Chemical Signatures of Population III Stars in Damped Lyman-$α$ Absorption Systems at $z \approx 6$

Abstract

Recently, Sodini et al. (2024) presented a sample of OI damped Lyman-$α$ absorption system (DLA) analogs at $z\sim6$ that contain possible chemical signatures of Population III (Pop III) stars. In this paper, we use an N-body simulation-based semi-analytic model of the first stars and galaxies to predict the impact of Pop III stars on high-redshift DLAs. These Pop III DLA predictions are the first to include a number of important physical effects such as Lyman-Werner (LW) feedback, reionization, and external metal enrichment (all of which account for three-dimensional spatial fluctuations caused by halo clustering). We predict the abundance of DLAs as a function of their carbon-to-oxygen ratios ([C/O]). We find that our fiducial model is strongly ruled out by the data as it contains too few high-[C/O] DLAs, which have metals primarily from Pop III stars. However, increasing the delay time between Pop III and metal-enriched star formation due to supernovae feedback leads to better agreement with the data. Our results suggest that DLA analogs at $z\sim6$ are a promising probe of Pop III star formation for two key reasons. First, for reasonable parameter choices there are significant numbers of DLAs with metals primarily originating from Pop III stars. Second, we find that the number of DLAs with substantial Pop III contributions depends strongly on the Pop III star formation efficiency and the delay time between Pop III and metal-enriched star formation.

Figures (6)

• Figure 1: The carbon-to-oxygen elemental abundance ratio of supernovae ejecta as a function of stellar mass for our assumed metal yields. Metal-enriched yields are assumed to follow 2006ApJ...653.1145K with a fixed metallicity of the stars (either $Z=0.004$ or $Z=0.02$). Pop III yields are taken from 2023MNRAS.526.4467J with $v_{\rm rot}/v_{\rm crit} = 0.6$. There is only one Pop III point due to the simplifying assumption that all Pop III stars have the same metal yields. The shaded rectangle around the Pop III point is included to emphasize the general uncertainty for Pop III stars (but we note that the [C/O] values do not change by more than roughly ten percent in the specific case of rapidly rotating Pop III stars from Tables 2 and 3 in 2023MNRAS.526.4467J, which vary stellar mass and rotation speed). The dashed lines indicate the [C/O] produced by a massive cluster of metal-enriched stars, sampling over our assumed Kroupa IMF.
• Figure 2: The metal-enriched (solid curves) and Pop III (dashed curves) SFRDs computed from our 10 N-body simulations (using an average weighted by their assigned $f(v_{\rm bc})$ values). The fiducial model parameters are listed in Table \ref{['table']}. For the other curves, only one parameter is varied from the fiducial values as indicated in the legend. "V2020 $M_{\rm crit}$" uses the formula from 2020ApJ...897...95V (based on simulations of 2001ApJ...548..509M), and the rest of the simulations use the fitting form from the more recent simulation of 2021ApJ...917...40K. The curve labeled "increased $M_{\rm ion}$" has been assigned $5\times 10^8 \left ( \frac{1+z}{11} \right )^{-3/2}~M_\odot$.
• Figure 3: The covering factor per comoving Mpc along the line of sight for halos as a function of their mass at $z=5.9$ (computed from Eq. \ref{['eqn:A_eff']}). This gives the relative contribution of halos with different masses to the observed DLAs. We show the halo densities with and without stars. The former are computed with our fiducial model parameterization.
• Figure 4: Carbon-to-oxygen abundance ratios in the ISM for all of the halos in our model with stars at $z=5.9$ as a function of the ratio of cumulative Pop III mass to total cumulative stellar mass ($M_{\rm *, III}/[M_{\rm *, III}+M_{\rm *, II}]$). These masses include all stars formed within the halo or its progenitors. Colors indicate the virial mass of the halos. Results are shown for all of our single-parameter variations around the fiducial model. The "V2020 $M_{\rm crit}$" and "increased $M_{\rm ion}$" models are the same as described in the caption of Figure \ref{['fig:SFRDs']}. Note that all halos with star formation are included, but that masses above $10^9~M_\odot$ are suppressed in the color bar as they are all tightly clustered near zero Pop III fraction and [C/O]$\sim -0.45$.
• Figure 5: Carbon-to-oxygen abundance ratios in the ISM for all of the halos with stars in our model at $z=5.9$ as a function of total cumulative stellar mass ($M_{\rm *, III}+M_{\rm *, II}$). Colors indicate cumulative fraction of Pop III stars ($M_{\rm *, III}/[M_{\rm *, III}+M_{\rm *, II}]$). Results are shown for all of our single-parameter variations around the fiducial model. The "V2020 $M_{\rm crit}$" and "increased $M_{\rm ion}$" models are the same as described in the caption of Figure \ref{['fig:SFRDs']}.