The circumgalactic medium of high redshift galaxies

We study the properties of the circumgalactic medium (CGM) of high-$z$ galaxies in the metal enrichment simulations presented in Pallottini et al. 2014. At $z=4$, we find that the simulated CGM gas density profiles are self-similar, once scaled with the virial radius of the parent dark matter halo. We also find a simple analytical expression relating the neutral hydrogen equivalent width (${\rm EW}_{\rm HI}$) of CGM absorbers as a function of the line of sight impact parameter ($b$). We test our predictions against mock spectra extracted from the simulations, and show that the model reproduces the ${\rm EW}_{\rm HI}(b)$ profile extracted from the synthetic spectra analysis. When compared with available data, our CGM model nicely predicts the observed ${\rm EW}_{\rm HI}(b)$ in $z\lesssim2$ galaxies, and supports the idea that the CGM profile does not evolve with redshift.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter

The nature of the Lyman Alpha Emitter CR7: a persisting puzzle

The peculiar emission properties of the $z \sim 6.6$ Ly$\alpha$ emitter CR7 have been initially interpreted with the presence of either a direct collapse black hole (DCBH) or a substantial mass of Pop III stars. Instead, updated photometric observations by Bowler et al. (2016) seem to suggest that CR7 is a more standard system. Here we confirm that the original DCBH hypothesis is consistent also with the new data. Using radiation-hydrodynamic simulations, we reproduce the new IR photometry with two models involving a Compton-thick DCBH of mass $\approx 7 \times 10^6 \, \mathrm{M_{\odot}}$ accreting (a) metal-free ($Z=0$) gas with column density $N_H = 8 \times 10^{25} \, \mathrm{cm^{-2}}$, or (b) low-metallicity gas ($Z = 5 \times 10^{-3} \, \mathrm{Z_{\odot}}$) with $N_H = 3 \times 10^{24} \, \mathrm{cm^{-2}}$. The best fit model reproduces the photometric data to within $1 \sigma$. Such metals can be produced by weak star-forming activity occurring after the formation of the DCBH. The main contribution to the Spitzer/IRAC $3.6 \, \mathrm{\mu m}$ photometric band in both models is due to HeI/HeII $\lambda 4714, 4687$ emission lines, while the contribution of [OIII] $\lambda 4959, 5007$ emission lines, if present, is sub-dominant. Spectroscopic observations with JWST will be required to ultimately clarify the nature of CR7.Comment: Accepted for publication in MNRAS Letter

Scintillation in the InterGalactic Medium

The scintillation phenomenon consists in a light wave, coming from a compact enough source, that, passing through a ionized inhomogeneous medium, is subject to a random variation of the refraction index, and thus suffers multiple phase changes which lead to fluctuations in all the wave related quantities; the aim of this Thesis is to exploit the effects of this process when the light comes from a distant quasar and the random medium is given by the InterGalactic Medium, namely the baryon mass fraction of the universe laying between galaxies that is not locked up in stellar objects. The motivation that drives us toward this purpose is that either until now in this specific context a full investigation has not been developed in the literature, and either the IGM scintillation can, as we eventually show, be relevant in explaining some of the current discrepancies between theory and observations; furthermore this can prove to be a useful investigation tool, complementary to the ones available. To achieve the result proposed we have decided to employ the standard cosmological model, which gives the proper tools to analyze the environment we want to deal with, which is described in Chapter 1, and to numerically simulate the IGM, with the tools provided and depicted in Chapter 2. Next, in Chapter 3, we review the scintillation physical basis and its application to the InterStellar Medium, and then we naturally extend the ISM results to the IGM case. In Chapter 4 we expose and discuss the results of the simulation and finally, in Chapter 5, we apply our model to the simulation and analyze the behaviour of the IGM scintillation, confronting it with the observations. To conclude, in Chapter 6 we summarize the results obtained and the possible future developments

Ly{\alpha} emission from galaxies in the Epoch of Reionization

The intrinsic strength of the Ly$\alpha$ line in young, star-forming systems makes it a special tool for studying high-redshift galaxies. However, interpreting observations remains challenging due to the complex radiative transfer involved. Here, we combine state-of-the-art hydrodynamical simulations of 'Althaea', a prototypical Lyman Break Galaxy (LBG, stellar mass $M_{\star}$$\simeq$ $10^{10}{\rm M}_{\odot})$ at $z=7.2$, with detailed radiative transfer computations of dust/continuum, [CII] 158 $\mu$m, and Ly$\alpha$ to clarify the relation between the galaxy properties and its Ly$\alpha$ emission. Althaea exhibits low ($f_\alpha< 1\%$) Ly$\alpha$ escape fractions and Equivalent Widths, EW $\lesssim 6$ Angstrom for the simulated lines of sight, with a large scatter. The correlation between escape fraction and inclination is weak, as a result of the rather chaotic structure of high-redshift galaxies. Low $f_\alpha$ values persist even if we artificially remove neutral gas around star forming regions to mimick the presence of HII regions. The high attenuation is primarily caused by dust clumps co-located with young stellar clusters. We can turn Althaea into a Lyman Alpha Emitter (LAE) only if we artificially remove dust from the clumps, yielding EWs up to $22$ Angstrom. Our study suggests that the LBG-LAE duty-cycle required by recent clustering measurements poses the challenging problem of a dynamically changing dust attenuation. Finally, we find an anti-correlation between the magnitude of Ly$\alpha$-[CII] line velocity shift and Ly$\alpha$ luminosity.Comment: published in MNRA

Molecular clumps photoevaporation in ionized regions

We study the photoevaporation of molecular clumps exposed to a UV radiation field including hydrogen-ionizing photons ($h\nu > 13.6$ eV) produced by massive stars or quasars. We follow the propagation and collision of shock waves inside clumps and take into account self-shielding effects, determining the evolution of clump size and density with time. The structure of the ionization-photodissociation region (iPDR) is obtained for different initial clump masses ($M=0.01 - 10^4\,{\rm M}_\odot$) and impinging fluxes ($G_0=10^2 - 10^5$ in units of the Habing flux). The cases of molecular clumps engulfed in the HII region of an OB star and clumps carried within quasar outflows are treated separately. We find that the clump undergoes in both cases an initial shock-contraction phase and a following expansion phase, which lets the radiation penetrate in until the clump is completely evaporated. Typical evaporation time-scales are $\simeq 0.01$ Myr in the stellar case and 0.1 Myr in the quasar case, where the clump mass is 0.1 ${\rm M}_\odot$ and $10^3\,{\rm M}_\odot$ respectively. We find that clump lifetimes in quasar outflows are compatible with their observed extension, suggesting that photoevaporation is the main mechanism regulating the size of molecular outflows.Comment: 13 pages, 12 figures, accepted for publication in MNRA

Intensity mapping of [CII] from early galaxies

The intensity mapping of the [CII] 157.7 $\rm \mu$m fine-structure emission line represents an ideal experiment to probe star formation activity in galaxies, especially in those that are too faint to be individually detected. Here, we investigate the feasibility of such an experiment for $z > 5$ galaxies. We construct the $L_{\rm CII} - M_{\rm h}$ relation from observations and simulations, then generate mock [CII] intensity maps by applying this relation to halo catalogs built from large scale N-body simulations. Maps of the extragalactic far-infrared (FIR) continuum, referred to as "foreground", and CO rotational transition lines and [CI] fine-structure lines referred to as "contamination", are produced as well. We find that, at 316 GHz (corresponding to $z_{\rm CII} = 5$), the mean intensities of the extragalactic FIR continuum, [CII] signal, all CO lines from $J=1$ to 13 and two [CI] lines are $\sim 3\times10^5$ Jy sr$^{-1}$, $\sim 1200$ Jy sr$^{-1}$, $\sim 800$ Jy sr$^{-1}$ and $\sim 100$ Jy sr$^{-1}$, respectively. We discuss a method that allows us to subtract the FIR continuum foreground by removing a spectrally smooth component from each line of sight, and to suppress the CO/[CI] contamination by discarding pixels that are bright in contamination emission. The $z > 5$ [CII] signal comes mainly from halos in the mass range $10^{11-12} \,M_\odot$; as this mass range is narrow, intensity mapping is an ideal experiment to investigate these early galaxies. In principle such signal is accessible to a ground-based telescope with a 6 m aperture, 150 K system temperature, a $128\times128$ pixels FIR camera in 5000 hr total integration time, however it is difficult to perform such an experiment by using currently available telescopes.Comment: 12 pages, 10 figures. Accepted for publication in MNRA

On the stunning abundance of super-early, luminous galaxies revealed by JWST

The earliest JWST observations have revealed an unexpected abundance of super-early (z &gt; 10), massive (M∗ ≈ 109M☉ ) galaxies at the bright-end (MUV ≈ −21) of the ultraviolet luminosity function (UV LF). We present a minimal physical model that explains the observed galaxy abundance at z = 10-14. The model primarily combines (i) the halo mass function, with (ii) an obscured star formation fraction prescription that is consistent with findings of the ALMA REBELS dusty galaxy survey. It has been successfully tested on well-known UV LFs up to z = 7. We argue that the weak evolution from z = 7 to z ≈ 14 of the LF bright-end can arise from a conspiracy between a decreasing dust attenuation, making galaxies brighter, that almost exactly compensates for the increasing shortage of their host halos. Our minimal model naturally reproduces the z = 10-14 LF if galaxies at z ∼&gt; 11 contain a negligible amounts of dust. We speculate that dust could have been efficiently ejected during the very first phases of galaxy build-up.</p

Quiescent low-mass galaxies observed by JWST in the Epoch of Reionization

The surprising JWST discovery of a quiescent, low-mass ($M_\star=10^{8.7} \rm M_\odot$) galaxy at redshift $z=7.3$ (JADES-GS-z7-01-QU) represents a unique opportunity to study the imprint of feedback processes on early galaxy evolution. We build a sample of 130 low-mass ($M_\star\lesssim 10^{9.5} \rm M_\odot$) galaxies from the SERRA cosmological zoom-in simulations, which show a feedback-regulated, bursty star formation history (SFH). The fraction of time spent in an active phase increases with the stellar mass from $f_{duty}\approx 0.6$ at $M_\star\approx 10^{7.5} \rm M_\odot$ to $\approx 0.99$ at $M_\star\geq 10^{9} \rm M_\odot$, and it is in agreement with the value $f_{duty}\approx 0.75$ estimated for JADES-GS-z7-01-QU. On average, 30% of the galaxies are quiescent in the range $6 < z < 8.4$; they become the dominant population at $M_\star\lesssim 10^{8.3} \rm M_\odot$. However, none of these quiescent systems matches the Spectral Energy Distribution of JADES-GS-z7-01-QU, unless their SFH is artificially truncated a few Myr after the main star formation peak. As supernova feedback can only act on a longer timescale ($\gtrsim 30 \rm \, Myr$), this implies that the observed abrupt quenching must be caused by a faster physical mechanism, such as radiation-driven winds.Comment: submitted to ApJ Letters, 5 pages, 4 figure

Dwarf satellites of high-z Lyman Break Galaxies: a free lunch for JWST

We show that the James Webb Space Telescope will be able to detect dwarf satellites of high-$z$ Lyman Break Galaxies (LBGs). To this aim, we use cosmological simulations following the evolution of a typical $M_\star\simeq10^{10}\rm M_\odot$ LBG up to $z\simeq6$, and analyse the observational properties of its five satellite dwarf galaxies ($10^7{\rm M_\odot}<M_\star<10^9{\rm M_\odot}$). Modelling their stellar emission and dust attenuation, we reconstruct their rest-frame UV-optical spectra for $6<z<6.5$. JWST/NIRCam synthetic images show that the satellites can be spatially resolved from their host, and their emission is detectable by planned deep surveys. Moreover, we build synthetic spectral energy distributions and colour-magnitude diagrams for the satellites. We conclude that the color $\rm F200W-F356W$ is a powerful diagnostic tool for understanding their physical properties once they have been identified. For example, $\rm F200W-F356W~\lesssim-0.25$ can be used to identify star-bursting ($\rm SFR\sim5~M_\odot yr^{-1}$), low-mass ($M_\star\lesssim5\times 10^8\rm M_\odot$) systems, with $\sim80\%$ of their stars being young and metal-poor ($\log(Z_\star/Z_\odot) < -0.5$).Comment: 8 pages, 3 figures, accepted for publication in ApJ

Dynamical Properties of Molecular-forming Gas Clumps in Galaxies at the Epoch of Reionization

We study the properties of molecular-forming gas clumps (MGCs) at the epoch of reionization using cosmological zoom-in simulations. We identify MGCs in a z=6 prototypical galaxy ("Althaea") using an H2 density-based clump finder. We compare their mass, size, velocity dispersion, gas surface density, and virial parameter (alpha_vir) to observations. In Althaea, the typical MGC mass and size are Mgas=10^6.5 Msun and R=45-100 pc, which are comparable to those found in nearby spirals and starburst galaxies. MGCs are highly supersonic and supported by turbulence, with rms velocity dispersions of sigma_gas=20-100 km s^-1 and pressure of P/k_B=10^7.6 K cm^-3 (i.e., >1000x with respect to the Milky Way), similar to those found in nearby and z~2 gas-rich starburst galaxies. In addition, we perform stability analysis to understand the origin and dynamical properties of MGCs. We find that MGCs are globally stable in the main disk of Althaea. Densest regions where star formation is expected to take place in clouds and cores on even smaller scales instead have lower alpha_vir and Toomre-Q values. Detailed studies of the star-forming gas dynamics at the epoch of reionization thus require a spatial resolution of <40 pc (=0.01"), which is within reach with the Atacama Large (sub-)Millimeter Array and the Next Generation Very Large Array.Comment: 16 pages, 10 figures, ApJ accepte