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Register a new userThe effect of thermally pulsating asymptotic giant branch stars on the evolution of the rest-frame near-infrared galaxy luminosity function
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We address the fundamental question of matching the rest-frame K-band luminosity function (LF) of galaxies over the Hubble time using semi-analytic models, after modification of the stellar population modelling. We include the Maraston evolutionary synthesis models, that feature a higher contribution by the Thermally Pulsating - Asymptotic Giant Branch (TP-AGB) stellar phase, into three different semi-analytic models, namely the De Lucia and Blaizot version of the Munich model, MORGANA and the Menci model. We leave all other input physics and parameters unchanged. We find that the modification of the stellar population emission can solve the mismatch between models and the observed rest-frame K-band luminosity from the brightest galaxies derived from UKIDSS data at high redshift. For all explored semi-analytic models this holds at the redshifts - between 2 and 3 - where the discrepancy was recently pointed out. The reason for the success is that at these cosmic epochs the model galaxies have the right age (~1 Gyr) to contain a well-developed TP-AGB phase which makes them redder without the need of changing their mass or age. At the same time, the known overestimation of the faint end is enhanced in the K-band when including the TP-AGB contribution. At lower redshifts (z<2) some of the explored models deviate from the data. This is due to too short merging timescales and inefficient radio-mode AGN feedback. Our results show that a strong evolution in mass predicted by hierarchical models is compatible with no evolution on the bright-end of the K-band LF from z=3 to the local universe. This means that, at high redshifts and contrary to what is commonly accepted, K-band emission is not necessarily a good tracer of galaxy mass.
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We study the debated contribution from thermally pulsing asymptotic giant branch (TP-AGB) stars in evolutionary population synthesis models. We investigate the Spectral Energy Distributions (SEDs) of a sample of 51 spectroscopically confirmed, high-z ($1.3<z_{rm spec}<2.7$), galaxies using three evolutionary population synthesis models with strong, mild and light TP-AGB. Our sample is the largest of spectroscopically confirmed galaxies on which such models are tested so far. Galaxies were selected as passive, but we model them using a variety of star formation histories in order not to be dependent on this pre-selection. We find that the observed SEDs are best fitted with a significant contribution of TP-AGB stars or with substantial dust attenuation. Without including reddening, TP-AGB-strong models perform better and deliver solutions consistent within $1sigma$ from the best-fit ones in the vast majority of cases. Including reddening, all models perform similarly. Using independent constraints from observations in the mid- and far-IR, we show that low/negligible dust attenuation, i.e. $E(B-V)lesssim 0.05$ , should be preferred for the SEDs of passively-selected galaxies. Given that TP-AGB-light models give systematically older ages for passive galaxies, we suggest number counts of passive galaxies at higher redshifts as a further test to discriminate among stellar population models.
We present a robust measurement and analysis of the rest-frame ultraviolet (UV) luminosity function at z=4-8. We use deep Hubble Space Telescope imaging over the CANDELS/GOODS fields, the Hubble Ultra Deep Field and the Year 1 Hubble Frontier Field deep parallel observations. These surveys provides an effective volume of 0.6-1.2 x 10^6 Mpc^3 over this epoch, allowing us to perform a robust search for faint (M_UV=-18) and bright (M_UV < -21) galaxies. We select candidate galaxies using a well-tested photometric redshift technique with careful screening of contaminants, finding a sample of 7446 galaxies at 3.5<z<8.5, with >1000 galaxies at z~6-8. We measure the luminosity function using a Markov Chain Monte Carlo analysis to measure robust uncertainties. At the faint end our results agree with previous studies, yet we find a higher abundance of UV-bright galaxies at z>6, with M* ~ -21 at z>5, different than that inferred based on previous trends at lower redshift. At z=8, a single power-law provides an equally good fit to the UV luminosity function, while at z=6 and 7, an exponential cutoff at the bright-end is moderately preferred. We compare to semi-analytical models, and find that the lack of evolution in M* is consistent with models where the impact of dust attenuation on the bright-end of the luminosity function decreases at higher redshift. We measure the evolution of the cosmic star-formation rate density, correcting for dust attenuation, and find that it declines as (1+z)^(-4.3 +/- 0.5) at z>4, consistent with observations at z>9. Our observations are consistent with a reionization history that starts at z>10, completes at z>6, and reaches a midpoint (x_HII = 0.5) at 6.7<z<9.4. Finally, our observations predict that the abundance of bright z=9 galaxies is likely higher than previous constraints, though consistent with recent estimates of bright z~10 galaxies. [abridged]
We present the rest-frame J- and H-band luminosity function (LF) of field galaxies, based on a deep multi-wavelength composite sample from the MUSYC, FIRES and FIREWORKS survey public catalogues, covering a total area of 450 arcmin^2. The availability of flux measurements in the Spitzer IRAC 3.6, 4.5, 5.8, and 8 um channels allows us to compute absolute magnitudes in the rest-frame J and H bands up to z=3.5 minimizing the dependence on the stellar evolution models. We compute the LF in the four redshift bins 1.5<z<2.0, 2.0<z<2.5, 2.5<z<3.0 and 3.0<z<3.5. Combining our results with those already available at lower redshifts, we find that (1) the faint end slope is consistent with being constant up to z=3.5, with alpha=-1.05+/-0.03 for the rest-frame J band and alpha=-1.15+/-0.02 for the rest-frame H band; (2) the normalization phi* decreases by a factor of 6 between z=0 and z~1.75 and by a factor 3 between z~1.75 and z=3.25; (3) the characteristic magnitude M* shows a brightening from z=0 to z~2 followed by a slow dimming to z=3.25. We finally compute the luminosity density (LD) in both rest-frame J and H bands. The analysis of our results together with those available in the literature shows that the LD is approximately constant up to z~1, and it then decreases by a factor of 6 up to z=3.5.
We present the dust ejecta of the new stellar models for the Thermally Pulsing Asymptotic Giant Branch (TP-AGB) phase computed with the COLIBRI code. We use a formalism of dust growth coupled with a stationary wind for both M and C-stars. In the original version of this formalism, the most efficient destruction process of silicate dust in M-giants is chemisputtering by H2 molecules. For these stars we find that dust grains can only form at relatively large radial distances (r~5 R*), where they cannot be efficiently accelerated, in agreement with other investigations. In the light of recent laboratory results, we also consider the alternative case that the condensation temperature of silicates is determined only by the competition between growth and free evaporation processes (i.e. no chemisputtering). With this latter approach we obtain dust condensation temperatures that are significantly higher (up to Tcond~1400 K) than those found when chemisputtering is included (Tcond~900 K), and in better agreement with condensation experiments. As a consequence, silicate grains can remain stable in inner regions of the circumstellar envelopes (r~2 R*), where they can rapidly grow and can be efficiently accelerated. With this modification, our models nicely reproduce the observed trend between terminal velocities and mass loss rates of Galactic M-giants. For C-stars the formalism is based on the homogeneous growth scheme where the key role is played by the carbon over oxygen excess. The models reproduce fairly well the terminal velocities of Galactic stars and there is no need to invoke changes in the standard assumptions. At decreasing metallicity the carbon excess becomes more pronounced and the efficiency of dust formation increases. This trend could be in tension with recent observational evidence in favour of a decreasing efficiency, at decreasing metallicity.
We extend the formalism presented in our recent calculations of dust ejecta from the Thermally Pulsing Asymptotic Giant Branch (TP-AGB) phase, to the case of super-solar metallicity stars. The TP-AGB evolutionary models are computed with the COLIBRI code. We adopt our preferred scheme for dust growth. For M-giants, we neglect chemisputtering by H$_2$ molecules and, for C-stars we assume a homogeneous growth scheme which is primarily controlled by the carbon over oxygen excess. At super-solar metallicities, dust forms more efficiently and silicates tend to condense significantly closer to the photosphere (r~1.5 R$_*$) - and thus at higher temperatures and densities - than at solar and sub-solar metallicities (r~2-3 R$_*$). In such conditions, the hypothesis of thermal decoupling between gas and dust becomes questionable, while dust heating due to collisions plays an important role. The heating mechanism delays dust condensation to slightly outer regions in the circumstellar envelope. We find that the same mechanism is not significant at solar and sub-solar metallicities. The main dust products at super-solar metallicities are silicates. We calculate the total dust ejecta and dust-to-gas ejecta, for various values of the stellar initial masses and initial metallicities Z=0.04, 0.06. Merging these new calculations with those for lower metallicities it turns out that, contrary to what often assumed, the total dust-to-gas ejecta of intermediate-mass stars exhibit only a weak dependence on the initial metal content.
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