arXiv subjects

Chris Simpson

Publications and source records attributed to Chris Simpson.

At least 19 recordsLinked to original sources

arXiv2026

GOATS: The next generation software infrastructure for time-domain astronomy at Gemini/NOIRLab. Application to alerts from Vera C. Rubin Observatory's Legacy Survey of Space and Time

Time-domain and multimessenger astronomy (MMA/TDA) targets demand rapid-response follow-up observations. In many cases, it is the only way to make discoveries and advance our understanding of the astrophysical phenomena, for example, kilonovae accompanying gravitational waves from compact object mergers, shock breakout in supernovae, prompt emission from GRBs, etc. Presently the MMA/TDA follow-up workflow requires wrangling disparate software packages and user interfaces. We present an end-to-end software tool for the community, the Gemini Observation and Analysis of Targets System (GOATS), which unifies and simplifies the workflow, particularly for Gemini follow-up observations. GOATS achieves this by integrating services from Gemini Observatory and its parent organization, NSF NOIRLab. From a single platform, GOATS enables enhanced target selection via NOIRLab's ANTARES alert broker, triggering of Gemini (and other facilities within the Astronomical Event Observatory Network), automated data retrieval from the Gemini Observatory Archive, and interactive data reduction and analysis through Gemini's DRAGONS software and NOIRLab's Astro Data Lab science platform. GOATS was successfully deployed in an end-to-end demonstration of real-time follow-up of Rubin/LSST alerts with NOIRLab facilities. As part of this demonstration, we selected targets from the Rubin alert stream and triggered follow-up observations within minutes of the Rubin detections. We obtained spectra for several targets and classified them as supernova of various types (Ia, IIP, Ib/c) with redshifts ranging from 0.05 to 0.35. By eliminating the need to manually connect tools and automating repetitive tasks, GOATS lowers the entry barrier and allows users to focus on the scientific interpretation of the observation results.

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arXiv2025

A Time-Resolved High-Resolution Spectroscopic Analysis of Ionized Calcium and Dynamical Processes in the Ultra-Hot Jupiter HAT-P-70 b

We present the first transmission spectroscopy study of an exoplanet atmosphere with the high-resolution mode of the new Gemini High-resolution Optical SpecTrograph (GHOST) instrument at the Gemini South Observatory. We observed one transit of HAT-P-70 b - an ultra-hot Jupiter with an inflated radius - and made a new detection of the infrared Ca II triplet in its transmission spectrum. The depth of the strongest line implies that a substantial amount of Ca II extends to at least 47% above the bulk planetary radius. The triplet lines are blueshifted between ~ 3 to 5 km/s, indicative of strong dayside-to-nightside winds common on highly irradiated gas giants. Comparing the transmission spectrum with atmospheric models that incorporate non-local thermodynamic equilibrium effects suggests that the planetary mass is likely between 1 to 2 $M_{\rm J}$, much lighter than the upper limit previously derived from radial velocity measurements. Importantly, thanks to the the high signal-to-noise ratio achieved by GHOST/Gemini South, we are able to measure the temporal variation of these signals. Absorption depths and velocity offsets of the individual Ca II lines remain mostly consistent across the transit, except for the egress phases, where weaker absorption and stronger blueshifts are observed, highlighting the atmospheric processes within the trailing limb alone. Our study demonstrates the ability of GHOST to make time-resolved detections of individual spectral lines, providing valuable insights into the 3D nature of exoplanet atmospheres by probing different planetary longitudes as the tidally locked planet rotates during the transit.

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arXiv2024

GHOST Reduced Data Products for the Gemini Observatory Community and Beyond

The Gemini High-resolution Optical SpecTrograph (GHOST) at Gemini South started regular queue operations in early 2024, bringing a long-sought open-access capability to the astronomy community. This research note briefly describes an effort to provide easy-to-access reduced spectra for GHOST programs from all Gemini partner countries and encourage prompt data exploration and analysis. Since March 2024, over 4500 spectra have been reduced and made available to principal investigators (PIs). The aim is to increase demand for GHOST and expedite the publication of scientific results.

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arXiv2024

High-Resolution Dayside Spectroscopy of WASP-189b: Detection of Iron during the GHOST/Gemini South System Verification Run

With high equilibrium temperatures and tidally locked rotation, ultra-hot Jupiters (UHJs) are unique laboratories within which to probe extreme atmospheric physics and chemistry. In this paper, we present high-resolution dayside spectroscopy of the UHJ WASP-189b obtained with the new Gemini High-resolution Optical SpecTrograph (GHOST) at the Gemini South Observatory. The observations, which cover three hours of post-eclipse orbital phases, were obtained during the instrument's System Verification run. We detect the planet's atmosphere via the Doppler cross-correlation technique, and recover a detection of neutral iron in the planet's dayside atmosphere at a significance of 7.5$\sigma$ in the red-arm of the data, verifying the presence of a thermal inversion. We also investigate the presence of other species in the atmosphere and discuss the implications of model injection/recovery tests. These results represent the first atmospheric characterization of an exoplanet with GHOST's high-resolution mode, and demonstrate the potential of this new instrument in detecting and studying ultra-hot exoplanet atmospheres.

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arXiv2024

GHOST Commissioning Science Results III: Characterizing an iron-poor damped Lyman $\alpha$ system

The Gemini High-resolution Optical SpecTrograph (GHOST) is a new echelle spectrograph available on the Gemini-South telescope as of Semester 2024A. We present the first high resolution spectrum of the quasar J1449-1227 (redshift z_em=3.27) using data taken during the commissioning of GHOST. The observed quasar hosts an intervening iron-poor ([Fe/H] = -2.5) damped Lyman alpha (DLA) system at redshift z=2.904. Taking advantage of the high spectral resolving power of GHOST (R~55000), we are able to accurately model the metal absorption lines of the metal-poor DLA and find a supersolar [Si/Fe], suggesting the DLA gas is in an early stage of chemical enrichment. Using simple ionization models, we find that the large range in the C IV/Si IV column density ratio of individual components within the DLA's high ionization absorption profile can be reproduced by several metal-poor Lyman limit systems surrounding the low-ionization gas of the DLA. It is possible that this metal-poor DLA resides within a complex system of metal-poor galaxies or filaments with inflowing gas. The high spectral resolution, wavelength coverage and sensitivity of GHOST makes it an ideal spectrograph for characterizing the chemistry and kinematics of quasar absorption lines.

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arXiv2024

The Science Performance of the Gemini High Resolution Optical Spectrograph

The Gemini High Resolution Optical Spectrograph (GHOST) is a fiber-fed spectrograph system on the Gemini South telescope that provides simultaneous wavelength coverage from 348 - 1061nm, and designed for optimal performance between 363 - 950nm. It can observe up to two objects simultaneously in a 7.5 arcmin diameter field of regard at R = 56,000 or a single object at R = 75,000. The spectral resolution modes are obtained by using integral field units to image slice a 1.2" aperture by a factor of five in width using 19 fibers in the high resolution mode and by a factor of three in width using 7 fibers in the standard resolution mode. GHOST is equipped with hardware to allow for precision radial velocity measurements, expected to approach meters per second precision. Here, we describe the basic design and operational capabilities of GHOST, and proceed to derive and quantify the key aspects of its on-sky performance that are of most relevance to its science users.

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arXiv2023

SPLUS J142445.34-254247.1: An R-Process Enhanced, Actinide-Boost, Extremely Metal-Poor star observed with GHOST

We report on the chemo-dynamical analysis of SPLUS J142445.34-254247.1, an extremely metal-poor halo star enhanced in elements formed by the rapid neutron-capture process. This star was first selected as a metal-poor candidate from its narrow-band S-PLUS photometry and followed up spectroscopically in medium-resolution with Gemini South/GMOS, which confirmed its low-metallicity status. High-resolution spectroscopy was gathered with GHOST at Gemini South, allowing for the determination of chemical abundances for 36 elements, from carbon to thorium. At [Fe/H]=-3.39, SPLUS J1424-2542 is one of the lowest metallicity stars with measured Th and has the highest logeps(Th/Eu) observed to date, making it part of the "actinide-boost" category of r-process enhanced stars. The analysis presented here suggests that the gas cloud from which SPLUS J1424-2542 was formed must have been enriched by at least two progenitor populations. The light-element (Z<=30) abundance pattern is consistent with the yields from a supernova explosion of metal-free stars with 11.3-13.4 Msun, and the heavy-element (Z>=38) abundance pattern can be reproduced by the yields from a neutron star merger (1.66Msun and 1.27Msun) event. A kinematical analysis also reveals that SPLUS J1424-2542 is a low-mass, old halo star with a likely in-situ origin, not associated with any known early merger events in the Milky Way.

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arXiv2023

SN 2022jox: An extraordinarily ordinary Type II SN with Flash Spectroscopy

We present high cadence optical and ultraviolet observations of the Type II supernova (SN), SN 2022jox which exhibits early spectroscopic high ionization flash features of \ion{H}{1}, \ion{He}{2}, \ion{C}{4}, and \ion{N}{4} that disappear within the first few days after explosion. SN 2022jox was discovered by the Distance Less than 40 Mpc (DLT40) survey $\sim$0.75 days after explosion with followup spectra and UV photometry obtained within minutes of discovery. The SN reached a peak brightness of M$_V \sim$ $-$17.3 mag, and has an estimated $^{56}$Ni mass of 0.04 M$_{\odot}$, typical values for normal Type II SNe. The modeling of the early lightcurve and the strong flash signatures present in the optical spectra indicate interaction with circumstellar material (CSM) created from a progenitor with a mass loss rate of $\dot{M} \sim 10^{-3}-10^{-2}\ M_\odot\ \mathrm{yr}^{-1}$. There may also be some indication of late-time CSM interaction in the form of an emission line blueward of H$\alpha$ seen in spectra around 200 days. The mass-loss rate is much higher than the values typically associated with quiescent mass loss from red supergiants, the known progenitors of Type II SNe, but is comparable to inferred values from similar core collapse SNe with flash features, suggesting an eruptive event or a superwind in the progenitor in the months or years before explosion.

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arXiv2023

Probing the early Milky Way with GHOST spectra of an extremely metal-poor star in the Galactic disk

Pristine_183.6849+04.8619 (P1836849) is an extremely metal-poor ([Fe/H]$=-3.3\pm0.1$) star on a prograde orbit confined to the Galactic disk. Such stars are rare and may have their origins in protogalactic fragments that formed the early Milky Way, in low mass satellites accreted later, or forming in situ in the Galactic plane. Here we present a chemo-dynamical analysis of the spectral features between $3700-11000$\r{A} from a high-resolution spectrum taken during Science Verification of the new Gemini High-resolution Optical SpecTrograph (GHOST). Spectral features for many chemical elements are analysed (Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Ni), and valuable upper limits are determined for others (C, Na, Sr, Ba). This main sequence star exhibits several rare chemical signatures, including (i) extremely low metallicity for a star in the Galactic disk, (ii) very low abundances of the light $\alpha$-elements (Na, Mg, Si) compared to other metal-poor stars, and (iii) unusually large abundances of Cr and Mn, where [Cr, Mn/Fe]$_{\rm NLTE}>+0.5$. A comparison to theoretical yields from supernova models suggests that two low mass Population III objects (one 10 M$_\odot$ supernova and one 17 M$_\odot$ hypernova) can reproduce the abundance pattern well (reduced $\chi^2<1$). When this star is compared to other extremely metal-poor stars on quasi-circular, prograde planar orbits, differences in both chemistry and kinematics imply there is little evidence for a common origin. The unique chemistry of P1836849 is discussed in terms of the earliest stages in the formation of the Milky Way.

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arXiv2023

DRAGONS -- A Quick Overview

DRAGONS (Data Reduction for Astronomy from Gemini Observatory North and South) is a platform for the reduction and processing of astronomical data. The Python-based, open-source package includes infrastructure for automation and algorithms for the processing of imaging and spectroscopic data, up to the analysis-ready stage. DRAGONS currently focuses on the reduction of Gemini data, although it allows for support of data from other instruments and telescopes through third-party extensions. Its latest release (v3.1) enables automated reduction of all currently-active Gemini imaging facility instruments, as well as optical longslit spectroscopic data, acquired with GMOS.

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arXiv2023

GHOST Commissioning Science Results: Identifying a new chemically peculiar star in Reticulum II

The Gemini High-resolution Optical SpecTrograph (GHOST) is the newest high resolution spectrograph to be developed for a large aperture telescope, recently deployed and commissioned at the Gemini-South telescope. In this paper, we present the first science results from the GHOST spectrograph taking during its commissioning runs. We have observed the bright metal-poor benchmark star HD 122563, along with two stars in the ultra faint dwarf galaxy, Ret II, one of which was previously identified as a candidate member, but did not have a previous detailed chemical abundance analysis. This star (GDR3 0928) is found to be a bona fide member of Ret II, and from a spectral synthesis analysis, it is also revealed to be a CEMP-r star, with significant enhancements in the several light elements (C, N, O, Na, Mg, and Si), in addition to featuring an r-process enhancement like many other Ret II stars. The light-element enhancements in this star resemble the abundance patterns seen in the CEMP-no stars of other ultra faint dwarf galaxies, and are thought to have been produced by an independent source from the r-process. These unusual abundance patterns are thought to be produced by faint supernovae, which may be produced by some of the earliest generations of stars.

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arXiv2021

From starburst to quiescence: post-starburst galaxies and their large-scale clustering over cosmic time

We present the first study of the large-scale clustering of post-starburst (PSB) galaxies in the high redshift Universe ($0.5 10^{14}$M$_{\odot}$), suggesting they are likely to be infalling satellite galaxies in cluster-like environments. High-mass PSB galaxies are more weakly clustered at low redshifts, but they reside in higher mass haloes with increasing look-back time, suggesting strong redshift-dependent halo downsizing. These key results are consistent with previous results suggesting that two main channels are responsible for the rapid quenching of galaxies. While high-redshift ($z>1$) galaxies appear to be quenched by secular feedback mechanisms, processes associated with dense environments are likely to be the key driver of rapid quenching in the low-redshift Universe ($z<1$). Finally, we show that the clustering of photometrically selected PSBs are consistent with them being direct descendants of highly dust-enshrouded sub-millimetre galaxies (SMGs), providing tantalising evidence for the oft-speculated evolutionary pathway from starburst to quiescence.

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arXiv2020

An ALMA survey of the S2CLS UDS field: Optically invisible submillimetre galaxies

We analyse a robust sample of 30 near-infrared-faint (K>25.3, 5 sigma) submillimetre galaxies selected across a 0.96 deg^2 field, to investigate their properties and the cause of their lack of detectable optical/near-infrared emission. Our analysis exploits precise identifications based on ALMA 870um continuum imaging, combined with the very deep near-infrared imaging from the UKIDSS-UDS survey. We estimate that K>25.3 submillimetre galaxies represent 15+/-2 per cent of the total population brighter than S870=3.6mJy, with an expected surface density of ~450/deg^2 above S870>1mJy. As such they pose a source of contamination in surveys for both high-redshift "quiescent" galaxies and very-high-redshift Lyman-break galaxies. We show that these K-faint submillimetre galaxies are simply the tail of the broader submillimetre population, with comparable dust and stellar masses to K<25.3 mag submillimetre galaxies, but lying at significantly higher redshifts (z=3.44+/-0.06 versus z=2.36+/-0.11) and having higher dust attenuation (Av=5.2+/-0.3 versus Av=2.9+/-0.1). We investigate the origin of the strong dust attenuation and find indications that these K-faint galaxies have smaller dust continuum sizes than the K<25.3 galaxies, as measured by ALMA, which suggests their high attenuation is related to their compact sizes. We find a correlation of dust attenuation with star-formation rate surface density (Sigma_SFR), with the K-faint submillimetre galaxies representing the higher-Sigma_SFR and highest-Av galaxies. The concentrated, intense star-formation activity in these systems is likely to be associated with the formation of spheroids in compact galaxies at high redshifts, but as a result of their high obscuration these are completely missed in UV, optical and even near-infrared surveys.

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arXiv2018

Compact star-forming galaxies preferentially quenched to become PSBs in $z<1$ clusters

We analyse the structure of galaxies with high specific star formation rate (SSFR) in cluster and field environments in the redshift range $0.5<z<1.0$. Recent studies have shown that these galaxies are strongly depleted in dense environments due to rapid environmental quenching, giving rise to post-starburst galaxies (PSBs). We use effective radii and S\'ersic indices as tracers of galaxy structure, determined using imaging from the UKIDSS Ultra Deep Survey (UDS). We find that the high-SSFR galaxies that survive into the cluster environment have, on average, larger effective radii than those in the field. We suggest that this trend is likely to be driven by the most compact star-forming galaxies being preferentially quenched in dense environments. We also show that the PSBs in clusters have stellar masses and effective radii that are similar to the missing compact star-forming population, suggesting that these PSBs are the result of size-dependent quenching. We propose that both strong stellar feedback and the stripping of the extended halo act together to preferentially and rapidly quench the compact and low-mass star-forming systems in clusters to produce PSBs. We test this scenario using the stacked spectra of 124 high-SSFR galaxies, showing that more compact galaxies are more likely to host outflows. We conclude that a combination of environmental and secular processes is the most likely explanation for the appearance of PSBs in galaxy clusters.

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arXiv2018

The structure of post-starburst galaxies at $0.5 < z < 2$: evidence for two distinct quenching routes at different epochs

We present an analysis of the structure of post-starburst (PSB) galaxies in the redshift range $0.5 < z < 2$, using a photometrically-selected sample identified in the Ultra Deep Survey (UDS) field. We examine the structure of $\sim80$ of these transient galaxies using radial light $\mu(r)$ profiles obtained from CANDELS $\textit{Hubble Space Telescope}$ near-infrared/optical imaging, and compare to a large sample of $\sim2000$ passive and star-forming galaxies. For each population, we determine their typical structural properties (effective radius $r_{\rm e}$, S\'ersic index $n$) and find significant differences in PSB structure at different epochs. At high redshift ($z > 1$), PSBs are typically massive ($M_* > 10^{10}\rm\,M_{\odot}$), very compact and exhibit high S\'ersic indices, with structures that differ significantly from their star-forming progenitors but are similar to massive passive galaxies. In contrast, at lower redshift ($0.5 < z < 1$), PSBs are generally of low mass ($M_* < 10^{10}\rm\,M_{\odot}$) and exhibit compact but less concentrated profiles (i.e. lower S\'ersic indices), with structures similar to low-mass passive discs. Furthermore, for both epochs we find remarkably consistent PSB structure across the optical/near-infrared wavebands (which largely trace different stellar populations), suggesting that any preceding starburst and/or quenching in PSBs was not strongly centralized. Taken together, these results imply that PSBs at $z > 1$ have been recently quenched during a major disruptive event (e.g. merger or protogalactic collapse) which formed a compact remnant, while at $z < 1$ an alternative less disruptive process is primarily responsible. Our results suggest that high-$z$ PSBs are an intrinsically different population to those at lower redshifts, and indicate different quenching routes are active at different epochs.

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arXiv2018

The enhancement of rapidly quenched galaxies in distant clusters at 0.5<z<1.0

We investigate the relationship between environment and galaxy evolution in the redshift range $0.5 < z < 1.0$. Galaxy overdensities are selected using a Friends-of-Friends algorithm, applied to deep photometric data in the Ultra-Deep Survey (UDS) field. A study of the resulting stellar mass functions reveals clear differences between cluster and field environments, with a strong excess of low-mass rapidly quenched galaxies in cluster environments compared to the field. Cluster environments also show a corresponding deficit of young, low-mass star-forming galaxies, which show a sharp radial decline towards cluster centres. By comparing mass functions and radial distributions, we conclude that young star-forming galaxies are rapidly quenched as they enter overdense environments, becoming post-starburst galaxies before joining the red sequence. Our results also point to the existence of two environmental quenching pathways operating in galaxy clusters, operating on different timescales. Fast quenching acts on galaxies with high specific star-formation rates, operating on timescales shorter than the cluster dynamical time ($ < 1$ Gyr). In contrast, slow quenching affects galaxies with moderate specific star-formation rates, regardless of their stellar mass, and acts on longer timescales ($\gtrsim 1$ Gyr). Of the cluster galaxies in the stellar mass range $9.0 < \log(M_{*}/M_{\odot}) < 10.5$ quenched during this epoch, we find that 73% were transformed through fast quenching, while the remaining 27% followed the slow quenching route.

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arXiv2017

Massive post-starburst galaxies at z > 1 are compact proto-spheroids

We investigate the relationship between the quenching of star formation and the structural transformation of massive galaxies, using a large sample of photometrically-selected post-starburst galaxies in the UKIDSS UDS field. We find that post-starburst galaxies at high-redshift ($z>1$) show high S\'ersic indices, significantly higher than those of active star-forming galaxies, but with a distribution that is indistinguishable from the old quiescent population. We conclude that the morphological transformation occurs before (or during) the quenching of star formation. Recently quenched galaxies are also the most compact; we find evidence that massive post-starburst galaxies (M$_{\ast}> 10^{10.5} ~$M$_{\odot}$) at high redshift ($z>1$) are on average smaller than comparable quiescent galaxies at the same epoch. Our findings are consistent with a scenario in which massive passive galaxies are formed from three distinct phases: (1) gas-rich dissipative collapse to very high densities, forming the proto-spheroid; (2) rapid quenching of star formation, to create the "red nugget" with post-starburst features; (3) a gradual growth in size as the population ages, perhaps as a result of minor mergers.

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arXiv2017

Extragalactic radio surveys in the pre-Square Kilometre Array era

The era of the Square Kilometre Array is almost upon us, and pathfinder telescopes are already in operation. This brief review summarizes our current knowledge of extragalactic radio sources, accumulated through six decades of continuum surveys at the low-frequency end of the electromagnetic spectrum and the extensive complementary observations at other wavelengths necessary to gain this understanding. The relationships between radio survey data and surveys at other wavelengths are discussed. Some of the outstanding questions are identified and prospects over the next few years are outlined.

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