The First Galaxies and the Likely Discovery of Their Fossils in the Local Group

Figure 5

(a) Average star formation efficiency (i.e., fraction of the collapsed baryon mass converted into stars), ⟨𝑓_∗⟩=⟨𝑀_∗/𝑀_bar⟩ (thick curves), and gas fraction (i.e., fraction of the collapsed baryon mass retained in the gas phase) ⟨𝑓_g⟩=⟨𝑀_g/𝑀_bar⟩ (thin curves), of the first galaxies as a function of their halo mass for run S2 in RGS08 at 𝑧=14.5,12.5,10.2, and 8.3. For comparison, the symbol with error bar shows the expected star formation efficiency (roughly 𝑀_∗∼30-300 M⊙ divided by the baryonic mass of the halo) in the first mini halo of mass 10⁶ M⊙ simulated by Abel et al. [71]. The dot-dashed line shows a power-law fit for the mean SFE, ⟨𝑓_∗⟩(𝑀_dm,𝑧)∝𝑀²_dm. The SFE is nearly independent of redshift apart from an increase in halos with 𝑀_dm<10⁷ M⊙ at 𝑧∼8. (b) Same as (a) but showing the star formation efficiency, 𝑓_∗, for individual galaxies in the simulation at 𝑧=10.2. Circles, from smaller to the larger, refer to galaxies with gas fractions 𝑓_g<0.1% (blue), 0.1%<𝑓_g<1% (cyan), 1%<𝑓_g<10% (red), and 𝑓_g>10% (green), respectively.

(a)

(b)