The first galaxies in the Hubble Frontier Fields

In this thesis we have exploited the power of gravitational lensing of massive clusters to probe galaxy evolution with the galaxy stellar mass functions (GSMF) and UV luminosity functions (UV LF) at $z=6-9$. Throughout the thesis the data utilized is from the Hubble Frontier Fields (HFF) program. We present new measurements of the evolution of the GSMF and UV LF for galaxies from $z=6-9$ within the HFF cluster MACSJ0416.1-2403 and its parallel field. To obtain these results, we have developed a novel method to subtract the massive foreground galaxies that lie close to the critical line from the MACSJ0416.1-2403 cluster, allowing for a deeper and cleaner detection of the faintest systems at $z\geq6$. We derive the stellar masses of our sample by fitting synthetic stellar population models to their observed spectral energy distribution (SED) with the inclusion of nebular emission lines. This is the deepest and most distant mass function measured to date and probes down to a level of M$_{*} = 10^{6.8}M_{\odot}$. The main result of this study is that the low-mass end of our stellar mass functions to these limits and redshifts are measured to be $\alpha=-1.98_{-0.07}^{+0.07}$ at $z=6$ and $\alpha=-2.38_{-0.88}^{+0.72}$ at $z=9$ and we find no evidence of any turnover in the mass range probed. The faint end slope of the UV LF for these system are also measured to be $\alpha=-2.03_{-0.10}^{+0.12}$ at $z=6$ and $\alpha=-2.20_{-0.47}^{+0.51}$ at $z=9$, without any evidence of a turnover in the luminosity range probed. Our $M_{\mathrm{UV}}-M_{*}$ relation exhibit shallower slopes than previously observed and are in accordance with a constant mass-to-light ratio. Integrating our GSMF, we find that the stellar mass density increases from log$_{10}\rho_{*}=5.61_{-0.90}^{+0.92}$ M$_{\odot}$Mpc$^{-3}$ at $z=9$ to log$_{10}\rho_{*}=6.79_{-0.12}^{+0.13}$ M$_{\odot}$Mpc$^{-3}$ at $z=6$. We also find that there is a surprisingly high amount of stellar mass density for galaxies in the early universe up to $z \sim 9$. We estimate the dust-corrected star formation rates (SFRs) to calculate the specific star formation rates ($\mathrm{sSFR}=\mathrm{SFR/M_{*}}$) of our sample, and find that for a fixed stellar mass of $5\times10^{9}M_{\odot}$, sSFR $\propto(1+z)^{2.01\pm0.16}$. From our new measurements, we also estimate the UV luminosity density ($\rho_{\textrm{UV}}$) and find that our results support a smooth decline of $\rho_{\textrm{UV}}$ towards high redshifts. Finally, we use the same dataset to investigate the evolution of the galaxy rest-frame UV colours (UV spectral slope $\beta$) for our sample of high redshift galaxies at $z=6-9$. We measure the UV spectral slope $\beta$ by fitting the observed spectral energy distribution to a set of synthetic stellar population models and estimate the value of $\beta$ from the best-fit model spectrum. With this method, we find no correlation between $\beta$ and rest-frame UV magnitude $M_{1500}$ at all redshifts probed in this work. However, a possible weak evolution of the median $\beta$ values (from $\beta=-2.24$ at $z\sim6$ to $\beta=-2.52$ at $z\sim9$) for galaxies at all luminosities from $z=6-9$ is observed, likely due to increased dust extinction. Furthermore, we find that at $z=7$, the bluest value of our sample is $\beta=-2.31\pm0.31$, which is redder than previously reported values at this redshift in the literature. Similarly, with the help of our SED fitting method, we determine the UV slopes for the first time at $z\sim9$ and find that our bluest data point has a value of $\beta=-2.63\pm0.21$, indicating no evidence as yet for extreme stellar populations at $z>6$. Examining the $\beta$ to stellar mass relation, we find a strong correlation between $\beta$ with stellar mass, in that lower mass galaxies exhibit bluer UV slopes. We also find that low mass galaxies at $\log M/M_{\odot}9$ appear to exhibit a nearly constant $\beta$ at each redshift. We also investigate, for the first time, the correlation between $\beta$ and SFR and find that there is a strong correlation between $\beta$ and SFR, in that galaxies with low SFRs exhibit bluer slopes, and they also appear to get bluer with increasing redshift

Hubble Asteroid Hunter: II. Identifying strong gravitational lenses in HST images with crowdsourcing

The Hubble Space Telescope (HST) archives constitute a rich dataset of high resolution images to mine for strong gravitational lenses. While many HST programs specifically target strong lenses, they can also be present by coincidence in other HST observations. We aim to identify non-targeted strong gravitational lenses in almost two decades of images from the ESA it Hubble Space Telescope archive (eHST), without any prior selection on the lens properties. We used crowdsourcing on the Hubble Asteroid Hunter (HAH) citizen science project to identify strong lenses, alongside asteroid trails, in publicly available large field-of-view HST images. We visually inspected 2354 objects tagged by citizen scientists as strong lenses to clean the sample and identify the genuine lenses. We report the detection of 252 strong gravitational lens candidates, which were not the primary targets of the HST observations. 198 of them are new, not previously reported by other studies, consisting of 45 A grades, 74 B grades and 79 C grades. The majority are galaxy-galaxy configurations. The newly detected lenses are, on average, 1.3 magnitudes fainter than previous HST searches. This sample of strong lenses with high resolution HST imaging is ideal to follow-up with spectroscopy, for lens modelling and scientific analyses. This paper presents an unbiased search of lenses, which enabled us to find a high variety of lens configurations, including exotic lenses. We demonstrate the power of crowdsourcing in visually identifying strong lenses and the benefits of exploring large archival datasets. This study shows the potential of using crowdsourcing in combination with artificial intelligence for the detection and validation of strong lenses in future large-scale surveys such as ESA's future mission Euclid or in JWST archival images.Comment: 24 page, 14 figures, 5 tables, accepted for publication in A&A June 28 202

Evolution of the galaxy stellar mass functions and UV luminosity functions at z=6-9 in the Hubble Frontier Fields

© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. We present new measurements of the evolution of the galaxy stellar mass functions (GSMFs) and ultraviolet luminosity functions (UV LFs) for galaxies from z = 6-9 within the Frontier Field cluster MACSJ0416.1-2403 and its parallel field. To obtain these results, we derive the stellar masses of our sample by fitting synthetic stellar population models to their observed spectral energy distribution with the inclusion of nebular emission lines. This is the deepest and farthest in distance mass function measured to date and probes down to a level of M∗ = 106.8 M⊙. The main result of this study is that the low-mass end of our GSMF to these limits and redshifts appears to become steeper from-1.98-0.07/+0.07 at z = 6 to-2.38-0.88/+0.72 at z = 9, steeper than previously observed mass functions at slightly lower redshifts, and we find no evidence of turnover in the mass range probed. We furthermore demonstrate that the UV LF for these systems also appears to show a steepening at the highest redshifts, without any evidence of turnover in the luminosity range probed. Our MUV-M∗ relation exhibit shallower slopes than previously observed and are in accordance with a constant mass-to-light ratio. Integrating our GSMF, we find that the stellar mass density increases by a factor of ∼ 15-6+21 from z = 9 to z = 6. We estimate the dust-corrected star formation rates (SFRs) to calculate the specific SFRs (sSFR = SFR/M∗) of our sample, and find that for a fixed stellar mass of 5 × 109 M⊙, sSFR ∝ (1 + z)2.01 ± 0.16. Finally, from our new measurements, we estimate the UV luminosity density (ρUV) and find that our results support a smooth decline of ρUV towards high redshifts

Building the First Galaxies -- Chapter 2. Starbursts Dominate The Star Formation Histories of 6 < z <12 Galaxies

We use SEDz* -- a code designed to chart star formation histories (SFHs) of 6<z<12 galaxies -- to analyze the SEDs of 894 galaxies with deep JWST/NIRCam imaging by JADES in the GOODS-S field. We show how SEDz* matches observed SEDs using stellar-population templates, graphing the contribution of each epoch-by-epoch to confirm the robustness of the technique. Very good SED fits for most SFHs demonstrates the compatibility of the templates with stars in the first galaxies -- as expected, because their light is primarily from main-sequence A-stars, free of post-main-sequence complexity and insensitive to heavy-element compositions. We confirm earlier results from Dressler(2023): (1) Four types of star formation histories: SFH1 -- burst; SFH2 -- stochastic; SFH3 -- `contiguous' (3-epochs); and SFH4 -- `continuous' (4-6 epochs); (2) Starbursts -- both single and multiple -- are predominate (~70%) in this critical period of cosmic history, although longer SFHs (0.5-1.0 Gyr) contribute one-third of the accumulated stellar mass. These 894 SFHs contribute log M/Msun = 11.14, 11.09, 11.00, and 10.60 for SFH1-4, respectively, adding up to 4x10^11 Msun by z=6 for this field. We suggest that the absence of rising SFHs could be explained as an intense dust-enshrouded phase of star formation lasting tens of Myr that preceded each of the SFHs we measure. We find no strong dependencies of SFH type with the large-scale environment, however, the discovery of a compact group of 30 galaxies, 11 of which had first star formation at z=11-12, suggests that long SFHs could dominate in rare, dense environments.Comment: Accepted for publication in the Astrophysical Journa

MUSE spectroscopy and deep observations of a unique compact JWST target, lensing cluster CLIO

We present the results of a VLT MUSE/FORS2 and Spitzer survey of a unique compact lensing cluster CLIO at z = 0.42, discovered through the GAMA survey using spectroscopic redshifts. Compact and massive clusters such as this are understudied, but provide a unique prospective on dark matter distributions and for finding background lensed high-z galaxies. The CLIO cluster was identified for follow-up observations due to its almost unique combination of high-mass and dark matter halo concentration, as well as having observed lensing arcs from ground-based images. Using dual band optical and infra-red imaging from FORS2 and Spitzer, in combination with MUSE optical spectroscopy we identify 89 cluster members and find background sources out to z = 6.49. We describe the physical state of this cluster, finding a strong correlation between environment and galaxy spectral type. Under the assumption of an NFW profile, we measure the total mass of CLIO to be M200 = (4.49 ± 0.25) × 1014 M⊙. We build and present an initial strong-lensing model for this cluster, and measure a relatively low intracluster light (ICL) fraction of 7.21 ± 1.53 per cent through galaxy profile fitting. Due to its strong potential for lensing background galaxies and its low ICL, the CLIO cluster will be a target for our 110 h James Webb Space Telescope ‘Webb Medium-Deep Field’ (WMDF) GTO program.Publisher PDFPeer reviewe

Seeing sharper and deeper: JWST's first glimpse of the photometric and spectroscopic properties of galaxies in the epoch of reionisation

We analyse the photometric and spectroscopic properties of four galaxies in the epoch of reionisation (EoR) within the SMACS 0723 JWST Early Release Observations field. Given the known spectroscopic redshifts of these sources, we investigated the accuracy with which photometric redshifts can be derived using NIRCam photometry alone, finding that F115W imaging is essential to distinguish between z~8 galaxies with high equivalent width (EW) [O III] {\lambda}5007 emission and z~10 Balmer break galaxies. We find that all four sources exhibit strong (> 0.6 mag) F356W-F444W colours, which sit at the extreme end of theoretical predictions from numerical simulations. We find that these galaxies deviate (by roughly 0.5 dex) from the local correlation between [O III] {\lambda}5007/H\beta and [Ne III] {\lambda}3869/[O II], which is consistent with the predictions from simulations of high-redshift galaxies. We measure the [O III] {\lambda}5007 rest-frame equivalent widths both directly from the spectroscopy, and indirectly as inferred from the strong F356W-F444W colours, finding large [O III] {\lambda}5007 EWs of 400-1000 {\AA}. The [O III] {\lambda}5007 and H\beta EWs are consistent with those seen in extreme, intensely star-forming dwarf galaxies in the local Universe. Our structural analysis indicates that these galaxies are resolved, exhibiting irregular shapes with bright clumps and colour gradients. In line with the predictions from the FLARES hydrodynamic simulations, such intense star formation and extreme nebular conditions are likely the norm, rather than the exception, in the EoR. Finally, although star-forming galaxies and AGN often occupy similar regions within the [O III] {\lambda}5007/H\beta-[O II]/H{\delta} plane, we find that AGN exhibit distinct, red colours in the F150W-F200W, F200W-F277W plane.Comment: 14 pages, 8 figure

First insights into the ISM at z > 8 with JWST: possible physical implications of a high [O III] λ4363/[O III] λ5007

© 2022 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.We present a detailed analysis of the rest-frame optical emission line ratios for three spectroscopically confirmed galaxies at z > 7.5. The galaxies were identified in the James Webb Space Telescope (JWST) Early Release Observations field SMACS J0723.3 − 7327. By quantitatively comparing Balmer and oxygen line ratios of these galaxies with various low-redshift ‘analogue’ populations (e.g. Green Peas, Blueberries, etc.), we show that no single analogue population captures the diversity of line ratios of all three galaxies observed at z > 7.5. We find that S06355 at z = 7.67 and S10612 at z = 7.66 are similar to local Green Peas and Blueberries. In contrast, S04590 at z = 8.50 appears to be significantly different from the other two galaxies, most resembling extremely low-metallicity systems in the local Universe. Perhaps the most striking spectral feature in S04590 is the curiously high [O III] λ4363/[O III] λ5007 ratio (RO3) of 0.048 (or 0.055 when dust-corrected), implying either extremely high electron temperatures, >3 × 104 K, or gas densities >104 cm−3. Observed line ratios indicate that this galaxy is unlikely to host an AGN. Using photoionization modelling, we show that the inclusion of high-mass X-ray binaries or a high cosmic ray background in addition to a young, low-metallicity stellar population can provide the additional heating necessary to explain the observed high RO3 while remaining consistent with other observed line ratios. Our models represent a first step at accurately characterizing the dominant sources of photoionization and heating at very high redshifts, demonstrating that non-thermal processes may become important as we probe deeper into the Epoch of Reionization.Peer reviewe