Measurements of the Dust Properties in z ≃ 1–3 Submillimeter Galaxies with ALMA

Abstract

We present Atacama Large Millimeter/submillimeter Array (ALMA) 2 mm continuum observations of a complete and unbiased sample of 99 870 μm selected submillimeter galaxies (SMGs) in the Extended Chandra Deep Field South (ALESS). Our observations of each SMG reach average sensitivities of 53 μJy beam −1 . We measure the flux densities for 70 sources, for which we obtain a typical 870 μm-to-2 mm flux ratio of 14 ± 5. We do not find a redshift dependence of this flux ratio, which would be expected if the dust emission properties of our SMGs were the same at all redshifts. By combining our ALMA measurements with existing Herschel/SPIRE observations, we construct a (biased) subset of 27 galaxies for which the cool dust emission is sufficiently well sampled to obtain precise constraints on their dust properties using simple isothermal models. Thanks to our new 2 mm observations, the dust emissivity index is well constrained and robust against different dust opacity assumptions. The median dust emissivity index of our SMGs is β ; 1.9 ± 0.4, consistent with the emissivity index of dust in the Milky Way and other local and high-redshift galaxies, as well as classical dust-grain model predictions. We also find a negative correlation between the dust temperature and β, similar to low-redshift observational and theoretical studies. Our results indicate that β ; 2 in high-redshift dusty star-forming galaxies, implying little evolution in dust-grain properties between our SMGs and local dusty galaxy samples, and suggesting that these high-mass and highmetallicity galaxies have dust reservoirs driven by grain growth in their interstellar medium.

Searching Far and Long. I. Pilot ALMA 2 mm Follow-up of Bright Dusty Galaxies as a Redshift Filter

“…While the redshift distribution of galaxies selected at 2 mm is expected to be relatively high with 〈z〉 ≈ 3.6 (Casey et al 2021), the selection of these targets at 850 μm leads to a lower median redshift, consistent with what has been previously found for 850 μm-selected DSFGs (e.g., Dudzevičiūtė et al 2020;da Cunha et al 2021). Though we expect the follow up 2 mm observations to filter out lower-redshift sources (Casey et al 2021;Zavala et al 2021;Manning et al 2022), our source selection here is based on 850 μm, and thus for this study the redshift distribution is reflective of the 850 μm DSFG redshift distribution.…”

“…To test the impact of SED fitting on our results, we also fit our SEDs using isothermal modified blackbody models as in da Cunha et al (2021). Given the known degeneracies and bias from opacity assumptions, we toggled λ 0 over multiple fits.…”

“…For λ 0 = 200 μm (same assumption as in our modeling), we find a median of β = 2.1 ± 0.5, statistically consistent within error with our β ≈ 2.4 measurement. Note that while in both SEDfitting routines we assume isothermal dust, a primary difference between methods is that the Drew & Casey (2022) model accounts for the range in temperature on the Wiens portion of the SED with a power law, whereas da Cunha et al (2021) do not attempt to fit any points below the 70 μm rest-frame with their simple single-component model. The discrepancy in results from these two methods demonstrates that care should be taken when directly comparing emissivity spectral indices obtained using different fitting methods.…”

An ALMA Spectroscopic Survey of the Brightest Submillimeter Galaxies in the SCUBA-2-COSMOS field (AS2COSPEC): Survey Description and First Results

“…These efforts have pointed to the need to better understand the properties of dust, and their relationships to metal and gas masses. For example recent measurements of dust mass of SMGs have arrived to an average of about 6 − 7 × 10 8 M (Dudzevičiūtė et al 2020;da Cunha et al 2021), somewhat higher than what is seen in most models. In our forthcoming paper (Liao et al in preparation) we will show that our sample SMGs have even higher dust masses.…”

“…Galaxies that are bright in far-infrared and submillimeter host sites of both vigorous star formation and active black hole growth (Sanders et al 1988). Predominantly at z = 1 − 4 (e.g., Chapman et al 2005;Ivison et al 2007;Wardlow et al 2011;Casey et al 2014;Hodge & da Cunha 2020), recent studies focusing on submillimeter galaxies that are bright at 850 µm (S 850 1 mJy), or SMGs, found that they are some of the most massive systems during these epochs, sitting at the massive end of the stellar mass functions (Dye et al 2008;Hainline et al 2011;Micha lowski et al 2012;Koprowski et al 2016;Dudzevičiūtė et al 2020), molecular gas mass functions (Bothwell et al 2013;Birkin et al 2021), dust mass functions (da Cunha et al 2015(da Cunha et al , 2021, and also the halo mass functions (Hickox et al 2012;Chen et al 2016;Wilkinson et al 2017;An et al 2019;Stach et al 2021). The large cold molecular gas reservoir (∼ 10 11 M ) estimated from their molecular line measurements is believed to provide the gas supply over ∼100 Myr timescales for their extensive starformation rates (SFRs) of ∼100-1000 M yr −1 , similar to the levels of star formation seen in nearby (hyper-)ultra-luminous infrared galaxies (HyLIRGs/ULIRGs; Sanders & Mirabel 1996).…”