per unit area for positive detections is <dot{psi}*> = 11.3 × 10-3 Modot year-1 kpc-2 for the CNM solution and 0.21 Modot year-1 kpc-2 for the WNM solution; by comparison with the Galaxy dot{psi}* approx 4 × 10-3 Modot year-1 kpc-2. However, the WNM solution is unlikely to be correct because the bolometric background intensity produced exceeds the observational limits (see Wolfe, Gawiser & Prochaska 2003). This conclusion is consistent with recent evidence from individual systems; Howk, Wolfe & Prochaska (2005) showed that the low upper limit on the optical-depth ratio of Si II* lambda 1264.7 to C II* lambda 1335.7 in a high-redshift damped Lyalpha system results in an upper limit of 800 K for the temperature in the gas producing C II*, which implies that C II* absorption arises in CNM gas in this system. Furthermore, a pure WNM solution would significantly overpredict the observed rest-frame-UV luminosity from DLA2206 - 19A based on its measured ellc, implying that C II* absorption in this damped Lyalpha system arises in a CNM. It is significant that the predicted value for CNM, JnuCII* = 1.7+2.7-1.0 × 10-18 ergs cm-2 s-1 Hz-1 sr-1, is the largest mean intensity inferred from the entire C II* sample. This may help explain why DLA2206 - 19A is one of the rare damped Lyalpha systems detected in emission.

On the other hand, the gas detected in absorption is more likely to be a WNM for damped Lyalpha systems with upper limits on C II* absorption. In this case the gas could be a pure single-phase WNM heated by background radiation alone. These damped Lyalpha systems would then be objects without significant star formation at the absorption epochs. Or the gas could be the WNM branch of a two-phase medium in which the SFR per unit area is similar to that found for the CNM solutions (Wolfe et al. 2004). While the absence of 21 cm absorption at z > 3 (Kanekar & Chengalur 2003) and the large C II/C I ratios (Liszt 2002) are consistent with the WNM hypothesis, both phenomena are also naturally explained within the context of the two-phase hypothesis. Clearly, it is important to determine which of these explanations is the correct one.

8.3. Implications

The C II* technique can also be used to obtain quantities with cosmological significance. Specifically, the SFR per unit comoving volume is given by dot{rho}* = <dot{psi}*(z)>(A* / Ap)(dN / dX)(H0 / c), where A* is the intrinsic cross-sectional area occupied by stars emitting FUV radiation and Ap is the projection of the intrinsic H I area, AHI, on the sky. 8 In the uniform disk model, neutral gas and stars are uniformly distributed across AHI, yielding the values for dot{rho}* (z) given in Table 2, which also summarizes the systematic uncertainties discussed at length in Wolfe, Gawiser & Prochaska (2003).

TABLE 2. Global properties

Property 0.0-1.6 1.6-4.5

dN / dX .... 0.077 ± 0.016 b
103 Omegag(z) 0.96 ± 0.23 a 0.92 ± 0.21 b
log10 dot{rho}*(z) <   -1.40 -0.70 ± 0.28c
< Z > -0.81 ± 0.032 -1.33 ± 0.09

a Due to large uncertainties in individual measurements, error in mean given by propagation of experimental errors.
b Because of systematic decrease in dN / dX and Omegag(z) with time, error in mean determined by standard deviation.
cComputed by averaging over the "WD low" and other dust models discussed in Wolfe et al. (2003a) (see their Table 1), and by assuming that both DLAs with detected and undetected C II* absorption have same mean SFR per unit area. If the SFR per unit area of the non-detections were significantly lower, dot{rho}* would decrease by about 0.3 dex.

The total mass of stars produced in damped Lyalpha systems as a function of redshift can be computed by integrating over the cosmic star-formation history of damped Lyalpha systems. The mass density of stars predicted by today, including the contribution from "normal" galaxies at z < 1.6, is consistent with the total current density of stars in spiral and elliptical galaxies, but this does not reveal the precise population of stars produced by high-redshift damped Lyalpha systems. The same star-formation history will also deplete the neutral gas reservoir at high redshift in about t* approx 2 Gyr, i.e., Omegag(z) would vanish by z approx 2 if star formation starts at z approx 5. Instead Omegag(z) drops by a factor of two during this time, which argues for replenishment of neutral gas at an accretion rate rhoa(z) approx 0.5 rho*(z).

Wolfe, Gawiser & Prochaska (2003) also calculated the mass density of metals produced in damped Lyalpha systems by using the conversion of SFR to metal formation rate suggested by Madau et al. (1996) and Pettini (1999), dot{rho}metals = (1/42) dot{rho}*. Extrapolating to the present day, including the contribution from star formation in normal galaxies at z < 1.6, yields a mass of metals a few times larger than that found in spiral bulges today, which seems feasible. Integrating under the DLA cosmic star-formation history at z geq 2.5 predicts a factor of 30 higher cosmic mean metallicity due to metal enrichment of neutral gas than is observed in damped Lyalpha systems at z = 2.5 (Prochaska et al. 2003a). This sort of "missing metals" problem was first identified by Pettini (1999) for Lyman Break Galaxy SFRs compared to damped Lyalpha system metallicities, but that problem could be solved by assuming that damped Lyalpha systems are not the descendants of the star-forming Lyman Break Galaxies. The damped Lyalpha system "missing metals" problem appears to be fundamental and may illuminate a fundamental flaw in our understanding of the relationship between metal enrichment and star formation at high redshift. The problem is a significant challenge not only to the C II* method of measuring damped Lyalpha SFRs, but to most hierarchical models, which also produce too many metals (see Nagamine, Springel & Hernquist 2004b; Somerville, Primack & Faber 2001). 9 Given the other successes of the method it seems likely that the problem has a physical solution.

The simplest solution would be to hypothesize that the damped Lyalpha systems forming stars at z geq 2.5 are no longer damped Lyalpha systems at z = 2.5 but have used up most of their neutral gas and now have the expected metallicities; the Lyman Break Galaxies are an obvious candidate for their descendants. This is inconsistent, however, with timescale of approx 2 Gyr to use up most of the neutral gas implied by the observed SFRs, which is too long. Ejecting the metals into the IGM does not solve the problem, as the IGM metallicity predicted at z = 2.5 would be [M/H] = -1.5, i.e., about a factor of 30 higher than that observed in the Lyman alpha forest, unless the ejected gas is so hot that most metals are in an unobservable ionization state, with the possible exception of oxygen, which might be detected in O VI lambda lambda1031.9, 1037.6. An alternative possibility is that the metals are sequestered, with the most attractive solution being a model in which damped Lyalpha systems represent neutral gas on the outskirts of actively star-forming "bulge" regions with the majority of the produced metals remaining in these regions rather than polluting the damped Lyalpha gas or general IGM (Wolfe, Gawiser & Prochaska 2003). This hypothesis necessitates the conclusion that metal-rich bulge regions are underrepresented (or completely unrepresented) in the H I-weighted cosmic mean metallicity measured from DLAs. That would not be surprising given that the bulges may well have used up most of their neutral gas and/or have sufficient dust content to dim background QSOs out of optically selected samples.



7 Due to its extreme sensitivity to dust extinction, Lyalpha emission provides a lower limit to the star formation rate as long as the AGN contribution to the emission line is negligible. Back.

8 Note that Ap equals A(N, X) (see eq. 1) averaged over the column-density interval N(H I) = [Nmin, Nmax] Back.

9 The model of Pei et al.(1999) avoids this problem by invoking significant obscuration corrections and low yields. Back.

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