On the [CII]-SFR relation in high redshift galaxies

After two ALMA observing cycles, only a handful of [CII] $158\,\mu m$ emission line searches in z>6 galaxies have reported a positive detection, questioning the applicability of the local [CII]-SFR relation to high-z systems. To investigate this issue we use the Vallini et al. 2013 (V13) model, based on high-resolution, radiative transfer cosmological simulations to predict the [CII] emission from the interstellar medium of a z~7 (halo mass $M_h=1.17\times10^{11}M_{\odot}$) galaxy. We improve the V13 model by including (a) a physically-motivated metallicity (Z) distribution of the gas, (b) the contribution of Photo-Dissociation Regions (PDRs), (c) the effects of Cosmic Microwave Background on the [CII] line luminosity. We study the relative contribution of diffuse neutral gas to the total [CII] emission ($F _{diff}/F_{tot}$) for different SFR and Z values. We find that the [CII] emission arises predominantly from PDRs: regardless of the galaxy properties, $F _{diff}/F_{tot}\leq 10$% since, at these early epochs, the CMB temperature approaches the spin temperature of the [CII] transition in the cold neutral medium ($T_{CMB}\sim T_s^{CNM}\sim 20$ K). Our model predicts a high-z [CII]-SFR relation consistent with observations of local dwarf galaxies ($0.02<Z/Z_{\odot}<0.5$). The [CII] deficit suggested by actual data ($L_{CII}<2.0\times 10^7 L_{\odot}$ in BDF3299 at z~7.1) if confirmed by deeper ALMA observations, can be ascribed to negative stellar feedback disrupting molecular clouds around star formation sites. The deviation from the local [CII]-SFR would then imply a modified Kennicutt-Schmidt relation in z>6 galaxies. Alternatively/in addition, the deficit might be explained by low gas metallicities ($Z<0.1 Z_{\odot}$).Comment: 9 pages, 6 figures, replaced with the version accepted for pubblication in Ap

Neural networks : solving the chemistry of the interstellar medium

Non-equilibrium chemistry is a key process in the study of the interstellar medium (ISM), in particular the formation of molecular clouds and thus stars. However, computationally, it is among the most difficult tasks to include in astrophysical simulations, because of the typically high (&gt;40) number of reactions, the short evolutionary time-scales (about 104 times less than the ISM dynamical time), and the characteristic non-linearity and stiffness of the associated ordinary differential equations&nbsp;system (ODEs). In this proof of concept work, we show that Physics Informed Neural Networks (PINN) are a viable alternative to traditional ODE time integrators for stiff thermochemical systems, i.e. up to molecular hydrogen formation (9 species and 46 reactions). Testing different chemical networks in a wide range of densities (−2 &lt; log n/cm−3 &lt; 3) and temperatures (1 &lt; log T/K &lt; 5), we find that a basic architecture can give a comfortable convergence only for simplified chemical systems: to properly capture the sudden chemical and thermal variations, a Deep Galerkin Method is needed. Once trained (∼103 GPUhr), the PINN well reproduces the strong non-linear nature of the solutions (errors ≲10&nbsp;per&nbsp;cent⁠) and can give speed-ups up to a factor of ∼200 with respect to traditional ODE solvers. Further, the latter have completion times that vary by about ∼30&nbsp;per&nbsp;cent for different initial n and T, while the PINN method gives negligible variations. Both the speed-up and the potential improvement in load balancing imply that PINN-powered simulations are a very palatable way to solve complex chemical calculation in astrophysical and cosmological problems

CO line emission from galaxies in the Epoch of Reionization

We study the CO line luminosity ($L_{\rm CO}$), the shape of the CO Spectral Line Energy Distribution (SLED), and the value of the CO-to-$\rm H_2$ conversion factor in galaxies in the Epoch of Reionization (EoR). To this aim, we construct a model that simultaneously takes into account the radiative transfer and the clumpy structure of giant molecular clouds (GMCs) where the CO lines are excited. We then use it to post-process state-of-the-art zoomed, high resolution ($30\, \rm{pc}$), cosmological simulation of a main-sequence ($M_{*}\approx10^{10}\, \rm{M_{\odot}}$, $SFR\approx 100\,\rm{M_{\odot}\, yr^{-1}}$) galaxy, "Alth{\ae}a", at $z\approx6$. We find that the CO emission traces the inner molecular disk ($r\approx 0.5 \,\rm{kpc}$) of Alth{\ae}a with the peak of the CO surface brightness co-located with that of the [CII] 158$\rm \mu m$ emission. Its $L_{\rm CO(1-0)}=10^{4.85}\, \rm{L_{\odot}}$ is comparable to that observed in local galaxies with similar stellar mass. The high ($\Sigma_{gas} \approx 220\, \rm M_{\odot}\, pc^{-2}$) gas surface density in Alth{\ae}a, its large Mach number (\mach$\approx 30$), and the warm kinetic temperature ($T_{k}\approx 45 \, \rm K$) of GMCs yield a CO SLED peaked at the CO(7-6) transition, i.e. at relatively high-$J$, and a CO-to-$\rm H_2$ conversion factor $\alpha_{\rm CO}\approx 1.5 \, \rm M_{\odot} \rm (K\, km\, s^{-1}\, pc^2)^{-1}$ lower than that of the Milky Way. The ALMA observing time required to detect (resolve) at 5$\sigma$ the CO(7-6) line from galaxies similar to Alth{\ae}a is $\approx13$ h ($\approx 38$ h).Comment: 16 pages, 14 figures, accepted for publication in MNRA

Mapping metals at high redshift with far-infrared lines

Cosmic metal enrichment is one of the key physical processes regulating galaxy formation and the evolution of the intergalactic medium (IGM). However, determining the metal content of the most distant galaxies has proven so far almost impossible; also, absorption line experiments at $z\sim6$ become increasingly difficult because of instrumental limitations and the paucity of background quasars. With the advent of ALMA, far-infrared emission lines provide a novel tool to study early metal enrichment. Among these, the [CII] line at 157.74 $\mu$m is the most luminous line emitted by the interstellar medium of galaxies. It can also resonant scatter CMB photons inducing characteristic intensity fluctuations ($\Delta I/I_{CMB}$) near the peak of the CMB spectrum, thus allowing to probe the low-density IGM. We compute both [CII] galaxy emission and metal-induced CMB fluctuations at $z\sim 6$ by using Adaptive Mesh Refinement cosmological hydrodynamical simulations and produce mock observations to be directly compared with ALMA BAND6 data ($\nu_{obs}\sim 272$ GHz). The [CII] line flux is correlated with $M_{UV}$ as $\log(F_{peak}/\mu{\rm Jy})=-27.205-2.253\,M_{UV}-0.038\,M_{UV}^2$. Such relation is in very good agreement with recent ALMA observations (e.g. Maiolino et al. 2015; Capak et al. 2015) of $M_{UV}<-20$ galaxies. We predict that a $M_{UV}=-19$ ($M_{UV}=-18$) galaxy can be detected at $4\sigma$ in $\simeq40$ (2000) hours, respectively. CMB resonant scattering can produce $\simeq\pm 0.1\,\mu$Jy/beam emission/absorptions features that are very challenging to be detected with current facilities. The best strategy to detect these signals consists in the stacking of deep ALMA observations pointing fields with known $M_{UV}\simeq-19$ galaxies. This would allow to simultaneously detect both [CII] emission from galactic reionization sources and CMB fluctuations produced by $z\sim6$ metals.Comment: 13 pages, 6 figure

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

We study the kinematical properties of galaxies in the Epoch of Reionization via the [CII] 158$\mu$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 $> 5\sigma$; 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.Comment: 14 pages, 12 figures, accepted for publication in MNRA

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 $\Sigma_{\rm [OIII]}/\Sigma_{\rm [CII]} \simeq 10$. 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.Comment: 22 pages, 14 figures, accepted by MNRA

Photoevaporation of Jeans-unstable molecular clumps

We study the photoevaporation of Jeans-unstable molecular clumps by isotropic FUV (6 eV &lt; h\u3bd &lt; 13.6 eV) radiation, through 3D radiative transfer hydrodynamical simulations implementing a non-equilibrium chemical network that includes the formation and dissociation of H2. We run a set of simulations considering different clump masses (M=10 - 200 M_{odot }) and impinging fluxes (G0 = 2 7 103 to 8 7 104 in Habing units). In the initial phase, the radiation sweeps the clump as an R-type dissociation front, reducing the H2 mass by a factor 40 - 90{{ per cent}}. Then, a weak (M 3ceq 2) shock develops and travels towards the centre of the clump, which collapses while losing 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 H2 mass available for star formation: for example, for G0 &lt; 104 more than 10 per cent of the initial clump mass survives. Finally, for massive clumps ({ 73 } 100 M_{odot }) the H2 mass increases by 25 - 50{{ per cent}} during the collapse, mostly because of the rapid density growth that implies a more efficient H2 self-shielding

Dusty galaxies in the Epoch of Reionization: simulations

The recent discovery of dusty galaxies well into the Epoch of Reionization (redshift $z>6$) poses challenging questions about the properties of the interstellar medium in these pristine systems. By combining state-of-the-art hydrodynamic and dust radiative transfer simulations, we address these questions focusing on the recently discovered dusty galaxy A2744_YD4 ($z=8.38$, Laporte et al. 2017}). We show that we can reproduce the observed spectral energy distribution (SED) only using different physical values with respect to the inferred ones by Laporte et al(2017), i.e. a star formation rate of $\mathrm{SFR} = 78\;\rm M_\odot \rm yr^{-1}$, a factor $\approx 4$ higher than deduced from simple Spectral Energy Distribution fitting. In this case we find: (a) dust attenuation (corresponding to $\tau_V=1.4$) is consistent with a Milky Way extinction curve; (b) the dust-to-metal ratio is low, $f_\mathrm{d} \sim 0.08$, implying that early dust formation is rather inefficient; (c) the luminosity-weighted dust temperature is high, $T_d=91\pm 23\, \rm K$, as a result of the intense ($\approx 100\times$ MW) interstellar radiation field; (d) due to the high $T_d$, the ALMA Band 7 detection can be explained by a limited dust mass, $M_d=1.6\times 10^6$M$_\odot$. Finally, the high dust temperatures might solve the puzzling low infrared excess recently deduced for high-$z$ galaxies from the IRX-$\beta$ relation.Comment: 15 pages, accepted for publication in MNRA

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-$\beta$ relation, (b) alleviates the problem of the uncomfortably large dust masses deduced from observations of some EoR galaxies.Comment: 14 pages, 6 figures, accepted for publication in MNRA

Dust temperature in {ALMA} [C~ii]-detected high-z galaxies

At redshift z &gt; 5, the far-infrared (FIR) continuum spectra of main-sequence galaxies are sparsely sampled, often with a single data point. The dust temperature T-d(,SED), thus has to be assumed in the FIR continuum fitting. This introduces large uncertainties regarding the derived dust mass (M-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 observation limitations, we introduce a new method that combines dust continuum information with the overlying [C II] 158 mu m line emission. By breaking the M-d T-d(,SED) degeneracy, with our method, we can reliably constrain the dust temperature with a single observation at 158 mu m. This method can be applied to all Atacama Large Millimeter Array (ALMA) and NOEMA [C II] observations, and exploited in ALMA Large Programs such as ALPINE and REBELS targeting [C II] emitters at high-z. We also provide a physical interpretation of the empirical relation recently found between molecular gas mass and [C II] luminosity. We derive an analogous relation linking the total gas surface density and [C II] surface brightness. By combining the two, we predict the cosmic evolution of the surface density ratio Sigma(H2) / Sigma(gas). We find that Sigma(H2)/ Sigma(gas) slowly increases with redshift, which is compatible with current observations at 0 &lt; z &lt; 4