arXiv subjects

S. Gallerani

Publications and source records attributed to S. Gallerani.

At least 37 records · Page 2Linked to original sources

arXiv2021

The role of SPICA-like missions and the Origins Space Telescope in the quest for heavily obscured AGN and synergies with Athena

In the BH-galaxy co-evolution framework, most of the star-formation (SF) and the black hole (BH) accretion is expected to take place in highly obscured conditions. Thus, obscured AGN are difficult to identify in optical or X-ray bands, but shine bright in the IR. Moreover, X-ray background (XRB) synthesis models predict that a large fraction of the yet-unresolved XRB is due to the most obscured (Compton thick, CT) of these AGN. In this work, we investigate the synergies between putative IR missions (using SPICA, proposed for ESA/M5 but withdrawn in October 2020, and Origins Space Telescope, OST, as `templates') and the X-ray mission Athena, which should fly in early 2030s, in detecting and characterizing AGN, with a particular focus on the most obscured ones. Using an XRB synthesis model, we estimated the number of AGN and the number of those which will be detected in the X-rays. For each AGN we associated an optical-to-FIR SED from observed AGN with both X-ray data and SED decomposition, and used these SEDs to check if the AGN will be detected by SPICA-like or OST at IR wavelengths. We expect that, with the deepest Athena and SPICA-like (or OST) surveys, we will be able to detect in the IR more than $90\,\%$ of all the AGN (down to L$_{2-10\text{keV}} \sim 10^{42}\,$erg/s and up to $z \sim 10$) predicted by XRB synthesis modeling, and we will detect at least half of them in the X-rays. Athena will be extremely powerful in detecting and discerning moderate- and high-luminosity AGN. We find that the most obscured and elusive CT-AGN will be exquisitely sampled by SPICA-like mission or OST and that Athena will allow a fine characterization of the most-luminous ones. This will provide a significant step forward in the process of placing stronger constraints on the yet-unresolved XRB and investigating the BH accretion rate evolution up to very high redshift ($z \ge 4$).

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arXiv2021

High [OIII]/[CII] surface brightness ratios trace early starburst galaxies

We study the impact of deviations from the Kennicutt-Schmidt relation (quantified by the `burstiness' parameter $κ_s$), gas metallicity ($Z$), and density ($n$) on the observed [OIII]88$μ$m/[CII]158$μ$m surface brightness ratios ($Σ_{[OIII]}/Σ_{[CII]}$) in nine galaxies at $z\approx6-9$. We first discuss possible biases in the measured $Σ_{[OIII]}/Σ_{[CII]}$ ratios by comparing the data with zoom-in cosmological simulations, and then use a Markov Chain Monte Carlo algorithm to derive the best fit values of ($κ_s, Z, n$). We find that (i) the strongest dependence of $Σ_{[OIII]}/Σ_{[CII]}$ is on $κ_s$; (ii) high ratios identify starburst galaxies with short gas depletion times ($t_{dep}=6-49\,\rm Myr$); (iii) a secondary dependence on density is found, with $Σ_{[OIII]}/Σ_{[CII]}$ anticorrelating with $n$ as a result of the lower [OIII] critical density, (iv) the ratio only weakly depends on $Z$. The nine galaxies are significantly enriched (Z=0.2-0.5 $Z_\odot$), and dense ($n=10^{1-3} {\rm cm}^{-3}$). This lends further support to the starburst scenario in which a rapid enrichment of the interstellar medium is expected.

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arXiv2021

Infrared emission of $z \sim 6$ galaxies: AGN imprints

We investigate the infrared (IR) emission of high-redshift ($z\sim 6$), highly star-forming (${ {\rm SFR} > 100}$ $M_{\rm \odot} {\rm yr}^{-1}$) galaxies, with/without Active Galactic Nuclei (AGN), using a suite of cosmological simulations featuring dust radiative transfer. Synthetic Spectral Energy Distributions (SEDs) are used to quantify the relative contribution of stars/AGN to dust heating. In dusty (${M_{\rm d}\gtrsim 3\times 10^7 M_{\rm \odot}}$) galaxies, $\gtrsim 50-90 \%$ of the UV radiation is obscured by dust inhomogeneities on scales ${\gtrsim 100}$ pc. In runs with AGN, a clumpy, warm ($\approx 250$ K) dust component co-exists with a colder ($\approx 60$ K) and more diffuse one, heated by stars. Warm dust provides up to ${50 \%}$ of the total IR luminosity, but only $\lesssim 0.1 \%$ of the total mass content. The AGN boosts the MIR flux by ${10-100 \times}$ with respect to star forming galaxies, without significantly affecting the FIR. Our simulations successfully reproduce the observed SED of bright (${M_{\rm UV}\sim -26}$) ${z\sim 6}$ quasars, and show that these objects are part of complex, dust-rich merging systems, containing multiple sources (accreting BHs and/or star forming galaxies) in agreement with recent HST and ALMA observations. Our results show that the proposed ORIGINS missions will be able to investigate the MIR properties of dusty star forming galaxies and to obtain good quality spectra of bright quasars at $z\sim 6$. Finally, the MIR-to-FIR flux ratio of faint (${M_{\rm UV}\sim -24}$) AGN is ${>10\times}$ higher than for normal star forming galaxies. This implies that combined JWST/ORIGINS/ALMA observations will be crucial to identify faint and/or dust-obscured AGN in the distant Universe.

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arXiv2021

Dust temperature in ALMA $\hbox{[C $\scriptstyle\rm II $]}$-detected high-$z$ galaxies

At redshift $z>5$ the far-infrared (FIR) continuum spectra of main-sequence galaxies are sparsely sampled, often with a single data point. The dust temperature $T_{\rm d, SED}$ thus has to be assumed in the FIR continuum fitting. This introduces large uncertainties regarding the derived dust mass ($M_{\rm d}$), FIR luminosity, and obscured fraction of the star formation rate. These are crucial quantities to quantify the effect of dust obscuration in high-$z$ galaxies. To overcome observations limitations, we introduce a new method that combines dust continuum information with the overlying $\hbox{[C $\scriptstyle\rm II $]} 158μ$m line emission. By breaking the $M_{\rm d} - T_{\rm d, SED}$ degeneracy, with our method, we can reliably constrain the dust temperature with a single observation at $158μ$m. This method can be applied to all ALMA and NOEMA $\hbox{[C $\scriptstyle\rm II $]}$ observations and exploited in ALMA Large Programs such as ALPINE and REBELS targeting $\hbox{[C $\scriptstyle\rm II $]}$ emitters at high-$z$. We also provide a physical interpretation of the empirical relation recently found between $molecular$ gas mass and $\hbox{[C $\scriptstyle\rm II $]}$ luminosity. We derive an analogous relation linking the $total$ gas surface density and $\hbox{[C $\scriptstyle\rm II $]}$ surface brightness. By combining the two, we predict the cosmic evolution of the surface density ratio $Σ_{\rm H_2} / Σ_{\rm gas}$. We find that $Σ_{\rm H_2} / Σ_{\rm gas}$ slowly increases with redshift, which is compatible with current observations at $0 < z < 4$.

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arXiv2020

Missing [CII] emission from early galaxies

ALMA observations have revealed that [CII] 158$μ$m line emission in high-z galaxies is ~2-3$\times$ more extended than the UV continuum emission. Here we explore whether surface brightness dimming (SBD) of the [CII] line is responsible for the reported [CII] deficit, and the large $L_{\rm [OIII]}/L_{\rm [CII]}$ luminosity ratio measured in early galaxies. We first analyse archival ALMA images of nine z>6 galaxies observed in both [CII] and [OIII]. After performing several uv-tapering experiments to optimize the identification of extended line emission, we detect [CII] emission in the whole sample, with an extent systematically larger than the [CII] emission. Next, we use interferometric simulations to study the effect of SBD on the line luminosity estimate. About 40% of the extended [CII] component might be missed at an angular resolution of 0.8$^{\prime\prime}$, implying that $L_{\rm [CII]}$ is underestimated by a factor $\approx2$ in data at low (<7) signal-to-noise ratio . By combining these results, we conclude that $L_{\rm [CII]}$ of z>6 galaxies lies, on average, slightly below the local $L_{\rm [CII]}-SFR$ relation ($Δ^{z=6-9}=-0.07\pm0.3$), but within the intrinsic dispersion of the relation. SBD correction also yields $L_{\rm [OIII]}/L_{\rm [CII]}<10$, i.e. more in line with current hydrodynamical simulations.

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arXiv2020

Velocity dispersion in the interstellar medium of early galaxies

We study the structure of spatially resolved, line-of-sight velocity dispersion for galaxies in the Epoch of Reionization (EoR) traced by [CII] $158μ\rm{m}$ line emission. Our laboratory is a simulated prototypical Lyman-break galaxy, "Freesia", part of the SERRA suite. The analysis encompasses the redshift range 6 < z < 8, when Freesia is in a very active assembling phase. We build velocity dispersion maps for three dynamically distinct evolutionary stages (Spiral Disk at z=7.4, Merger at z=8.0, and Disturbed Disk at z=6.5) using [CII] hyperspectral data cubes. We find that, at a high spatial resolution of 0.005" ($\simeq 30 pc$), the luminosity-weighted average velocity dispersion is $σ_{\rm{CII}}$~23-38 km/s with the highest value belonging to the highly-structured Disturbed Disk stage. Low resolution observations tend to overestimate $σ_{\rm CII}$ values due to beam smearing effects that depend on the specific galaxy structure. For an angular resolution of 0.02" (0.1"), the average velocity dispersion is 16-34% (52-115%) larger than the actual one. The [CII] emitting gas in Freesia has a Toomre parameter $\mathcal{Q}$~0.2 and a rotational-to-dispersion ratio of $v_{\rm c}/σ$~ 7 similar to that observed in z=2-3 galaxies. The primary energy source for the velocity dispersion is due to gravitational processes, such as merging/accretion events; energy input from stellar feedback is generally subdominant (< 10%). Finally, we find that the resolved $σ_{\rm{CII}} - Σ_{\rm SFR}$ relation is relatively flat for $0.02<Σ_{\rm SFR}/{{\rm M}_{\odot}} \mathrm{yr}^{-1} {\mathrm kpc}^{-2} < 30$, with the majority of data lying on the derived analytical relation $σ\propto Σ_{\rm SFR}^{5/7}$. At high SFR, the increased contribution from stellar feedback steepens the relation, and $σ_{\rm{CII}}$ rises slightly.

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arXiv2020

Early galaxy growth: mergers or gravitational instability?

We investigate the spatially-resolved morphology of galaxies in the early Universe. We consider a typical redshift z = 6 Lyman Break galaxy, "Althaea" from the SERRA hydrodynamical simulations. We create mock rest-frame ultraviolet, optical, and far-infrared observations, and perform a two-dimensional morphological analysis to de-blend the galaxy disk from substructures (merging satellites or star-forming regions). We find that the [CII]158um emitting region has an effective radius 1.5 - 2.5 times larger than the optical one, consistent with recent observations. This [CII] halo in our simulated galaxy arises as the joint effect of stellar outflows and carbon photoionization by the galaxy UV field, rather than from the emission of unresolved nearby satellites. At the typical angular resolution of current observations (> 0.15") only merging satellites can be detected; detection of star-forming regions requires resolutions of < 0.05". The [CII]-detected satellite has a 2.5 kpc projected distance from the galaxy disk, whereas the star-forming regions are embedded in the disk itself (distance < 1 kpc). This suggests that multi-component systems reported in the literature, which have separations > 2 kpc, are merging satellites, rather than galactic substructures. Finally, the star-forming regions found in our mock maps follow the local L[CII] - SFR_UV relation of galaxy disks, although sampling the low-luminosity, low-SFR tail of the distribution. We show that future JWST observations, bridging UV and [CII] datasets, will be exceptionally suited to characterize galaxy substructures thanks to their exquisite spatial resolution and sensitivity to both low-metallicity and dust-obscured regions that are bright at infrared wavelengths.

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arXiv2020

Warm dust in high-z galaxies: origin and implications

ALMA observations have revealed the presence of dust in galaxies in the Epoch of Reionization (redshift $z>6$). However, the dust temperature, $T_d$, remains unconstrained, and this introduces large uncertainties, particularly in the dust mass determinations. Using an analytical and physically-motivated model, we show that dust in high-$z$, star-forming giant molecular clouds (GMC), largely dominating the observed far-infrared luminosity, is warmer ($T_d > 60\ \mathrm{K}$) than locally. This is due to the more compact GMC structure induced by the higher gas pressure and turbulence characterizing early galaxies. The compactness also delays GMC dispersal by stellar feedback, thus $\sim 40\%$ of the total UV radiation emitted by newly born stars remains obscured. A higher $T_d$ has additional implications: it (a) reduces the tension between local and high-$z$ IRX-$β$ relation, (b) alleviates the problem of the uncomfortably large dust masses deduced from observations of some EoR galaxies.

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arXiv2020

Star formation law in the EoR from [CII] and CIII] lines

We present a novel method to simultaneously characterise the star formation law and the interstellar medium properties of galaxies in the Epoch of Reionization (EoR) through the combination of [CII] 158$μ$m (and its known relation with star formation rate) and CIII]$λ$1909Å emission line data. The method, based on a Markov Chain Monte Carlo algorithm, allows to determine the target galaxy average density, $n$, gas metallicity, $Z$, and burstiness parameter, $κ_s$, quantifying deviations from the Kennicutt-Schmidt relation. As an application, we consider COS-3018 (z=6.854), the only EoR Lyman Break Galaxy so far detected in both [CII] and CIII]. We show that COS-3018 is a moderate starburst ($κ_s \approx 3$), with $Z\approx 0.4\, Z_{\odot}$, and $n \approx 500\, {\rm cm^{-3}}$. Our method will be optimally applied to joint ALMA and JWST targets.

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arXiv2019

Constraints on high-J CO emission lines in $z\sim 6$ quasars

We present Atacama Large Millimiter/submillimiter Array (ALMA) observations of eight highly excited CO (J$_{\rm up}>8$) lines and continuum emission in two $z\sim6$ quasars: SDSS J231038.88+185519.7 (hereafter J2310), for which CO(8-7), CO(9-8), and CO(17-16) lines have been observed, and ULAS J131911.29+095951.4 (J1319), observed in the CO(14-13), CO(17-16) and CO(19-18) lines. The continuum emission of both quasars arises from a compact region ($< 0.9$ kpc). By assuming a modified black-body law, we estimate dust masses of Log$(M_{\rm dust}/M_{\odot})=8.75\pm0.07$ and Log$(M_{\rm dust}/M_{\odot})=8.8\pm0.2$ and dust temperatures of $T_{\rm dust}=76\pm3~{\rm K}$ and $T_{\rm dust}=66^{+15}_{-10}~{\rm K}$, respectively for J2310 and J1319. Only CO(8-7) and CO(9-8) in J2310 are detected, while $3σ$ upper limits on luminosities are reported for the other lines of both quasars. The CO line luminosities and upper limits measured in J2310 and J1319 are consistent with those observed in local AGN and starburst galaxies, and other $z\sim 6$ quasars, except for SDSS J1148+5251 (J1148), the only quasar at $z=6.4$ with a previous CO(17-16) line detection. By computing the CO SLEDs normalised to the CO(6-5) line and FIR luminosities for J2310, J1319, and J1149, we conclude that different gas heating mechanisms (X-ray radiation and/or shocks) may explain the different CO luminosities observed in these $z\sim6$ quasar. Future J$_{\rm up}>8$ CO observations will be crucial to understand the processes responsible for molecular gas excitation in luminous high-$z$ quasars.

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arXiv2019

Impact of X-rays on CO emission from high-z galaxies

We study the impact of active galactic nuclei (AGN) on the CO Spectral Line Energy Distribution (SLED) of high-$z$ galaxies. In particular, we want to assess if the CO SLED can be used as a probe of AGN activity. To this purpose, we develop a semi-analytical model that takes into account the radiative transfer and the clumpy structure of giant molecular clouds where the CO lines are excited, their distribution in the galaxy disk, and the torus obscuration of the AGN radiation. We study the joint effect on the CO SLED excitation of (i) the X-ray luminosity from the AGN ($L_{X}$), (ii) the size of the molecular disk, (iii) the inclination angle ($Ω$) of the torus with respect to the molecular disk, and (iv) the GMC mean density. We also discuss the possibility of an enhanced Cosmic Ray Ionization Rate (CRIR). We find that the X-ray Dominated Region (XDR) generated by the AGN in every case enhances the CO SLED for $J>5$, with increasing excitation of high-$J$ CO lines for increasing X-ray luminosity. Because high-$z$ galaxies are compact, the XDR region typically encloses the whole disk, thus its effect can be more important with respect to lower redshift objects. The impact of the XDR can be disentangled from an enhanced CRIR either if $L_X>10^{44} \rm \,erg\, s^{-1}$, or if $Ω\geq 60^{\circ}$. We finally provide predictions on the CO(7-6)/[CII] and CO(17-16)/[CII] ratios as a function of $L_X$, which can be relevant for ALMA follow up of galaxies and quasars previously detected in [CII].

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arXiv2019

A physical model for [CII] line emission from galaxies

A tight relation between the [CII]158$μ$m line luminosity and star formation rate is measured in local galaxies. At high redshift ($z>5$), though, a much larger scatter is observed, with a considerable (15-20\%) fraction of the outliers being [CII]-deficient. Moreover, the [CII] surface brightness ($Σ_{\rm CII}$) of these sources is systematically lower than expected from the local relation. To clarify the origin of such [CII]-deficiency we have developed an analytical model that fits local [CII] data, and has been validated against radiative transfer simulations performed with CLOUDY. The model predicts an overall increase of $Σ_{\rm CII}$ with the surface star formation rate ($Σ_*$). However, for $Σ_* > 1 M_\odot~{\rm yr}^{-1}~{\rm kpc}^{-2}$, $Σ_{\rm CII}$ saturates. We conclude that underluminous [CII] systems can result from a combination of three factors: (a) large upward deviations from the Kennicutt-Schmidt relation ($κ_s \gg 1$), parameterized by the "burstiness" parameter $κ_s$; (b) low metallicity; (c) low gas density, at least for the most extreme sources (e.g. CR7). Observations of [CII] emission alone cannot break the degeneracy among the above three parameters; this requires additional information coming from other emission lines (e.g. [OIII]88$μ$m, CIII]1909A, CO lines). Simple formulae are given to interpret available data for low and high-$z$ galaxies.

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arXiv2019

Photoevaporation of Jeans-unstable molecular clumps

We study the photoevaporation of Jeans-unstable molecular clumps by isotropic FUV (6 eV $< {\rm h}ν$ < 13.6 eV) radiation, through 3D radiative transfer hydrodynamical simulations implementing a non-equilibrium chemical network that includes the formation and dissociation of H$_2$. We run a set of simulations considering different clump masses ($M=10-200$ M$_\odot$) and impinging fluxes ($G_0=2\times 10^3-8\times 10^4$ in Habing units). In the initial phase, the radiation sweeps the clump as an R-type dissociation front, reducing the H$_2$ mass by a factor 40-90%. Then, a weak ($\mathcal{M}\simeq 2$) shock develops and travels towards the centre of the clump, which collapses while loosing mass from its surface. All considered clumps remain gravitationally unstable even if radiation rips off most of the clump mass, showing that external FUV radiation is not able to stop clump collapse. However, the FUV intensity regulates the final H$_2$ mass available for star formation: for example, for $G_0 < 10^4$ more than 10% of the initial clump mass survives. Finally, for massive clumps ($\sim 100$ M$_\odot$) the H$_2$ mass increases by 25-50% during the collapse, mostly because of the rapid density growth that implies a more efficient H$_2$ self-shielding.

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arXiv2019

Kinematics of $z\geq 6$ galaxies from [CII] line emission

We study the kinematical properties of galaxies in the Epoch of Reionization via the [CII] 158$μ$m line emission. The line profile provides information on the kinematics as well as structural properties such as the presence of a disk and satellites. To understand how these properties are encoded in the line profile, first we develop analytical models from which we identify disk inclination and gas turbulent motions as the key parameters affecting the line profile. To gain further insights, we use "Althaea", a highly-resolved ($30\, \rm pc$) simulated prototypical Lyman Break Galaxy, in the redshift range $z = 6-7$, when the galaxy is in a very active assembling phase. Based on morphology, we select three main dynamical stages: I) Merger , II) Spiral Disk, and III) Disturbed Disk. We identify spectral signatures of merger events, spiral arms, and extra-planar flows in I), II), and III), respectively. We derive a generalised dynamical mass vs. [CII]-line FWHM relation. If precise information on the galaxy inclination is (not) available, the returned mass estimate is accurate within a factor $2$ ($4$). A Tully-Fisher relation is found for the observed high-$z$ galaxies, i.e. $L_{\rm[CII]}\propto (FWHM)^{1.80\pm 0.35}$ for which we provide a simple, physically-based interpretation. Finally, we perform mock ALMA simulations to check the detectability of [CII]. When seen face-on, Althaea is always detected at $> 5σ$; in the edge-on case it remains undetected because the larger intrinsic FWHM pushes the line peak flux below detection limit. This suggests that some of the reported non-detections might be due to inclination effects.

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arXiv2019

Deep into the structure of the first galaxies: SERRA views

We study the formation and evolution of a sample of Lyman Break Galaxies in the Epoch of Reionization by using high-resolution ($\sim 10 \,{\rm pc}$), cosmological zoom-in simulations part of the SERRA suite. In SERRA, we follow the interstellar medium (ISM) thermo-chemical non-equilibrium evolution, and perform on-the-fly radiative transfer of the interstellar radiation field (ISRF). The simulation outputs are post-processed to compute the emission of far infrared lines ([CII], [NII], and [OIII]). At $z=8$, the most massive galaxy, `Freesia', has an age $t_\star \simeq 409\,{\rm Myr}$, stellar mass $M_{\star} \simeq 4.2\times 10^9 {\rm M}_{\odot}$, and a star formation rate ${\rm SFR} \simeq 11.5\,{\rm M}_{\odot}{\rm yr}^{-1}$, due to a recent burst. Freesia has two stellar components (A and B) separated by $\simeq 2.5\, {\rm kpc}$; other 11 galaxies are found within $56.9 \pm 21.6 \, {\rm kpc}$. The mean ISRF in the Habing band is $G = 7.9\, G_0$ and is spatially uniform; in contrast, the ionisation parameter is $U = 2^{+20}_{-2} \times 10^{-3}$, and has a patchy distribution peaked at the location of star-forming sites. The resulting ionising escape fraction from Freesia is $f_{\rm esc}\simeq 2\%$. While [CII] emission is extended (radius 1.54 kpc), [OIII] is concentrated in Freesia-A (0.85 kpc), where the ratio $Σ_{\rm [OIII]}/Σ_{\rm [CII]} \simeq 10$. As many high-$z$ galaxies, Freesia lies below the local [CII]-SFR relation. We show that this is the general consequence of a starburst phase (pushing the galaxy above the Kennicutt-Schmidt relation) which disrupts/photodissociates the emitting molecular clouds around star-forming sites. Metallicity has a sub-dominant impact on the amplitude of [CII]-SFR deviations.

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arXiv2018

Witnessing galaxy assembly at the edge of the reionization epoch

We report the discovery of Serenity-18, a galaxy at z=5.939 for which we could measure the content of molecular gas, M(H_2)~ 5 x10^9 M_sun, traced by the CO(6-5) emission, together with the metal-poor ([Fe/H]=-3.08 +- 0.12, [Si/H]=-2.86 +- 0.14) gas clump/filament which is possibly feeding its growth. The galaxy has an estimated star formation rate of ~100 M_sun yr^{-1}, implying that it is a typical main sequence galaxy at these redshifts. The metal-poor gas is detected through a damped Lyman-alpha absorber (DLA) observed at a spatial separation of 40 kpc and at the same redshift of Serenity-18, along the line of sight to the quasar SDSS J2310+1855 (z_em = 6.0025). The chemical abundances measured for the damped Lyman-alpha system are in very good agreement with those measured for other DLAs discovered at similar redshifts, indicating an enrichment due to massive PopII stars. The galaxy/Damped system we discovered is a direct observational evidence of the assembly of a galaxy at the edge of the reionization epoch.

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arXiv2018

Probing the High-Redshift Universe with SPICA: Toward the Epoch of Reionization and Beyond

With the recent discovery of a dozen dusty star-forming galaxies and around 30 quasars at z>5 that are hyper-luminous in the infrared ($μ$$L_{\rm IR}>10^{13}$ L$_{\odot}$, where $μ$ is a lensing magnification factor), the possibility has opened up for SPICA, the proposed ESA M5 mid-/far-infrared mission, to extend its spectroscopic studies toward the epoch of reionization and beyond. In this paper, we examine the feasibility and scientific potential of such observations with SPICA's far-infrared spectrometer SAFARI, which will probe a spectral range (35-230 $μ$m) that will be unexplored by ALMA and JWST. Our simulations show that SAFARI is capable of delivering good-quality spectra for hyper-luminous infrared galaxies (HyLIRGs) at z=5-10, allowing us to sample spectral features in the rest-frame mid-infrared and to investigate a host of key scientific issues, such as the relative importance of star formation versus AGN, the hardness of the radiation field, the level of chemical enrichment, and the properties of the molecular gas. From a broader perspective, SAFARI offers the potential to open up a new frontier in the study of the early Universe, providing access to uniquely powerful spectral features for probing first-generation objects, such as the key cooling lines of low-metallicity or metal-free forming galaxies (fine-structure and H2 lines) and emission features of solid compounds freshly synthesized by Population III supernovae. Ultimately, SAFARI's ability to explore the high-redshift Universe will be determined by the availability of sufficiently bright targets (whether intrinsically luminous or gravitationally lensed). With its launch expected around 2030, SPICA is ideally positioned to take full advantage of upcoming wide-field surveys such as LSST, SKA, Euclid, and WFIRST, which are likely to provide extraordinary targets for SAFARI.

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arXiv2018

The dense molecular gas in the $\rm z\sim6$ QSO SDSS J231038.88+185519.7 resolved by ALMA

We present ALMA observations of the CO(6-5) and [CII] emission lines and the sub-millimeter continuum of the $z\sim6$ quasi-stellar object (QSO) SDSS J231038.88+185519.7. Compared to previous studies, we have analyzed a synthetic beam that is ten times smaller in angular size, we have achieved ten times better sensitivity in the CO(6-5) line, and two and half times better sensitivity in the [CII] line, enabling us to resolve the molecular gas emission. We obtain a size of the dense molecular gas of $2.9\pm0.5$ kpc, and of $1.4\pm0.2$ kpc for the 91.5 GHz dust continuum. By assuming that CO(6-5) is thermalized, and by adopting a CO--to--$H_2$ conversion factor $\rm α_{CO} = 0.8~ M_{\odot}~K^{-1}~ (km/s)^{-1} ~pc^{2}$, we infer a molecular gas mass of $\rm M(H_2)=(3.2 \pm0.2) \times 10^{10}\rm M_{\odot}$. Assuming that the observed CO velocity gradient is due to an inclined rotating disk, we derive a dynamical mass of $\rm M_{dyn}~sin^2(i) = (2.4\pm0.5) \times 10^{10}~ M_{\odot}$, which is a factor of approximately two smaller than the previously reported estimate based on [CII]. Regarding the central black hole, we provide a new estimate of the black hole mass based on the C~IV emission line detected in the X-SHOOTER/VLT spectrum: $\rm M_{BH}=(1.8\pm 0.5) \times 10^{9}~ M_{\odot}$. We find a molecular gas fraction of $\rm μ=M(H_2)/M^*\sim4.4$, where $\rm M^*\approx M_{dyn} - M(H_2)-M(BH)$. We derive a ratio $v_{rot}/σ\approx 1-2$ suggesting high gas turbulence, outflows/inflows and/or complex kinematics due to a merger event. We estimate a global Toomre parameter $Q\sim 0.2-0.5$, indicating likely cloud fragmentation. We compare, at the same angular resolution, the CO(6-5) and [CII] distributions, finding that dense molecular gas is more centrally concentrated with respect to [CII]. We find that the current BH growth rate is similar to that of its host galaxy.

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