Molecular gas budget and characterization of intermediate-mass star-forming galaxies at z ≈ 2–3

Abstract

Star-forming galaxies (SFGs) with stellar masses below 1010 M⊙ make up the bulk of the galaxy population at z > 2. The properties of the cold gas in these galaxies can only be probed in very deep observations or by targeting strongly lensed galaxies. Here we report the results of a pilot survey using the Atacama Compact Array of molecular gas in the most strongly magnified galaxies selected as giant arcs in optical data. The selection in rest-frame ultraviolet (UV) wavelengths ensures that sources are regular SFGs, without a priori indications of intense dusty starburst activity. We conducted Band 4 and Band 7 observations to detect mid-J CO, [C I] and thermal continuum as molecular gas tracers from four strongly lensed systems at z ≈ 2 − 3: our targets are SGAS J1226651.3+215220 (A and B), SGAS J003341.5+024217 and the Sunburst Arc. The measured molecular mass was then projected onto the source plane with detailed lens models developed from high resolution Hubble Space Telescope observations. Multiwavelength photometry was then used to obtain the intrinsic stellar mass and star formation rate via spectral energy distribution modeling. In only one of the sources are the three tracers robustly detected, while in the others they are either undetected or detected in continuum only. The implied molecular gass masses range from 4 × 109 M⊙ in the detected source to an upper limit of ≲109 M⊙ in the most magnified source. The inferred gas fraction and gas depletion timescale are found to lie approximately 0.5–1.0 dex below the established scaling relations based on previous studies of unlensed massive galaxies, but in relative agreement with existing literature about UV-bright lensed galaxies at these high redshifts. Our results indicate that the cold gas content of intermediate to low mass galaxies should not be extrapolated from the trends seen in more massive high-z galaxies. The apparent gas deficit is robust against biases in the stellar mass or star formation rate. However, we find that in this mass-metallicity range, the molecular gas mass measurements are severely limited by uncertainties in the current tracer-to-gas calibrations.

Molecular gas properties of Planck-selected protocluster candidates at z~1.3-3

“…Such a behavior has been interpreted as lensing magnification acting on a population with intrinsically steep number counts and detected above a constant CO flux threshold (Harris et al 2012). Note that our lens compilation, that includes lensed SFGs with low IR luminosities (Dessauges-Zavadsky et al 2015), at intermediate masses (Solimano et al 2021), and with extreme IR luminosities (Harrington et al 2018) produces a different distribution than the flat one reported in some previous works. An analysis of the L CO(1−0) -FWHM relation in lensed SFGs is beyond the scope of this work, but understanding the origin of such a flat trend might help to interpret our sample as well, and vice versa, even if our sources are not affected by lensing.…”

“…detected SMGs by Bothwell et al (2013), the values measured in several cluster and protocluster members at 1.4<z<3.1 from the literature (see Table C.1), and those measured in lensed SFGs at 1.4<z<3.0 from the HATLAS, GEMS, SPT, and PCCS samples, and in intermediate mass or low L IR lensed SFGs (Harris et al 2012;Saintonge et al 2013;Dessauges-Zavadsky et al 2015;Cañameras et al 2015;Aravena et al 2016;Yang et al 2017;Harrington et al 2018;Solimano et al 2021). We also report the median values of each subsample 15 .…”

Molecular gas properties of Planck-selected protocluster candidates at z ≃ 1.3–3

“…Such a behavior has been interpreted as lensing magnification acting on a population with intrinsically steep number counts and detected above a constant CO flux threshold (Harris et al 2012). It is relevant to point out that our lens compilation, which includes lensed SFGs with low IR luminosities (Dessauges-Zavadsky et al 2015), at intermediate masses (Solimano et al 2021), and with extreme IR luminosities (Harrington et al 2018) produces a different distribution than the flat one reported in some previous works. An analysis of the L CO(1−0) -FWHM relation in lensed SFGs is beyond the scope of this work, but understanding the origin of this flat trend might help to interpret our sample as well, and vice versa, even if our sources are not affected by lensing.…”

“…For each line, we report the measured width, whereas the luminosity is converted to the CO(1−0) luminosity using the line ratios from Bothwell et al (2013). We also show for comparison the relation between the CO(1−0) luminosity and FWHM derived for unlensed CO-detected SMGs by Bothwell et al (2013), the values measured in several cluster and protocluster members at 1.4 < z < 3.1 from the literature (see Table C.1), and those measured in lensed SFGs at 1.4 < z < 3.0 from the HAT-LAS, GEMS, SPT, and PCCS samples, and in intermediate mass or low L IR lensed SFGs (Harris et al 2012;Saintonge et al 2013;Dessauges-Zavadsky et al 2015;Cañameras et al 2015;Aravena et al 2016;Yang et al 2017;Harrington et al 2018;Solimano et al 2021). We also report the median values of each subsample 15 .…”

“…Black diamonds represent CO-detected cluster and protocluster galaxies at 1.4 < z < 3.1 and log(L IR /L ) > 11.44 + 0.5 × z from the literature, and the large filled orange diamond the corresponding median value. Green open triangles represent the observed values for lensed SFGs at 1.4 < z < 3.1 (Harris et al 2012;Saintonge et al 2013;Dessauges-Zavadsky et al 2015;Cañameras et al 2015;Aravena et al 2016;Yang et al 2017;Harrington et al 2018;Solimano et al 2021) and the large full green triangle the corresponding median value. The dashed purple line is the best-fit relation for unlensed high-z SMGs from Bothwell et al (2013), and the red dashed line is the same relation but multiplied by a factor of 5 to match the PHz-IRAM median value.…”