arXiv subjects

Andrea Ferrara

Publications and source records attributed to Andrea Ferrara.

At least 145 records · Page 8Linked to original sources

arXiv2016

Coevolution of metallicity and star formation in galaxies to z=3.7: I. A fundamental plane

With the aim of understanding the coevolution of star formation rate (SFR), stellar mass (M*), and oxygen abundance (O/H) in galaxies up to redshift z=3.7, we have compiled the largest available dataset for studying Metallicity Evolution and Galaxy Assembly (MEGA); it comprises roughly 1000 galaxies with a common O/H calibration and spans almost two orders of magnitude in metallicity, a factor of 10^6 in SFR, and a factor of 10^5 in stellar mass. From a Principal Component Analysis, we find that the 3-dimensional parameter space reduces to a Fundamental Plane of Metallicity (FPZ) given by 12+log(O/H) = -0.14 log (SFR) + 0.37 log (M*) + 4.82. The mean O/H FPZ residuals are small (0.16 dex) and consistent with trends found in smaller galaxy samples with more limited ranges in M*, SFR, and O/H. Importantly, the FPZ is found to be redshift-invariant within the uncertainties. In a companion paper, these results are interpreted with an updated version of the model presented by Dayal et al. (2013).

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arXiv2016

Coevolution of metallicity and star formation in galaxies to z=3.7: II. A theoretical model

Recent work suggests that galaxy evolution, and the build-up of stellar mass (M*) over cosmic time, is characterized by changes with redshift of star formation rate (SFR) and oxygen abundance (O/H). In a companion paper, we have compiled a large dataset to study Metallicity Evolution and Galaxy Assembly (MEGA), consisting of roughly 1000 galaxies to z=3.7 with a common O/H calibration. Here we interpret the MEGA scaling relations of M*, SFR, and O/H with an updated version of the model presented by Dayal et al. (2013). This model successfully reproduces the observed O/H ratio of 80,000 galaxies selected from the Sloan Digital Sky Survey to within 0.05-0.06 dex. By extending the model to the higher redshift MEGA sample, we find that although the specific mass loading of outflows does not change measurably during the evolution, the accretion rate and gas content of galaxies increase significantly with redshift. These two effects can explain, either separately or possibly in tandem, the observed lower metal abundance of high-z galaxies.

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arXiv2016

The problematic growth of dust in high redshift galaxies

Dust growth via accretion of gas species has been proposed as the dominant process to increase the amount of dust in galaxies. We show here that this hypothesis encounters severe difficulties that make it unfit to explain the observed UV and IR properties of such systems, particularly at high redshifts. Dust growth in the diffuse ISM phases is hampered by (a) too slow accretion rates; (b) too high dust temperatures, and (c) the Coulomb barrier that effectively blocks accretion. In molecular clouds these problems are largely alleviated. Grains are cold (but not colder than the CMB temperature, (Tcmb = 20 K at redshift z=6). However, in dense environments accreted materials form icy water mantles, perhaps with impurities. Mantles are immediately photo-desorbed as grains return to the diffuse ISM at the end of the cloud lifetime, thus erasing any memory of the growth. We conclude that dust attenuating stellar light at high-$z$ must be ready-made stardust largely produced in supernova ejecta.

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arXiv2016

Empowering line intensity mapping to study early galaxies

Line intensity mapping is a superb tool to study the collective radiation from early galaxies. However, the method is hampered by the presence of strong foregrounds, mostly produced by low-redshift interloping lines. We present here a general method to overcome this problem which is robust against foreground residual noise and based on the cross-correlation function $ψ_{αL}(r)$ between diffuse line emission and Ly$α$ emitters (LAE). We compute the diffuse line (Ly$α$ is used as an example) emission from galaxies in a $(800{\rm Mpc})^3$ box at $z = 5.7$ and $6.6$. We divide the box in slices and populate them with $14000(5500)$ LAEs at $z = 5.7(6.6)$, considering duty cycles from $10^{-3}$ to $1$. Both the LAE number density and slice volume are consistent with the expected outcome of the Subaru HSC survey. We add gaussian random noise with variance $σ_{\rm N}$ up to 100 times the variance of the Ly$α$ emission, $σ_α$, to simulate foregrounds and compute $ψ_{αL}(r)$. We find that the signal-to-noise of the observed $ψ_{αL}(r)$ does not change significantly if $σ_{\rm N} \le 10 σ_α$ and show that in these conditions the mean line intensity, $I_{Lyα}$, can be precisely recovered independently of the LAE duty cycle. Even if $σ_{\rm N} = 100 σ_α$, $I_α$ can be constrained within a factor $2$. The method works equally well for any other line (e.g. HI 21 cm, [CII], HeII) used for the intensity mapping experiment.

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arXiv2016

Observational challenges in Ly-alpha intensity mapping

Intensity mapping (IM) is sensitive to the cumulative line emission of galaxies. As such it represents a promising technique for statistical studies of galaxies fainter than the limiting magnitude of traditional galaxy surveys. The strong hydrogen Ly-alpha line is the primary target for such an experiment, as its intensity is linked to star formation activity and the physical state of the interstellar (ISM) and intergalactic (IGM) medium. However, to extract the meaningful information one has to solve the confusion problems caused by interloping lines from foreground galaxies. We discuss here the challenges for a Ly-alpha IM experiment targeting z > 4 sources. We find that the Ly-alpha power spectrum can be in principle easily (marginally) obtained with a 40 cm space telescope in a few days of observing time up to z < 8 (z = 10) assuming that the interloping lines (e.g. H-alpha, [O II], [O III] lines) can be efficiently removed. We show that interlopers can be removed by using an ancillary photometric galaxy survey with limiting AB mag 26 in the NIR bands (Y, J, H, or K). This would enable detection of the Ly-alpha signal from 5 < z < 9 faint sources. However, if a [C II] IM experiment is feasible, by cross-correlating the Ly-alpha with the [C II] signal the required depth of the galaxy survey can be decreased to AB mag 24. This would bring the detection at reach of future facilities working in close synergy.

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arXiv2016

First Identification of Direct Collapse Black Hole Candidates in the Early Universe in CANDELS/GOODS-S

The first black hole seeds, formed when the Universe was younger than 500 Myr, are recognized to play an important role for the growth of early (z ~ 7) super-massive black holes. While progresses have been made in understanding their formation and growth, their observational signatures remain largely unexplored. As a result, no detection of such sources has been confirmed so far. Supported by numerical simulations, we present a novel photometric method to identify black hole seed candidates in deep multi-wavelength surveys. We predict that these highly-obscured sources are characterized by a steep spectrum in the infrared (1.6-4.5 micron), i.e. by very red colors. The method selects the only 2 objects with a robust X-ray detection found in the CANDELS/GOODS-S survey with a photometric redshift z > 6. Fitting their infrared spectra only with a stellar component would require unrealistic star formation rates (>2000 solar masses per year). To date, the selected objects represent the most promising black hole seed candidates, possibly formed via the direct collapse black hole scenario, with predicted mass >10^5 solar masses. While this result is based on the best photometric observations of high-z sources available to date, additional progress is expected from spectroscopic and deeper X-ray data. Upcoming observatories, like the JWST, will greatly expand the scope of this work.

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arXiv2016

Detecting high-$z$ galaxies in the Near Infrared Background

Emission from high-$z$ galaxies must unquestionably contribute to the near-infrared background (NIRB). However, this contribution has so far proven difficult to isolate even after subtracting the resolved galaxies to deep levels. Remaining NIRB fluctuations are dominated by unresolved low-$z$ galaxies on small angular scales, and by an unidentified component with unclear origin on large scales ($\approx 1000''$). In this paper, by analyzing mock maps generated from semi-numerical simulations and empirically determined $L_{\rm UV} - M_{\rm h}$ relations, we find that fluctuations associated with galaxies at $5 < z < 10$ amount to several percent of the unresolved NIRB flux fluctuations. We investigate the properties of this component for different survey areas and limiting magnitudes. In all cases, we show that this signal can be efficiently, and most easily at small angular scales, isolated by cross-correlating the source-subtracted NIRB with Lyman Break Galaxies (LBGs) detected in the same field by {\tt HST} surveys. This result provides a fresh insight into the properties of reionization sources.

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arXiv2016

Cosmic Dawn Intensity Mapper

Cosmic Dawn Intensity Mapper is a "Probe Class" mission concept for reionization studies of the universe. It will be capable of spectroscopic imaging observations between 0.7 to 6-7 microns in the near-Infrared. The primary observational objective is pioneering observations of spectral emission lines of interest throughout the cosmic history, but especially from the first generation of distant, faint galaxies when the universe was less than 800 million years old. With spectro-imaging capabilities, using a set of linear variable filters (LVFs), CDIM will produce a three-dimensional tomographic view of the epoch of reionization (EoR). CDIM will also study galaxy formation over more than 90% of the cosmic history and will move the astronomical community from broad-band astronomical imaging to low-resolution (R=200-300) spectro-imaging of the universe.

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arXiv2016

Updated analysis of near-infrared background fluctuations

The power spectrum of Near InfraRed Background (NIRB) fluctuations measured at 3.6 $μ$m by {\tt Spitzer} shows a clustering excess over the known galaxies signal that has been interpreted in terms of early ($z\simgt 13$), accreting (direct collapse) black holes (DCBH) or low-$z$ intrahalo light (IHL). In addition, these fluctuations correlate with the cosmic X-ray background (CXB) measured at (0.5-2) keV, supporting the black hole explanation. This scenario has been questioned by the recent detection of a correlation between the two {\tt CIBER} 1.1/1.6 $μ$m bands with the 3.6 $μ$m {\tt Spitzer} one. This correlation is hardly explained by early DCBHs that, due to intergalactic absorption, cannot contribute to the shortest wavelength bands. Here we show that the new correlation is caused instead by a Diffuse Galactic Light (DGL) component arising from Galactic stellar light scattered by dust. The black hole interpretation of the excess remains perfectly valid and, actually, the inclusion of DGL allows less demanding (by up to about 30\%) requirements on the DCBH abundance/mass.

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arXiv2015

Radio recombination lines from obscured quasars with the SKA

We explore the possibility of detecting hydrogen radio recombination lines from 0 < z < 10 quasars. We compute the expected Hnalpha flux densities as a function of absolute magnitude and redshift by considering (i) the range of observed AGN spectral indices from UV to X-ray bands, (ii) secondary ionizations from X-ray photons, and (iii) stimulated emission due to nonthermal radiation. All these effects are important to determine the line fluxes. We find that the combination of slopes: alpha_X,hard = -1.11, alpha_X,soft = -0.7, alpha_EUV = -1.3, alpha_UV = -1.7, maximizes the expected flux, f_Hnalpha = 10 microJy for z = 7 quasars with M_AB = -27 in the n = 50 lines; allowed SED variations produce variations by a factor of 3 around this value. Secondaries boost the line intensity by a factor of 2 to 4, while stimulated emission in high-z quasars with M_AB = -26 provides an extra boost to RRL flux observed at nu = 1 GHz if recombinations arise in HII regions with T_e = 10^3-5 K, n_e = 10^3-5 cm^-3. We compute the sensitivity required for a 5sigma detection of Hnalpha lines using the SKA, finding that the SKA-MID could detect sources with M_AB < -27 (M_AB < -26) at z < 8 (z < 3) in less than 100 hrs of observing time. These observations could open new paths to searches for obscured SMBH progenitors, complementing X-ray, optical/IR and sub-mm surveys.

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arXiv2015

Metal Enrichment in the Reionization Epoch

The presence of elements heavier than helium ("metals") is of fundamental importance for a large number of astrophysical processes occurring in planet, star and galaxy formation; it also affects cosmic structure formation and evolution in several ways. Even a small amount of heavy elements can dramatically alter the chemistry of the gas, opening the path to complex molecules. Metals might enhance the ability of the gas to radiate away its thermal energy, thus favoring the formation of gravitationally bound objects; they can also condensate in a solid phase (dust grains), partly or totally blocking radiation from luminous sources. Finally, they represent useful tracers of energy deposition by stars and probe the physical properties of the environment by absorption or emission lines. Last, but certainly not least, life -- as we know it on Earth -- is tightly related to the presence of at least some of the heavy elements. In this pedagogical review I will concentrate on the connection between early metal enrichment and cosmic reionization. As we will see these two processes are intimately connected and their joint study might turn out to be fundamental in understanding the overall evolution of the Universe during the first billion years after the Big Bang, an epoch corresponding to redshifts z>6.

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arXiv2015

The chemical evolution of self-gravitating primordial disks

Numerical simulations show the formation of self-gravitating primordial disks during the assembly of the first structures in the Universe, in particular during the formation of Pop.~III and supermassive stars. Their subsequent evolution is expected to be crucial to determine the mass scale of the first cosmological objects, which depends on the temperature of the gas and the dominant cooling mechanism. Here, we derive a one-zone framework to explore the chemical evolution of such disks and show that viscous heating leads to the collisional dissociation of an initially molecular gas. The effect is relevant on scales of 10 AU (1000 AU) for a central mass of 10 M_sun (10^4 M_sun) at an accretion rate of 10^{-1} M_sun yr^{-1}, and provides a substantial heat input to stabilize the disk. If the gas is initially atomic, it remains atomic during the further evolution, and the effect of viscous heating is less significant. The additional thermal support is particularly relevant for the formation of very massive objects, such as the progenitors of the first supermassive black holes. The stabilizing impact of viscous heating thus alleviates the need for a strong radiation background as a means of keeping the gas atomic.

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arXiv2015

Cosmic reionization after Planck

Cosmic reionization holds the key to understand structure formation in the Universe, and can inform us about the properties of the first sources, as their star formation efficiency and escape fraction of ionizing photons. By combining the recent release of Planck electron scattering optical depth data with observations of high-redshift quasar absorption spectra, we obtain strong constraints on viable reionization histories. We show that inclusion of Planck data favors a reionization scenario with a single stellar population. The mean $x_{\rm HI}$ drops from $\sim0.8$ at $z=10.6$ to $\sim10^{-4}$ at $z=5.8$ and reionization is completed around $5.8\lesssim z\lesssim8.5$ (2-$σ$), thus indicating a significant reduction in contributions to reionization from high redshift sources. We can put independent constraints on the escape fraction $f_{\rm esc}$ of ionizing photons by incorporating the high-redshift galaxy luminosity function data into our analysis. We find a non-evolving $f_{\rm esc}$ of $\sim10\%$ in the redshift range $z=6-9$.

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arXiv2015

Shining in the Dark: the Spectral Evolution of the First Black Holes

Massive Black Hole (MBH) seeds at redshift $z \gtrsim 10$ are now thought to be key ingredients to explain the presence of the super-massive ($10^{9-10} \, \mathrm{M_{\odot}}$) black holes in place $ < 1 \, \mathrm{Gyr}$ after the Big Bang. Once formed, massive seeds grow and emit copious amounts of radiation by accreting the left-over halo gas; their spectrum can then provide crucial information on their evolution. By combining radiation-hydrodynamic and spectral synthesis codes, we simulate the time-evolving spectrum emerging from the host halo of a MBH seed with initial mass $10^5 \, \mathrm{M_{\odot}}$, assuming both standard Eddington-limited accretion, or slim accretion disks, appropriate for super-Eddington flows. The emission occurs predominantly in the observed infrared-submm ($1-1000 \, \mathrm{μm}$) and X-ray ($0.1 - 100 \, \mathrm{keV}$) bands. Such signal should be easily detectable by JWST around $\sim 1 \, \mathrm{μm}$ up to $z \sim 25$, and by ATHENA (between $0.1$ and $10 \, \mathrm{keV}$, up to $z \sim 15$). Ultra-deep X-ray surveys like the Chandra Deep Field South could have already detected these systems up to $z \sim 15$. Based on this, we provide an upper limit for the $z \gtrsim 6$ MBH mass density of $ρ_{\bullet} \lesssim 2.5 \times 10^{2} \, \mathrm{M_{\odot} \, Mpc^{-3}}$ assuming standard Eddington-limited accretion. If accretion occurs in the slim disk mode the limits are much weaker, $ρ_{\bullet} \lesssim 7.6 \times 10^{3} \, \mathrm{M_{\odot} \, Mpc^{-3}}$ in the most constraining case.

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arXiv2015

The Growth Efficiency of High-Redshift Black Holes

The observational evidence that Super-Massive Black Holes ($M_{\bullet} \sim 10^{9-10} \, \mathrm{M_{\odot}}$) are already in place less than $1 \, \mathrm{Gyr}$ after the Big Bang poses stringent time constraints on the growth efficiency of their seeds. Among proposed possibilities, the formation of massive ($\sim 10^{3-6} \, \mathrm{M_{\odot}}$) seeds and/or the occurrence of super-Eddington ($\dot{M}>\dot{M}_{Edd}$) accretion episodes may contribute to the solution of this problem. In this work we analytically and numerically investigate the accretion flow onto high-redshift ($z \sim 10$) black holes to understand the physical requirements favoring rapid and efficient growth. Our model identifies a "feeding-dominated" accretion regime and a "feedback-limited" one, the latter being characterized by intermittent (duty cycles ${\cal D} \lesssim 0.5$) and inefficient growth, with recurring outflow episodes. We find that low-mass seeds ($\lesssim 10^{3-4} \, \mathrm{M_{\odot}}$) evolve in the feedback-limited regime, while more massive seeds ($\gtrsim 10^{5-6} \, \mathrm{M_{\odot}}$) grow very rapidly as they are found in the feeding-dominated regime. In addition to the standard accretion model with a fixed matter-energy conversion factor ($ε= 0.1$), we have also explored slim disk models, appropriate for super-Eddington accretion, where radiation is trapped in the disk and the radiative efficiency is reduced ($ε\lesssim 0.04$), which may ensure a continuous growth with $\dot{M} \gg \dot{M}_{Edd}$ (up to $\sim 300\dot{M}_{Edd}$ in our simulations). Under these conditions, outflows play a negligible role and a black hole can accrete $80\%-100\%$ of the gas mass of the host halo ($\sim 10^7 \, \mathrm{M_{\odot}}$) in $\sim 10 \, \mathrm{Myr}$, while in feedback-limited systems we predict that black holes can accrete only up to $\sim 15\%$ of the available mass.

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arXiv2015

Reionization constraints on primordial magnetic fields

We study the impact of the extra density fluctuations induced by primordial magnetic fields on the reionization history in the redshift range: $6 < z < 10$. We perform a comprehensive MCMC physical analysis allowing the variation of parameters related to primordial magnetic fields (strength, $B_0$, and power-spectrum index $n_{\scriptscriptstyle \rm B}$), reionization, and $Λ$CDM cosmological model. We find that magnetic field strengths in the range: $B_0 \simeq 0.05{-}0.3$ nG (for nearly scale-free power spectra) can significantly alter the reionization history in the above redshift range and can relieve the tension between the WMAP and quasar absorption spectra data. Our analysis puts upper-limits on the magnetic field strength $B_0 < 0.358, 0.120, 0.059$ nG (95 % c.l.) for $n_{\scriptscriptstyle \rm B} = -2.95, -2.9, -2.85$, respectively. These represent the strongest magnetic field constraints among those available from other cosmological observables.

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arXiv2015

Intensity mapping of [CII] from early galaxies

The intensity mapping of the [CII] 157.7 $\rm μ$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.

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arXiv2015

Gravitational Waves from Direct Collapse Black Holes Formation

The possible formation of Direct Collapse Black Holes (DCBHs) in the first metal-free atomic cooling halos at high redshifts ($z > 10$) is nowadays object of intense study and several methods to prove their existence are currently under development. The abrupt collapse of a massive ($\sim 10^4 - 10^5 \, \mathrm{M_{\odot}}$) and rotating object is a powerful source of gravitational waves emission. In this work, we employ modern waveforms and the improved knowledge on the DCBHs formation rate to estimate the gravitational signal emitted by these sources at cosmological distances. Their formation rate is very high ($\sim 10^4 \, \mathrm{yr^{-1}}$ up to $z\sim20$), but due to a short duration of the collapse event ($\sim 2-30\, \mathrm{s}$, depending on the DCBH mass) the integrated signal from these sources is characterized by a very low duty-cycle (${\cal D}\sim 10^{-3}$), i.e. a shot-noise signal. Our results show that the estimated signal lies above the foreseen sensitivity of the Ultimate-DECIGO observatory in the frequency range $(0.8-300) \, \mathrm{mHz}$, with a peak amplitude $Ω_{gw} = 1.1 \times 10^{-54}$ at $ν_{max} = 0.9 \, \mathrm{mHz}$ and a peak Signal-to-Noise Ratio $\mathrm{SNR}\sim 22$ at $ν= 20 \, \mathrm{mHz}$. This amplitude is lower than the Galactic confusion noise, generated by binary systems of compact objects in the same frequency band. For this reason, advanced techniques will be required to separate this signal from background and foreground noise components. As a proof-of-concept, we conclude by proposing a simple method, based on the auto-correlation function, to recognize the presence of a ${\cal D} \ll 1$ signal buried into the continuous noise. The aim of this work is to test the existence of a large population of high-z DCBHs, by observing the gravitational waves emitted during their infancy.

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