Population III star clusters in the reionized Universe

Abstract

In reionized regions of the Universe, gas can only collapse to form stars in dark matter (DM) haloes which grow to be sufficiently massive. If star formation is prevented in the minihalo progenitors of such DM haloes at redshifts z≳ 20, then these haloes will not be self‐enriched with metals and so may host Population (Pop) III star formation. We estimate an upper limit for the abundance of Pop III star clusters which thus form in the reionized Universe, as a function of redshift. Depending on the minimum DM halo mass for star formation, between of the order of 1 and of the order of 1000, Pop III star clusters per square degree may be observable at 2 ≲z≲ 7. Thus, there may be a sufficient number density of Pop III star clusters for detection in surveys such as the Deep‐Wide Survey (DWS) to be conducted by the James Webb Space Telescope. We predict that Pop III clusters formed after reionization are most likely to be found at z≳ 3 and within ∼40 arcsec (∼1 Mpc comoving) of DM haloes with masses of ∼1011 M⊙, the descendants of the haloes at z∼ 20 which host the first galaxies that begin reionization. However, if star formation is inefficient in the haloes hosting Pop III clusters due to the photoionizing background radiation, these clusters may not be bright enough for detection by the Near‐Infrared Camera which will conduct the DWS. None the less, if the stellar initial mass function (IMF) is top‐heavy the clusters may have sufficiently high luminosities in both Lyα and He iiλ1640 to be detected and for constraints to be placed on the Pop III IMF. While a small fraction of DM haloes with masses as high as ∼109 M⊙ at redshifts z≲ 4 are not enriched due to star formation in their progenitors, external metal enrichment due to galactic winds is likely to preclude Pop III star formation in a large fraction of otherwise unenriched haloes, perhaps even preventing star formation in pristine haloes altogether after reionization is complete at z∼ 6.

Challenges for $Λ$CDM: An update

“…about around the end of the cosmic dark ages) (for models indicating late, z ∼ 2 − 7, Pop III star formation, see Refs. [1281,1282]).…”

Probing the high-redshift universe with SPICA: Toward the epoch of reionisation and beyond

“…This leads to a scenario in which the first galaxies that appeared in the Universe (in the chronological sense) were mostly Pop II galaxies; Pop III galaxies would appear subsequently in underdense regions where the star formation in minihalos was suppressed by radiation emitted by stars/galaxies formed earlier in overdense regions. For this reason, Pop III galaxies may be considered as the second-generation galaxies containing first-generation stars (e.g., Johnson, Greif, & Bromm 2008;Trenti & Stiavelli 2009;Johnson 2010;Stiavelli & Trenti 2010;Johnson, Dalla Vecchia, & Khochfar 2013).…”

“…Although detecting genuine Pop III objects at high redshift will likely require strong boosting of H 2 line luminosities by some mechanism as well as lensing amplification, SAFARI may be able to probe the properties of such objects through the observations of similarly metal-poor objects that may exist at lower redshift. For example, a number of studies have suggested that Pop III star formation may continue towards low redshift and maybe even down to z ∼ 3 (e.g., Scannapieco, Schneider, & Ferrara 2003;Jimenez & Haiman 2006;Schneider et al 2006a;Tornatore, Ferrara, & Schneider 2007;Ricotti, Gnedin, & Shull 2008;Trenti & Stiavelli 2009;Johnson 2010;Johnson et al 2013;Pallottini et al 2014Pallottini et al , 2015a. According to the recent Renaissance Simulations (Xu et al 2016), only 6% of the volume and 13% of the gas mass are enriched to [Z/H] > −4 at z = 7.6 in the comoving survey volume of 220 Mpc 3 , indicating that there is a large amount of pristine gas available for Pop III star formation at z < 7.6 (although much of it likely resides in low-density diffuse IGM).…”

What is the maximum mass of a Population III galaxy?

“…This is because the lifetimes of massive (M 100 M⊙) Pop III stars are only a few Myr (Schaerer 2002), leading to prompt enrichment of metals via supernovae winds and a transition to Population II (Pop II) stars. However, it has been proposed that Pop III stars could form in much more massive halos if star formation is suppressed in all progenitor halos via strong ionizing radiation (Johnson 2010;Visbal et al 2016;Yajima & Khochfar 2016).…”

“…For the three halos tested, we find that an ionizing photon flux of at least ≈ 10 6 s −1 cm −2 is required to suppress star formation significantly above the atomic cooling threshold. At high-redshift, this is most likely to occur in close proximity to large star-forming galaxies (Johnson 2010;Visbal et al 2016).…”