Hello! I am a research fellow working on the chemical evolution of galaxies at the University of Surrey. I obtained my PhD at the Max Planck Institute for Astrophysics (MPA) in 2014, and my Masters degree in physics from Imperial College London.
My main research interests are in galaxy formation modelling and observations, in particular the chemical evolution of star-forming galaxies, ellipticals, and galaxy clusters. I am a main developer of the L-Galaxies semi-analytic model of galaxy evolution, and use this model to interpret spectroscopic data from large galaxy surveys and integral field units (IFUs).
Areas of specialism
The 25 Al(p, γ) reaction has long been highlighted as a possible means to bypass the production of 26 Al cosmic γ rays in classical nova explosions. However, uncertainties in the properties of key resonant states in 26 Si have hindered our ability to accurately model the influence of this reaction in such environments. We report on a detailed γ-ray spectroscopy study of 26 Si and present evidence for the existence of a new, likely ℓ = 1, resonance in the 25 Al + p system at Er = 153.9(15) keV. This state is now expected to provide the dominant contribution to the 25 Al(p, γ) stellar reaction rate over the temperature range, T ∼ 0.1 − 0.2 GK. Despite a significant increase in the rate at low temperatures, we find that the final ejected abundance of 26 Al from classical novae remains largely unaffected even if the reaction rate is artificially increased by a factor of 10. Based on new, Galactic chemical evolution calculations, we estimate that the maximum contribution of novae to the observed Galactic abundance of 26 Al is ∼0.2 M⊙. Finally, we briefly highlight the important role that Super-AGB stars may play in the production of 26 Al.
We present an analysis of the iron abundance in the hot gas surrounding galaxy groups and clusters. To do this, we first compile and homogenize a large data set of 79 low-redshift ((z) over tilde = 0.03) systems (159 individual measurements) from the literature. Our analysis accounts for differences in aperture size, solar abundance, and cosmology, and scales all measurements using customized radial profiles for the temperature (T), gas density (rho(gas)), and iron abundance (Z(Fe)). We then compare this data set to groups and clusters in the L-GALAXIES galaxy evolution model. Our homogenized data set reveals a tight T-Z(Fe) relation for clusters, with a scatter in ZFe of only 0.10 dex and a slight negative gradient. After examining potential measurement biases, we conclude that some of this negative gradient has a physical origin. Our model suggests greater accretion of hydrogen in the hottest systems, via stripping from infalling satellites, as a cause. In groups, L-GALAXIES over-estimates ZFe, indicating that metal-rich gas removal (via e.g. AGN feedback) is required. L-GALAXIES is consistent with the observed ZFe in the intracluster medium (ICM) of the hottest clusters at z = 0, and shows a similar rate of ICM enrichment as that observed from at least z similar to 1.3 to the present day. This is achieved without needing to modify any of the galactic chemical evolution (GCE) model parameters. However, the ZFe in intermediate-T clusters could be under-estimated in our model. We caution that modifications to the GCE modelling to correct this disrupt the agreement with observations of galaxies' stellar components.
We study relations between stellar mass, star formation and gas-phase metallicity in a sample of 177 071 unique emission line galaxies from the Sloan Digital Sky Survey Data Release 7, as well as in a sample of 43 767 star-forming galaxies at z = 0 from the cosmological semi-analytic model L-GALAXIES. We demonstrate that metallicity is dependent on star formation rate at fixed mass, but that the trend is opposite for low and for high stellar mass galaxies. Low-mass galaxies that are actively forming stars are more metal poor than quiescent low-mass galaxies. High-mass galaxies, on the other hand, have lower gas-phase metallicities if their star formation rates are small. Remarkably, the same trends are found for our sample of model galaxies. By examining the evolution of the stellar component, gas and metals as a function of time in these galaxies, we gain some insight into the physical processes that may be responsible for these trends. We find that massive galaxies with low gas-phase metallicities have undergone a gas-rich merger in the past, inducing a starburst which exhausted their cold gas reservoirs and shutdown star formation. Thereafter, these galaxies were able to accrete metal-poor gas, but this gas remained at too low a density to form stars efficiently. This led to a gradual dilution in the gas-phase metallicities of these systems over time. These model galaxies are predicted to have lower-than-average gas-to-stellar mass ratios and higher-than-average central black hole masses. We use our observational sample to confirm that real massive galaxies with low gas-phase metallicities also have very massive black holes. We propose that accretion may therefore play a significant role in regulating the gas-phase metallicities of present-day massive galaxies.
We contrast the latest observations of the cosmic metal density in neutral gas (.met,neu) with three cosmological galaxy evolution simulations: L-GALAXIES 2020, TNG100, and EAGLE. We find that the fraction of total metals that are in neutral gas is ](neu), we determine two possible ways in which the absolute rho(met,neu) observed in DLAs at high redshift can be matched by simulations: (i) the rho(SFR) at z greater than or similar to 3 is greater than inferred from current FUV observations, or (ii) current high-redshift DLA metallicity samples have a higher mean host mass than the overall galaxy population. If the first is correct, TNG100 would match the ensemble data best, however there would be an outstanding tension between the currently observed rho(SFR) and rho(met,neu). If the second is correct, L-GALAXIES 2020 would match the ensemble data best, but would require an increase in neutral gas mass inside subhaloes above z similar to 2.5. If neither is correct, EAGLE would match the ensemble data best, although at the expense of overestimating [< M/H >](neu). Modulo details related to numerical resolution and HI mass modelling in simulations, these incompatibilities highlight current tensions between key observed cosmic properties at high redshift.
We reveal multiple components of an interacting galaxy system at z approximate to 3.35 through a detailed analysis of the exquisite high-resolution Keck/HIRES spectrum of the afterglow of a gamma-ray burst (GRB). Through Voigt-profile fitting of absorption lines from the Lyman series, we constrain the neutral hydrogen column density to N-HI
We study the presence and importance of stellar migration in the evolution of 17 Milky-Way like disc galaxies with stellar mass 10 < log(M-*/M-circle dot) < 11 from the Auriga suite of zoom-in cosmological hydrodynamical simulations. We compare the birth radii of the stars to their radii at z = 0 for each system and present mean values of the strength of stellar migration as a function of radius and stellar age which vary between 1-4 kpc. We also investigate the effect of migration on age and metallicity radial profiles in the discs. We find several cases of age gradient flattening due to migration, but significant changes to metallicity profiles only for older stellar populations and discs that develop a strong bar. Furthermore, we study stellar migration from the perspective of the change of the galactocentric radius (Delta R) and orbital guiding centre radius (Delta R-g) of stellar particles between given time intervals. We find that stars migrate approximately as a diffusion process only in the outer parts of the discs and for particular galaxies that have a weak bar. Strongly barred galaxies in our sample show larger stellar migration but its timestep evolution is slower-than-diffusion. Finally, we give parametrizations that encapsulate the dependence of the strength of the radial migration as a function of time and radius, for incorporation into (semi-)analytic models of galaxy evolution.
We study the radial motions of cold, star-forming gas in the secular evolution phase of a set of 14 magnetohydrodynamical cosmological zoom-in simulations of Milky Way-mass galaxies. We study the radial transport of material within the disc plane in a series of concentric rings. For the gas in each ring at a given time we compute two quantities as a function of time and radius: (1) the radial bulk flow of the gas and (2) the radial spread of the gas relative to the bulk flow. Averaging the data from all the haloes, we find that the radial spread increases with radius in the form of a power law with strong secondary dependencies on the fraction of accreted material and the local radial velocity dispersion of the gas. We find that the bulk motion of gas is well described in the inner disc regions by a radially independent mean inwards flow speed of -2.4km s(-1). The spread around this value relates to the change in angular momentum of the gas and also the amount of accreted material. These scalings from fully cosmological, MHD simulations of galaxy formation can then be used in semi-analytic models to better parametrize the radial flow of gas in discs.
We perform a comparison, object by object and statistically, between the Munich semi-analytical model, L-GALAXIES, and the IllustrisTNG hydrodynamical simulations. By running L-GALAXIES on the IllustrisTNG dark matter-only merger trees, we identify the same galaxies in the two models. This allows us to compare the stellar mass, star formation rate, and gas content of galaxies, as well as the baryonic content of subhaloes and haloes in the two models. We find that both the stellar mass functions and the stellar masses of individual galaxies agree to better than similar to 0.2 dex. On the other hand, specific star formation rates and gas contents can differ more substantially. At z = 0, the transition between low-mass star-forming galaxies and high-mass quenched galaxies occurs at a stellar mass scale similar to 0.5 dex lower in IllustrisTNG than that in L-GALAXIES. IllustrisTNG also produces substantially more quenched galaxies at higher redshifts. Both models predict a halo baryon fraction close to the cosmic value for clusters, but IllustrisTNG predicts lower baryon fractions in group environments. These differences are primarily due to differences in modelling feedback from stars and supermassive black holes. The gas content and star formation rates of galaxies in and around clusters and groups differ substantially, with IllustrisTNG satellites less star forming and less gas rich. We show that environmental processes such as ram-pressure stripping are stronger and operate to larger distances and for a broader host mass range in IllustrisTNG. We suggest that the treatment of galaxy evolution in the semi-analytic model needs to be improved by prescriptions that capture local environmental effects more accurately.
We report on the detection of a bright, short, structured X-ray burst coming from the supernova remnant RCW 103 on 2016 June 22 caught by the Swift/Burst Alert Telescope (BAT) monitor, and on the follow-up campaign made with Swift/X-ray Telescope, Swift/UV/Optical Telescope, and the optical/near-infrared (NIR) Gamma-Ray burst Optical and Near-infrared Detector. The characteristics of this flash, such as duration and spectral shape, are consistent with typical short bursts observed from soft gamma repeaters. The BAT error circle at 68 per cent confidence range encloses the point-like X-ray source at the centre of the nebula, 1E 161348-5055. Its nature has been long debated due to a periodicity of 6.67 h in X-rays, which could indicate either an extremely slow pulsating neutron star, or the orbital period of a very compact X-ray binary system. We found that 20 min before the BAT trigger, the soft X-ray emission of 1E 161348-5055 was a factor of similar to 100 higher than measured 2 yr earlier, indicating that an outburst had already started. By comparing the spectral and timing characteristics of the source in the 2 yr before the outburst and after the BAT event, we find that, besides a change in luminosity and spectral shape, also the 6.67 h pulsed profile has significantly changed with a clear phase shift with respect to its low-flux profile. The UV/optical/NIR observations did not reveal any counterpart at the position of 1E 161348-5055. Based on these findings, we associate the BAT burst with 1E 161348-5055, we classify it as a magnetar, and pinpoint the 6.67 h periodicity as the magnetar spin period.
Recent improvements in the age dating of stellar populations and single stars allow us to study the ages and abundance of stars and galaxies with unprecedented accuracy. We here compare the relation between age and alpha-element abundances for stars in the solar neighborhood to that of local, early-type galaxies. We find these two relations to be very similar. Both fall into two regimes with a shallow slope for ages younger than similar to 9 Gyr and a steeper slope for ages older than that value. This quantitative similarity seems surprising because of the different types of galaxies and scales involved. For the sample of early-type galaxies we also show that the data are inconsistent with literature delay-time distributions of either single- or double-Gaussian shape. The data are consistent with a power-law delay-time distribution. We thus confirm that the delay-time distribution inferred for the Milky Way from chemical evolution arguments must also apply to massive early-type galaxies. We also offer a tentative explanation for the seeming universality of the age-[alpha/Fe] relation: it is the manifestation of averaging different stellar populations with varying chemical evolution histories.
We update the treatment of chemical evolution in the Munich semi-analytic model, L-Galaxies. Our new implementation includes delayed enrichment from stellar winds, Type II supernovae (SNe-II) and Type Ia supernovae (SNe-Ia), as well as metallicity-dependent yields and a reformulation of the associated supernova feedback. Two different sets of SN-II yields and three different SN-Ia delay-time distributions (DTDs) are considered, and 11 heavy elements (including O, Mg and Fe) are self-consistently tracked. We compare the results of this new implementation with data on (a) local, star-forming galaxies, (b) Milky Way disc G dwarfs and (c) local, elliptical galaxies. We find that the z = 0 gas-phase mass-metallicity relation is very well reproduced for all forms of DTD considered, as is the [Fe/H] distribution in the Milky Way disc. The [O/Fe] distribution in the Milky Way disc is best reproduced when using a DTD with < 50 per cent of SNe-Ia exploding within similar to 400 Myr. Positive slopes in the mass-[alpha/Fe] relations of local ellipticals are also obtained when using a DTD with such a minor 'prompt' component. Alternatively, metal-rich winds that drive light alpha elements directly out into the circumgalactic medium also produce positive slopes for all forms of DTD and SN-II yields considered. Overall, we find that the best model for matching the wide range of observational data considered here should include a power-law SN-Ia DTD, SN-II yields that take account of prior mass-loss through stellar winds and some direct ejection of light alpha elements out of galaxies.
We present a modified version of the L-GALAXIES 2020 semi-analytic model of galaxy evolution, which includes significantly increased direct metal enrichment of the circumgalactic medium (CGM) by supernovae (SNe). These more metal-rich outflows do not require increased mass-loading factors, in contrast to some other galaxy evolution models. This modified L-GALAXIES 2020 model is able to simultaneously reproduce the gas-phasemetallicity (Z(g)) and stellarmetallicity (Z(*)) radial profiles observed in nearby disc galaxies by MaNGA and MUSE, as well as the observed mass - metallicity relations for gas and stars at z = 0 and their evolution back to z similar to 2-3. A direct CGM enrichment fraction of similar to 90 per cent for SNe-II is preferred. We find that massive disc galaxies have slightly flatter Z(g) profiles than their lower-mass counterparts in L-GALAXIES 2020, due to more efficient enrichment of their outskirts via inside-out growth and metal-rich accretion. Such a weak, positive correlation between stellar mass and Z(g) profile slope is also seen in our MaNGA-DR15 sample of 571 star-forming disc galaxies, although below log(10)(M-*/M-circle dot) similar to 10.0 this observational result is strongly dependent on the metallicity diagnostic and morphological selection chosen. In addition, a lowered maximum SN-II progenitor mass of 25M(circle dot), reflecting recent theoretical and observational estimates, can also provide a good match to observed Z(g) and Z(*) profiles at z = 0 in L-GALAXIES 2020. However, this model version fails to reproduce an evolution in Z(g) at fixed mass over cosmic time, or the magnesium abundances observed in the intracluster medium (ICM).
Short gamma-ray bursts (SGRBs) are produced by the coalescence of compact binary systems which are remnants of massive stars. GRB 160410A is classified as a short-duration GRB with extended emission and is currently the farthest SGRB with a redshift determined from an afterglow spectrum and also one of the brightest SGRBs to date. The fast reaction to the Neil Gehrels Swift Observatory alert allowed us to obtain a spectrum of the afterglow using the X-shooter spectrograph at the Very Large Telescope (VLT). The spectrum shows several absorption features at a redshift of z = 1.7177, in addition, we detect two intervening systems at z = 1.581 and z = 1.444. The spectrum shows Ly alpha in absorption with a column density of log (N(H i)/cm(2)) = 21.2 +/- 0.2 which, together with Fe ii, C ii, Si ii, Al ii, and O i, allow us to perform the first study of chemical abundances in a SGRB host galaxy. We determine a metallicity of [X/H] = -2.3 +/- 0.2 for Fe ii and -2.5 +/- 0.2 for Si ii and no dust depletion. We also find no evidence for extinction in the afterglow spectral energy distribution modelling. The environment has a low degree of ionization and the C iv and Si iv lines are completely absent. We do not detect an underlying host galaxy down to deep limits. Additionally, we compare GRB 160410A to GRB 201221D, another high-z short GRB that shows absorption lines at z = 1.045 and an underlying massive host galaxy.
In this study we investigate the relation between stellar mass, dust extinction and star formation rate (SFR) using ~150,000 star-forming galaxies from the SDSS DR7. We show that the relation between dust extinction and SFR changes with stellar mass. For galaxies at the same stellar mass dust extinction is anti-correlated with the SFR at stellar masses 10^10 M_solar is shown to extend to the population of quiescent galaxies suggesting that the physical processes responsible for the observed relation between stellar mass, dust extinction and SFR may be related to the processes leading to the shut down of star formation in galaxies.
Host galaxy properties provide strong constraints on the stellar progenitors of superluminous supernovae. By comparing a sample of 19 low-redshift (z < 0.3) superluminous supernova hosts to galaxy populations in the local Universe, we show that sub-solar metallicities seem to be a requirement. All superluminous supernovae in hosts with high measured gas-phase metallicities are found to explode at large galactocentric radii, indicating that the metallicity at the explosion site is likely lower than the integrated host value. We found that superluminous supernova hosts do not always have star formation rates higher than typical star-forming galaxies of the same mass. However, we confirm that high absolute specific star formation rates are a feature of superluminous supernova host galaxies, but interpret this as simply a consequence of the anticorrelation between gas-phase metallicity and specific star formation rate and the requirement of on-going star formation to produce young, massive stars greater than similar to 10-20 M-circle dot. Based on our sample, we propose an upper limit of similar to 0.5 Z(circle dot) for forming superluminous supernova progenitors (assuming an N2 metallicity diagnostic and a solar oxygen abundance of 8.69). Finally, we show that if magnetar powering is the source of the extreme luminosity, then the required initial spins appear to be correlated with metallicity of the host galaxy. This correlation needs further work, but if it applies, it is a powerful link between the supernova parameters and nature of the progenitor population.
We implement a detailed dustmodel into the L-Galaxies semi-analytical model which includes: injection of dust by type II and type Ia supernovae (SNe) and AGB stars; grain growth in molecular clouds; and destruction due to supernova-induced shocks, star formation, and reheating. Our grain growth model follows the dust content in molecular clouds and the inter-cloud medium separately, and allows growth only on pre-existing dust grains. At early times, this can make a significant difference to the dust growth rate. Above z similar to 8, type II SNe are the primary source of dust, whereas below z similar to 8, grain growth in molecular clouds dominates, with the total dust content being dominated by the latter below z similar to 6. However, the detailed history of galaxy formation is important for determining the dust content of any individual galaxy. We introduce a fit to the dust-to-metal (DTM) ratio as a function of metallicity and age, which can be used to deduce the DTM ratio of galaxies at any redshift. At z less than or similar to 3, we find a fairly flat mean relation between metallicity and the DTM, and a positive correlation between metallicity and the dust-to-gas (DTG) ratio, in good agreement with the shape and normalization of the observed relations. We also match the normalization of the observed stellar mass-dust mass relation over the redshift range of 0-4, and to the dust mass function at z = 0. Our results are important in interpreting observations on the dust content of galaxies across cosmic time, particularly so at high redshift.
Context. Several issues regarding the nature of dust at high redshift remain unresolved: its composition, its production and growth mechanisms, and its effect on background sources. Aims. We provide a more accurate relation between dust depletion levels and dust-to-metals ratio (DTM), and to use the DTM to investigate the origin and evolution of dust in the high-redshift Universe via gamma-ray burst damped Lyman-alpha absorbers (GRB-DLAs). Methods. We use absorption-line measured metal column densities for a total of 19 GRB-DLAs, including five new GRB afterglow spectra from VLT/X-Shooter. We use the latest linear models to calculate the dust depletion strength factor in each DLA. Using these values we calculate total dust and metal column densities to determine a DTM. We explore the evolution of DTM with metallicity, and compare it to previous trends in DTM measured with different methods. Results. We find significant dust depletion in 16 of our 19 GRB-DLAs, yet 18 of the 19 have a DTM significantly lower than the Milky Way. We find that DTM is positively correlated with metallicity, which supports a dominant ISM grain-growth mode of dust formation. We find a substantial discrepancy between the dust content measured from depletion and that derived from the total V-band extinction, AV, measured by fitting the afterglow SED. We advise against using a measurement from one method to estimate that from the other until the discrepancy can be resolved.
We present an analysis of the formation and chemical evolution of stellar haloes around (a) Milky Way analogue (MWA) galaxies and (b) galaxy clusters in the L-GALAXIES 2020 semi-analytic model of galaxy evolution. Observed stellar halo properties are better reproduced when assuming a gradual stripping model for the removal of cold gas and stars from satellites, compared to an instantaneous stripping model. The slope of the stellar mass-metallicity relation for MWA stellar haloes is in good agreement with that observed in the local Universe. This extends the good agreement between L-GALAXIES 2020 and metallicity observations from the gas and stars inside galaxies to those outside. Halo stars contribute on average only similar to 0.1 per cent of the total circumgalactic medium enrichment by z = 0 in MWAs, ejecting predominantly carbon produced by asymptotic giant branch stars. Around a quarter of MWAs have a single `significant progenitor' with a mean mass of similar to 2.3 x 10(9) M-circle dot, similar to that measured for Gaia Enceladus. For galaxy clusters, L-GALAXIES 2020 shows good correspondence with observations of stellar halo mass fractions, but slightly overpredicts stellar masses. Assuming a Navarro-Frenk-White profile for the stellar halo mass distribution provides the best agreement. On average, the intracluster stellar component is responsible for 5.4 per cent of the total intracluster medium iron enrichment, exceeding the contribution from the brightest cluster galaxy by z = 0. We find that considering gradual stripping of satellites and realistic radial profiles is crucial for accurately modelling stellar halo formation on all scales in semi-analytic models.
We present a variation of the recently updated Munich semi-analytical galaxy formation model, L-Galaxies, with a new gas stripping method. Extending earlier work, we directly measure the local environmental properties of galaxies to formulate a more accurate treatment of ram-pressure stripping for all galaxies. We fully recalibrate the modified L-Galaxies model using a Markov Chain Monte Carlo (MCMC) method with the stellar mass function and quenched fraction of galaxies as a function of stellar mass at 0
We investigate whether dilution in some elliptical galaxies is the cause of a positive correlation between specific star formation rate (sSFR) and gas-phase metallicity (Z(g)) at high stellar mass in the local Universe. In the Munich semi-analytic model of galaxy formation, L-GALAXIES, massive, low-sSFR, elliptical galaxies are seen to undergo a gradual dilution of their interstellar medium, via accretion of metal-poor gas in cold-gas clumps and low-mass satellites. This occurs after a merger-induced starburst and the associated supernova feedback have quenched most of the original gas reservoir. Signatures of this evolution are present in these model galaxies at z = 0, including low gas fractions, large central black holes, old ages, and importantly, low (Z(g)-Z(*)). Remarkably, all of these properties are also found in massive, low-sSFR, elliptical galaxies in the sloan digital sky survey data release 7 (SDSS-DR7). This provides strong, indirect evidence that gradual dilution is also occurring in nearby ellipticals in the real Universe. This scenario provides an explanation for the positive correlation between SFR and Z(g) measured in high-M-* galaxies, and therefore has consequences for the local fundamental metallicity relation, which assumes a weak anticorrelation above similar to 10(10.5) M-circle dot.
We have updated our radially resolved semi-analytic models (SAMs) of galaxy formation, which track both the atomic and molecular gas phases of the interstellar medium. The models are adapted from those of Guo et al. using similar methodology as by Fu et al. and are run on halo merger trees from the Millennium and Millennium-II simulations with the following main changes. (1) We adopt a simple star formation law Sigma(SFR) Sigma(H2). (2) We inject the heavy elements produced by supernovae directly into the halo hot gas, instead of first mixing them with the cold gas in the disc. (3) We include radial gas inflows in discs using a model of the form v(inflow) = alpha r. The models are used to study the radial profiles of star formation rate and gas-phase metallicity in present-day galaxies. The surface density profiles of molecular gas in L-* galaxies place strong constraints on inflow velocities, favouring models where v(inflow) similar to 7 km s(-1) at a galactocentric radius of 10 kpc. Radial gas inflow has little influence on gas-phase and stellar metallicity gradients, which are affected much more strongly by the fraction of metals that are directly injected into the halo gas, rather than mixed with the cold gas. Metals ejected out of the galaxy in early epochs result in late infall of pre-enriched gas and flatter present-day gas-phase metallicity gradients. A prescription in which 80 per cent of the metals are injected into the halo gas results in good fits to the flat observed metallicity gradients in galaxies with stellar masses greater than 10(10) M-circle dot, as well as the relations between gas-phase metallicity and specific star formation rate in the outer parts of galactic discs. We examine the correlation between the gas-phase metallicity gradient and global galaxy properties, finding that it is most strongly correlated with the bulge-to-total ratio of the galaxy. This is because gas is consumed when the bulge forms during galaxy mergers, and the gas-phase metallicity gradient is then set by newly accreted gas.
Context. Large-scale surveys over the last years have revealed about 300 quasi-stellar objects (QSOs) at redshifts above 6. Follow-up observations have identified surprising properties, such as the very high black hole (BH) masses, spatial correlations with surrounding cold gas of the host galaxy, and high CIV-MgII Velocity shifts. In particular, the discovery of luminous high-redshift quasars suggests that at least some BHs likely have high masses at birth and grow efficiently. Aims. Our aim is to quantify quasar pairs at high redshift for a large sample of objects. This provides a new key constraint on a combination of parameters related to the origin and assembly for the most massive BHs: formation efficiency and clustering, growth efficiency, and the relative contribution of BH mergers. Methods. We observed 116 spectroscopically confirmed QSOs around redshift 6 with the simultaneous seven-channel imager Gamma-ray Burst Optical/Near-infrared Detector in order to search for companions. Applying colour-colour cuts identical to those which led to the spectroscopically confirmed QSOs, we performed Le PHARE fits to the 26 best QSO pair candidates, and obtained spectroscopic observations for 11 of them. Results. We do not find any QSO pair with a companion brighter than M-1450(AB) < -26 mag within our 0.1-3.3 h(-1) cMpc search radius, in contrast to the serendipitous findings in the redshift range 4-5. However, a small fraction of such pairs at this luminosity and redshift is consistent with indications from present-day cosmological-scale galaxy evolution models. In turn, the incidence of L- and T-type brown dwarfs, which occupy a similar colour space to z similar to 6 QSOs, is higher than expected, by a factor of 5 and 20, respectively.
We have updated the Munich galaxy formation model, L-GALAXIES, to follow the radial distributions of stars and atomic and molecular gas in galaxy discs. We include an H-2-based star-formation law, as well as a detailed chemical-enrichment model with explicit mass-dependent delay times for SN-II, SN-Ia, and AGB stars. Information about the star formation, feedback, and chemical-enrichment histories of discs is stored in 12 concentric rings. The new model retains the success of its predecessor in reproducing the observed evolution of the galaxy population, in particular, stellar mass functions and passive fractions over the redshift range 0
We introduce a local background environment (LBE) estimator that can be measured in and around every galaxy or its dark matter subhalo in high-resolution cosmological simulations. The LBE is designed to capture the influence of environmental effects such as ram-pressure stripping (RPS) on the formation and evolution of galaxies in semi-analytical models. We define the LBE directly from the particle data within an adaptive spherical shell, and devise a Gaussian mixture estimator to separate background particles from previously unidentified subhalo particles. Analysing the LBE properties, we find that the LBE of satellite galaxies is not at rest with respect to their host halo, in contrast to typical assumptions. The orientations of the velocities of a subhalo and its LBE are well aligned in the outer infall regions of haloes, but decorrelated near halo centre. Significantly, there is no abrupt change in LBE velocity or density at the halo virial radius. This suggests that stripping should also happen beyond this radius. Therefore, we use the time-evolving LBE of galaxies to develop a method to better account for RPS of hot gas within the Munich semi-analytical model, L-GALAXIES. Overall, our new approach results in a significant increase in gas stripping across cosmic time. Central galaxies, as well as satellites beyond the virial radius, can lose a significant fraction of their hot halo gas. As a result, the gas fractions and star formation rates of satellite galaxies are suppressed relative to the fiducial model, although the stellar masses and global stellar mass functions are largely unchanged.
Hot ionized gas is important in the baryon cycle of galaxies and contributes the majority of their 'missing baryons'. Until now, most semi-analytic models of galaxy formation have paid little attention to hot gaseous haloes and their X-ray emission. In this paper, we adopt the one-dimensional model from Sharma et al. instead of the isothermal sphere to describe the radial distribution of hot gas in the L-Galaxies semi-analytic model. The hot gas halo can be divided into two parts according to the ratio of the local thermal instability time-scale and the free-fall time-scale: a cool core with t(TI)/t(ff) = 10 and a stable outer halo with t(TI)/t(ff) > 10. We update the prescriptions of cooling, feedback, and stripping based on the new hot gas profiles, and then reproduce several X-ray observational results like the radial profiles of hot gas density, and the scaling relations of X-ray luminosity and temperature. We find: (1) Consistent with observations, flatter density profiles in halo centers produce lower X-ray emission than an isothermal sphere; (2) Cool core regions prone to precipitation have higher gas temperature than the virial temperature, and a larger T-X/T-200 ratio in smaller haloes leads to a steeper slope in the L-X-T-X relation; (3) The ionized gas in the unbounded reservoir and low-temperature intergalactic gas in low-mass haloes could be the main components of the halo 'missing baryons'. Our model outputs can predict the observations of hot gas in the nearby universe and produce mock surveys of baryons probed by future X-ray telescopes.
We investigate electron temperature (Te) and gas-phase oxygen abundance (Z_Te) measurements for galaxies in the local Universe (z < 0.25). Our sample comprises spectra from a total of 264 emission-line systems, ranging from individual HII regions to whole galaxies, including 23 composite HII regions from "star-forming main sequence" galaxies in the MaNGA survey. We utilise 130 of these systems with directly measurable Te(OII) to calibrate a new metallicity-dependent Te(OIII) - Te(OII) relation that provides a better representation of our varied dataset than existing relations from the literature. We also provide an alternative Te(OIII) - Te(NII) calibration. This new Te method is then used to obtain accurate Z_Te estimates and form the mass - metallicity relation (MZR) for a sample of 118 local galaxies. We find that all the Te(OIII) - Te(OII) relations considered here systematically under-estimate Z_Te for low-ionisation systems by up to 0.6 dex. We determine that this is due to such systems having an intrinsically higher O+ abundance than O++ abundance, rendering Z_Te estimates based only on [OIII] lines inaccurate. We therefore provide an empirical correction based on strong emission lines to account for this bias when using our new Te(OIII) - Te(OII) and Te(OIII) - Te(NII) relations. This allows for accurate metallicities (1sigma = 0.08 dex) to be derived for any low-redshift system with an [OIII]4363 detection, regardless of its physical size or ionisation state. The MZR formed from our dataset is in very good agreement with those formed from direct measurements of metal recombination lines and blue supergiant absorption lines, in contrast to most other Te-based and strong-line-based MZRs. Our new Te method therefore provides an accurate and precise way of obtaining Z_Te for a large and diverse range of star-forming systems in the local Universe.
Radial metallicity trends provide a key indicator of physical processes such as star formation and radial gas migration within a galaxy. Large integral field unit surveys allow for detailed studies of these radial variations, with recent observations detecting central dips in the metallicity, which may trace the impact of various evolutionary processes. However, the origin of these dips has not been conclusively determined, with suggestions that they may be diagnostic dependent. In this paper, we use the Sloan Digital Sky Survey IV Mapping Nearby Galaxies at Apache Point Observatory survey to investigate whether the observed dips represent genuine decreases in the central metallicity, or whether they could be an artefact of the diagnostic used. Using a sub-sample of 758 local star-forming galaxies at low inclinations, we investigate in detail the impact of using different strong line diagnostics on the shapes of the returned profiles, and the prevalence of dips. We find no clear evidence of the dips being caused by changing values of the ionization parameter within galaxies. To investigate physical causes, we explore both global and spatially resolved parameters, finding that galaxies exhibiting central dips in the O3N2 metallicity profile have on average lower H alpha equivalent width values out to R/R-e similar to 1.5, and higher values of D-N(4000) in the central regions. We additionally find a higher prevalence of dips in galaxies with high stellar mass, and lower values of global specific star formation rate, suggesting a possible link to central quenching. Nevertheless, these results are dependent on the diagnostic used, suggesting caution should be taken when interpreting observed features in galaxy metallicity gradients.
For an up-to-date list of my publications, see the NASA-ADS listing here.