Simulating intergalactic quasar scintillation

Intergalactic scintillation of distant quasars is sensitive to free electrons and therefore complements Ly$\alpha$ absorption line experiments probing the neutral intergalactic medium (IGM). We present a new scheme to compute IGM refractive scintillation effects on distant sources in combination with Adaptive Mesh Refinement cosmological simulations. First we validate our model by reproducing the well-known interstellar scintillation (ISS) of Galactic sources. The simulated cosmic density field is then used to infer the statistical properties of intergalactic scintillation. Contrary to previous claims, we find that the scattering measure of the simulated IGM at $z<2$ is \langle \mbox{SM}_{\equ}\rangle=3.879, i.e. almost 40 times larger than for the usually assumed smooth IGM. This yield an average modulation index ranging from 0.01 ($\nu_s=5$ GHz) up to 0.2 ($\nu_s=50$ GHz); above \nu_{s}\gsim30 GHz the IGM contribution dominates over ISS modulation. We compare our model with data from a $0.3\leq z\leq 2$ quasar sample observed at \nu_{\obs}=8.4 GHz. For this high frequency ($10.92\leq \nu_s \leq 25.2$), high galactic latitude sample ISS is negligible, and IGM scintillation can reproduce the observed modulation with a 4% accuracy, without invoking intrinsic source variability. We conclude by discussing the possibility of using IGM scintillation as a tool to pinpoint the presence of intervening high-$z$ groups/clusters along the line of sight, thus making it a probe suitably complementing Sunyaev-Zeldovich data recently obtained by \textit{Planck}.Comment: 14 pages, 13 figures, accepted for publication in MNRA

Missing cosmic metals revealed by X-ray absorption towards distant sources

The census of heavy elements (metals) produced by all stars through cosmic times up to present-day is limited to ~50%; of these only half are still found within their parent galaxy. The majority of metals is expelled from galaxies into the circumgalactic (or even more distant, intergalactic) space by powerful galactic winds, leaving unpleasant uncertainty on the amount, thermal properties and distribution of these key chemical species. These dispersed metals unavoidably absorb soft X-ray photons from distant sources. We show that their integrated contribution can be detected in the form of increasing X-ray absorption with distance, for all kinds of high-energy cosmic sources. Based on extensive cosmological simulations, we assess that $\sim$ 10\% of all cosmic metals reside in the intergalactic medium. Most of the X-ray absorption arises instead from a few discrete structures along the line of sight. These extended structures, possibly pin-pointing galaxy groups, contain million degree, metal-enriched gas, 100-1,000 times denser than the cosmic mean. An additional ~10% of cosmic metals could reside in this phase.Comment: Accepted for publication in Astronomy & Astrophysics. 9 pages, 4 figures, 1 tabl

Stochastic star formation in early galaxies: JWST implications

The star formation rate (SFR) in high redshift galaxies is expected to be time-variable due to competing physical processes. Such stochastic variability might boost the luminosity of galaxies, possibly explaining the over-abundance seen at $z\gtrsim 10$ by JWST. We aim at quantifying the amplitude and timescales of such variability, and identifying the key driving physical processes. We select 245 $z=7.7$ galaxies with stellar mass $5\times 10^{6}\lesssim M_\star/{\rm M}_\odot\lesssim 5\times 10^{10}$ from SERRA, a suite of high-resolution, radiation-hydrodynamic cosmological simulations. After fitting the average SFR trend, $\langle {\rm SFR} \rangle$, we quantify the time-dependent variation, $\delta(t) \equiv \log [\rm SFR/\langle {\rm SFR} \rangle]$ for each system, and perform a periodogram analysis to search for periodicity modulations. We find that $\delta(t)$ is distributed as a zero-mean Gaussian, with standard deviation $\sigma_\delta \simeq 0.24$ (corresponding to a UV magnitude s.d. $\sigma_{\rm UV} \simeq 0.61$) that is independent of $M_\star$. However, the modulation timescale increases with stellar mass: $t_\delta \sim (9, 50, 100)\, \rm Myr$ for $M_\star \sim (0.1, 1, 5)\times 10^9\, {\rm M}_\odot$, respectively. These timescales are imprinted on the SFR by different processes: (i) photoevaporation, (ii) supernova explosions, and (iii) cosmological accretion dominating in low, intermediate, and high mass systems, respectively. The predicted SFR variations cannot account for the required $z\gtrsim 10$ UV luminosity function boost. Other processes, such as radiation-driven outflows clearing the dust, must then be invoked to explain the enhanced luminosity of super-early systems.Comment: 5 pages, 4 figures, submitted to A&

Simulating cosmic metal enrichment by the first galaxies

We study cosmic metal enrichment via AMR hydrodynamical simulations in a (10 Mpc/h)$^3$ volume following the Pop III-Pop II transition and for different Pop III IMFs. We have analyzed the joint evolution of metal enrichment on galactic and intergalactic scales at z=6 and z=4. Galaxies account for <9% of the baryonic mass; the remaining gas resides in the diffuse phases: (a) voids, i.e. regions with extremely low density ($\Delta$<1), (b) the true intergalactic medium (IGM, 1<$\Delta$<10) and (c) the circumgalactic medium (CGM, 10<$\Delta<10^{2.5}$), the interface between the IGM and galaxies. By z=6 a galactic mass-metallicity relation is established. At z=4, galaxies with a stellar mass $M_*=10^{8.5}M_\odot$ show log(O/H)+12=8.19, consistent with observations. The total amount of heavy elements rises from $\Omega^{SFH}_Z=1.52\, 10^{-6}$ at z=6 to 8.05 $10^{-6}$ at z=4. Metals in galaxies make up to ~0.89 of such budget at z=6; this fraction increases to ~0.95 at z=4. At z=6 (z=4) the remaining metals are distributed in CGM/IGM/voids with the following mass fractions: 0.06/0.04/0.01 (0.03/0.02/0.01). Analogously to galaxies, at z=4 a density-metallicity ($\Delta$-Z) relation is in place for the diffuse phases: the IGM/voids have a spatially uniform metallicity, Z~$10^{-3.5}$Zsun; in the CGM Z steeply rises with density up to ~$10^{-2}$Zsun. In all diffuse phases a considerable fraction of metals is in a warm/hot (T>$10^{4.5}$K) state. Due to these physical conditions, CIV absorption line experiments can probe only ~2% of the total carbon present in the IGM/CGM; however, metal absorption line spectra are very effective tools to study reionization. Finally, the Pop III star formation history is almost insensitive to the chosen Pop III IMF. Pop III stars are preferentially formed in truly pristine (Z=0) gas pockets, well outside polluted regions created by previous star formation episodes.Comment: 23 pages, 18 figures, 3 tables, Accepted for publication in MNRA

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

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

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

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

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

The stellar populations of high-redshift dwarf galaxies

We use high-resolution ($\approx 10$ pc), zoom-in simulations of a typical (stellar mass $M_\star\simeq10^{10}M_\odot$) Lyman Break Galaxy (LBG) at $z\simeq 6$ to investigate the stellar populations of its six dwarf galaxy satellites, whose stellar [gas] masses are in the range $\log (M_\star/M_\odot) \simeq 6-9$ [$\log (M_{gas}/M_\odot) \simeq4.3-7.75$]. The properties and evolution of satellites show no dependence on the distance from the central massive LBG ($< 11.5$ kpc). Instead, their star formation and chemical enrichment histories are tightly connected their stellar (and sub-halo) mass. High-mass dwarf galaxies ($\rm M_\star \gtrsim 5\times 10^8 M_\odot$) experience a long history of star formation, characterised by many merger events. Lower-mass systems go through a series of short star formation episodes, with no signs of mergers; their star formation activity starts relatively late ($z\approx 7$), and it is rapidly quenched by internal stellar feedback. In spite of the different evolutionary patterns, all satellites show a spherical morphology, with ancient and more metal-poor stars located towards the inner regions. All six dwarf satellites experienced high star formation rate ($\rm >5\,M_\odot yr ^{-1}$) bursts, which can be detected by JWST while targeting high-$z$ LBGs.Comment: 17 pages, 14 figures. To be published in MNRA