Unveiling the warm and dense ISM in z > 6 quasar host galaxies via water vapor emission

Abstract

Water vapor (H 2 O) is one of the brightest molecular emitters after carbon monoxide (CO) in galaxies with high infrared (IR) luminosity, and allows us to investigate the warm dense phase of the interstellar medium (ISM) where star formation occurs. However, due to the complexity of its radiative spectrum, H 2 O is not frequently exploited as an ISM tracer in distant galaxies. Therefore, H 2 O studies of the warm and dense gas at high-z remains largely unexplored. In this work we present observations conducted with the Northern Extended Millimeter Array (NOEMA) toward three z > 6 IR-bright quasars J2310+1855, J1148+5251, and J0439+1634 targeted in their multiple para-/ortho-H 2 O transitions (3 12 − 3 03 , 1 11 − 0 00 , 2 20 − 2 11 , and 4 22 − 4 13 ), as well as their far-IR (FIR) dust continuum. By combining our data with previous measurements from the literature we estimate dust masses and temperatures, continuum optical depths, IR luminosities, and the star-formation rates from the FIR continuum. We model the H 2 O lines using the MOLPOP-CEP radiative transfer code and find that water vapor lines in our quasar host galaxies are primarily excited in warm dense (gas kinetic temperature and density of T kin = 50 K, n H 2 ∼ 10 4.5 − 10 5 cm −3 ) molecular medium with water vapor column density of N H 2 O ∼ 2 × 10 17 − 3 × 10 18 cm −3 . High-J H 2 O lines are mainly radiatively pumped by the intense optically-thin far-IR radiation field associated with a warm dust component with temperatures of T dust ∼ 80 − 190 K that account for < 5 − 10% of the total dust mass. In the case of J2310+1855, our analysis points to a relatively high value of the continuum optical depth at 100 µm (τ 100 ∼ 1). Our results are in agreement with expectations based on the H 2 O spectral line energy distribution of local and high-z ultra-luminous IR galaxies and active galactic nuclei (AGN). The analysis of the Boltzmann diagrams highlights the interplay between collisions and IR pumping in populating the high H 2 O energy levels and allows us to directly compare the excitation conditions in the targeted quasar host galaxies. In addition, the observations enable us to sample the high-luminosity part of the H 2 O-total-IR (TIR) luminosity relations (L H 2 O − L TIR ). Overall, our results point to supralinear trends suggesting that H 2 O-TIR relations are likely driven by IR pumping rather than the mere co-spatiality between the FIR-continuum-and the line-emitting regions. The observed L H 2 O /L TIR ratios in our z > 6 quasars do not show any strong deviations with respect to those measured in star-forming galaxies and AGN at lower redshift. This supports the idea that H 2 O can be likely used to trace the star-formation buried deep within the dense molecular clouds.

The X–shooter/ALMA Sample of Quasars in the Epoch of Reionization. II. Black Hole Masses, Eddington Ratios, and the Formation of the First Quasars

“…Indeed, given the extreme brightness of the quasar emission at these wavelengths, the detection of the stellar component has proven to be prohibitive with large aperture ground-based telescopes (e.g., Targett et al 2012) and even with HST (e.g., Mechtley et al 2012;Marshall et al 2020a). On the other hand, the interstellar medium (ISM) of tens of quasar host galaxies has been detected at millimeter-wavelengths in the dust continuum and in the [C II] 158 μm emission line (e.g., Wang et al 2016bWang et al , 2019cDecarli et al 2017Decarli et al , 2018Venemans et al 2018;Wang et al 2019a;Rojas-Ruiz et al 2021;Khusanova et al 2022) and, in some cases, also in the [O III] 88 μm, [O I] 146 μm, [C I] 369 μm, CO, and water lines (e.g., Wang et al 2011aWang et al , 2011bWang et al , 2016bVenemans et al 2017a;Walter et al 2018;Novak et al 2019;Shao et al 2019;Yang et al 2019b;Wang et al 2019a;Pensabene et al 2021;Decarli et al 2022;Li et al 2022;Meyer et al 2022a;Pensabene et al 2022). In particular, Atacama Large Millimeter/submillimeter Array (ALMA) [C II] 158 μm observations at a resolution of 1 kpc are providing detailed information on star formation rates, the presence of molecular outflows, dynamical masses, and morphology of early massive galaxies hosting SMBHs (Venemans et al 2019(Venemans et al , 2020Novak et al 2020;Neeleman et al 2021;Walter et al 2022).…”

Molecular Outflow in the Reionization-epoch Quasar J2054-0005 Revealed by OH 119 μm Observations

“…Quasars at z ∼ 6, powered by AGN and star formation, are thus believed to be an important evolutionary phase in massive galaxy evolution (e.g., Carilli & Walter 2013;Casey et al 2014;Lapi et al 2018). To understand how massive galaxies evolved, it is important to reveal the physical conditions of the interstellar medium (ISM) in quasars at the epoch of reionization (EoR; 6  z  20), and this has been at the focus of recent research (e.g., Venemans et al 2017Venemans et al , 2018Walter et al 2018;Hashimoto et al 2019b;Novak et al 2019;Li et al 2020aLi et al , 2020bNeeleman et al 2021;Meyer et al 2022;Pensabene et al 2022;Decarli et al 2023). Many of these quasars exhibit high far-infrared luminosities (L FIR  10 13 L ☉ ) that indicate dust heating by extreme star formation and AGN radiation (e.g., Walter et al 2009;Wang et al 2013;Leipski et al 2014;Venemans et al 2018Venemans et al , 2020.…”

The Pan-STARRS1 z > 5.6 Quasar Survey. II. Discovery of 55 Quasars at 5.6 < z < 6.5

“…The quasars presented in Bañados et al (2016) nearly doubled the number of z > 5.6 quasars known at the time, enabling a transition from studies of individual sources to statistical analyses of the quasar population in early cosmic times. For example, the much enlarged quasar sample enabled (i) the measurement of the nuclear chemical enrichment, black hole mass, and Eddington ratio distributions (e.g., Schindler et al 2020;Farina et al 2022;Lai et al 2022;Wang et al 2022), (ii) the characterization of their X-ray (e.g., Vito et al 2019) and radio (e.g., Liu et al 2021) properties, (iii) the search for signatures of outflows and black hole feedback (e.g., Meyer et al 2019;Novak et al 2020;Bischetti et al 2022), (iv) a census of (atomic/molecular) gas and dust in the quasar hosts (e.g., Decarli et al 2018;Venemans et al 2018;Li et al 2020;Pensabene et al 2021;Decarli et al 2022), including the serendipitous discovery of star-forming companion galaxies (Decarli et al 2017), (v) the search for extended Lyα nebular emission (e.g., Farina et al 2019) and its connection to [C II] gas (Drake et al 2022), (vi) the quantification of the properties of water reservoirs in these quasars (e.g., Pensabene et al 2022), (vii) the identification of a population of particularly young quasars (Eilers et al 2020), (viii) the first constraints on quasar clustering at z ∼ 6 (Greiner et al 2021), (ix) the study of the environments in which these quasars reside (e.g., Mazzucchelli et al 2017a;Farina et al 2017;Meyer et al 2020Meyer et al , 2022, (x) the study of heavy elements in intervening absorption systems at z > 5 (e.g., Chen et al 2017;Cooper et al 2019), (xi) quantitative constraints on the thermal state of the intergalactic medium at z > 5 (e.g., Gaikwad et al 2020), and (xii) constraints on the end phases of cosmic reionization (e.g., Bosman et al 2022). In addition to these populat...…”