arXiv subjects

Richard S Ellis

Publications and source records attributed to Richard S Ellis.

14 recordsLinked to original sources

arXiv2025

Galaxies and Black Holes in the First Billion Years

I present written notes from three lectures given at the 54th Saas-Fee Advanced Course of the Swiss Society of Astrophysics and Astronomy in January 2025 entitled "Galaxies and Black Holes in the First Billion Years as seen by the JWST". I focused my lectures on progress in studies of cosmic reionisation, the properties of galaxies in the reionisation era, topics related to the redshift frontier and the search for Population III stars. The lectures were given to graduate students in astrophysics and cover both pedagogical material as well as observational results from the first two and half years of JWST science operations. The pace of discovery with JWST is, of course, rapid and so my lectures discuss long-term goals, analysis methods and their assumptions and limitations in the hope that the underlying material will retain value in the near future. In this written version, the visual material is that presented at Saas-Fee in January 2025 but I have provided updates on progress from the literature up to August 2025. The material is aimed at early career researchers and should not be considered as a scholarly review of the entire JWST literature on high redshift galaxies

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arXiv2024

Ciii]$\lambda1909$ emission as an alternative to Ly$α$ in the reionization era: the dependence of Ciii] and Ly$α$ at $3<z<4$ from the VANDELS survey

The velocity offset of Ly$α$ emission from a galaxy's systemic redshift is an excellent tracer of conditions that enable the escape of Ly$α$ photons from the galaxy, and potentially hydrogen ionizing Lyman continuum photons. However at $z\geq6$, Ly$α$ is often heavily attenuated by the neutral intergalactic medium. Here we investigate the utility of Ciii],$λ\lambda1907,1909$ emission, usually the brightest UV line after Ly$α$, as a proxy estimating the Ly$α$ velocity offset ($Δv_{\rm{Ly}α}$). To do so, we use analogues of reionization era galaxies based upon 52 star-forming galaxies with robust Ciii] detections drawn from the VANDELS survey. Our sample spans a broad UV magnitude range of $-18.5 < M_{\rm{UV}} < -22.0$, with a sample average value of EW(Ciii]) $=5.3$\,Å. We find a slight increase of EW(Ciii]) with increasing EW(Ly$α$), but find a large range of EW(Ciii]) $\sim$1-13\,Å particularly at EW(Ly$α$) $<10$\,Å. Using the Ciii] line peak as the systemic redshift, we calculate $Δv_{\rm{Ly}α}$ and recover the previously reported trend of decreasing $Δv_{\rm{Ly}α}$ with increasing EW(Ly$α$). Interestingly, we find an anti-correlation between $Δv_{\rm{Ly}α}$ and EW(Ciii]), which also displays a dependence on the UV absolute magnitude. We derive a multi-variate fit to obtain $Δv_{\rm{Ly}α}$ using both EW(Ciii]) and $M_{\rm{UV}}$, finding that $Δv_{\rm{Ly}α}$ is more strongly dependent on EW(Ciii]), with a weaker but non-negligible dependence on $M_{\rm{UV}}$. We find that for a fixed EW(Ciii]), UV-bright Ly$α$ emitting galaxies show smaller values of $Δv_{\rm{Ly}α}$,which suggests that such galaxies may be undergoing more bursty star-formation compared to the UV-fainter ones, akin to a population of extremely UV-bright galaxies identified at $z>10$.

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arXiv2012

The 2012 Hubble Ultra Deep Field (UDF12): Observational Overview

We present the 2012 Hubble Ultra Deep Field campaign (UDF12), a large 128-orbit Cycle 19 \HST\ program aimed at extending previous WFC3/IR observations of the UDF by quadrupling the exposure time in the F105W filter, imaging in an additional F140W filter, and extending the F160W exposure time by 50%. The principal scientific goal of this project is to determine whether galaxies reionized the universe; our observations are designed to provide a robust determination of the star formation density at $z$$\,\gtrsim\,$8, improve measurements of the ultraviolet continuum slope at $z$$\,\sim\,7\,-\,$8, facilitate the construction of new samples of $z$$\,\sim\,9\,-\,$10 candidates, and enable the detection of sources up to $z$$\,\sim\,$12. For this project we committed to combining these and other WFC3/IR imaging observations of the UDF area into a single homogeneous dataset, to provide the deepest near-infrared observations of the sky currently achievable. In this paper we present the observational overview of the project, motivated by its scientific goals, and describe the procedures used in reducing the data as well as the final products that are produced. We have used the most up up-to-date methods for calibrating and combining the images, in particular paying attention to correcting several instrumental effects. We release the full combined mosaics, comprising a single, unified set of mosaics of the UDF, providing the deepest near-infrared blank-field view of the universe obtained to date, reaching magnitudes as deep as AB$\,\sim\,$30 in the near-infrared, and yielding a legacy dataset on this field of lasting scientific value to the community.

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arXiv2012

The Abundance of Star-Forming Galaxies in the Redshift Range 8.5 to 12: New Results from the 2012 Hubble Ultra Deep Field Campaign

We present the results of the deepest search to date for star-forming galaxies beyond a redshift z~8.5 utilizing a new sequence of near-infrared Wide Field Camera 3 images of the Hubble Ultra Deep Field. This `UDF12' campaign completed in September 2012 doubles the earlier exposures with WFC3/IR in this field and quadruples the exposure in the key F105W filter used to locate such distant galaxies. Combined with additional imaging in the F140W filter, the fidelity of high redshift candidates is greatly improved. Using spectral energy distribution fitting techniques on objects selected from a deep multi-band near-infrared stack we find 7 promising z>8.5 candidates. As none of the previously claimed UDF candidates with 8.5 10 galaxies with JWST.

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arXiv2008

Conference Summary

This first Subaru international conference has highlighted the remarkably diverse and significant contributions made using the 8.2m Subaru telescope by both Japanese astronomers and the international community. As such, it serves as a satisfying tribute to the pioneering efforts of Professors Keiichi Kodaira and Sadanori Okamura whose insight and dedication is richly rewarded. Here I try to summarize the recent impact of wide field science in extragalactic astronomy and cosmology and take a look forward to the key questions we will address in the near future.

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arXiv2007

Observations of the High Redshift Universe

(Abridged) In these lectures aimed for non-specialists, I review progress in understanding how galaxies form and evolve. Both the star formation history and assembly of stellar mass can be empirically traced from redshifts z~6 to the present, but how the various distant populations inter-relate and how stellar assembly is regulated by feedback and environmental processes remains unclear. I also discuss how these studies are being extended to locate and characterize the earlier sources beyond z~6. Did early star-forming galaxies contribute significantly to the reionization process and over what period did this occur? Neither theory nor observations are well-developed in this frontier topic but the first results presented here provide important guidance on how we will use more powerful future facilities.

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arXiv2001

Conference Summary

Concluding remarks at the international workshop ``Tracing Cosmic Evolution with Galaxy Clusters", held at Sesto Pusteria, Italy, July 3-6, 2001.

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arXiv2001

Galaxy Formation and Evolution: Recent Progress

In this series of lectures I review recent observational progress in constraining models of galaxy formation and evolution highlighting the importance advances in addressing questions of the assembly history and origin of the Hubble sequence in the context of modern pictures of structure formation.

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arXiv2001

Cross-Roads in Studies of Galaxy Evolution

Studies of galaxy evolution at optical and near-infrared wavelengths have reached an interesting point in their historical development and the arrival of a new millenium provides an appropriate occasion to review the overall direction in which this subject is moving. The aim of this short essay is to emphasize the qualitatively different datasets now arriving and the more detailed and challenging scientific analyses needed for future progress.

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arXiv1998

NGST and the Physical Exploration of Galaxy Evolution During the Era 2<z<5

NGST offers unprecedented opportunities for charting the `dark ages' beyond the limits of the deep optical surveys conducted with HST. An equally important motivation, however, is a detailed physical understanding of the later stages of cosmic history including the period 2<z<5 when most galaxies are thought to assemble and undergo dramatic changes in their star formation rate, chemical content and, ultimately, their morphological characteristics. An important question we need to resolve is the extent to which established massive systems might exist at moderate redshifts. In the context of recent observational and theoretical progress in this area, I review the role NGST could play in this redshift range taking into account the likely progress made in the next decade with the new generation of 8-m class ground-based telescopes

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arXiv1998

The Formation and Evolution of Galaxies

Galaxies represent the visible fabric of the Universe and there has been considerable progress recently in both observational and theoretical studies. The underlying goal is to understand the present-day diversity of galaxy forms, masses and luminosities. Popular models predict the bulk of the population assembled recently, in apparent agreement with optical observations. However, numerous uncertainties remain, including the role that dust might play in obscuring star-forming systems. Astronomers now seek more detailed tests to verify that the Hubble sequence of types arises from transformations driven by the dynamical assembly of smaller systems. Multi-wavelength surveys and studies of the resolved internal properties of distant galaxies promise answers to these fundamental questions.

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arXiv1998

The Formation and Evolution of Galaxies: Perspectives on the Origin of the Hubble Sequence

I review recent observational progress concerning the evolution of the morphological distribution of galaxies in the rich cluster environment and in the faint field population. By coupling HST imagery with ground-based spectroscopic diagnostics, evidence accumulates that galaxy morphology can be a transient phenomenon reflecting various changes in the star formation rate. Possible physical processes responsible for these changes are discussed. Future progress in understanding them will depend on securing 2-D spectroscopic data for representative systems.

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arXiv1997

HDF: Introduction and Motivation

Although it is just over a year since the data was made public, the HDF exposure has stimulated considerable progress towards our understanding of the faint galaxy population. I present a brief personal account of the history of faint galaxy studies culminating in the HDF, and describe what I consider to be the main highlights thus far from this remarkable image. The HDF has given considerable impetus to studies of galaxy evolution and this has led to the emergence of a convincing empirical framework. Further exploitation of deep HST images in conjunction with ground-based 2-D spectroscopy will assist in the physical understanding of the evolutionary processes involved.

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arXiv1994

The Morphological Evolution of Field Galaxies

I review two observational programs which, together, promise to unravel the detailed astrophysical evolution of normal field galaxies over the last 5-7 Gyr. Systematic ground-based spectroscopy of faint galaxies have revealed an increasing faint end slope for the luminosity function with redshift. The trend is strongest for galaxies undergoing intense star-formation. Deep images taken with the repaired HST can be used to count galaxies as a function of morphological type. Regular `Hubble sequence' galaxies follow the no-evolution prediction, but irregular/peculiar sources have a steeper count slope and provide the excess population. Although the overlap between the spectral and HST samples is currently small, plans to merge similar datasets should reveal the physical explanation for the demise of star formation in faint blue galaxies since $z\simeq$0.5-1.

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