Trading oxygen for iron II. Oxygen- versus iron-dependent cosmic star formation history

TL;DR

This paper argues that oxygen and iron track different cosmic enrichment histories and should be treated separately in galaxy evolution modeling. It builds an observationally motivated framework to derive independent iron- and oxygen-based cosmic star formation histories by leveraging the redshift-invariant $[O/Fe]-sSFR$ relation and updated high-redshift constraints from JWST. The study finds that near-solar $[O/Fe]$ star formation is rare (at least 70% of stellar mass forms with non-solar O/Fe), and that the SFRD-weighted mean metallicity is typically lower in $[Fe/H]$ than in $[O/H]$, with the offset peaking around $z\sim3$ and then plateauing. These results bear on interpreting spectra and rates of metal-poor transients and highlight biases introduced by solar-scaling in common SPS and galaxy evolution diagnostics.

Abstract

Due to their different nucleosynthetic origin, a stellar population produces oxygen (O) and iron (Fe) on different timescales and their relative abundance can deviate strongly from solar. Galaxy formation models should treat these elements separately, as they play a distinct role in shaping physical phenomena. For example, oxygen mainly sets the gas cooling rate, while the iron abundance sets stellar atmosphere opacities impacting stellar evolution, spectra and feedback. Observations of star-forming galaxies usually only constrain gas-phase oxygen abundance, vastly limiting our capabilities of separating the cosmic evolution of oxygen and iron. Here, we present an observationally-motivated framework to scale the cosmic evolution of O and Fe abundances. We apply the relation between the alpha-enhancement and galaxies' specific star formation rate ([O/Fe]-sSFR; Chruslinska et al. 2024) to derive the Fe and O-dependent cosmic star formation history (cSFH). We find that star formation with near-solar O/Fe is rare: at least 70% of the integrated cosmic stellar mass forms at non-solar O/Fe. The cosmic average metallicity is generally lower in [Fe/H] than in [O/H] (by up to a factor 3), with the offset increasing from redshifts z=0 to z~3 and then approaching the core-collapse O/Fe ratio. We validate our results against samples that probe the Fe-dependent cSFH in different regimes such as absorption-derived <[Fe/H]> from long gamma-ray bursts. Our results impact the interpretations of stellar and galaxy spectra and the predicted rates of transients, especially those linked to metal-poor progenitors (e.g., black hole mergers).

Figures (16)

• Figure 1: The [O/Fe] - sSFR relation of galaxies. Colored lines show three model variations considered in this paper (purple - "slow" Fe enrichment, orange - "fast" Fe enrichment and green - "mixed" Fe enrichment), which span the range allowed by observational constraints discussed in Chruslinska24_OFe and shown as solid/hatched areas (see text for the details). The horizontal black line indicates the average abundance ratio of the Milky Way thick disc/halo dwarf stars with [Fe/H] $<$–2 from Amarsi19. The horizontal orange line indicates our reference solar log$_{10}$(O/Fe)$|_{\odot}$=1.33 dex GrevesseSauval98. The zero point on the Asplund09 solar scale is shown as a gray dashed line.
• Figure 2: Evolution of the gas-phase $Z_{O/H}$-MZR in our model against $z>3$ data. MZR($z$>3) predicted based on the known evolution of the SFMR and $z$-invariant FMR (modelled as in Chruslinska21) is consistent with the current data, but additional evolution may be present. Solid lines - MZR at $z$=0, 1,3 10 assuming a $z$-invariant FMR. Dashed line - MZR at $z$=10, assuming additional evolution in the FMR normalization at $z>3$. Data points - $z>3$ samples from Nakajima23 (circles, $z$-bin average), Curti24 (triangles, small - individual, big - $z$-bin average) and Chemerynska24 (squares, assuming calibration from Nakajima22, faint errors extend to values estimated using Sanders24 calibration). Data points are shown in blue for $z<6.5$ and in black for $z>=6.5$. Shown for a variation described in Sec. \ref{['sec: method']}, but for MZR($z$=0) from Curti20 (i.e. without +0.2 dex offset, to compare with the data as reported).
• Figure 3: Distribution of the cosmic SFRD at different [Fe/H] and $z$ (color scale), shown for an example variation described in Sec. \ref{['sec: method']} Black/brown lines: SFRD-weighted average [O/H] / [Fe/H] at each $z$. Black/brown contours enclose 90% of the SFRD to further show the offset between the $f_{\rm SFR}$([O/H],$z$) and $f_{\rm SFR}$([Fe/H],$z$). White solid lines: [Fe/H] below which $M_{*}<10^{8}{M}_{\odot}$ galaxies contribute at least 10% of the SFRD (thin line) and below which contribution of $M_{*}>10^{8}{M}_{\odot}$ galaxies is negligible (thick line). Data points: cross-check samples introduced in Sec. \ref{['sec: Fe-cross-check-samples']}. Hexagons: $\langle \mathrm{[Fe/H]} \rangle_{\rm HI}$ of LGRB hosts binned in $z$. Brown/black stars: [Fe/H]/[O/H] of individual LGRB hosts. Crosses: mean [Fe/H] of RR Lyrae stars in local dwarf galaxies (small/big: $M_{*}<10^{8}{M}_{\odot}$ / $M_{*}\gtrsim 10^{8}{M}_{\odot}$, offset for visibility) but formed at $z>2-5$ (horizontal arrows). Brown/black dots: [O/H]/[Fe/H] of MW globular clusters. [O/H] and [Fe/H] of the LMC and SMC on our solar scale are indicated for the reference.
• Figure 4: Top: The gray area indicates that, across our models, the fraction of cosmic stellar mass formed since $z$=10 with near-solar [O/Fe] (between –0.15 and 0.05 dex) is at most 30%. Lines show the fraction of the total SFRD($z$) formed with near-solar [O/Fe]. Bottom: SFRD weighted mean [O/Fe] as a function of redshift/lookback time. At $z<4$ ($z>4$), the line styles distinguish between the assumptions about the SFMR ($\alpha_{\rm GSMF}$). Colors indicate the [O/Fe]--sSFR relation and the colored areas indicate the range spanned by models with a fixed [O/Fe]--sSFR relation. Green dotted line corresponds to our example variation.
• Figure 5: Contribution to the cosmic SFRD (SFR density per log $M_{*}$ bin) versus galaxy log$_{10}$(M$_{*}$) at several $z$ (colors; bottom and top curves correspond to $z$=0 and $z$=2, respectively, reflecting the cSFH evolution). Filled circles mark the log$_{10}$(M$_{*}$) at which the contribution peaks at each $z$ (ill-defined for $z>$4). At each marked mass we list the typical abundances ([O/Fe],[O/H],[Fe/H]) of star-forming gas in our model. The thick dashed curve highlights $z\approx$0.4, corresponding to a lookback time of 4.5 Gyr (the epoch of Solar birth). Vertical dashed lines show log$_{10}$(M$_{*}$) within $\pm$0.1 dex of the $z=0.4$ peak; abundances for these two masses are reported at the top. This brackets MW-mass galaxies that, at Solar birth, were forming stars with close to solar abundance ratios. Shown for our example variation.