We use our state-of-the-art Galaxy Evolution and Assembly (GAEA) semi-analytic model to study how and on which time-scales star formation is suppressed in satellite galaxies. Our fiducial stellar ...feedback model, implementing strong stellar driven outflows, reproduces relatively well the variations of passive fractions as a function of galaxy stellar mass and halo mass measured in the local Universe, as well as the `quenching' time-scales inferred from the data. We show that the same level of agreement can be obtained by using an alternative stellar feedback scheme featuring lower ejection rates at high redshift, and modifying the treatment for hot gas stripping. This scheme overpredicts the number densities of low-to-intermediate mass galaxies. In addition, a good agreement with the observed passive fractions can be obtained only by assuming that cooling can continue on satellites, at the rate predicted considering halo properties at infall, even after their parent dark matter substructure is stripped below the resolution of the simulation. For our fiducial model, the better agreement with the observed passive fractions can be ascribed to: (i) a larger cold gas fraction of satellites at the time of accretion, and (ii) a lower rate of gas reheating by supernovae explosions and stellar winds with respect to previous versions of our model. Our results suggest that the abundance of passive galaxies with stellar mass larger than ∼ 10^{10} M_{⊙} is primarily determined by the self-regulation between star formation and stellar feedback, with environmental processes playing a more marginal role.
Abstract
We update our recently published model for GAlaxy Evolution and Assembly (GAEA), to include a self-consistent treatment of the partition of cold gas in atomic and molecular hydrogen. Our ...model provides significant improvements with respect to previous ones used for similar studies. In particular, GAEA (i) includes a sophisticated chemical enrichment scheme accounting for non-instantaneous recycling of gas, metals and energy; (ii) reproduces the measured evolution of the galaxy stellar mass function; (iii) reasonably reproduces the observed correlation between galaxy stellar mass and gas metallicity at different redshifts. These are important prerequisites for models considering a metallicity-dependent efficiency of molecular gas formation. We also update our model for disc sizes and show that model predictions are in nice agreement with observational estimates for the gas, stellar and star-forming discs at different cosmic epochs. We analyse the influence of different star formation laws including empirical relations based on the hydrostatic pressure of the disc, analytic models and prescriptions derived from detailed hydrodynamical simulations. We find that modifying the star formation law does not affect significantly the global properties of model galaxies, neither their distributions. The only quantity showing significant deviations in different models is the cosmic molecular-to-atomic hydrogen ratio, particularly at high redshift. Unfortunately, however, this quantity also depends strongly on the modelling adopted for additional physical processes. Useful constraints on the physical processes regulating star formation can be obtained focusing on low-mass galaxies and/or at higher redshift. In this case, self-regulation has not yet washed out differences imprinted at early time.
In this paper, we investigate the implications of the integrated galaxy-wide stellar initial mass function (IGIMF) approach in the framework of the semi-analytical model GAEA (GAlaxy Evolution and ...Assembly), which features a detailed treatment of chemical enrichment and stellar feedback. The IGIMF provides an analytic description of the dependence of the stellar IMF shape on the rate of star formation in galaxies. We find that our model with a universal IMF predicts a rather flat a/Fe-stellar mass relation. The model assuming the IGIMF, instead, is able to reproduce the observed increase of a-enhancement with stellar mass, in agreement with previous studies. This is mainly due to the fact that massive galaxies are characterized by larger star formation rates at high redshift, leading to stronger a-enhancement with respect to low-mass galaxies. At the same time, the IGIMF hypothesis does not affect significantly the trend for shorter star formation time-scales for more massive galaxies. We argue that in the IGIMF scenario the a/Fe ratios are good tracers of the highest star formation events. The final stellar masses and mass-to-light ratio of our model massive galaxies are larger than those estimated from the synthetic photometry assuming a universal IMF, providing a self-consistent interpretation of similar recent results, based on dynamical analysis of local early-type galaxies.
Abstract
In this paper we discuss the contribution of different astrophysical sources to the ionization of neutral hydrogen at different redshifts. We critically revise the arguments in favour ...of/against a substantial contribution of active galactic nuclei (AGNs) and/or Lyman break galaxies (LBGs) to the reionization of the Universe at z > 5. We consider extrapolations of the high-z quasi-stellar objects (QSO) and LBG luminosity functions (LFs) and their redshift evolution as well as indirect constraints on the space density of lower luminosity AGNs based on the galaxy stellar mass function. Since the hypothesis of reionization due to LBGs alone requires a significant contribution of faint dwarf galaxies and a LyC photon escape fraction (f
esc) of the order of ∼20 per cent, in tension with present observational constraints, we examine under which hypothesis AGNs and LBGs may provide a combined relevant contribution to reionization. We show that a relatively steep faint end of the AGN LF, consistent with present constraints, provides a relevant (although sub-dominant) contribution, thus allowing us to recover the required ionizing photon rates with f
esc ∼ 5 per cent up to z ∼ 7. At higher redshifts, we test the case for a luminosity-dependent f
esc scenario and we conclude that, if the observed LBGs are indeed characterized by very low f
esc, values of the order of f
esc ∼ 70 per cent are needed for objects below our detection threshold, for this galaxy population to provide a substantial contribution to reionization. Clearly, the study of the properties of faint sources (both AGNs and LBGs) is crucial.
It has been widely claimed that several lines of observational evidence point towards a ‘downsizing’ of the process of galaxy formation over cosmic time. This behaviour is sometimes termed ...‘antihierarchical’, and contrasted with the ‘bottom-up’ (small objects form first) assembly of the dark matter structures in cold dark matter (CDM) models. In this paper, we address three different kinds of observational evidence that have been described as ‘downsizing’: the stellar mass assembly (i.e. more massive galaxies assemble at higher redshift with respect to low-mass ones), star formation rate (SFR) (i.e. the decline of the specific star formation rate is faster for more massive systems) and the ages of the stellar populations in local galaxies (i.e. more massive galaxies host older stellar populations). We compare a broad compilation of available data sets with the predictions of three different semi-analytic models of galaxy formation within the ΛCDM framework. In the data, we see only weak evidence at best of ‘downsizing’ in stellar mass and in SFR. Despite the different implementations of the physical recipes, the three models agree remarkably well in their predictions. We find that, when observational errors on stellar mass and SFR are taken into account, the models acceptably reproduce the evolution of massive galaxies (M > 1011 M⊙ in stellar mass), over the entire redshift range that we consider (0 ≲z≲ 4). However, lower mass galaxies, in the stellar mass range 109–1011 M⊙, are formed too early in the models and are too passive at late times. Thus, the models do not correctly reproduce the downsizing trend in stellar mass or the archaeological downsizing, while they qualitatively reproduce the mass-dependent evolution of the SFR. We demonstrate that these discrepancies are not solely due to a poor treatment of satellite galaxies but are mainly connected to the excessively efficient formation of central galaxies in high-redshift haloes with circular velocities ∼100–200 km s−1. We conclude that some physical processes operating on these mass scales – most probably star formation and/or supernova feedback – are not yet properly treated in these models.
Abstract
The origins of Lyman continuum (LyC) photons responsible for the reionization of the universe are as of yet unknown and highly contested. Detecting LyC photons from the Epoch of Reionization ...is not possible due to absorption by the intergalactic medium, which has prompted the development of several indirect diagnostics to infer the rate at which galaxies contribute LyC photons to reionize the universe by studying lower-redshift analogs. We present the Low-redshift Lyman Continuum Survey (LzLCS) comprising measurements made with the Hubble Space Telescope Cosmic Origins Spectrograph for a
z
= 0.2–0.4 sample of 66 galaxies. After careful processing of the far-UV spectra, we obtain a total of 35 Lyman continuum emitters (LCEs) detected with 97.725% confidence, nearly tripling the number of known local LCEs. We estimate escape fractions from the detected LyC flux and upper limits on the undetected LyC flux, finding a range of LyC escape fractions up to 50%. Of the 35 LzLCS LCEs, 12 have LyC escape fractions greater than 5%, more than doubling the number of known local LCEs with cosmologically relevant LyC escape.
We use a state-of-the-art semi-analytic model to study the size and the specific angular momentum of galaxies. Our model includes a specific treatment for the angular momentum exchange between ...different galactic components. Disc scale radii are estimated from the angular momentum of the gaseous/stellar disc, while bulge sizes are estimated assuming energy conservation. The predicted size-mass and angular momentum-mass relations are in fair agreement with observational measurements in the local Universe, provided a treatment for gas dissipation during major mergers is included. Our treatment for disc instability leads to unrealistically small radii of bulges formed through this channel, and predicts an offset between the size-mass relations of central and satellite early-type galaxies, that is not observed. The model reproduces the observed dependence of the size-mass relation on morphology, and predicts a strong correlation between specific angular momentum and cold gas content. This correlation is a natural consequence of galaxy evolution: gas-rich galaxies reside in smaller haloes, and form stars gradually until present day, while gas-poor ones reside in massive haloes, that formed most of their stars at early epochs, when the angular momentum of their parent haloes is low. The dynamical and structural properties of galaxies can be strongly affected by a different treatment for stellar feedback, as this would modify their star formation history. A higher angular momentum for gas accreted through rapid mode does not affect significantly the properties of massive galaxies today, but has a more important effect on low-mass galaxies at higher redshift.
On the evolution of the cosmic ionizing background Fontanot, Fabio; Cristiani, Stefano; Pfrommer, Christoph ...
Monthly notices of the Royal Astronomical Society,
03/2014, Letnik:
438, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We study the observed cosmic ionizing background as a constraint on the nature of the sources responsible for the reionization of the Universe. In earlier work, we showed that extrapolations of the ...ultraviolet (UV) luminosity function (LF) of Lyman-break galaxies (LBGs) at fixed Lyman continuum photon escape fraction (f
esc) are not able to reproduce the redshift evolution of this background. Here, we employ extrapolations of the high-z LFs to describe the contribution of LBGs to the ionizing photon rate, taking into account the smoothing of the baryonic perturbations, due to the background itself (i.e. the filtering mass), as well as a possible sharp increase of f
esc in dwarf galaxies. Under the hypothesis of a dominant contribution of LBGs to cosmic reionization, our results suggest that sources fainter than the current observational limits should be characterized by f
esc values of the order of ∼0.1-0.3 (larger than the current estimates for bright galaxies) to account for a z 6 reionization and the measured evolution of cosmic ionizing background, at the same time. The contribution to the background from quasars turns out to be relevant at z 3. Overall, our results support the case for dedicated observations of faint galaxies in the rest-frame UV, in order to better determine their physical properties. Observed f
esc values outside our proposed range bear relevant consequences on the nature of the astrophysical sources responsible for cosmic reionization and/or its build-up process.
We test the luminosity function of Milky Way satellites as a constraint for the nature of dark matter particles. We perform dissipationless high-resolution N-body simulations of the evolution of ...Galaxy-sized halo in the standard cold dark matter model and in four warm dark matter (WDM) scenarios, with a different choice for the WDM particle mass mw. We then combine the results of the numerical simulations with semi-analytic models for galaxy formation, to infer the properties of the satellite population. Quite surprisingly, we find that even WDM models with relatively low mw values (2–5keV) are able to reproduce the observed abundance of ultra faint (Mv < −9) dwarf galaxies, as well as the observed relation between luminosity and mass within 300pc. Our results suggest a lower limit of 1keV for thermal WDM, in broad agreement with previous results from other astrophysical observations such as Lyman α forest and gravitational lensing.
We present the Model for the Rise of Galaxies and Active Nuclei (morgana), a new code for the formation and evolution of galaxies and active galactic nuclei (AGNs). Starting from the merger trees of ...dark matter (DM) haloes and a model for the evolution of substructure within the haloes, the complex physics of baryons is modelled with a set of state-of-the-art models that describe the mass, metal and energy flows between the various components (baryonic halo, bulge, disc) and phases (cold and hot gas, stars) of a galaxy. These flows are then numerically integrated to produce predictions for the evolution of galaxies. The processes of shock-heating and cooling, star formation, feedback, galactic winds and superwinds, accretion on to black holes and AGN feedback are described by new models. In particular, the evolution of the halo gas explicitly follows the thermal and kinetic energies of the hot and cold phases, while star formation and feedback follow the results of the multiphase model recently proposed by Monaco. The increased level of sophistication of these models allows to move from a phenomenological description of gas physics, based on simple scalings with the depth of the DM halo potential, towards a fully physically motivated one. We deem that this is fully justified by the level of maturity and rough convergence reached by the latest versions of numerical and semi-analytic models of galaxy formation. The comparison of the predictions of morgana with a basic set of galactic data reveals from the one hand an overall rough agreement, and from the other hand highlights a number of well- or less-known problems: (i) producing the cut-off of the luminosity function requires to force the quenching of the late cooling flows by AGN feedback, (ii) the normalization of the Tully-Fisher relation of local spirals cannot be recovered unless the DM haloes are assumed to have a very low concentration, (iii) the mass function of H i gas is not easily fitted at small masses, unless a similarly low concentration is assumed, (iv) there is an excess of small elliptical galaxies at z= 0. These discrepancies, more than the points of agreement with data, give important clues on the missing ingredients of galaxy formation.
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