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Firmani C. | Firmani Claudio | Firmani Donatella

C David J. Fernández | C David J Fernández | C Susanth | c Aleksandar Ili\' | c Marko Kosti\' | c Stevan Pilipovi\' | C Jiji | C Swathi | C | C Dharmani Bhaveshkumar

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

Firmani C., Avila-Reese V.
02 Apr 2013 astro-ph.CO arxiv.org/abs/1304.0765

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.

The stellar and dark halo mass assembly of galaxies

Avila-Reese V., Firmani C.
22 Mar 2011 astro-ph.CO arxiv.org/abs/1103.4329

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).

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

Avila-Reese V., , , Valenzuela O., Firmani C., , Ceverino D.
22 Mar 2011 astro-ph.CO arxiv.org/abs/1103.4422

(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.

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

Rodriguez-Puebla A., Avila-Reese V., Firmani C., Colin P.
21 Mar 2011 astro-ph.CO arxiv.org/abs/1103.4151

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.

Galaxy downsizing evidenced by hybrid evolutionary tracks

Firmani C., Avila-Reese V.
27 Aug 2010 astro-ph.CO arxiv.org/abs/1008.4796

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)

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

Firmani C., Avila-Reese V., Rodriguez-Puebla A.
28 Sep 2009 astro-ph.CO arxiv.org/abs/0909.5188

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)

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

Firmani C., Avila-Reese V.
31 Mar 2009 astro-ph.CO arxiv.org/abs/0903.5325

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.

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

Ghirlanda G., Nava L., Ghisellini G., Celotti A., Firmani C.
05 Feb 2009 astro-ph.HE arxiv.org/abs/0902.0983

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.

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

Firmani C., Cabrera J. I., Avila-Reese V., Ghisellini G., Ghirlanda G., Nava L., Bosnjak Z.
10 Nov 2008 astro-ph arxiv.org/abs/0811.1578

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, integratedAmati' 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.

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

Avila-Reese V., Zavala J., Firmani C.,
03 Jul 2008 astro-ph arxiv.org/abs/0807.0636

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]