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A. Lapi

Publications and source records attributed to A. Lapi.

123 records · Page 7Linked to original sources
arXiv2007

Observability of the virialization phase of spheroidal galaxies with radio arrays

In the standard galaxy formation scenario plasma clouds with a high thermal energy content must exist at high redshifts since the proto-galactic gas is shock heated to the virial temperature, and extensive cooling, leading to efficient star formation, must await the collapse of massive halos (as indicated by the massive body of evidence, referred to as "downsizing"). Massive plasma clouds are potentially observable through the thermal and kinetic Sunyaev-Zel'dovich effects and their free-free emission. We find that the detection of substantial numbers of galaxy-scale thermal SZ signals is achievable by blind surveys with next generation radio telescope arrays such as EVLA, ALMA and SKA. This population is even detectable with the 10% SKA, and wide field of view options at high frequency on any of these arrays would greatly increase survey speed. An analysis of confusion effects and of the contamination by radio and dust emissions shows that the optimal frequencies are those in the range 10-35 GHz. Predictions for the redshift distributions of detected sources are also worked out.

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arXiv2007

The Baryonic and Dark Matter Properties of High Redshift Gravitationally Lensed Disk Galaxies

We present a detailed study of the structural properties of four gravitationally lensed disk galaxies at z=1. Modelling the rotation curves on sub-kpc scales we derive the values for the disk mass, the reference dark matter density and core radius, and the angular momentum per unit mass. The derived models suggest that the rotation curve profile and amplitude are best fit with a dark matter component similar to those of local spiral galaxies. The stellar component also has a similar length scale, but with substantially smaller masses than similarly luminous disk galaxies in the local universe. Comparing the average dark matter density inside the optical radius we find that the disk galaxies at z=1 have larger densities (by up to a factor of 7) than similar disk galaxies in the local Universe. Furthermore, the angular momentum per unit mass versus reference velocity is well matched to the local relation, suggesting that the angular momentum of the disk remains constant between high redshifts and the present day. Though statistically limited, these observations point towards a spirals' formation scenario in which stellar disks are slowly grown by the accretion of angular momentum conserving material.

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arXiv2007

The Role of the Dust in Primeval Galaxies: A Simple Physical Model for Lyman Break Galaxies and Lyman Alpha Emitters

We explore the onset of star formation in the early Universe, exploiting the observations of high-redshift Lyman-break galaxies (LBGs) and Lyman alpha emitters (LAEs), in the framework of the galaxy formation scenario elaborated by Granato et al. (2004) already successfully tested against the wealth of data on later evolutionary stages. Complementing the model with a simple, physically plausible, recipe for the evolution of dust attenuation in metal poor galaxies we reproduce the luminosity functions (LFs) of LBGs and of LAEs at different redshifts. This recipe yields a much faster increase with galactic age of attenuation in more massive galaxies, endowed with higher star formation rates. These objects have therefore shorter lifetimes in the LAE and LBG phases, and are more easily detected in the dusty submillimeter bright phase. The short UV bright lifetimes of massive objects strongly mitigate the effect of the fast increase of the massive halo density with decreasing redshift, thus accounting for the weaker evolution of the LBG LF, compared to that of the halo mass function, and the even weaker evolution between z~6 and z~3 of the LAE LF. LAEs are on the average expected to be younger, with lower stellar masses, and associated to less massive halos than LBGs. Finally, we show that the intergalactic medium can be completely reionized at redshift z~6-7 by massive stars shining in protogalactic spheroids with halo masses from a few 10^10 to a few 10^11 M_sun, showing up as faint LBGs with magnitude in the range -17<M_1350<-20, without resorting to any special stellar initial mass function.

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arXiv2007

The Universal Rotation Curve of Spiral Galaxies. II The Dark Matter Distribution out to the Virial Radius

In the current LambdaCDM cosmological scenario, N-body simulations provide us with a Universal mass profile, and consequently a Universal equilibrium circular velocity of the virialized objects, as galaxies. In this paper we obtain, by combining kinematical data of their inner regions with global observational properties, the Universal Rotation Curve (URC) of disk galaxies and the corresponding mass distribution out to their virial radius. This curve extends the results of Paper I, concerning the inner luminous regions of Sb-Im spirals, out to the edge of the galaxy halos.

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arXiv2006

A highly obscured and strongly clustered galaxy population discovered with the Spitzer Space Telescope

The ~800 optically unseen (R>25.5) 24mum-selected sources in the complete Spitzer First Look Survey sample (Fadda et al. 2006) with F[24mum]>0.35 mJy are found to be very strongly clustered. If, as indicated by several lines of circumstantial evidence, they are ultraluminous far-IR galaxies at z ~ [1.6-2.7], the amplitude of their spatial correlation function is very high. The associated comoving clustering length is estimated to be r_0=14.0_{-2.4}^{+2.1} Mpc, value which puts these sources amongst the most strongly clustered populations of our known universe. Their 8mum-24mum colours suggest that the AGN contribution dominates above F[24mum] ~ 0.8 mJy, consistent with earlier analyses. The properties of these objects (number counts, redshift distribution, clustering amplitude) are fully consistent with those of proto-spheroidal galaxies in the process of forming most of their stars and of growing their active nucleus, as described by the Granato et al. (2004) model. In particular, the inferred space density of such galaxies at z ~ 2 is much higher than what expected from most semi-analytic models. Matches of the observed projected correlation function w(θ) with models derived within the so-called Halo Occupation Scenario show that these sources have to be hosted by haloes more massive than ~10^{13.4} M_\odot. This value is significantly higher than that for the typical galactic haloes hosting massive elliptical galaxies, suggesting a duration of the starburst phase of massive high-redshift dusty galaxies of T_B ~ 0.5 Gyr.

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arXiv2006

Searching for AGN-driven Shocks in Galaxy Clusters

Shocks and blastwaves are expected to be driven driven into the intracluster medium filling galaxy groups and clusters by powerful outbursts of active galactic nuclei or quasars in the member galaxies; the first footprints of shock fronts have been tentatively traced out with X-ray imaging. We show how overpressures in the blasts behind the shock can prove the case and also provide specific marks of the nuclear activity: its strength, its current stage, and the nature of its prevailing output. We propose to detect these marks with the aimed pressure probe constituted by the resolved Sunyaev-Zel'dovich effect. We compute and discuss the outcomes to be expected in nearby and distant sources at different stages of their activity.

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arXiv2006

The growth of the nuclear black holes in submillimeter galaxies

We show that the ABC scenario we proposed for the co-evolution of spheroids and QSOs predicts accretion rates and masses of supermassive black holes in sub-mm galaxies in keeping with recent X-ray determinations. These masses are well below the local values, and those predicted by alternative models. The observed column densities may be mostly due to ISM in the galaxy. The contribution of the associated nuclear activity to the X-ray background is likely negligible, while they may contribute a sizeable fraction $\sim 10 %$ to hard-X cumulative counts at the faintest observed fluxes.

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arXiv2006

A Physical Model for Co-evolution of QSOs and of their Spheroidal Hosts

At variance with most semi-analytic models, in the Anti-hierarchical Baryon Collapse scenario (Granato et al. 2001, 2004) the main driver of the galaxy formation and evolution is not the merging sequence but are baryon processes. This approach emphasizes, still in the framework of the hierarchical clustering paradigm for dark matter halos, feedback processes from supernova explosions and from active nuclei, that tie together star formation in spheroidal galaxies and the growth of black holes at their centers. We review some recent results showing the remarkably successful predictive power of this scenario, which allows us to account for the evolution with cosmic time of a broad variety of properties of galaxies and active nuclei, which proved to be very challenging for competing models.

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arXiv2006

New Relationships between Galaxy Properties and Host Halo Mass, and the Role of Feedbacks in Galaxy Formation

We present new relationships between halo masses M_h and several galaxy properties, including r*-band luminosities L_r, stellar (M_star) and baryonic masses, stellar velocity dispersions (sigma), and black hole masses (M_BH). Approximate analytic expressions are given. In the galaxy halo mass range 3*10^(10) M_sun <= M_h <= 3*10^(13) M_sun the M_h-L_r, M_star-M_h, and M_BH-M_h are well represented by a double power law, with a break at M_h_break~3*10^(11) M_sun, corresponding to a mass in stars M_star~ 1.2*10^(10) M_sun, to a r*-band luminosity L_r~ 5*10^9 L_sun, to a stellar velocity dispersion sigma ~ 88 km/s, and to a black hole mass M_BH~ 9*10^(6) M_sun. The sigma-M_h relation can be approximated by a single power law, though a double power law is a better representation. We interpret these relations in terms of the effect of feedback from supernovae and from the active nucleus on the interstellar medium. We argue that the break of the power laws occurs at a mass which marks the transition between the dominance of the stellar and the AGN feedback.

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arXiv2005

Intracluster Entropy from Joint X-ray and Sunyaev-Zel'dovich Observations

The temperature and density of the hot diffuse medium pervading galaxy groups and clusters combine into one significant quantity, the entropy. Here we express the entropy levels and profiles in model-independent forms by joining two observables, the X-ray luminosity and the change in the CMB intensity due to the Sunyaev-Zel'dovich (SZ) effect. Thus we present both global scaling relations for the entropy levels from clusters and groups, and a simple expression yielding the entropy profiles in individual clusters from resolved X-ray surface brightness and SZ spatial distributions. We propose that our approach provides two useful tools for comparing large data samples with models, in order to probe the processes that govern the thermal state of the hot intracluster medium. The feasibility of using such a diagnostic for the entropy is quantitatively assessed, based on current X-ray and upcoming SZ measurements.

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arXiv2005

Intracluster and Intragroup Entropy from Quasar Activity

We investigate how the hierarchical merging of dark matter halos, the radiative cooling of baryons, and the energy feedback from supernovae and active galactic nuclei or quasars combine to govern the amount and the thermal state of the hot plasma pervading groups and clusters of galaxies. We show that, by itself, supernova preheating of the external gas flowing into clusters falls short of explaining the observed X-ray scaling relations of the plasma luminosity L_X or the plasma entropy K versus the X-ray temperature T. To account for the scaling laws from rich to poor clusters takes preheating enhanced by the energy input from active galactic nuclei. In groups, on the other hand, the internal impacts of powerful quasars going off in member galaxies can blow some plasma out of the structure. So they depress L_X and raise K to the observed average levels; meanwhile, the sporadic nature of such impulsive events generates the intrinsic component of the wide scatter apparent in the data. The same quasar feedback gives rise in groups to entropy profiles as steep as observed, a feature hard to explain with simple preheating schemes. Finally, we argue a close connection of the L_X-T or the K-T relation with the M_{\bullet}-σcorrelation between the host velocity dispersion and the masses of the black holes, relics of the quasar activity.

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arXiv2004

The Sunyaev-Zeldovich effect as a probe of the galaxy formation process

The Sunyaev-Zeldovich (SZ) effect has proven to be an extremely powerful tool to study the physical and evolutionary properties of rich clusters of galaxies. Upcoming SZ experiments, with their much improved sensitivity and angular resolution, will provide unique information also on phases of galaxy evolution characterized by the presence of large amounts of hot proto-galactic gas. We present a preliminary analysis of the SZ signals that can be expected at the collapse of the proto-galaxy when, according to the standard scenario, the gas is heated at its virial temperature, and during episodes of strong energy injections from the active nucleus. The contributions of such signals to excess power on arc-minute scales recently found by CBI and BIMA experiments are briefly discussed.

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arXiv2003

Sunyaev-Zel'dovich Effects from Quasars in Galaxies and Groups

The energy fed by active galactic nuclei to the surrounding diffuse baryons changes their amount, temperature, and distribution; so in groups and in member galaxies it affects the X-ray luminosity and also the Sunyaev-Zel'dovich effect. Here we compute how the latter is enhanced by the transient blastwave driven by an active quasar, and is depressed when the equilibrium is recovered with a depleted density. We constrain such depressions and enhancements with the masses of relic black holes in galaxies and the X-ray luminosities in groups. We discuss how all these linked observables can tell the quasar contribution to the thermal history of the baryons pervading galaxies and groups.

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arXiv2002

AGN Feedback on the Intracluster Medium

Galaxy groups are quite underluminous in X-rays compared to clusters, so the intracluster medium has to be considerably underdense in the former. We consider this to be due to substantial energy fed back into the ICM when the baryons in the member galaxies condense into stars ending up in SNe, or accrete on to central supermassive black holes energizing AGNs. We compute the outflow and the blowout effects driven by the AGNs and the resulting, steep luminosity-temperature correlation L_X-T. We compare this with the SN contribution and with the X-ray data; the latter require the AGN energy to be coupled to the surrounding ICM at fractional levels around 5 10^{-2}. We link the L_X-T behavior with the parallel effects of the AGN feedback on the gas in the host galaxy; we find that these yield a correlation steep up to M_{\bullet}\propto σ^5 between the galactic velocity dispersions and the central BH masses.

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