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C. Firmani

Publications and source records attributed to C. Firmani.

At least 19 recordsLinked to original sources
arXiv2013

Mass function and assembly of dark halos: an approach to inventory isolated overdense regions in random fields

In order to attain a statistical description of the evolution of cosmic density fluctuations in agreement with results from the numerical simulations, we introduce a probability conditional formalism (CF) based on an inventory of isolated overdense regions in a density random field. This formalism is a useful tool for describing at the same time the mass function (MF) of dark haloes, their mass aggregation histories (MAHs) and merging rates (MRs). The CF focuses on virialized regions in a self-consistent way rather than in mass elements, and it offers an economical description for a variety of random fields. Within the framework of the CF, we confirm that, for a Gaussian field, it is not possible to reproduce at the same time the MF, MAH, and MR of haloes, both for a constant and moving barrier. Then, we develop an inductive method for constraining the cumulative conditional probability from a given halo MF description, and thus, using the CF, we calculate the halo MAHs and MRs. By applying this method to the MF measured in numerical simulations by Tinker et al. 2008, we find that a reasonable solution, justified by a mass conservation argument, is obtained if ones introduce a rescaling -increment by ~30% - of the virial mass used in simulations and a (slight) deviation from Gaussianity. Thus, both the MAH and MR obtained by a Monte Carlo merger tree agree now with the predictions of numerical simulations. We discuss on the necessity of rescaling the virial mass in simulations when comparing with analytical approaches on the ground of the matter not accounted as part of the halos and the halo mass limit due to numerical. Our analysis supports the presence of a diffuse dark matter component that is not taken into account in the measured halo MFs inasmuch as it is not part of the collapsed structures.

astro-ph.CO↗
arXiv2011

Galaxy downsizing evidenced by hybrid evolutionary tracks

An unified picture of stellar and halo mass build-up as a function of mass is presented. Inferred stellar-dark halo mass relations of galaxies, Ms-Mh, out to z=4 together with average LCDM halo mass aggregation histories (MAHs) are used for inferring average Ms growth histories, the Galaxian Hybrid Evolutionary Tracks (GHETs). The more massive the galaxy, the earlier transited in average from an active regime of Ms growth to a passive one: log(Mtran/Msun)=10.30+0.55z ("population downsizing"), where Mtran is the typical transition stellar mass. This result agrees with independent observational determinations based on the evolution of the galaxy stellar mass function decomposition into blue and red galaxies. The specific star formation rate, SSFR, predicted from the derivative of the GHET is consistent with direct measures of the SSFR for galaxies at different z's. The average GHETs of galaxies smaller than Mtran at z=0 (Ms~10^10.3 Msun) did not reach the quiescent regime, and for them, the lower the mass, the faster the later Ms growth rate ("downsizing in SSFR"). The GHETs allow to predict the transition rate in number density of active to passive population; the predicted values agree with direct estimates of growth rate in number density for the (massive) red population up to z~1. We show that LCDM-based models of disk galaxy evolution are able to reproduce the low-mass side of the Ms-Mh relation at z~0, but at higher z's disagree strongly with the GHETs: models do not reproduce the downsizing in SSFR and the high SSFR of low mass galaxies. (Abridged)

astro-ph.CO↗
arXiv2011

On the stellar and baryonic mass fractions of central blue and red galaxies

By means of the abundance matching technique, we infer the local stellar and baryonic mass-halo mass (Ms-Mh and Mb-Mh) relation for central blue and red galaxies separately in the mass range Ms~10^8.5-10^12.0 Msun. The observational inputs are the SDSS central blue and red Galaxy Stellar Mass Functions reported in Yang et al. 2009, and the measured local gas mass-Ms relations for blue and red galaxies. For the Halo Mass Function associated to central blue galaxies, the distinct LCDM one is used and set up to exclude: (i) the observed group/cluster mass function (blue galaxies are rare as centers of groups/clusters), and (ii) halos with a central major merger at resdshifts z<0.8 (dry late major mergers destroy the disks of blue galaxies). For red galaxies, we take the complement of this function to the total. The obtained mean Ms-Mh and Mb-Mh relations of central blue and red galaxies do not differ significantly from the respective relations for all central galaxies. For Mh>10^11.5 Msun, the Ms's of red galaxies tend to be higher than those of blue ones for a given Mh, the difference not being larger than 1.7. For Mh<10^11.5 Msun, this trend is inverted. For blue (red) galaxies: (a) the maximum value of fs=Ms/Mh is 0.021^{+0.016}_{-0.009} (0.034{+0.026}_{-0.015}) and it is attained atlog(Mh/Msun)~12.0 (log(Mh/Msun)~11.9); (b) fs\propto Mh (fs\propto Mh^3) at the low-mass end while at the high-mass end, fs\propto Mh^-0.4 (fs\propto Mh^-0.6). The baryon mass fractions, fb=Mb/Mh, of blue and red galaxies reach maximum values of fb=0.028^{+0.018}_{-0.011} and fb=0.034^{+0.025}_{-0.014}, respectively. For Mh<10^11.3 Msun, a much steeper dependence of fb on Mh is obtained for the red galaxies than for the blue ones. We discuss on the differences found in the fs-Mh and fb-Mh relations between blue and red galaxies in the light of of semi-empirical galaxy models.

astro-ph.CO↗
arXiv2011

The specific star formation rate and stellar mass fraction of low-mass central galaxies in cosmological simulations

(Abridged) By means of high-resolution cosmological simulations in the context of the LCDM scenario, the specific star formation rate (SSFR=SFR/Ms, Ms is the stellar mass)--Ms and stellar mass fraction (Fs=Ms/Mh, Mh is the halo mass)--Ms relations of low-mass galaxies (2.5< Mh/10^10 Msun <50 at redshift z=0) at different epochs are predicted. The Hydrodynamics ART code was used and some variations of the sub-grid parameters were explored. Most of simulated galaxies, specially those with the highest resolutions, have significant disk components and their structural and dynamical properties are in reasonable agreement with observations of sub-M* field galaxies. However, the SSFRs are 5-10 times smaller than the averages of several (compiled and homogenized here) observational determinations for field blue/star-forming galaxies at z<0.3 (at low masses, most of observed field galaxies are actually blue/star-forming). This inconsistency seems to remain even at z~1.5 though less drastic. The Fs of simulated galaxies increases with Mh as semi-empirical inferences show, but in absolute values the former are ~5-10 times larger than the latter at z=0; this difference increases probably to larger factors at z~1-1.5. The inconsistencies reported here imply that simulated low-mass galaxies (0.2<Ms/10^9 Msun <30 at z=0) assembled their stellar masses much earlier than observations suggest. This confirms the predictions previously found by means of LCDM-based models of disk galaxy formation and evolution for isolated low-mass galaxies (Firmani & Avila-Reese 2010), and highlight that our implementation of astrophysics into simulations and models are still lacking vital ingredients.

astro-ph.CO↗
arXiv2011

The stellar and dark halo mass assembly of galaxies

The emerging empirical picture of galaxy stellar mass (Ms) assembly shows that galaxy population buildup proceeds from top to down in Ms. By connecting galaxies to LCDM halos and their histories, individual (average) Ms growth tracks can be inferred. These tracks show that massive galaxies assembled their Ms the earlier the more massive the halo, and that less massive galaxies are yet actively growing in Ms, the more active the less massive is the halo. The predicted star formation rates as a function of mass and the downsizing of the typical mass that separate active galaxies from the passive ones agree with direct observational determinations. This implies that the LCDM scenario is consistent with these observations. The challenge is now to understand the baryonic physics that drives the significant and systematical shift of the stellar mass assembly of galaxies from the mass assembly of their corresponding halos (from halo upsizing to galaxy downsizing).

astro-ph.CO↗
arXiv2010

Can galaxy outflows and re-accretion produce the downsizing in specific star formation rate of late-type galaxies?

The observations show that less massive the galaxies are, the higher on average is their specific star formation rate (SSFR = SFR/Ms, Ms is the stellar mass). Such a trend, called the 'SSFR downsizing' (SSFR-DS) phenomenon, is seen for local and high-z (back to z~1-2) galaxy samples. We use observational data related only to disc galaxies and explore the average SSFR change with z for different masses. For Ms in the range ~10^9.5-10^10.5 Msun, the SSFR increases with (1+z) to a power that barely depends on Ms, and at all z's smaller galaxies have ever higher SSFRs. The latter strongly disagree with the LCDM hierarchical mass accretion rates. By means of self-consistent models of disc galaxy evolution inside growing LCDM halos, the effects that disc feedback-driven outflows and gas re-accretion have on the galaxy SSFR histories are explored. The parameters of the outflow and re-accretion schemes are tuned to reproduce the z~0 Mh-Ms relation inferred from observations. In the case of outflows only, the SSFR of individual model galaxies is roughly proportional to (1+z)^2.2 for all the masses with a normalization factor that depends on mass as Ms^0.1, i.e more massive galaxies have slightly larger SSFRs, contrary to the observed strong SSFR-DS trend. For the re-accretion cases, the dependence on z remains approximately the same as without re-infall, but the correlation on Ms even increases for most of the reasonable values of the model parameters. The comparison of models and observations in the SSFR-Ms plane at z~0 shows the divergent trend in SSFR as lower are the masses (upsizing vs downsizing). We explain why the models show the reported trends, and conclude that the SSFR-DS for low-mass galaxies poses a challenge for LCDM-based disc galaxy evolution models. (Abridged)

astro-ph.CO↗
arXiv2009

The size evolution of galaxy discs formed within Lambda Cold Dark Matter haloes

By means of galaxy evolutionary models, we explore the direct consequences of the LCDM cosmogony on the size evolution of galactic discs, avoiding intentionally the introduction of intermediate (uncertain) astrophysical processes. Based on the shape of the rotation curves and guided by a simplicity criterion, we adopt an average galaxy mass baryon fraction of 0.03. In order to study general behaviors, only models with the average initial conditions are analyzed. The stellar and B-band effective radii, R* and RB, of individual galaxies grow significantly with time (inside-out disc formation) with laws that are weakly dependent on mass, M*,or luminosity, LB. However, the change of R* with z at fixed M* is slow; for z<2.5, R*(M*=const) ~ (1+z)^-0.4. On the other hand, the change of RB with z at a fixed LB is strong and resembles the RB decreasing law of the individual models; roughly RB(LB=const) ~ (1+z)^-0.85 for z<0.75, and ~(1+z)^-1.1 for z>0.75. We find also that at z=0, R* ~ M*^0.38 and RB ~ LB^0.40, remaining the slopes of these relations almost the same up to z ~ 3. Our predictions are in reasonable agreement with observational inferences on the typical radius change with z of late-type galaxies more luminous (massive) than high values imposed by the selection effects. The models seem also to be consistent, within the large scatter, with the RB and LB values obtained from non complete samples of sub-L* late-type galaxies with available rest-frame photometric information at different z's. The properties and evolution of the LCDM haloes seem to be the main drivers of galaxy disc size evolution. Nevertheless, the models reveal a potential difficulty in explaining the observed steepening of the RB-LB relation with respect to the R*-M* one, an effect related to the well established color-magnitude relation.

astro-ph.CO↗
arXiv2009

Short versus Long Gamma-Ray Bursts: spectra, energetics, and luminosities

We compare the spectral properties of 79 short and 79 long Gamma-Ray Bursts (GRBs) detected by BATSE and selected with the same limiting peak flux. Short GRBs have a low-energy spectral component harder and a peak energy slightly higher than long GRBs, but no difference is found when comparing short GRB spectra with those of the first 1-2 sec emission of long GRBs. These results confirm earlier findings for brighter GRBs. The bolometric peak flux of short GRBs correlates with their peak energy in a similar way to long bursts. Short and long GRBs populate different regions of the bolometric fluence-peak energy plane, short bursts being less energetic by a factor similar to the ratio of their durations. If short and long GRBs had similar redshift distributions, they would have similar luminosities yet different energies, which correlate with the peak energy E_peak for the population of long GRBs. We also test whether short GRBs are consistent with the E_peak-E_iso and E_peak-L_iso correlations for the available sample of short (6 events) and long (92 events) GRBs with measured redshifts and E_peak,obs: while short GRBs are inconsistent with the E_peak-E_iso correlation of long GRBs, they could follow the E_peak-L_iso correlation of long bursts. All the above indications point to short GRBs being similar to the first phases of long bursts. This suggests that a similar central engine (except for its duration) operates in GRBs of different durations.

astro-ph.HE↗
arXiv2008

Time-resolved spectral correlations of long-duration Gamma-Ray Bursts

For a sample of long GRBs with known redshift, we study the distribution of the evolutionary tracks on the rest-frame luminosity-peak energy Liso-Ep' diagram. We are interested in exploring the extension of the `Yonetoku' correlation to any phase of the prompt light curve, and in verifying how the high-signal prompt duration time, Tf, in the rest frame correlates with the residuals of such correlation (Firmani et al. 2006). For our purpose, we analyse separately two samples of time-resolved spectra corresponding to 32 GRBs with peak fluxes >1.8 phot cm^-2 s^-1 from the Swift-BAT detector, and 7 bright GRBs from the CGRO-BATSE detector previously processed by Kaneko et al. (2006). After constructing the Liso-Ep' diagram, we discuss the relevance of selection effects, finding that they could affect significantly the correlation. However, we find that these effects are much less significant in the Liso x Tf-Ep' diagram, where the intrinsic scatter reduces significantly. We apply further corrections for reducing the intrinsic scatter even more. For the sub-samples of GRBs (7 from Swift and 5 from CGRO) with measured jet break time, we analyse the effects of correcting Liso by jet collimation. We find that (i) the scatter around the correlation is reduced, and (ii) this scatter is dominated by the internal scatter of the individual evolutionary tracks. These results suggest that the time, integrated `Amati' and `Ghirlanda' correlations are consequences of the time resolved features, not of selection effects, and therefore call for a physical origin. We finally remark the relevance of looking inside the nature of the evolutionary tracks.

astro-ph↗
arXiv2008

On the baryonic, stellar, and luminous scaling relations of disk galaxies

We explore how the slopes and scatters of the scaling relations of disk galaxies (Vm-L[-M], R-L[-M], and Vm-R) do change when moving from B to K bands and to stellar and baryonic quantities. For our compiled sample of 76 normal, non-interacting high and low surface brightness galaxies, we find some changes, which evidence evolution effects, mainly related to gas infall and star formation (SF). We also explore correlations among the (B-K) color, stellar mass fraction fs, mass M (luminosity L), and surface density (SB), as well as correlations among the residuals of the scaling relations. Some of our findings are: (i) the scale length Rb is a third parameter in the baryonic TF relation and the residuals of this relation follow a trend (slope ~-0.15) with the residuals of the Rb-Mb relation; for the stellar and K band cases, R is not anymore a third parameter and the mentioned trend disappears; (ii) among the TFRs, the B-band TFR is the most scattered; in this case, the color is a third parameter; (iii) the LSB galaxies break some observed trends, which suggest a threshold in the gas surface density Sg, below which the SF becomes independent of the gas infall rate and Sg. Our results are interpreted and discussed in the light of LCDM-based models of galaxy evolution. The models explain not only the baryonic scaling relations, but also most of the processes responsible for the observed changes in the slopes, scatters, and correlations among the residuals when changing to stellar and luminous quantities. The baryon fraction is required to be smaller than 0.05 on average. We detect some potential difficulties for the models: the observed color-M and surface density-M correlations are steeper, and the intrinsic scatter in the baryonic TFR is smaller than those predicted. [abridged]

astro-ph↗
arXiv2008

The Epeak-Eiso plane of long Gamma Ray Bursts and selection effects

We study the distribution of long Gamma Ray Bursts in the Ep-Eiso and in the Ep,obs-Fluence planes through an updated sample of 76 bursts, with measured redshift and spectral parameters, detected up to September 2007. We confirm the existence of a strong rest frame correlation Ep ~ Eiso^0.54+-0.01. Contrary to previous studies, no sign of evolution with redshift of the Ep-Eiso correlation (either its slope and normalisation) is found. The 76 bursts define a strong Ep,obs-Fluence correlation in the observer frame (Ep,obs ~ F^0.32+-0.05) with redshifts evenly distributed along this correlation. We study possible instrumental selection effects in the observer frame Ep,obs-Fluence plane. In particular, we concentrate on the minimum peak flux necessary to trigger a given GRB detector (trigger threshold) and the minimum fluence a burst must have to determine the value of Ep,obs (spectral analysis threshold). We find that the latter dominates in the Ep,obs-Fluence plane over the former. Our analysis shows, however, that these instrumental selection effects do not dominate for bursts detected before the launch of the Swift satellite, while the spectral analysis threshold is the dominant truncation effect of the Swift GRB sample (27 out of 76 events). This suggests that the Ep,obs-Fluence correlation defined by the pre--Swift sample could be affected by other, still not understood, selection effects. Besides we caution about the conclusions on the existence of the Ep,obs-Fluence correlation based on our Swift sample alone.

astro-ph↗
arXiv2008

Gamma-Ray Bursts, new cosmological beacons

Long Gamma-Ray Bursts (GRBs) are the brightest electromagnetic explosions in the Universe, associated to the death of massive stars. As such, GRBs are potential tracers of the evolution of the cosmic massive star formation, metallicity, and Initial Mass Function. GRBs also proved to be appealing cosmological distance indicators. This opens a unique opportunity to constrain the cosmic expansion history up to redshifts 5-6. A brief review on both subjects is presented here.

astro-ph↗
arXiv2007

Re-born fireballs in Gamma-Ray Bursts

We consider the interaction between a relativistic fireball and material assumed to be still located just outside the progenitor star. Only a small fraction of the expected mass is sufficient to efficiently decelerate the fireball, leading to dissipation of most of its kinetic energy. Since the scattering optical depths are still large at distances comparable to the progenitor radius, the dissipated energy is trapped in the system, accelerating it to relativistic velocities. The process resembles the birth of another fireball at radii R~1e11 cm, not far from the transparency radius, and with a starting bulk Lorentz factors Gamma_c~10. As seen in the observer frame, this "re--generated" fireball appears collimated within an angle theta_j=1/Gamma_c. If the central engine works intermittently, the funnel can, at least partially, refill and the process can repeat itself. We discuss how this idea can help solving some open issues of the more conventional internal shock scenario for interpreting the Gamma-Ray Burst properties.

astro-ph↗
arXiv2007

Spectral analysis of Swift long GRBs with known redshift

We study the spectral and energetics properties of 47 long-duration gamma-ray bursts (GRBs) with known redshift, all of them detected by the Swift satellite. Due to the narrow energy range (15-150 keV) of the Swift-BAT detector, the spectral fitting is reliable only for fitting models with 2 or 3 parameters. As high uncertainty and correlation among the errors is expected, a careful analysis of the errors is necessary. We fit both the power law (PL, 2 parameters) and cut--off power law (CPL, 3 parameters) models to the time-integrated spectra of the 47 bursts, and present the corresponding parameters, their uncertainties, and the correlations among the uncertainties. The CPL model is reliable only for 29 bursts for which we estimate the nuf_nu peak energy Epk. For these GRBs, we calculate the energy fluence and the rest- frame isotropic-equivalent radiated energy, Eiso, as well as the propagated uncertainties and correlations among them. We explore the distribution of our homogeneous sample of GRBs on the rest-frame diagram E'pk vs Eiso. We confirm a significant correlation between these two quantities (the "Amati" relation) and we verify that, within the uncertainty limits, no outliers are present. We also fit the spectra to a Band model with the high energy power law index frozen to -2.3, obtaining a rather good agreement with the "Amati" relation of non-Swift GRBs.

astro-ph↗
arXiv2007

Black-body components in Gamma-Ray Bursts spectra?

We study 7 Gamma Ray Bursts (GRBs), detected both by the BATSE instrument, on-board the Compton Gamma Ray Observatory, and by the Wide Field Camera (WFC), on-board BeppoSAX. These bursts have measured spectroscopic redshifts and are a sizeable fraction of the bursts defining the correlation between the peak energy E_peak (i.e. the peak of the vFv spectrum) and the total prompt isotropic energy E_iso (the so called "Amati" relation). Recent theoretical interpretations of this correlation assume that black-body emission dominates the time resolved spectra of GRBs, even if, in the time integrated spectrum, its presence may be hidden by the change of its temperature and by the dilution of a possible non-thermal power law component. We perform a time resolved spectral analysis, and show that the sum of a power-law and a black-body gives acceptable fits to the time dependent spectra within the BATSE energy range, but overpredicts the flux in the WFC X-ray range. Moreover, a fit with a cutoff power-law plus a black-body is consistent with the WFC data, but the black-body component contributes a negligible fraction of the total flux. On the contrary, we find that fitting the spectra with a Band model or a simple cutoff power-law model yields an X-ray flux and spectral slope which well matches the WFC spectra.

astro-ph↗
arXiv2007

The role of afterglow break-times as GRB jet angle indicators

The early X-ray light curve of Gamma Ray Bursts (GRBs) is complex, and shows a typical steep-flat-steep behaviour. The time T_a at which the flat (plateau) part ends may bear some important physical information, especially if it plays the same role of the so called jet break time t_jet. To this aim, stimulated by the recent analysis of Willingale et al., we have assembled a sample of GRBs of known redshifts, spectral parameters of the prompt emission, and T_a. By using T_a as a jet angle indicator, and then estimating the collimation corrected prompt energetics, we find a correlation between the latter quantity and the peak energy of the prompt emission. However, this correlation has a large dispersion, similar to the dispersion of the Amati correlation and it is not parallel to the Ghirlanda correlation. Furthermore, we show that the correlation itself results mainly from the dependence of the jet opening angle on the isotropic prompt energy, with the time T_a playing no role, contrary to what we find for the jet break time t_jet. We also find that for the bursts in our sample T_a weakly correlates with E_iso of the prompt emission, but that this correlation disappears when considering all bursts of known redshift and T_a. There is no correlation between T_a and the isotropic energy of the plateau phase.

astro-ph↗
arXiv2007

Long Gamma-Ray Burst prompt emission properties as a cosmological tool

Recently, a tight correlation among three quantities that characterize the prompt emission of long Gamma-Ray Bursts (GRBs) with known redshift z, was discovered (Firmani et al. 2006). We use this correlation to construct the Hubble diagram (HD) with a sample of 19 GRBs in the broad range of z=0.17-4.5, and carry out a full statistical analysis to constrain cosmological parameters (CPs). To optimally solve the problem of circularity, a Bayesian approach is applied. The main result is that the concordance LambdaCDM cosmology is fully consistent with the GRB data at the level of several tests. If we assume the Lambda cosmology, then we find Om_M=0.31^{+0.09}_{-0.08} and Om_Lambda=0.80^{+0.20}_{-0.30}$ (1sigma); the flat-geometry case is within 1sigma. Assuming flatness, we find Om_M=0.29^{+0.08}_{-0.06}, and fixing Om_M=0.28, we obtain a dark energy equation of state parameter w=-1.07^{+0.25}_{-0.38}, i.e. the ambdaCDM model (w=-1) is within 1sigma. Given the low number of usable GRBs we cannot yet constrain well the possible evolution of w=w(z). However, the case w(z)=-1 (LambdaCDM) is consistent at the 68.3% CL with GRBs. It is shown also how a broad range of z's in the used sample improves the determination of CPs from the HD, which is the case of GRBs as distance indicators.

astro-ph↗