Publications by Rebecca Bowler


The rest-frame UV luminosity function at z≃4 : a significant contribution of AGN to the bright-end of the galaxy population

Monthly Notices of the Royal Astronomical Society Oxford University Press 494 (2020) 1771-1783

N Adams, R Bowler, M Jarvis, B Haussler, R McLure, A Bunker, J Dunlop, A Verma

We measure the rest-frame UV luminosity function (LF) at z ∼ 4 self-consistently over a wide range in absolute magnitude (−27 . MUV . −20). The LF is measured with 46,904 sources selected using a photometric redshift approach over ∼ 6 deg2 of the combined COSMOS and XMM-LSS fields. We simultaneously fit for both AGN and galaxy LFs using a combination of Schechter or Double Power Law (DPL) functions alongside a single power law for the faint-end slope of the AGN LF. We find a lack of evolution in the shape of the bright-end of the LBG component when compared to other studies at z ' 5 and evolutionary recipes for the UV LF. Regardless of whether the LBG LF is fit with a Schechter function or DPL, AGN are found to dominate at MUV < −23.5. We measure a steep faint-end slope of the AGN LF with αAGN = −2.09+0.35 −0.38 (−1.66+0.29 −0.58) when fit alongside a Schechter function (DPL) for the galaxies. Our results suggest that if AGN are morphologically selected it results in a bias to lower number densities. Only by considering the full galaxy population over the transition region from AGN to LBG domination can an accurate measurement of the total LF be attained.


A lack of evolution in the very bright-end of the galaxy luminosity function from z ≃ 8-10

Monthly Notices of the Royal Astronomical Society Oxford University Press 493 (2020) 2059-2084

R Bowler, M Jarvis, JS Dunlop, HJ McCracken

We utilize deep near-infrared survey data from the UltraVISTA fourth data release (DR4) and the VIDEO survey, in combination with overlapping optical and Spitzer data, to search for bright star-forming galaxies at z ≳ 7.5. Using a full photometric redshift fitting analysis applied to the ∼6 deg2 of imaging searched, we find 27 Lyman break galaxies (LBGs), including 20 new sources, with best-fitting photometric redshifts in the range 7.4 < z < 9.1. From this sample, we derive the rest-frame UV luminosity function at z = 8 and z = 9 out to extremely bright UV magnitudes (MUV ≃ −23) for the first time. We find an excess in the number density of bright galaxies in comparison to the typically assumed Schechter functional form derived from fainter samples. Combined with previous studies at lower redshift, our results show that there is little evolution in the number density of very bright (MUV ∼ −23) LBGs between z ≃ 5 and z ≃ 9. The tentative detection of an LBG with best-fitting photometric redshift of z = 10.9 ± 1.0 in our data is consistent with the derived evolution. We show that a double power-law fit with a brightening characteristic magnitude (ΔM*/Δz ≃ −0.5) and a steadily steepening bright-end slope (Δβ/Δz ≃ −0.5) provides a good description of the z > 5 data over a wide range in absolute UV magnitude (−23 < MUV < −17). We postulate that the observed evolution can be explained by a lack of mass quenching at very high redshifts in combination with increasing dust obscuration within the first ∼1Gyr of galaxy evolution.


Augmenting machine learning photometric redshifts with Gaussian mixture models

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2020)

P Hatfield, I Almosallam, M Jarvis, N Adams, R Bowler, Z Gomes, S Roberts, C Schreiber

&lt;jats:title&gt;Abstract&lt;/jats:title&gt; &lt;jats:p&gt;Wide-area imaging surveys are one of the key ways of advancing our understanding of cosmology, galaxy formation physics, and the large-scale structure of the Universe in the coming years. These surveys typically require calculating redshifts for huge numbers (hundreds of millions to billions) of galaxies - almost all of which must be derived from photometry rather than spectroscopy. In this paper we investigate how using statistical models to understand the populations that make up the colour-magnitude distribution of galaxies can be combined with machine learning photometric redshift codes to improve redshift estimates. In particular we combine the use of Gaussian Mixture Models with the high performing machine learning photo-z algorithm GPz and show that modelling and accounting for the different colour-magnitude distributions of training and test data separately can give improved redshift estimates, reduce the bias on estimates by up to a half, and speed up the run-time of the algorithm. These methods are illustrated using data from deep optical and near infrared data in two separate deep fields, where training and test data of different colour-magnitude distributions are constructed from the galaxies with known spectroscopic redshifts, derived from several heterogeneous surveys.&lt;/jats:p&gt;


Obscured star formation in bright z ≃ 7 Lyman-break galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press 481 (2018) 1631-1644

R Bowler, N Bourne, J Dunlop, R McLure, D McLeod

We present Atacama Large Millimeter/Submillimeter Array observations of the rest-frame far-infrared (FIR) dust continuum emission of six bright Lyman-break galaxies (LBGs) at z ≃ 7. One LBG is detected (5.2σ at peak emission), whilst the others remain individually undetected at the 3σ level. The average FIR luminosity of the sample is found to be LFIR≃2×1011L⊙⁠, corresponding to an obscured star formation rate (SFR) that is comparable to that inferred from the unobscured UV emission. In comparison to the infrared excess (IRX=LFIR/LUV⁠)–β relation, our results are consistent with a Calzetti-like attenuation law (assuming a dust temperature of T = 40–50 K). We find a physical offset of 3kpc between the dust continuum emission and the rest-frame UV light probed by Hubble Space Telescope imaging for galaxy ID65666 at z=7.17+0.09−0.06⁠. The offset is suggestive of an inhomogeneous dust distribution, where 75 per cent of the total star formation activity (SFR≃70M⊙/yr⁠) of the galaxy is completely obscured. Our results provide direct evidence that dust obscuration plays a key role in shaping the bright end of the observed rest-frame UV luminosity function at z ≃ 7, in agreement with cosmological galaxy formation simulations. The existence of a heavily obscured component of galaxy ID65666 indicates that dusty star-forming regions, or even entire galaxies, that are ‘UV dark’ are significant even in the z ≃ 7 galaxy population.


The environment and host haloes of the brightest z~6 Lyman-break galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press 477 (2018) 3760-3774

P Hatfield, R Bowler, M Jarvis, C Hale

By studying the large-scale structure of the bright high-redshift Lyman-break galaxy (LBG) population it is possible to gain an insight into the role of environment in galaxy formation physics in the early Universe. We measure the clustering of a sample of bright ($-22.7


Dust attenuation in 2 < z < 3 star-forming galaxies from deep ALMA observations of the Hubble Ultra Deep Field

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 476 (2018) 3991-4006

RJ McLure, JS Dunlop, F Cullen, N Bourne, PN Best, S Khochfar, RAA Bowler, AD Biggs, JE Geach, D Scott, MJ Michalowski, W Rujopakarn, E van Kampen, A Kirkpatrick, A Pope


Extragalactic optical and near-infrared foregrounds to 21-cm epoch of reionisation experiments

Proceedings of the International Astronomical Union Cambridge University Press 12 (2018) 183-190

MJ Jarvis, RAA Bowler, PW Hatfield

Foreground contamination is one of the most important limiting factors in detecting the neutral hydrogen in the epoch of reionisation. These foregrounds can be roughly split into galactic and extragalactic foregrounds. In these proceedings we highlight information that can be gleaned from multi-wavelength extragalactic surveys in order to overcome this issue. We discuss how clustering information from the lower-redshift, foreground galaxies, can be used as additional information in accounting for the noise associated with the foregrounds. We then go on to highlight the expected contribution of future optical and near-infrared surveys for detecting the galaxies responsible for ionising the Universe. We suggest that these galaxies can also be used to reduce the systematics in the 21-cm epoch of reionisation signal through cross-correlations if enough common area is surveyed.


Unveiling the nature of bright z ≃ 7 galaxies with the Hubble Space Telescope

Monthly Notices of the Royal Astronomical Society Oxford University Press 466 (2017) 3612-3635

R Bowler, JS Dunlop, RJ McLure, DJ McLeod

We present new Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) imaging of 25 extremely luminous (−23.2 ≤ MUV ≲ −21.2) Lyman-break galaxies (LBGs) at z ≃ 7. The sample was initially selected from 1.65 deg2 of ground-based imaging in the UltraVISTA/COSMOS and UDS/SXDS fields, and includes the extreme Lyman α emitters, ‘Himiko’ and ‘CR7’. A deconfusion analysis of the deep Spitzer photometry available suggests that these galaxies exhibit strong rest-frame optical nebular emission lines (EW0(Hβ + [O iii]) &gt; 600 Å). We find that irregular, multiple-component morphologies suggestive of clumpy or merging systems are common (fmulti &gt; 0.4) in bright z ≃ 7 galaxies, and ubiquitous at the very bright end (MUV &lt; −22.5). The galaxies have half-light radii in the range r1/2 ∼ 0.5–3 kpc. The size measurements provide the first determination of the size–luminosity relation at z ≃ 7 that extends to MUV ∼ −23. We find the relation to be steep with r1/2 ∝ L1/2. Excluding clumpy, multicomponent galaxies however, we find a shallower relation that implies an increased star formation rate surface density in bright LBGs. Using the new, independent, HST/WFC3 data we confirm that the rest-frame UV luminosity function at z ≃ 7 favours a power-law decline at the bright end, compared to an exponential Schechter function drop-off. Finally, these results have important implications for the Euclid mission, which we predict will detect &gt;1000 similarly bright galaxies at z ≃ 7. Our new HST imaging suggests that the vast majority of these galaxies will be spatially resolved by Euclid, mitigating concerns over dwarf star contamination.


No evidence for Population III stars or a Direct Collapse Black Hole in the z = 6.6 Lyman-$α$ emitter 'CR7'

Monthly Notices of the Royal Astronomical Society Oxford University Press 469 (2017) 448-458

RAA Bowler, RJ McLure, JS Dunlop, DJ McLeod, ER Stanway, JJ Eldridge, MJ Jarvis

The z = 6.6 Lyman-$\alpha$ emitter 'CR7' has been claimed to have a Population III-like stellar population, or alternatively, be a candidate Direct Collapse Black Hole (DCBH). In this paper we investigate the evidence for these exotic scenarios using recently available, deeper, optical, near-infrared and mid-infrared imaging. We find strong Spitzer/IRAC detections for the main component of CR7 at 3.6 and 4.5 microns, and show that it has a blue colour ([3.6] - [4.5] $= -1.2\pm 0.3$). This colour cannot be reproduced by current Pop. III or pristine DCBH models. Instead, the results suggest that the [3.6] band is contaminated by the [OIII]4959,5007 emission line with an implied rest-frame equivalent width of EW_0 (H$\beta$ + [OIII]) $\gtrsim 2000$\AA. Furthermore, we find that new near-infrared data from the UltraVISTA survey supports a weaker He II 1640 emission line than previously measured, with EW_0 $= 40 \pm 30$\AA. For the fainter components of CR7 visible in Hubble Space Telescope imaging, we find no evidence that they are particularly red as previously claimed, and show that the derived masses and ages are considerably uncertain. In light of the likely detection of strong [OIII] emission in CR7 we discuss other more standard interpretations of the system that are consistent with the data. We find that a low-mass, narrow-line AGN can reproduce the observed features of CR7, including the lack of radio and X-ray detections. Alternatively, a young, low-metallicity (~1/200 solar) starburst, modelled including binary stellar pathways, can reproduce the inferred strength of the He II line and simultaneously the strength of the observed [OIII] emission, but only if the gas shows super-solar $\alpha$-element abundances (O/Fe ~ 5 O/Fe solar).


Characterizing the evolving K-band luminosity function using the Ultra VISTA, CANDELS and HUDF surveys

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 465 (2017) 672-687

A Mortlock, RJ McLure, RAA Bowler, DJ McLeod, E Marmol-Queralto, S Parsa, JS Dunlop, VA Bruce


The SCUBA-2 Cosmology Legacy Survey: the nature of bright submm galaxies from 2 deg(2) of 850-mu m imaging

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 469 (2017) 492-515

MJ Michalowski, JS Dunlop, MP Koprowski, M Cirasuolo, JE Geach, RAA Bowler, A Mortlock, KI Caputi, I Aretxaga, V Arumugam, C-C Chen, RJ McLure, M Birkinshaw, N Bourne, D Farrah, E Ibar, P van der Werf, M Zemcov


Changing physical conditions in star-forming galaxies between redshifts 0 < Z < 4: [O iii]/H beta evolution

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 460 (2016) 3002-3013

F Cullen, M Cirasuolo, LJ Kewley, RJ McLure, JS Dunlop, RAA Bowler


The SCUBA-2 Cosmology Legacy Survey: galaxies in the deep 850 mu m survey, and the star-forming 'main sequence'

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 458 (2016) 4321-4344

MP Koprowski, JS Dunlop, MJ Michalowski, I Roseboom, JE Geach, M Cirasuolo, I Aretxaga, RAA Bowler, M Banerji, N Bourne, KEK Coppin, S Chapman, DH Hughes, T Jenness, RJ McLure, M Symeonidis, P van der Werf


The galaxy luminosity function at z ≃ 6 and evidence for rapid evolution in the bright end from z ≃ 7 to 5

Monthly Notices of the Royal Astronomical Society Oxford University Press 452 (2015) 1817-1840

R Bowler, JS Dunlop, RJ McLure, HJ McCracken, B Milvang-Jensen, H Furusawa, Y Taniguchi, O Le Fèvre, JPU Fynbo, M Jarvis, B Häußler

We present the results of a search for bright (-22.7 ≤MUV ≤-20.5) Lyman-break galaxies at z≃6 within a total of 1.65 deg &lt; sup &gt; 2 &lt; /sup &gt; of imaging in theUltraVISTA/Cosmological Evolution Survey (COSMOS) and United Kingdom Infrared Telescope Deep Sky Survey (UKIDSS) Ultra Deep Survey (UDS) fields. The deep near-infrared imaging available in the two independent fields, in addition to deep optical (including z′-band) data, enables the sample of z ≃ 6 star-forming galaxies to be securely detected longward of the break (in contrast to several previous studies). We show that the expected contamination rate of our initial sample by cool Galactic brown dwarfs is ≲3 per cent and demonstrate that they can be effectively removed by fitting brown dwarf spectral templates to the photometry. At z ≃ 6, the galaxy surface density in the UltraVISTA field exceeds that in the UDS by a factor of ≃ 1.8, indicating strong cosmic variance even between degree-scale fields at z &gt; 5. We calculate the bright end of the restframe Ultraviolet (UV) luminosity function (LF) at z ≃ 6. The galaxy number counts are a factor of ~1.7 lower than predicted by the recent LF determination by Bouwens et al. In comparison to other smaller area studies, we find an evolution in the characteristic magnitude between z ≃ 5 and z ≃ 7 of δM* ~ 0.4, and show that a double power law or a Schechter function can equally well describe the LF at z = 6. Furthermore, the bright end of the LF appears to steepen from z ≃ 7 to z ≃ 5, which could indicate the onset of mass quenching or the rise of dust obscuration, a conclusion supported by comparing the observed LFs to a range of theoretical model predictions.


Star-forming galaxies in the first billion years

Astronomy & Geophysics Oxford University Press (OUP) 56 (2015) 3.39-3.43

R Bowler


The decomposed bulge and disc size mass relations of massive galaxies at 1 < z < 3 in CANDELS

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 444 (2014) 1660-1673

VA Bruce, JS Dunlop, RJ McLure, M Cirasuolo, F Buitrago, RAA Bowler, TA Targett, EF Bell, DH McIntosh, A Dekel, SM Faber, HC Ferguson, NA Grogin, W Hartley, DD Kocevski, AM Koekemoer, DC Koo, EJ McGrath


The bulge-disc decomposed evolution of massive galaxies at 1 < z < 3 in CANDELS

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 444 (2014) 1001-1033

VA Bruce, JS Dunlop, RJ McLure, M Cirasuolo, F Buitrago, RAA Bowler, TA Targett, EF Bell, DH McIntosh, A Dekel, SM Faber, HC Ferguson, NA Grogin, W Hartley, DD Kocevski, AM Koekemoer, DC Koo, EJ McGrath


A reassessment of the redshift distribution and physical properties of luminous (sub-)millimetre galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 444 (2014) 117-128

MP Koprowski, JS Dunlop, MJ Michalowski, M Cirasuolo, RAA Bowler


The colour distribution of galaxies at redshift five

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 440 (2014) 3714-3725

AB Rogers, RJ McLure, JS Dunlop, RAA Bowler, EF Curtis-Lake, P Dayal, SM Faber, HC Ferguson, SL Finkelstein, NA Grogin, NP Hathi, D Kocevski, AM Koekemoer, P Kurczynski


Line-driven radiative outflows in luminous quasars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 445 (2014) 359-377

RAA Bowler, PC Hewett, JT Allen, GJ Ferland

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