The Contribution of Stars, Dust, Neutral Gas and SMBHs in Galaxies to the Cosmic Baryon Inventory

TL;DR

This study delivers a self-consistent census of the galactic baryon budget from the GAMA and DEVILS surveys by jointly modeling stars, dust, neutral gas, and SMBHs with the ProSpect SED fitter. By deriving the cosmic stellar, dust, and neutral gas mass histories and comparing the dust-traced gas to HI benchmarks, the work reveals a CDMH that tracks the cosmic star formation history with a peak at early to intermediate epochs and a steady decline thereafter, and shows that the neutral gas inferred from dust is systematically lower than 21 cm measurements due to spatial differences. The authors implement a metallicity-dependent dust-to-gas ratio and a variable dust-to-hydrogen mass ratio to stabilize dust mass estimates, and they place the SMBH mass density on the same homogeneous footing for a truly self-consistent baryon inventory. The key finding is that the combined mass in stars, dust, neutral gas, and SMBHs within optical radii accounts for only about 5% of the cosmic baryon density, pointing to the remaining ~95% residing in ionised, diffuse gas in the interstellar, circumgalactic, and intergalactic media. This work provides a reproducible, data-driven framework for constraining feedback, chemical enrichment, and the life cycle of baryons across cosmic time.

Abstract

We compute the cosmic stellar, dust and neutral gas mass history at $0<z\lesssim3$ using ProSpect spectral energy distribution modelling of $\approx 800 \, 000$ galaxies in the Galaxy and Mass Assembly (GAMA) survey and the Deep Extragalactic VIsible Legacy Survey (DEVILS). The cosmic dust mass history broadly follows the shape of the cosmic star formation history; though, the decline is slower, suggestive of a slowing rate of dust growth and destruction as the star formation declines past its peak at $z\approx 2$. Neutral gas masses were estimated by scaling the dust masses by the metallicity-dependent dust-to-gas ratio. The neutral gas mass density as traced by the dust is an average of $\approx 0.6$ dex lower than that measured from $21$cm experiments, most likely due to differences in the spatial scales inhabited by dust and HI. Folding in measurements of the supermassive black hole mass density obtained previously with similar data and methods, we present a self-consistent census of the baryons confined to galaxies. Stars, neutral gas, SMBHs and dust contained within the optical radii of galaxies account for $\approx 5$ per cent of the baryons. Most of the remaining $\approx 95$ per cent of baryons must be ionised and dispersed throughout the interstellar, circumgalactic and intergalactic media within, around and between galaxies.

Figures (12)

• Figure 1: Fraction of sources with FIR photometric observations in the DEVILS sample.
• Figure 2: Posterior distributions of the dust mass from ProSpect fits to the inverse-variance weighted stack of a sample of $z\approx 0.05$ galaxies with $S/N > 5$ FIR Herschel$P100,P160,S250,S350,S500$ photometric measurements. The blue curve shows the results when including the FIR measurements and a Gaussian prior on $\alpha$ with mean $\mu=2$ and dispersion $\sigma=1$. The orange curve shows the results including when including the FIR measurements but no prior. The green curve shows the results when excluding the FIR measurements and using the prior. The red curve shows the results when excluding both the FIR measurements and the prior.
• Figure 3: Top: inverse-variance weighted stacked SEDs of the $218$ galaxies with $S/N > 5$ in the Herschel$P100,P160,S250,S350,S500$ photometric bands from both ProFound and LAMBDAR photometry. Bottom:ProSpect dust masses and $1\sigma$ uncertainties against MAGPHYS. The grey dotted line is the equality line. The inset panel is the histogram of the dust mass differences between ProSpect and MAGPHYS
• Figure 4: Top: MAGPHYS standard SED model. The thick grey line is for the total luminosity while the brown, dotted line is only the mass contributing part of the SED. Middle: variable DTH as a function of wavelength. The left axis denotes the DTH per wavelength while the right axis denotes the factor that $\mathrm{DTH=0.0073}$ is multiplied by. The regimes at which very small grains (VSGs), PAHs and big grains (BGs) dominate the emission are noted. Bottom: histogram comparison of dust masses between MAGPHYS. The light grey histogram is the same comparison in Figure \ref{['fig:magphys-prospect']}. The dark histogram is the comparison against MAGPHYS using the updated dust masses and the variable DTH.
• Figure 5: Comparison of the dust mass from fitting 218, FIR detected galaxies with single-temperature greybody functions. We show the distributions of the ratio of dust masses when varying the emissivity index, $\beta$, compared to a fixed value of $\beta=2$, which is similar to the assumed implementation in MAGPHYS.