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2.5. Summary of the Low Redshift Budget

Our estimates of the baryon budget at low redshift, expressed as fractions of the critical Einstein-de Sitter value, are listed in lines 1 to 7 in Table 3. We rewrite the Hubble constant from equation (1) as H0 = 70h70 km s-1 Mpc-1 to center it on a more likely value. The central values in the table are computed from the central values of the parameters entering the estimate. An upper bound is obtained by systematically choosing each parameter at the end of its reasonable-looking range that maximizes the contribution to the mean mass density, and likewise for the lower bound.

Line 7a is an estimate of the X-ray-detected plasma around groups (eq. [29]), line 7b is an estimate of the cooler component detected by resonance Lyman alpha absorption in the trace HI fraction (eq. [31]), and the sum is an estimate of the plasma associated with groups of galaxies. Both entries are quite uncertain, and may well have missed a substantial amount of cool plasma. Consistent with this, the entry in line 7' from the scaling from the plasma mass in clusters of galaxies (eq. [32]) is larger. We suspect line 7' is more reliable than the sum of lines 7a and 7b. In the sums over the budget we use line 7' for the maximum and central values, and lines 7a and 7b instead of 7' for the minimum.

The Baryon Budget

Component Central Maximum Minimum Grade a

Observed at z approx 0

1. Stars in spheroids 0.0026 h70-1 0.0043 h70-1 0.0014 h70-1 A
2. Stars in disks 0.00086 h70-1 0.00129 h70-1 0.00051 h70-1 A-
3. Stars in irregulars 0.000069 h70-1 0.000116 h70-1 0.000033 h70-1 B
4. Neutral atomic gas 0.00033 h70-1 0.00041 h70-1 0.00025 h70-1 A
5. Molecular gas 0.00030 h70-1 0.00037 h70-1 0.00023 h70-1 A-
6. Plasma in clusters 0.0026 h70-1.5 0.0044 h70-1.5 0.0014 h70-1.5 A
7a. Warm plasma in groups 0.0056 h70-1.5 0.0115 h70-1.5 0.0029 h70-1.5 B
7b. Cool plasma 0.002 h70-1 0.003 h70-1 0.0007 h70-1 C
7'. Plasma in groups 0.014 h70-1 0.030 h70-1 0.0072 h70-1 B
8. Sum (at h = 70 and z appeq 0) 0.021 0.041 0.007 ...

Gas components at z approx 3

9. Damped absorbers 0.0015 h70-1 0.0027 h70-1 0.0007 h70-1 A-
10. Lyman-alpha forest clouds 0.04 h70-1.5 0.05 h70-1.5 0.01 h70-1.5 B
11. Intercloud gas (HeII) ... 0.01 h70-1.5 0.0001 h70-1 B

Abundances of:

12. Deuterium 0.04 h70-2 0.054 h70-2 0.013 h70-2 A
13. Helium0.010 h70-2 0.027 h70-2 ... A
14. Nucleosynthesis 0.020 h70-2 0.027 h70-2 0.013 h70-2 ...

a Confidence of evaluation, from A (robust) to C (highly uncertain).

The true values for each line are not likely to be close to the maximum in all entries, or close to the minima in all entries, but we retain the whole range in quoting the sums in each column in line 8 because the sums are dominated by a few entries.

At h70 = 1 our estimate of the baryons in stars and remnants is Omega = 0.0035+0.0025-0.0014 times the critical density. This star mass is only about 17% of our sum over all detected baryons. In our budget, in the 2.2µm K band, 26% of the light is from disk and irregular galaxies, in agreement with the fraction of the star mass. (We use < B - K > = 4.27 for ellipticals and bulges, and 2.78 for disks and irregulars). In the B band, 61% of the mean luminosity density is from disks and irregular galaxies. This component represents only about 25% of the star mass and only about 4% of our estimated sum of all observed baryons.

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