Pratika Dayal

Simulating high‐redshift galaxies

Recent observations have gathered a considerable sample of high-redshift galaxy candidates and determined the evolution of their luminosity function (LF). To interpret these findings, we use cosmological SPH simulations including, in addition to standard physical processes, a detailed treatment of the Pop III−Pop II transition in early objects. The simulated high-z galaxies match remarkably well the amplitude and slope of the observed LF in the redshift range 5 9.4) start of their star formation activity; the specific star formation rate is almost independent of galactic stellar mass. These objects are enriched rapidly with metals and galaxies identified by HST/WFC3 () show metallicities ≈0.1xa0Z⊙ even at z= 7–8. Most of the simulated galaxies at z≈ 7 (noticeably the smallest ones) are virtually dust-free, and none of them has an extinction larger than E(B−V) = 0.01. The bulk (50 per cent) of the ionizing photons is produced by objects populating the faint end of the LF (), which JWST will resolve up to z= 7.3. Pop III stars continue to form essentially at all redshifts; however, at z= 6 (z= 10) the contribution of Pop III stars to the total galactic luminosity is always less than 5 per cent for (). The typical high-z galaxies closely resemble the GRB host galaxy population observed at lower redshifts, strongly encouraging the use of GRBs to detect the first galaxies.

The UV continua and inferred stellar populations of galaxies at z ≃ 7–9 revealed by the Hubble Ultra-Deep Field 2012 campaign

We use the new ultra-deep, near-infrared imaging of the Hubble Ultra-Deep Field (HUDF) provided by our UDF12 Hubble Space Telescope (HST) Wide Field Camera 3/IR campaign to explore the rest-frame ultraviolet (UV) properties of galaxies at redshifts z > 6.5. We present the first unbiased measurement of the average UV power-law index, 〈β〉, (fλ ∝ λ^β) for faint galaxies at z ≃ 7, the first meaningful measurements of 〈β〉 at z ≃ 8, and tentative estimates for a new sample of galaxies at z ≃ 9. Utilizing galaxy selection in the new F140W (J_140) imaging to minimize colour bias, and applying both colour and power-law estimators of β, we find 〈β〉 = −2.1 ± 0.2 at z ≃ 7 for galaxies with M_UV ≃ −18. This means that the faintest galaxies uncovered at this epoch have, on average, UV colours no more extreme than those displayed by the bluest star-forming galaxies at low redshift. At z ≃ 8 we find a similar value, 〈β〉 = −1.9 ± 0.3. At z ≃ 9, we find 〈β〉 = −1.8 ± 0.6, essentially unchanged from z ≃ 6 to 7 (albeit highly uncertain). Finally, we show that there is as yet no evidence for a significant intrinsic scatter in β within our new, robust z ≃ 7 galaxy sample. Our results are most easily explained by a population of steadily star-forming galaxies with either ≃ solar metallicity and zero dust, or moderately sub-solar (≃10–20u2009peru2009cent) metallicity with modest dust obscuration (AV ≃ 0.1–0.2). This latter interpretation is consistent with the predictions of a state-of-the-art galaxy-formation simulation, which also suggests that a significant population of very-low metallicity, dust-free galaxies with β ≃ −2.5 may not emerge until M_UV > −16, a regime likely to remain inaccessible until the James Webb Space Telescope.

The physics of the fundamental metallicity relation

We present a simple, redshift-independent analytic model that explains the local Fundamental Metallicity Relation (FMR), taking into account the physical processes of star formation, inflow of metal-poor intergalactic medium (IGM) gas, and the outflow of metal rich interstellar medium (ISM) gas. We show that the physics of the FMR can be summarised as follows: for massive galaxies with stellar mass M_* >= 10^11 solar masses, ISM metal enrichment due to star formation is compensated by inflow of metal poor IGM gas, leading to a constant value of the gas metallicity with star formation rate (SFR); outflows are rendered negligible as a result of the large potential wells of these galaxies. On the other hand, as a result of their smaller SFR, less massive galaxies produce less heavy elements that are also more efficiently ejected due to their shallow potential wells; as a result, for a given M_*, the gas metallicity decreases with SFR. For such galaxies, the outflow efficiency determines both the slope, and the knee of the metallicity-SFR relation. Without changing any parameters, this simple model is also successfully matched to the gas fraction - gas metallicity relation observed for a sample of about 260 nearby galaxies.

Lyman alpha emitter evolution in the reionization epoch

Combining cosmological smoothed particle hydrodynamics (SPH) simulations with a previously developed Lyα production/transmission model and the Early Reionization Model (ERM; reionization ends at redshift z ~ 7), we obtain Lyα and UV luminosity functions (LFs) for lyman alpha emitters (LAEs) at 5.7 ≤ z ≤ 7.6. Matching model results to observations at z ~ 5.7 requires escape fractions of Lyα, f α = 0.3, and UV (non-ionizing) continuum photons, f c = 0.22, corresponding to a colour excess, E(B - V) = 0.15. We find that (i) f c increases towards higher redshifts, due the decreasing mean dust content of galaxies, (ii) the evolution of f α /f c hints at the dust content of the interstellar medium becoming progressively inhomogeneous/clumped with decreasing redshift. Using the model assumptions, clustering of sources has little effect on the Lyα LF for a cosmic hydrogen neutral fraction χHI ≤ 10 -4 , a value attained at z ≤ 6.6 in the ERM. However, during the initial reionization phases (z ≳ 7), the clustering photoionization boost becomes important. We quantify the physical properties of observed LAEs and their redshift evolution, for which we give handy analytical fitting functions. Halo (stellar) masses are in the range 10.0 20 Myr at all redshifts, while the mean stellar metallicity increases from Z = 0.12 Z ⊙ at z ~7.6 to Z = 0.22 Z ⊙ at z 5.7; both t * and Z positively correlate with stellar mass. The brightest LAEs are all characterized by large M*and intermediate ages (≈200 Myr), while objects in the faint end of the Lyα LF show large age and star formation rate spreads. With no more free parameters, the spectral energy distributions of three LAE at z ~ 5.7 observed by Lai et al. (2007) are well reproduced by an intermediate age (182-220 Myr) stellar population and the above E(B - V) value. The model uncertainties, mostly related to the simplified treatment of dust and to the possible effects related to gas outflow/infall, are discussed along with their impact on the results.

The visibility of Lyman α emitters during reionization

We present the first Lyman α emitter (LAE) study that combines: (i) cosmological smoothed particle hydrodynamic (SPH) simulations run using GADGET-2, (ii) radiative transfer simulations (CRASH) and (iii) a previously developed LAE model. This complete LAE model accounts for the intrinsic LAE, Lyα/continuum luminosity, dust enrichment and Lyα transmission through the intergalactic medium (IGM) to quantify the effects of reionization, dust and velocity fields on the Lyα and UV luminosity functions (LFs). We find that a model neglecting dust sorely fails to reproduce either the slope or the magnitude of the observed Lyα and UV LFs. Clumped dust is required to simultaneously fit the observed UV and Lyα LFs, such that the intrinsic Lyα-to-continuum luminosity is enhanced by a factor f α/f c ∼ 1.5 (3.7) excluding (including) peculiar velocities. The higher value including velocity fields arises since LAEs reside in large potential wells and inflows decrease their Lyα transmission. For the first time, a degeneracy is found between the ionization state of the IGM and the clumping of dust inside high-redshift galaxies. The Lyα LF z ∼ 5.7 can be well reproduced (to within a 5σ error) by a wide range of IGM average neutral hydrogen fraction, 3.4 × 10 −3 < � χ H I� < 0.16, provided that the increase in the Lyα transmission through a more ionized IGM is compensated by a decrease in the Lyα escape fraction from the galaxy due to dust absorption. The physical properties of LAEs are presented along with a discussion of the assumptions adopted.

Signatures of reionization on Lyα emitters

We use a semi-analytic model of Lyα emitters (LAEs) to constrain the reionization history. By considering two physically motivated scenarios in which reionization ends either early [early reionization model (ERM), zi ≈7] or late [late reionization model (LRM), z i ≈ 6], we fix the global value of the intergalactic medium neutral fraction (e.g. ΧH = 3 x 10 -4 , 0.15 at z = 6.56 for the ERM and LRM, respectively) leaving only the star formation efficiency and the effective escape fraction of Lyα photons as free parameters. The ERM fits the observed LAE luminosity function (LF) at z = 5.7 and 6.56 requiring no redshift evolution or mass dependence of the star formation efficiency, and LAE star formation rates (SFR) of 3 < M * /M ⊙ yr -1 < 103, contributing ≈8 per cent of the cosmic SFR density at z = 5.7. The LRM requires a physically uncomfortable drop of ≈4.5 times in the SFR of the emitters from z = 6.5 to 5.7. Thus, the data seem to imply that the Universe was already highly ionized at z = 6.56. The mass-dependent Lyα transmissivity is 0.36 ≤Tα ≤ 0.51 (ERM) and T α ≤ 0.26 (LRM) at z = 6.56. The LF data at z = 4.5 imply an extra Lyα line damping factor of ≈ 0.25 possibly due to dust; the presence of a (clumpy) dust component with E(B - V) ≤ 0.28 is also required to reproduce the observed large Lya equivalent widths at the same redshift. Additional useful information can be extracted from the line profile (weighted) skewness, found to be Sw = 10-17 A for the two reionization models, which shows an interesting L α - Χ H1 anti-correlation, holding under the model assumptions. The shortcomings of the model and strategies to overcome them are discussed.

The cool side of Lyman alpha emitters

We extend a previous study of Lyman alpha emitters (LAEs) based on hydrodynamical cosmological simulations, by including two physical processes important for LAEs: (i) Lyα and continuum luminosities produced by cooling of collisionally excited HI in the galaxy and (ii) dust formation and evolution; we follow these processes on a galaxy-by-galaxy basis. H i cooling on average contributes 16-18 per cent of the Lyα radiation produced by stars, but this value can be much higher in low-mass LAEs and further increased if the HI is clumpy. The continuum luminosity is instead almost completely dominated by stellar sources. The dust content of galaxies scales with their stellar mass, M dust α M 0.7 * , and stellar metallicity, Z * , such that M dust ∝ Z 1.7 * . As a result, the massive galaxies have Lyα escape fraction as low as f α = 0.1, with a LAE-averaged value decreasing with redshift: 〈f α 〉 = (0.33, 0.23) at z = (5.7, 6.6). The ultraviolet (UV) continuum escape fraction shows the opposite trend with z, possibly resulting from clumpiness evolution. The model successfully reproduces the observed Lyα and UV luminosity functions at different redshifts and the Lyα equivalent width scatter to a large degree, although the observed distribution appears to be more more extended than the predicted one. We discuss possible reasons for such tension.

The colour distribution of galaxies at redshift five

We present the results of a study investigating the rest-frame ultra-violet (UV) spectral slopes of redshift z~5 Lyman-break galaxies (LBGs). By combining deep Hubble Space Telescope imaging of the CANDELS and HUDF fields with ground-based imaging from the UKIDSS Ultra Deep Survey (UDS), we have produced a large sample of z~5 LBGs spanning an unprecedented factor of >100 in UV luminosity. Based on this sample we find a clear colour-magnitude relation (CMR) at z~5, such that the rest-frame UV slopes (beta) of brighter galaxies are notably redder than their fainter counterparts. We determine that the z~5 CMR is well described by a linear relationship of the form: d beta = (-0.12 +/- 0.02) d Muv, with no clear evidence for a change in CMR slope at faint magnitudes (i.e. Muv > -18.9). Using the results of detailed simulations we are able, for the first time, to infer the intrinsic (i.e. free from noise) variation of galaxy colours around the CMR at z~5. We find significant (12 sigma) evidence for intrinsic colour variation in the sample as a whole. Our results also demonstrate that the width of the intrinsic UV slope distribution of z~5 galaxies increases from Delta(beta)=0.1 at Muv=-18 to Delta(beta)=0.4 at Muv=-21. We suggest that the increasing width of the intrinsic galaxy colour distribution and the CMR itself are both plausibly explained by a luminosity independent lower limit of beta=-2.1, combined with an increase in the fraction of red galaxies in brighter UV-luminosity bins.

Simulating the assembly of galaxies at redshifts z = 6–12

We use state-of-the-art simulations to explore the physical evolution of galaxies in the first billion years of cosmic time. First, we demonstrate that our model reproduces the basic statistical properties of the observed Lyman-break galaxy (LBG) population at z = 6 − 8, including the evolving ultra-violet (UV) luminosity function (LF), the stellar-mass density (SMD), and the average specific star-formation rates (sSFR) of LBGs with MUV −18 were in fact brighter at z ≃ 7 (and even at z ≃ 8) despite obviously being less massive at earlier times. At this end, the evolution of the UV LF involves a mix of positive and negative luminosity evolution (as low-mass galaxies temporarily brighten then fade) coupled with both positive and negative density evolution (as new low-mass galaxies form, and other low-mass galaxies are consumed by merging). We also predict the average sSFR of LBGs should rise from sSFR ≃ 4.5Gyr 1 at z ≃ 6 to sSFR ≃ 11Gyr 1 by

Essential physics of early galaxy formation

We present a theoretical model embedding the essential physics of early galaxy formation (z ≃ 5-12) based on the single premise that any galaxy can form stars with a maximal limiting efficiency that provides enough energy to expel all the remaining gas, quenching further star formation. This simple idea is implemented into a merger-tree-based semi-analytical model that utilizes two mass and redshift-independent parameters to capture the key physics of supernova feedback in ejecting gas from low- mass haloes, and tracks the resulting impact on the subsequent growth of more massive systems via halo mergers and gas accretion. Our model shows that: (i) the smallest haloes (halo mass Mh ≤ 1010 M☉) build up their gas mass by accretion from the intergalactic medium; (ii) the bulk of the gas powering star formation in larger haloes (Mh ≥ 1011.5 M☉) is brought in by merging progenitors; (iii) the faint-end UV luminosity function slope evolves according to α = -1.75 log z - 0.52. In addition, (iv) the stellar mass-to-light ratio is well fitted by the functional form log M* = -0.38MUV - 0.13 z + 2.4, which we use to build the evolving stellar mass function to compare to observations. We end with a census of the cosmic stellar mass density (SMD) across galaxies with UV magnitudes over the range -23 ≤ MUV ≤ -11 spanning redshifts 5 < z < 12; (v) while currently detected LBGs contain ≈50 per cent (10 per cent) of the total SMD at z = 5 (8), the James Webb Space Telescope will detect up to 25 per cent of the SMD at z ≃ 9.5.