IR SED and Dust Masses of Sub-solar Metallicity Galaxies at z~2.3

Abstract

We present results from ALMA 1.2 mm continuum observations of a sample of 27 star-forming galaxies at z = 2.1 − 2.5 from the MOSFIRE Deep Evolution Field (MOSDEF) survey. These galaxies have gas-phase metallicity and star-formation rate measurements from Hβ, [Oiii], Hα, and [Nii]. Using stacks of Spitzer, Herschel, and ALMA photometry (rest-frame ∼ 8 − 400 µm), we examine the IR SED of high-redshift subsolar metallicity (∼ 0.5 Z ) LIRGs. We find that the data agree well with an average SED template of higher luminosity local low-metallicity dwarf galaxies (reduced χ 2 of 1.8). When compared with the commonly used templates for solar-metallicity local galaxies or highredshift LIRGs and ULIRGs, even in the most favorable case (with reduced χ 2 of 2.8), the templates are rejected at > 98% confidence level. The broader and hotter IR SED of both the local dwarfs and high-redshift subsolar metallicity galaxies may result from different grain properties, a clumpy dust geometry, or a harder/more intense ionizing radiation field that heats the dust to higher temperatures. The obscured SFR indicated by the FIR emission of the subsolar metallicity galaxies is only ∼ 60% of the total SFR, which is considerably lower than that of the local LIRGs with ∼ 96 − 97% obscured fractions. Due to the evolving IR SED shape, the local LIRG templates fit to mid-IR data can overestimate the Rayleigh-Jeans tail measurements at z ∼ 2 by a factor of 2 − 20, and these templates underestimate IR luminosities if fit to the observed ALMA fluxes by > 0.4 dex. At a given stellar mass or metallicity, dust masses at z ∼ 2.3 are an order of magnitude higher than those at z ∼ 0. Given the predicted molecular gas mass fractions, the observed z ∼ 2.3 dust-to-stellar mass ratios suggest lower dust-to-molecular gas masses than in local galaxies at the same metallicity. CO observations are necessary to better constrain the molecular gas content of sub-solar metallicity galaxies at z > 1.

CO Emission, Molecular Gas, and Metallicity in Main-Sequence Star-Forming Galaxies at $z\sim2.3$

“…Dust appears to be a subdominant destination for metals at z ∼ 2, similarly to z ∼ 0. Our dust mass estimates, based on a dust-to-gas ratio, are ∼ 0.2 dex lower than those derived from ALMA dust-continuum measurements of z ∼ 2.3 galaxies at similar M * by Shivaei et al (2022). Even if M Z,dust was larger by that amount, it would remain smaller than M Z,ISM and M Z,stars .…”

“…It is probable that the rest-optical line emission originates from the UV-bright blue component since the Hα/Hβ ratio implies relatively low nebular reddening (E(B-V) gas =0.16). Dust continuum emission from ALMA Band 6 observations are offset from the blue component and are more spatially coincident with the red component (Shivaei et al 2022). Due to the large beam size of the Band 3 observations, we cannot determine whether the CO(3−2) emission is co-spatial with the blue or red component, though it is likely that the CO emission aligns with the dust emission.…”

“…Detecting CO(3−2) in main-sequence galaxies at masses significantly below 10 10 M would require prohibitively long integrations with ALMA and is thus not a feasible route to constrain the gas masses of these targets. More promising strategies to reach lower masses include measuring CO in gravitationally-lensed galaxies, where even modest magnification factors of 3 − 5 would lead to significant gains in efficiency, or deriving M mol from the Rayleigh-Jeans dust-continuum, which has proven to be detectable with ALMA in shorter integration times than are required for CO (e.g., Kaasinen et al 2019;Aravena et al 2020;Suzuki et al 2021;Shivaei et al 2022).…”