PHIBSS2: survey design and z = 0.5 – 0.8 results

Abstract

Following the success of the Plateau de Bure high-z Blue Sequence Survey (PHIBSS), we present the PHIBSS2 legacy program, a survey of the molecular gas properties of star-forming galaxies on and around the star-formation main sequence (MS) at different redshifts using IRAM’s NOrthern Extended Millimeter Array (NOEMA). This survey significantly extends the existing sample of star-forming galaxies with CO molecular gas measurements, probing the peak epoch of star formation (z = 1 − 1.6) as well as its building-up (z = 2 − 3) and winding-down (z = 0.5 − 0.8) phases. The targets are drawn from the well-studied GOODS, COSMOS, and AEGIS cosmological deep fields and uniformly sample the MS in the stellar mass (M⋆) – star formation rate (SFR) plane with log(M⋆/M⊙) = 10 − 11.8 and SFR = 3.5 − 500 M⊙ yr−1 without morphological selection, thus providing a statistically meaningful census of star-forming galaxies at different epochs. We describe the survey strategy and sample selection before focusing on the results obtained at redshift z = 0.5 − 0.8, where we report 60 CO(2-1) detections out of 61 targets. We determine molecular gas masses between 2 × 109 and 5 × 1010 M⊙ and separately obtain disc sizes and bulge-to-total (B/T) luminosity ratios from HST I-band images. The median molecular gas-to-stellar mass ratio μgas∼ = 0.28 ± 0.04, gas fraction fgas∼ = 0.22 ± 0.02, and depletion time $ \widetilde{t_{\mathrm{depl}}} = 0.84 \pm 0.07\,\mathrm{Gyr} $ as well as their dependence with stellar mass and offset from the MS follow published scaling relations for a much larger sample of galaxies spanning a significantly wider range of redshifts, the cosmic evolution of the SFR being mainly driven by that of the molecular gas fraction. The galaxy-averaged molecular Kennicutt–Schmidt (KS) relation between molecular gas and SFR surface densities is strikingly linear, pointing towards similar star formation timescales within galaxies at any given epoch. In terms of morphology, the molecular gas content, the SFR, the disc stellar mass, and the disc molecular gas fraction do not seem to correlate with B/T and the stellar surface density, which suggests an ongoing supply of fresh molecular gas to compensate for the build-up of the bulge. Our measurements do not yield any significant variation of the depletion time with B/T and hence no strong evidence for morphological quenching within the scatter of the MS.

Plateau de Bure High-z Blue Sequence Survey 2 (PHIBSS2): Search for Secondary Sources, CO Luminosity Functions in the Field, and the Evolution of Molecular Gas Density through Cosmic Time*

“…A single source appears in this map with SNR above the detection threshold we impose (see eg016-1 in Table A for physical properties). The central targeted source in the eg016 data cube has a SNR of 3.1 (see Table 3 in Freundlich et al 2019), which is below our detection threshold and is therefore not visible in this SNR map. and "likely signal".…”

“…PHIBSS and PHIBSS2 have been productive surveys with key results on their main objective, characterizing normal z ∼ 1 − 2 galaxies. Among other results, PHIBSS and PHIBSS2 have yielded scaling relations for main sequence galaxies at those redshifts, depletion times and molecular fractions (Genzel et al 2015;Tacconi et al 2018), and characterized molecular reservoirs for z < 1 galaxies (Freundlich et al 2019). However, these observations also have the potential to yield impactful "deep field" science.…”

“…These studies have shaped our understanding of the relation between molecular gas content and star formation in known populations of galaxies. Targeted CO studies find that z ∼ 2 galaxies have much larger molecular gas reservoirs than local galaxies (Greve et al 2005;Daddi et al 2010;Genzel et al 2010Genzel et al , 2015Tacconi et al 2010Tacconi et al , 2013Tacconi et al , 2018Freundlich et al 2019) and that the changes in growth history are largely driven by the cold molecular gas mass properties of galaxies. While these types of studies allow us to understand the properties of galaxy samples, they can potentially introduce unknown systematic biases through selection effects.…”

“…Observations of 12 CO (2 − 1), 12 CO (3 − 2), and 12 CO (6 − 5) transitions took place between October 2013 and June 2017. Observation times per target range from 0.6 to 30.3 hours, with a total of ∼ 1, 100 hours of 6-antenna equivalent on source integration time, and were mostly taken in C configuration to ensure that the galaxies are not spatially resolved (see Freundlich et al 2019, for more details on the data reduction process). Given the integration times and configurations, the synthesized beams range from 1 to 5 .…”

Merger induced clump formation in distant infrared luminous starburst galaxies

“…In order to check whether mergers are the physical cause of the increased clumpiness, we performed hydrodynamical simulations of collisions, choosing initial conditions that are typical of galaxies in our redshift range. In particular, we set a gas fraction of 30% (typical of z ∼ 0.7) (Combes et al 2013;Freundlich et al 2019). As mentioned in the Introduction, previous simulations of galaxy collisions with such high gas fraction did not reach the low gas temperatures needed for properly reproducing the gas distribution during merger events.…”