The Alma Spectroscopic Survey in the Hubble Ultra Deep Field: Implications for Spectral Line Intensity Mapping at Millimeter Wavelengths and CMB Spectral Distortions

Abstract

We present direct estimates of the mean sky brightness temperature in observing bands around 99 and 242 GHz due to line emission from distant galaxies. These values are calculated from the summed line emission observed in a blind, deep survey for spectral line emission from high redshift galaxies using ALMA (the ALMA spectral deep field observations "ASPECS" survey). In the 99 GHz band, the mean brightness will be dominated by rotational transitions of CO from intermediate and high redshift galaxies. In the 242 GHz band, the emission could be a combination of higher order CO lines, and possibly [C II] 158 μm line emission from very high redshift galaxies (z∼6-7). The mean line surface brightness is a quantity that is relevant to measurements of spectral distortions of the cosmic microwave background, and as a potential tool for studying large-scale structures in the early universe using intensity mapping. While the cosmic volume and the number of detections are admittedly small, this pilot survey provides a direct measure of the mean line surface brightness, independent of conversion factors, excitation, or other galaxy formation model assumptions. The mean surface brightness in the 99 GHZ band is: T B =0.94±0.09 μK. In the 242 GHz band, the mean brightness is: T B =0.55±0.033 μK. These should be interpreted as lower limits on the average sky signal, since we only include lines detected individually in the blind survey, while in a low resolution intensity mapping experiment, there will also be the summed contribution from lower luminosity galaxies that cannot be detected individually in the current blind survey.

The Alma Spectroscopic Survey in the Hubble Ultra Deep Field: Molecular Gas Reservoirs in High-Redshift Galaxies

“…The CO(7-6)/CO(3-2) ratio r 73 is typically 0.16-0.63 in high-density photon-dominated regions that are powered by star formation (as in ID.2), but it can reach values as high as r 73 =30 in the presence of intense X-ray illumination (Meijerink et al 2007). This might explain the higher CO excitation observed in high-z QSOs with respect to submillimeter galaxies (Carilli & Walter 2013). The lack of such highexcitation feature suggests that the central AGN activity in ID.2 has no major impact on its global CO properties.…”

“…In Figure 6 we compare the IR and CO(1-0) luminosities of our sources with respect to a compilation of galaxies both at low and high redshift from the review by Carilli & Walter (2013), and with the secure blind detections in Decarli et al (2014). For galaxies in our sample that are undetected in CO, we plot the corresponding 3σ limit on the line luminosities.…”

“…The IR luminosity of a galaxy (derived from the SED fitting) has been found to correlate with the CO luminosity (see also Section 5.2.1). Following the best fit of the relation in Carilli & Walter (2013), the IR-luminosity cut above corresponds to ¢ >-L 3 10 CO 1 0 9 ( ) Kkm s −1 pc 2 , i.e., it is similar to the line luminosity limit of our survey (see Paper I). Consequently, we should be able to detect CO, or at least place meaningful limits, on these 11 galaxies.…”

“…In other papers of this series, we present the data set and the catalog of blindly selected CO emitters (Paper I, Walter et al 2016), we study the properties of 1.2 mm detected sources (Paper II, Aravena et al 2016a), we discuss the inferred constraints on the luminosity functions of CO (Paper III, Decarli et al 2016a), and we search for [C II] emission in z=6-8 galaxies (Paper V, Aravena et al 2016b). Paper VI (Bouwens et al 2016) places our findings in the context of the dust extinction law for > z 2 galaxies, and Paper VII (Carilli et al 2016) uses ASPECS to place first direct constraints on intensity mapping experiments. Here we place the CO detections in the context of the properties of the associated galaxies.…”

“…This has been a demanding task in terms of telescope time. To date, only some several hundred sources at > z 1 have been detected in a molecular gas tracer (typically the rotational transitions of the carbon monoxide 12 CO molecule; e.g., Carilli & Walter 2013). This sample is dominated by "extreme" sources, such as QSO host galaxies (e.g., Bertoldi et al 2003;Walter et al 2003;Weiß et al 2007;Wang et al 2013) or submillimeter galaxies (e.g., Frayer et al 1998;Neri et al 2003;Greve et al 2005;Bothwell et al 2013;Riechers et al 2013;Aravena et al 2016), with IR luminosities  L 10 IR 12  L and star formation rates (SFRs) 100  M yr −1 .…”