ALBERT

Beschreibung: We investigate predictions from semi-analytic cosmological models of galaxy formation for the properties of star-forming galaxies (SFGs) and damped Ly α absorption systems (DLAS), and the relationship between these two populations. Our models reproduce fairly well the observed distributions of redshift, stellar mass, star formation rate (SFR), and dust extinction for z ~ 2 SFGs. We predict that DLA hosts span a broad range of properties, with broad and relatively flat distributions of stellar and halo mass, SFR, and luminosity. The photometric colours of DLA host galaxies trace the colours of galaxies with similar luminosities, but the majority are much fainter than the limits of most existing surveys of SFGs. Generally, DLA host galaxies and SFGs at z  = 2 follow similar trends between stellar mass, DLA cross-section, cold gas fraction, SFR, metallicity, and dust extinction as the global population of galaxies with the same stellar mass. Since DLAS select galaxies with larger cold gas masses, they tend to have larger cold gas fractions, lower metallicities, higher SFRs, and less dust extinction than galaxies at the same stellar mass. Our models reproduce the observed relations between impact parameter, column density, and metallicity, suggesting that the sizes of the gas discs giving rise to DLAS in our models are roughly correct. We find that molecular fractions and SFRs are in general significantly lower at the location of the DLA line of sight than the galaxy-averaged value.

Beschreibung: Now that Atacama Large (Sub)Millimeter Array is reaching its full capabilities, observations of sub-mm emission line deep fields become feasible. We couple a semi-analytic model of galaxy formation with a radiative transfer code to make predictions for the luminosity function of CO J =1–0 out to CO J = 6–5 and [C ii ] at redshifts z = 0–6. We find that (1) our model correctly reproduces the CO and [C ii ] emission of low- and high-redshift galaxies and reproduces the available constraints on the CO luminosity function at z ≤ 2.75; (2) we find that the CO and [C ii ] luminosity functions of galaxies increase from z = 6 to z = 4, remain relatively constant till z = 1 and rapidly decrease towards z = 0. The galaxies that are brightest in CO and [C ii ] are found at z ~ 2; (3) the CO J = 3–2 emission line is most favourable to study the CO luminosity and global H 2 mass content of galaxies, because of its brightness and observability with currently available sub-mm and radio instruments; (4) the luminosity functions of high-J CO lines show stronger evolution than the luminosity functions of low-J CO lines; (5) our model barely reproduces the available constraints on the CO and [C ii ] luminosity function of galaxies at z ≥ 1.5 and the CO luminosity of individual galaxies at intermediate redshifts. We argue that this is driven by a lack of cold gas in galaxies at intermediate redshifts as predicted by cosmological simulations of galaxy formation.

Beschreibung: We derive the total cold gas, atomic hydrogen, and molecular gas masses of approximately 24 000 galaxies covering four decades in stellar mass at redshifts 0.5 〈 z 〈 3.0, taken from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey survey. Our inferences are based on the inversion of a molecular hydrogen based star formation law, coupled with a prescription to separate atomic and molecular gas. We find that: (1) there is an increasing trend between the inferred cold gas (H i and H 2 ), H i , and H 2 mass and the stellar mass of galaxies down to stellar masses of 10 8 M already in place at z = 3; (2) the molecular fractions of cold gas increase with increasing stellar mass and look-back time; (3) there is hardly any evolution in the mean H i content of galaxies at fixed stellar mass; (4) the cold gas fraction and relative amount of molecular hydrogen in galaxies decrease at a relatively constant rate with time, independent of stellar mass; (5) there is a large population of low stellar mass galaxies dominated by atomic gas. These galaxies are very gas rich, but only a minor fraction of their gas is molecular; 6) the ratio between star formation rate (SFR) and inferred total cold gas mass (H i + H 2 ) of galaxies (i.e. star formation efficiency; SFE) increases with star formation at fixed stellar masses. Due to its simplicity, the presented approach is valuable to assess the impact of selection biases on small samples of directly observed gas masses and to extend scaling relations down to stellar mass ranges and redshifts that are currently difficult to probe with direct measurements of gas content.

Beschreibung: We conduct a detailed lensing, dynamics and stellar population analysis of nine massive lens early-type galaxies (ETGs) from the X-Shooter Lens Survey (XLENS). Combining gravitational lensing constraints from HST imaging with spatially-resolved kinematics and line-indices constraints from Very Large Telescope (VLT) X-Shooter spectra, we infer the low-mass slope and the low cut-off mass of the stellar initial mass function (IMF): $x_{250}=2.37^{+0.12}_{-0.12}$ and $M_{\mathrm{low}, 250}= 0.131^{+0.023}_{-0.026}\,\mathrm{M}_{\odot }$ , respectively, for a reference point with * 250 km s –1 and R eff 10 kpc. All the XLENS systems are consistent with an IMF slope steeper than Milky Way-like. We find no significant correlations between IMF slope and any other quantity, except for an anticorrelation between total dynamical mass density and low-mass IMF slope at the 87 per cent CL [d x /d log () =  $-0.19^{+0.15}_{-0.15}$ ]. This anticorrelation is consistent with the low-redshift lenses found by Smith et al. that have high velocity dispersions and high stellar mass densities but surprisingly shallow IMF slopes.

Beschreibung: We implement physically motivated recipes for partitioning cold gas into different phases (atomic, molecular, and ionized) in galaxies within semi-analytic models of galaxy formation based on cosmological merger trees. We then model the conversion of molecular gas into stars using empirical recipes motivated by recent observations. We explore the impact of these new recipes on the evolution of fundamental galaxy properties such as stellar mass, star formation rate (SFR), and gas and stellar phase metallicity. We present predictions for stellar mass functions, stellar mass versus SFR relations, and cold gas phase and stellar mass–metallicity relations for our fiducial models, from redshift z  ~ 6 to the present day. In addition we present predictions for the global SFR, mass assembly history, and cosmic enrichment history. We find that the predicted stellar properties of galaxies (stellar mass, SFR, metallicity) are remarkably insensitive to the details of the recipes used for partitioning gas into H  i and H 2 . We see significant sensitivity to the recipes for H 2 formation only in very low mass haloes ( $M_{\rm h} \lesssim 10^{10.5}\, {{\rm M}_{{\odot }}}$ ), which host galaxies with stellar masses $m_* \lesssim 10^8\, {{\rm M}_{{\odot }}}$ . The properties of low-mass galaxies are also quite insensitive to the details of the recipe used for converting H 2 into stars, while the formation epoch of massive galaxies does depend on this significantly. We argue that this behaviour can be interpreted within the framework of a simple equilibrium model for galaxy evolution, in which the conversion of cold gas into stars is balanced on average by inflows and outflows.

Beschreibung: We implement physically motivated recipes for partitioning cold gas into different phases (atomic, molecular, and ionized) in galaxies within semi-analytic models of galaxy formation based on cosmological merger trees. We then model the conversion of molecular gas into stars using empirical recipes motivated by recent observations. We explore the impact of these new recipes on the evolution of fundamental galaxy properties such as stellar mass, star formation rate (SFR), and gas and stellar phase metallicity. We present predictions for stellar mass functions, stellar mass versus SFR relations, and cold gas phase and stellar mass–metallicity relations for our fiducial models, from redshift z  ~ 6 to the present day. In addition we present predictions for the global SFR, mass assembly history, and cosmic enrichment history. We find that the predicted stellar properties of galaxies (stellar mass, SFR, metallicity) are remarkably insensitive to the details of the recipes used for partitioning gas into H  i and H 2 . We see significant sensitivity to the recipes for H 2 formation only in very low mass haloes ( $M_{\rm h} \lesssim 10^{10.5}\, {{\rm M}_{{\odot }}}$ ), which host galaxies with stellar masses $m_* \lesssim 10^8\, {{\rm M}_{{\odot }}}$ . The properties of low-mass galaxies are also quite insensitive to the details of the recipe used for converting H 2 into stars, while the formation epoch of massive galaxies does depend on this significantly. We argue that this behaviour can be interpreted within the framework of a simple equilibrium model for galaxy evolution, in which the conversion of cold gas into stars is balanced on average by inflows and outflows.

Beschreibung: We combine a semi-analytic model of galaxy formation, tracking atomic and molecular phases of cold gas, with a three-dimensional radiative-transfer and line tracing code to study the sub-mm emission from atomic and molecular species (CO, HCN, [C i ], [C ii ], [O i ]) in galaxies. We compare the physics that drives the formation of stars at the epoch of peak star formation (SF) in the Universe ( z  = 2.0) with that in local galaxies. We find that normal star-forming galaxies at high redshift have much higher CO-excitation peaks than their local counterparts and that CO cooling takes place at higher excitation levels. CO line ratios increase with redshift as a function of galaxy star-formation rate, but are well correlated with H 2 surface density independent of redshift. We find an increase in the [O i ]/[C ii ] line ratio in typical star-forming galaxies at z  = 1.2 and z  = 2.0 with respect to counterparts at z  = 0. Our model results suggest that typical star-forming galaxies at high redshift consist of much denser and warmer star-forming clouds than their local counterparts. Galaxies belonging to the tail of the SF activity peak at z  = 1.2 are already less dense and cooler than counterparts during the actual peak of SF activity ( z  = 2.0). We use our results to discuss how future ALMA surveys can best confront our predictions and constrain models of galaxy formation.

Beschreibung: Recent studies based on the integrated light of distant galaxies suggest that the initial mass function (IMF) might not be universal. Variations of the IMF with galaxy type and/or formation time may have important consequences for our understanding of galaxy evolution. We have developed a new stellar population synthesis (SPS) code specifically designed to reconstruct the IMF. We implement a novel approach combining regularization with hierarchical Bayesian inference. Within this approach, we use a parametrized IMF prior to regulate a direct inference of the IMF. This direct inference gives more freedom to the IMF and allows the model to deviate from parametrized models when demanded by the data. We use Markov chain Monte Carlo sampling techniques to reconstruct the best parameters for the IMF prior, the age and the metallicity of a single stellar population. We present our code and apply our model to a number of mock single stellar populations with different ages, metallicities and IMFs. When systematic uncertainties are not significant, we are able to reconstruct the input parameters that were used to create the mock populations. Our results show that if systematic uncertainties do play a role, this may introduce a bias on the results. Therefore, it is important to objectively compare different ingredients of SPS models. Through its Bayesian framework, our model is well suited for this.

Beschreibung: We investigate the properties of damped Lyman α absorption systems (DLAs) in semi-analytic models of galaxy formation, including new modelling of the partitioning of cold gas into atomic, molecular, and ionized phases, and a star formation recipe based on the density of molecular gas. We use three approaches for partitioning gas into atomic and molecular constituents: a pressure-based recipe and metallicity-based recipes with fixed and varying ultraviolet (UV) radiation fields. We identify DLAs by adopting an assumed gas density profile for galactic discs and passing lines of sight through our simulations to compute H i column densities. We find that models with ‘standard’ gas radial profiles – computed assuming that the average specific angular momentum of the gas disc is equal to that of the host dark matter halo – fail to reproduce the observed column density distribution of DLAs, regardless of the assumed gas partitioning. These models also fail to reproduce the distribution of velocity widths v of low-ionization state metal systems, overproducing low- v relative to high- v systems. Models with ‘extended’ radial gas profiles – corresponding to gas discs with higher specific angular momentum, or gas in an alternate extended configuration – are able to reproduce quite well the column density distribution of absorbers over the column density range 19 〈 log N H  i 〈 22.5 in the redshift range 2 〈 z  〈 3.5. The model with pressure-based gas partitioning and the metallicity-based recipe with a varying UV radiation field also reproduce the observed line density of DLAs, H i gas density, and v distribution at z  〈 3 well. However all of the models investigated here underproduce DLAs and the H i gas density at z  〉 3. This may indicate that DLAs at high redshift arise from a different physical phenomenon, such as outflows or filaments. If this is the case, the flatness in the number of DLAs and H i gas density over the redshift interval 0 〈 z  〈 5 may be due to a cosmic coincidence where the majority of DLAs at z  〉 3 arise from intergalactic gas in filaments or streams while those at z  〈 3 arise predominantly in galactic discs. We further investigate the dependence of DLA metallicity on redshift and v in our favoured models, and find good agreement with the observations, particularly when we include the effects of metallicity gradients.