The deduction of past star formation rates from rest-frame UV radiation in the Hubble and other deep fields as a function of red-shift is tied to "metal" production through the Lilly-Cowie theorem (Lilly & Cowie 1987):
(5) | |
(6) | |
(7) |
where (1 + a) 2.6 is a correction factor to allow for production of helium as well as conventional metals and (probably between about 1/2 and 1) allows for nucleosynthesis products falling back into black-hole remnants from the higher-mass stars. is the fraction of total energy output absorbed and re-radiated by dust and _{H} is the frequency at the Lyman limit (assuming a flat spectrum at lower frequencies). The advantage of this formulation is that the relationship is fairly insensitive to details of the IMF.
Eq (7) is the same as eq (13) of Madau et al (1996), so I refer to the metal-growth rate derived in this way as _{Z}(conventional).
Assuming a Salpeter IMF from 0.1 to 100 M_{} with all stars above 10 M_{} expelling their synthetic products in SN explosions, one then derives a conventional SFR density through multiplication with the magic number 42:
(8) |
In general, we shall have
(9) |
where is some factor. E.g., for the IMF adopted by FHP, = 0.67, whereas for the Kroupa-Scalo one (Kroupa et al 1993) = 2.5.
Finally, the present stellar density is derived by integrating over the past SFR and allowing for stellar mass loss in the meantime, and the metal density is related to this through the yield, p:
(10) | |
(11) |
(where is the lockup fraction), whence (if a = 1.6)
(12) |
which can be compared with Z_{} 1/60. It was pointed out by Madau et al (1996) that the Salpeter slope gives a better fit to the present-day stellar density than one gets from the steeper one - a result that is virtually independent of the low-mass cutoff if one assumes a power-law IMF.
Eq (8), duly corrected for absorption, forms the basis for numerous discussions of the cosmic past star-formation rate or "Madau plot". Among the more plausible ones are those given by Pettini (1999) shown in Fig 2 and by Rowan-Robinson (2000), which leads to similar results and is shown to explain the far IR data. Taking = 0.62 (corresponding to a Salpeter IMF that is flat below 0.7M_{}) rather than Pettini's value of 0.4 (for an IMF truncated at 1M_{}), and = 0.7, we get the data in the following table.
Figure 2. Global comoving star formation rate density vs. lookback time compiled from wide-angle ground-based surveys (Steidel et al. 1999 and references therein) assuming E-de S cosmology with h = 0.5, after Pettini (1999). Courtesy Max Pettini. |
Table 2 indicates that the known stars are roughly accounted for by the history shown in Fig 2 (or by Rowan-Robinson) and the metals also if is close to unity, i.e. the full range of stellar masses expel their nucleosynthesis products. At the very least, has to be 1/2, to account for metals in stars alone. The other point arising from the table, made by Pettini, is that at a red-shift of 2.5, 1/4 of the stars and metals have already been formed, but we do not know where the resulting metals reside.
z = 0 | z = 2.5 | |
_{*} = _{*}(conv.) dt | 3.6 × 10^{8} M_{} Mpc^{-3} | 9 × 10^{7} M_{} Mpc^{-3} |
_{*} = _{*} / 1.54 × 10^{11} h_{70}^{2} | .0024h_{70}^{-2} | 6 × 10^{-4} h_{70}^{-2} |
_{*}(FHP 98) | .0035h_{70}^{-1} | |
_{Z} = p _{*} = _{*} / (42 ) | 2.0 × 10^{7} M_{} Mpc^{-3} | 5 × 10^{6} M_{} Mpc^{-3} |
_{Z} (predicted) | 1.3 × 10^{-4} h_{70}^{-2} | 3.2 × 10^{-5} h_{70}^{-2} |
_{Z} (stars, Z = Z_{}) | 7 × 10^{-5} h_{70}^{-1} | |
_{Z} (hot gas, Z = 0.3Z_{}) | 1.0 × 10^{-4}h_{70}^{-1.5} | |
-> 0.5 1.3 | ||
_{Z} (DLA, Z = 0.07Z_{}) | 2 × 10^{-6} h_{70}^{-1} | |
_{Z} (Ly. forest, Z = 0.003Z_{}) | 1 × 10^{-6} h_{70}^{-2} | |
_{Z} (Ly. break gals, Z = 0.3Z_{}) | ? | |
_{Z} (hot gas) | ? | |