C. Firmani

On the Formation and Evolution of Disk Galaxies: Cosmological Initial Conditions and the Gravitational Collapse

We use a semianalytical approach and the standard σ8 = 1 cold dark matter (SCDM) cosmological model to study the gravitational collapse and virialization, the structure, and the global and statistical properties of isolated dark matter galactic halos that emerge from primordial Gaussian fluctuations. First, from the statistical properties of the primordial density fluctuation field, the possible mass aggregation histories (MAHs) are generated. Second, these histories are used as the initial conditions of the gravitational collapse. To calculate the structure of the virialized systems, we have generalized the secondary infall model to allow arbitrary MAHs and internal thermal motions. The average halo density profiles we obtained agree with the profile derived as a fitting formula to results of N-body cosmological simulations by Navarro, Frenk, & White. The comparison of the density profiles with the observational data is discussed, and some possible solutions to the disagreement found in the inner regions are proposed. The results of our approach, after considering the gravitational dragging of the baryon matter that forms a central disk in centrifugal equilibrium, show that the empirical Tully-Fisher (TF) relation and its scatter can be explained through the initial cosmological conditions, at least for the isolated systems. The σ8 = 1 SCDM model produces galaxies with high velocities when compared with observations, but when the SCDM power spectrum is normalized to σ8 = 0.57, an excellent agreement with the observable TF relation is found, suggesting that this relation is the natural extension to galactic scales of the observed galaxy distribution power spectrum. The theoretical TF scatter is close to the measured one. The slope of the TF relation is practically invariant with respect to the spin parameter λ.

Formation and Structure of Halos in a Warm Dark Matter Cosmology

Using high-resolution cosmological N-body simulations, we study how the density profiles of dark matter halos are affected by the filtering of the density power spectrum below a given scale length and by the introduction of a thermal velocity dispersion. In the warm dark matter (WDM) scenario, both the free-streaming scale, Rf, and the velocity dispersion, v, are determined by the mass, mW, of the WDM particle. We found that v is too small to affect the density profiles of WDM halos. Down to the resolution attained in our simulations (~0.01 virial radii), there is not any significant difference in the density profiles and concentrations of halos obtained in simulations with and without the inclusion of v. Resolved soft cores appear only when we artificially increase the thermal velocity dispersion to a value that is much higher than v. We show that the size of soft cores in a monolithic collapse is related to the tangential velocity dispersion. The density profiles of the studied halos with masses down to ~0.01 the filtering mass Mf can be described by the Navarro-Frenk-White shape; soft cores are not formed. Nevertheless, the concentrations of these halos are lower than those of the CDM counterparts and are approximately independent of mass. The cosmogony of halos with masses Mf is not hierarchical: they form through monolithic collapse and by fragmentation of larger structures. The formation epoch of these halos is slightly later than that of halos with masses ≈Mf. The lower concentrations of WDM halos with respect to their CDM counterparts can be accounted for by their late formation epoch. Overall, our results point to a series of advantages of a WDM model over the CDM one. In addition to solving the substructure problem, a WDM model with Rf ~ 0.16 Mpc (mW ≈ 0.75 keV; flat cosmology with ΩΛ = h = 0.7) also predicts concentrations, a Tully-Fisher relation, and formation epochs for small halos, which seems to be in better agreement with observations than CDM predictions.

Hα Rotation Curves: The Soft Core Question

We present high-resolution Hα rotation curves of four late-type dwarf galaxies and two low surface brightness (LSB) galaxies, for which accurate H I rotation curves are available from the literature. Observations are carried out at Telescopio Nazionale Galileo. For LSB F583-1 an innovative dispersing element was used, the Volume Phase Holographic, with a dispersion of about 0.35 A pixel-1. We find good agreement between the Hα data and the H I observations and conclude that the H I data for these galaxies suffer very little from beam smearing. We show that the optical rotation curves of these dark matter-dominated galaxies are best fitted by the Burkert profile. In the centers of galaxies, where the N-body simulations predict cuspy cores and fast rising rotation curves, our data seem to be in better agreement with the presence of soft cores.

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 s 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 Epeak for the population of long GRBs. We also test whether short GRBs are consistent with the Epeak−Eiso and Epeak−Liso correlations for the available sample of short (6 events) and long (92 events) GRBs with measured redshifts and E obs : while short GRBs are inconsistent with the Epeak−Eiso correlation of long GRBs, they could follow the Epeak−Liso 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.

High‐resolution imaging with a two‐dimensional resistive anode photon counter

We describe a method for achieving high spatial resolution readout of individual photoelectron events using microchannel plates and a resistive anode. Specifically, we employ a clamped pair of microchannel plates (’’V geometry’’) followed by a gap and a clamped triplet of microchannel plates (’’Z geometry’’) in cascade, in order to achieve a high (3×107) stable electron gain. This high gain in turn allows the position determination of each photoelectron event with a very high signal‐to‐noise ratio, thereby giving a theoretical spatial resolution limited chiefly by the microchannel plate channel spacing rather than by the anode signal‐to‐noise ratio. Our first model of this detector is a windowless vacuum‐ultraviolet image sensor, and demonstrates 500×500 pixel images (50 μm FWHM over a 25‐mm circular field of view). Our spatial resolution is presently limited by the microchannel structure and by the error distribution in our analog pulse ratio electronics. High‐current microchannel plates are employed to ...

Formation Rate, Evolving Luminosity Function, Jet Structure, and Progenitors for Long Gamma-Ray Bursts

We constrain the isotropic luminosity function (LF) and formation rate of long γ-ray bursts (GRBs) by fitting models jointly to both the observed differential peak-flux and redshift distributions. We find evidence supporting an evolving LF, where the luminosity scales as (1 + z)δ, with an optimal δ of 1.0 ± 0.2. For a single-power law LF, the best slope is approximately -1.57 with an upper luminosity of 1053.3 ergs s-1, while the best slopes for a double-power law LF are approximately -1.6 and -2.6, with a break luminosity of 1052.7 ergs s-1. Our finding implies a jet model intermediate between the universal structured (θ) ∝ θ-2 model and the quasi-universal Gaussian structured model. For the uniform-jet model our result is compatible with an angle distribution between 2° and 15°. Our best-constrained GRB formation rate histories increase from z = 0 to 2 by a factor of ~30 and then continue increasing slightly. We connect these histories to the cosmic star formation history and compare with observational inferences up to z ~ 6. GRBs could be tracing the cosmic rates of both the normal and obscured star formation regimes. We estimate a current GRB event rate in the Milky Way of ~5 × 10-5 yr-1 and compare it with the birthrate of massive close Wolf-Rayet + black hole binaries with orbital periods of hours. The agreement is rather good, suggesting that these systems could be the progenitors of the long GRBs.

Peak energy of the prompt emission of long gamma-ray bursts versus their fluence and peak flux

The spectral-energy (and luminosity) correlations in long gamma-ray bursts are being hotly debated to establish, first of all, their reality against possible selection effects. These are best studied in the observer planes, namely the peak energy E obs peak versus the fluence F or the peak flux P. In a recent paper, we have started to investigate this problem considering all bursts with known redshift and spectral properties. Here, we consider instead all bursts with known E obs peak , irrespective of redshift, adding to those a sample of 100 faint BATSE bursts representative of a larger population of 1000 objects. This allows us to construct a complete, fluence-limited, sample, tailored to study the selection/instrumental effects we consider. We found that the fainter BATSE bursts have smaller E obs peak than those of bright events. As a consequence, the E obs peak of these bursts is correlated with the fluence, though with a slope flatter than that defined by bursts with z. Selection effects, which are present, are shown not to be responsible for the existence of such a correlation. About six per cent of these bursts are surely outliers of the E peak -E iso correlation (updated in this paper to include 83 bursts), since they are inconsistent with it for any redshift. E obs peak also correlates with the peak flux, with a slope similar to the Ep eak -L iso correlation. In this case, there is only one sure outlier. The scatter of the E obs peak -P correlation defined by the BATSE bursts of our sample is significantly smaller than the E obs peak -F correlation of the same bursts, while for the bursts with known redshift the E peak -E iso correlation is tighter than the E peak -L iso one. Once a very large number of bursts with E obs peak and redshift will be available, we thus expect that the E peak -L iso correlation will be similar to that currently found, whereas it is very likely that the E peak -E iso correlation will become flatter and with a larger scatter.

The peak luminosity–peak energy correlation in gamma‐ray bursts

We derive the correlation between the peak luminosity Liso and the peak energy of the ν Fν spectrum Epeak using 25 long gamma-ray bursts (GRBs) with firm redshift measurements. We find that its slope is similar to that of the correlation between the time-integrated isotropic emitted energy Eiso and Epeak .F or the 16 GRBs in our sample with estimated jet opening angle, we compute the collimation-corrected peak luminosity L γ , and find that it correlates with Epeak. This correlation has, however, a scatter larger than that of the correlation between Epeak and E γ (the time-integrated emitted energy, corrected for collimation), which we ascribe to the fact that the opening angle is estimated through the global energetics. We have then selected a large sample of 442 GRBs with pseudo-redshifts, derived through the lag‐luminosity relation, to test the existence of the Liso‐Epeak correlation. With this sample we also explore the possibility of a correlation between time-resolved quantities, namely L p and the peak energy at the peak of emission E p .

Probing the existence of the Epeak–Eiso correlation in long gamma ray bursts

We probe the existence of the E peak‐E iso correlation in long gamma-ray bursts (GRBs) using a sample of 442 BATSE bursts with known E peak and with redshift estimated through the lag‐luminosity correlation. This sample confirms that the rest-frame peak energy is correlated with the isotropic equivalent energy. The distribution of the scatter of the points around the best-fitting line is similar to that obtained with the 27 bursts with spectroscopic redshifts. We interpret the scatter in the E peak‐E iso plane as due to the opening angle distribution of GRB jets. By assuming that the collimation corrected energy correlates with E peak we can derive the observed distribution of the jet opening angles, which turns out to be lognormal with a peak value of ∼ 6. ◦ 5. Ke yw ords: cosmology: observations ‐ distance scale ‐ gamma-rays: bursts.

Constraints on dark matter physics from dwarf galaxies through galaxy cluster haloes

One of the predictions of the standard cold dark matter model is that dark haloes have centrally divergent density profiles. An extensive body of rotation curve observations of dwarf and low surface brightness galaxies shows the dark haloes of those systems to be characterized by soft constant density central cores. Several physical processes have been proposed to produce soft cores in dark haloes, each one with different scaling properties. With the aim of discriminating among them we have examined the rotation curves of dark matter dominated dwarf and low surface brightness galaxies and the inner mass profiles of two clusters of galaxies lacking a central cD galaxy and with evidence of soft cores in the centre. The core radii and central densities of these haloes scale in a well defined manner with the depth of their potential wells, as measured through the maximum circular velocity. As a result of our analysis we identify self-interacting cold dark matter as a viable solution to the core problem, where a non-singular isothermal core is formed in the halo center surrounded by a Navarro, Frenk, & White profile in the outer parts. We show that this particular physical situation predicts core radii in agreement with observations. Furthermore, using the observed scalings, we derive an expression for the minimum cross section (�) which has an explicit dependence with the halo dispersion velocity (v). If mx is the mass of the dark matter particle: �/mx � 4 10 25 (100 kms 1 /v) cm 2 /Gev.