Using semi-analytical models of galaxy formation, the origin of boxy and discy elliptical galaxies is investigated. We find that the simple scenario, motivated by N-body simulations, in which the ...isophotal shape is only dependent on the mass ratio of the last major merger, is not able to reproduce the observation that the fraction of boxy and discy ellipticals depends on galaxy luminosity. The observations can, however, be reproduced with the following reasonable assumptions: (i) equal-mass mergers lead to boxy ellipticals and unequal-mass mergers produce discy ellipticals (as motivated by N-body simulations); (ii) major mergers between bulge-dominated galaxies result always in boxy ellipticals, independent of the mass ratio; (iii) merger remnants that subsequently accrete gas leading to a secondary stellar disc with more than 20 per cent of the total stellar fraction are always discy. This scenario indicates that the isophotal shapes of merger remnants are sensitive to the morphology of their progenitors and subsequent gas infall. Boxy and discy ellipticals can be divided into two subclasses, depending on their formation history. Boxy ellipticals are formed either by equal-mass mergers of disc galaxies or by major mergers of early-type galaxies. We find that discy ellipticals are indicators of unequal-mass mergers or late gas infall. Discy ellipticals with high luminosities are preferentially one-component systems that result from unequal-mass mergers, whereas low-luminosity discy ellipticals are more likely to harbour secondary disc components. In addition, the fraction of discy ellipticals with secular disc components should increase in regions with higher galaxy densities. Taking into account the conversion of cuspy cores into flat low-density cores by black hole merging, we find that discy ellipticals should contain central density cusps whereas boxy ellipticals should in general be characterized by flat cores. Only rare low-luminosity boxy ellipticals, resulting from equal-mass mergers of disc galaxies, could have power-law cores.
Observational and theoretical studies of extragalactic radio sources have suggested that an inhomogeneous environment may be responsible for observed arm-length asymmetries of jets and the properties ...of extended emission-line regions in high-redshift radio galaxies. We perform 3D hydrodynamic simulations of the interaction of a powerful extragalactic bipolar jet with a disc-shaped clumpy interstellar medium (ISM) of lognormal density distribution and analyse the asymmetry. Furthermore, we compute the relation between jet asymmetry and the ISM properties by means of Monte Carlo simulations based on a 1D propagation model for the jet through the dense medium. We find that the properties of the ISM can be related to a probability distribution of jet arm-length asymmetries: disc density and height are found to have the largest effect on the asymmetry for realistic parameter ranges, while the Fourier energy spectrum of the ISM and turbulent Mach number only have a smaller effect. The hydrodynamic simulations show that asymmetries generally may be even larger than expected from the 1D model due to the complex interaction of the jet and its bow shock with gaseous clumps, which goes much beyond simple energy disposal. From our results, observed asymmetries of medium-sized local radio galaxies may be explained by gas masses of 109- 1010 M⊙ in massive elliptical galaxies. Furthermore, the simulations provide a theoretical basis for the observed correlation that emission-line nebulae are generally found to be brighter on the side of the shorter lobe in high-redshift radio galaxies. This interaction of jets with the cold gas phase suggests that star formation in evolving high-redshift galaxies may be affected considerably by jet activity.
Long gamma-ray burst's (LGRB's) association with the death of massive stars suggests that they could be used to probe the cosmic star formation history (CSFH) with high accuracy, due to their high ...luminosities. We utilize cosmological simulations from the First Billion Years project to investigate the biases between the CSFH and the LGRB rate at ..., assuming various different models and constraints on the progenitors of LGRBs. We populate LGRBs using a selection based on environmental properties and demonstrate that the LGRB rate should trace the CSFH to high redshifts. The measured LGRB rate suggests that LGRBs have opening angles of ..., although the degeneracy with the progenitor model cannot rule out an underlying bias. We demonstrate that proxies that relate the LGRB rate with global LGRB host properties do not reflect the underlying LGRB environment, and are in fact a result of the host galaxy's spatial properties, such that LGRBs can exist in galaxies of solar metallicity. However, we find a class of host galaxies that have low stellar mass and are metal rich, and that their metallicity dispersions would not allow low-metallicity environments. Detection of hosts with this set of properties would directly reflect the progenitor's environment. We predict that 10 per cent of LGRBs per year are associated with this set of galaxies that would have forbidden line emission that could be detected by instruments on the James Webb Space Telescope. Such a discovery would place strong constraints on the collapsar model and suggests other avenues to be investigated. (ProQuest: ... denotes formulae/symbols omitted.)
Abstract
We present the results of a new study of dust attenuation at redshifts 3 < z < 4 based on a sample of 236 star-forming galaxies from the VANDELS spectroscopic survey. Motivated by results ...from the First Billion Years (FiBY) simulation project, we argue that the intrinsic spectral energy distributions (SEDs) of star-forming galaxies at these redshifts have a self-similar shape across the mass range 8.2 ≤ log (M⋆/M⊙) ≤ 10.6 probed by our sample. Using FiBY data, we construct a set of intrinsic SED templates which incorporate both detailed star formation and chemical abundance histories, and a variety of stellar population synthesis (SPS) model assumptions. With this set of intrinsic SEDs, we present a novel approach for directly recovering the shape and normalization of the dust attenuation curve. We find, across all of the intrinsic templates considered, that the average attenuation curve for star-forming galaxies at z ≃ 3.5 is similar in shape to the commonly adopted Calzetti starburst law, with an average total-to-selective attenuation ratio of RV = 4.18 ± 0.29. In contrast, we find that an average attenuation curve as steep as the SMC extinction law is strongly disfavoured. We show that the optical attenuation (AV) versus stellar mass (M⋆) relation predicted using our method is consistent with recent ALMA observations of galaxies at 2 < z < 3 in the Hubble Ultra Deep Field (HUDF), as well as empirical AV − M⋆ relations predicted by a Calzetti-like law. In fact, our results, combined with other literature data, suggest that the AV–M⋆ relation does not evolve over the redshift range 0 < z < 5, at least for galaxies with log(M⋆/M⊙) ≳ 9.5. Finally, we present tentative evidence which suggests that the attenuation curve may become steeper at lower masses log(M⋆/M⊙) ≲ 9.0.
Context. To answer questions on the start and duration of the epoch of reionisation, periods of galaxy mergers and properties of other cosmological encounters, the cosmic star formation history, ...\hbox{$\dot{\rho}_{*}$}ρ̇∗ or CSFH, is of fundamental importance. Using the association of long-duration gamma-ray bursts (LGRBs) with the death of massive stars and their ultra-luminous nature (>1052ergs-1), the CSFH can be probed to higher redshifts than current conventional methods. Unfortunately, no consensus has been reached on the manner in which the LGRB rate, \hbox{$\dot{\rho}_{\rm grb}$}ρ̇grb or LGRBR, traces the CSFH, leaving many of the questions mentioned mostly unexplored by this method. Aims. Observations by the gamma-ray burst near-infrared detector (GROND) over the past 4 years have, for the first time, acquired highly complete LGRB samples. Driven by these completeness levels and new evidence of LGRBs also occurring in more massive and metal rich galaxies than previously thought, the possible biases of the \hbox{$\dot{\rho}_{\rm grb}$}ρ̇grb-\hbox{$\dot{\rho}_{*}$}ρ̇∗ connection are investigated over a large range of galaxy properties. Methods. The CSFH is modelled using empirical fits to the galaxy mass function and galaxy star formation rates. Biasing the CSFH by means of metallicity cuts, mass range boundaries, and other unknown redshift dependencies of the form \hbox{$\dot{\rho}_{\rm grb}\propto\dot{\rho}_{*}(1+z)^{\delta}$} ρ̇grb ∝ ρ̇∗ (1 + z)δ, a \hbox{$\dot{\rho}_{\rm grb}$}ρ̇grb is generated and compared to the highly complete GROND LGRB sample. Results. It is found that there is no strong preference for a metallicity cut or fixed galaxy mass boundaries and that there are no unknown redshift effects (δ = 0), in contrast to previous work which suggest values of Z/Z⊙ ~ 0.1−0.3. From the best-fit models obtained, we predict that ~1.2% of the LGRB burst sample exists above z = 6. Conclusions. The linear relationship between \hbox{$\dot{\rho}_{\rm grb}$}ρ̇grb and \hbox{$\dot{\rho}_{*}$}ρ̇∗ suggested by our results implies that redshift biases present in previous LGRB samples significantly affect the inferred dependencies of LGRBs on their host galaxy properties. Such biases can lead to, for example, an interpretation of metallicity limitations and evolving LGRB luminosity functions.
Context. Massive ultra-compact galaxies (MUGs) are common at z = 2−3, but very rare in the nearby Universe. Simulations predict that the few surviving MUGs should reside in galaxy clusters, whose ...large relative velocities prevent them from merging, thus maintaining their original properties (namely stellar populations, masses, sizes and dynamical state). Aims. Our goal is to obtain a complete census of the MUG population at 0.02 < z < 0.3, determining the number density, population properties and environment. Methods. We have taken advantage of the high-completeness, large-area spectroscopic GAMA survey, complementing it with deeper imaging from the KiDS and VIKING surveys. We find a set of 22 bona-fide MUGs, defined as having high stellar mass (> 8 × 1010 M⊙) and compact size (Re < 2 kpc). An additional set of seven lower-mass objects (6 × 1010 < M⋆/M⊙ < 8 × 1010) are also potential candidates according to typical mass uncertainties. Results. The comoving number density of MUGs at low redshift (z < 0.3) is constrained at (1.0 ± 0.4)×10−6 Mpc−3, consistent with galaxy evolution models. However, we find a mixed distribution of old and young galaxies, with a quarter of the sample representing (old) relics. MUGs have a predominantly early or swollen disk morphology (Sérsic index 1 < n < 2.5) with high stellar surface densities (⟨Σe⟩∼1010 M⊙ Kpc−2). Interestingly, a large fraction feature close companions – at least in projection – suggesting that many (but not all) reside in the central regions of groups. Halo masses show these galaxies inhabit average-mass groups. Conclusions. As MUGs are found to be almost equally distributed among environments of different masses, their relative fraction is higher in more massive overdensities, matching the expectations that some of these galaxies fell in these regions at early times. However, there must be another channel leading some of these galaxies to an abnormally low merger history because our sample shows a number of objects that do not inhabit particularly dense environments.
We use GALEX near-UV (NUV) photometry of a sample of early-type galaxies selected in the SDSS (Sloan Digital Sky Survey) to study the UV color-magnitude relation (CMR). NUV - r color is an excellent ...tracer of even small amounts ( similar to 1% mass fraction) of recent ( unk1 Gyr) star formation, and so the NUV - r CMR allows us to study the effect of environment on the recent star formation history. We analyze a volume-limited sample of 839 visually inspected early-type galaxies in the redshift range 0.05 < z < 0.10 brighter than M sub(r) of -21.5 with any possible emission-line or radio-selected active galactic nuclei (AGNs) removed to avoid contamination. We find that contamination by AGN candidates and late-type interlopers highly bias any study of recent star formation in early-type galaxies and that, after removing those, our lower limit to the fraction of massive early-type galaxies showing signs of recent star formation is roughly 30% plus or minus 3%. This suggests that residual star formation is common even among the present day early-type galaxy population. We find that the fraction of UV-bright early-type galaxies is 25% higher in low-density environments. However, the density effect is clear only in the lowest density bin. The blue galaxy fraction for the subsample of the brightest early-type galaxies, however, shows a very strong density dependence, in the sense that the blue galaxy fraction is lower in a higher density region.
The star formation history of nearby early-type galaxies is investigated via numerical modelling. Idealized hydrodynamical N-body simulations with a star formation prescription are used to study the ...minor merger process (1/10 ≤M1/M2≤ 1/4; M1≤M2) between a giant galaxy (host) and a less massive spiral galaxy (satellite) with reasonable assumptions for the ages and metallicities of the merger progenitors. We find that the evolution of the star formation rate is extended over several dynamical times and shows peaks which correspond to pericentre passages of the satellite. The newly formed stars are mainly located in the central part of the satellite remnant while the older stars of the initial disc are deposited at larger radii in shell-like structures. After the final plunge of the satellite, star formation in the central part of the remnant can continue for several Gyr depending on the star formation efficiency. Although the mass fraction in new stars is small, we find that the half-mass radius differs from the half-light radius in the V and H bands. Moreover synthetic 2D images in J, H, NUV, Hβ and V bands, using the characteristic filters of the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope, reveal that residual star formation induced by gas-rich minor mergers can be clearly observed during and after the final plunge, especially in the near-ultraviolet band, for interacting systems at (z≤ 0.023) over moderate numbers of orbits (approximately two orbits correspond to typical exposure times of ∼3600 s). This suggests that WFC3 has the potential to resolve these substructures, characterize plausible past merger episodes and give clues to the formation of early-type galaxies.
We have studied similar to 2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range 0 < z < 0.11. Combining GALEX UV ...photometry with corollary optical data from the SDSS, we find that, at a 95% confidence level, at least similar to 30% of galaxies in this sample have UV to optical colors consistent with some recent star formation within the last Gyr. In particular, galaxies with an NUV - r color less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to NUV flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semianalytical Lambda CDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 < z < 0.11 have similar to 1%-3% of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is similar to 300-500 Myr. We also find that "monolithically" evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colors (NUV - r < 5.5) seen in a significant fraction ( similar to 30%) of our observed sample.
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