arXiv subjects

James E. Davies

Publications and source records attributed to James E. Davies.

13 recordsLinked to original sources

arXiv2026

Shedding the envelope: JWST reveals a kiloparsec-scale [OIII]-weak Balmer shell around a z=7.64 quasar

Luminous quasars at the redshift frontier z>7 serve as stringent probes of super-massive black hole formation and they are thought to undergo much of their growth obscured by dense gas and dust in their host galaxies. Fully characterizing the symbiotic evolution of SMBHs and hosts requires rest-frame optical observations that span spatial scales from the broad-line region to the ISM and CGM. JWST now provides the necessary spatially resolved spectroscopy to do so. But the physical conditions that regulate the interplay between SMBHs and their hosts at the highest redshifts, especially the nature of early feedback phases, remain unclear. We present JWST/NIRSpec IFU observations of J0313$-$1806 at z=7.64, the most distant luminous quasar known. From the restframe optical spectrum of the unresolved quasar, we derive a black hole mass of $M_\mathrm{BH}=(1.63 \pm 0.10)\times10^9 M_\odot$ based on H$β$ and an Eddington rate of $λ=L/L_\mathrm{Edd}=0.80\pm 0.05$, consistent with previous MgII-based estimates. J0313-1806 exhibits no detectable [O III] emission on nuclear scales. Most remarkably, we detect an ionized gas shell extending out to $\sim 1.8$ kpc traced by H$β$ emission that also lacks any significant [O III], with a $3σ$ upper limit on the [O III]$ λ$5007 to H$β$ flux ratio of $\log_{10} \left( F(\mathrm{[OIII]})/F(\mathrm{H}β)\right)=-1.15$. Through photoionization modelling, we demonstrate that the extended emission is consistent with a thin, clumpy outflowing shell where [OIII] is collisionally de-excited by dense gas. We interpret this structure as a fossil remnant of a recent blowout phase, providing evidence for episodic feedback cycles in one of the earliest quasars. These findings suggest that dense ISM phases may play a crucial role in shaping the spectral properties of quasars accross cosmic time.

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arXiv2025

Detecting galaxy-21-cm cross-correlation during reionization

The cosmic 21-cm signal promises to revolutionize studies of the Epoch of Reionization (EoR). Radio interferometers are aiming for a preliminary, low signal-to-noise (S/N) detection of the 21-cm power spectrum. Cross-correlating 21-cm with galaxies will be especially useful in these efforts, providing both a sanity check for initial 21-cm detection claims and potentially increasing the S/N due to uncorrelated residual systematics. Here we self-consistently simulate large-scale (1 Gpc) galaxy and 21-cm fields, computing their cross-power spectra for various choices of instruments as well as survey properties. We use 1080h observations with SKA-low AA* and HERA-350 as our benchmark 21-cm observations. We create mock Lyman alpha narrow-band, slitless and slit spectroscopic surveys, using benchmarks from instruments such as Subaru HyperSupremeCam, Roman grism, VLT MOONS, ELT MOSAIC, and JWST NIRCam. We forecast the resulting S/N of the galaxy-21-cm cross power spectrum, varying the galaxy survey area, depth and level of 21-cm foreground contamination for each pair of instruments. We find that the highest S/N is achievable through slitless, wide-area spectroscopic surveys, with the proposed Roman HLS survey resulting in a 55$σ$ detection of the cross power with 21-cm as observed with SKA-low AA* for our fiducial model. Narrow-band dropout surveys are unlikely to result in a detectable cross-power, due to their poor redshift localization. Slit spectroscopy can provide a high S/N detection of the cross power for SKA-low AA* observations. Specifically, the planned MOONRISE survey with MOONS on the VLT can result in a 3$σ$ detection, while a survey of comparable observing time using MOSAIC on the ELT can result in a 4$σ$ detection. Our results can be used to guide survey strategies, facilitating the detection of the galaxy-21-cm cross power spectrum.

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arXiv2025

Efficient simulation of discrete galaxy populations and associated radiation fields during the first billion years

Understanding the epochs of cosmic dawn and reionisation requires us to leverage multi-wavelength and multi-tracer observations, with each dataset providing a complimentary piece of the puzzle. To interpret such data, we update the public simulation code, 21cmFASTv4, to include a discrete source model based on stochastic sampling of conditional mass functions and semi-empirical galaxy relations. We demonstrate that our new galaxy model, which parametrizes the means and scatters of well-established scaling relations, is flexible enough to characterize very different predictions from hydrodynamic cosmological simulations of high-redshift galaxies. Combining a discrete galaxy population with approximate, efficient radiative transfer allows us to self-consistently forward-model galaxy surveys, line intensity maps (LIMs), and observations of the intergalactic medium (IGM). Not only does each observable probe different scales and physical processes, but cross-correlation will maximise the information gained from each measurement by probing the galaxy-IGM connection at high-redshift. We find that a stochastic source field produces significant shot-noise in 21cm and LIM power spectra. Scatter in galaxy properties can be constrained using UV luminosity functions and/or 21cm power spectra, especially if astrophysical scatter is higher than expected (as might be needed to explain recent JWST observations). Our modelling pipeline is both flexible and computationally efficient, facilitating high-dimensional, multi-tracer, field-level Bayesian inference of cosmology and astrophysics during the first billion years.

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arXiv2025

SKA-ESO Synergies for the Epoch of Reionization and Cosmic Dawn

Mapping out the first billion years using the 21-cm line with the Square Kilometer Array (SKA) will revolutionize our understanding of the cosmic dawn, reionization and the galaxies that drove these milestones. However, synergies with other telescopes in the form of cross correlations will be fundamental in making and confirming initial, low signal-to-noise claims of a detection. Participants of the 2023 European Southern Observatory (ESO) - SKA Observatory workshop discussed such synergies for Epoch of Reionization (EoR) and Cosmic Dawn (CD) science. Here we highlight some of the most promising candidates for cross-correlating SKA EoR/CD observations with ESO instruments such as the Multi-Object Optical and Near-infrared Spectrograph (MOONS), the MOSAIC multi-object spectrograph, and the ArmazoNes high Dispersion Echelle Spectrograph (ANDES).

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arXiv2024

The importance of stochasticity in determining galaxy emissivities and UV LFs during cosmic dawn and reionization

The stochastic nature of star formation and photon propagation in high-redshift galaxies can result in sizable galaxy-to-galaxy scatter in their properties. Ignoring this scatter by assuming mean quantities can bias estimates of their emissivity and corresponding observables. We construct a flexible, semi-empirical model, sampling scatter around the following mean relations: (i) the conditional halo mass function (CHMF); (ii) the stellar-to-halo mass relation (SHMR); (iii) galaxy star formation main sequence (SFMS); (iv) fundamental metallicity relation (FMR); (v) conditional intrinsic luminosity; and (vi) photon escape fraction. In our fiducial model, ignoring scatter in these galaxy properties overestimates the duration of the EoR, delaying its completion by up to $Δz$ ~ 2. We quantify the relative importance of each of the above sources of scatter in determining the ionizing, soft-band X-ray and Lyman Werner (LW) emissivities as a function of scale and redshift. We find that scatter around the SFMS is important for all bands, especially at the highest redshifts where the emissivity is dominated by the faintest, most "bursty" galaxies. Ignoring this scatter would underestimate the mean emissivity and its standard deviation computed over 5 cMpc regions by factors of up to $\sim$2-10 at $5< z < 15$. Scatter around the X-ray luminosity to star formation rate relation is important for determining X-ray emissivity, accounting for roughly half of its mean and standard deviation. The importance of scatter in the ionizing escape fraction depends on its functional form, while scatter around the SHMR contributes at the level of ~10-20%. Although scatter does flatten the UV luminosity functions, shifting the bright end by 1-2 magnitudes, the level of scatter in our fiducial model is insufficient to fully explain recent estimates from JWST photometry (consistent with previous studies).

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arXiv2023

Efficient Reionization in a Large Hydrodynamic Galaxy Formation Simulation

Accuracy in the topology and statistics of a simulated Epoch of Reionization (EoR) are vital to draw connections between observations and physical processes. While full radiative transfer models produce the most accurate reionization models, they are highly computationally expensive, and are infeasible for the largest cosmological simulations. Instead, large simulations often include EoR models that are pre-computed via the initial density field, or post-processed where feedback effects are ignored. We introduce Astrid-ES, a resimulation of the Astrid epoch of reionisation $20 > z > 5.5$ which includes an on-the-fly excursion-set reionization algorithm. Astrid-ES produces more accurate reionization histories without significantly impacting the computational time. This model directly utilises the star particles produced in the simulation to calculate the EoR history and includes a UV background which heats the gas particles after their reionization. We contrast the reionization topology and statistics in Astrid-ES with the previously employed parametric reionisation model, finding that in Astrid-ES, ionised regions are more correlated with galaxies, and the 21cm power-spectrum shows an increase in large scale power. We calculate the relation between the size of HII regions and the UV luminosity of the brightest galaxy within them. Prior to the overlap phase, we find a power-law fit of $\mathrm{log} (R) = -0.314 M_\mathrm{UV} - 2.550 \mathrm{log}(1+z) + 7.408$ with a standard deviation $σ_R < 0.15 \mathrm{dex}$ across all mass bins. We also examine the properties of halos throughout reionization, finding that while the properties of halos in the simulation are correlated with the redshift of reionisation, they are not greatly affected by reionisation itself.

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arXiv2019

Dark-Ages Reionisation & Galaxy Formation Simulation XVI: The Thermal Memory of Reionisation

Intergalactic medium temperature is a powerful probe of the epoch of reionisation, as information is retained long after reionisation itself. However, mean temperatures are highly degenerate with the timing of reionisation, with the amount heat injected during the epoch, and with the subsequent cooling rates. We post-process a suite of semi-analytic galaxy formation models to characterise how different thermal statistics of the intergalactic medium can be used to constrain reionisation. Temperature is highly correlated with redshift of reionisation for a period of time after the gas is heated. However as the gas cools, thermal memory of reionisation is lost, and a power-law temperature-density relation is formed, $T = T_0(1+δ)^{1-γ}$ with $γ\approx 1.5$. Constraining our model against observations of electron optical depth and temperature at mean density, we find that reionisation likely finished at $z_{\rm{reion}} = 6.8 ^{+ 0.5} _{-0.8}$ with a soft spectral slope of $α= 2.8 ^{+ 1.2} _{-1.0}$. By restricting spectral slope to the range $[0.5,2.5]$ motivated by population II synthesis models, reionisation timing is further constrained to $z_{\rm{reion}} = 6.9 ^{+ 0.4} _{-0.5}$. We find that, in the future, the degeneracies between reionisation timing and background spectrum can be broken using the scatter in temperatures and integrated thermal history.

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arXiv2016

Metallicity and Age of the Stellar Stream Around the Disk Galaxy NGC 5907

Stellar streams have become central to studies of the interaction histories of nearby galaxies. To characterize the most prominent parts of the stellar stream around the well-known nearby (d = 17 Mpc) edge-on disk galaxy NGC 5907, we have obtained and analyzed new, deep gri Subaru/Suprime-Cam and 3.6 micron Spitzer/Infrared Array Camera (IRAC) observations. Combining the near-infrared 3.6 micron data with visible-light images allows us to use a long wavelength baseline to estimate the metallicity and age of the stellar population along a ~60 kpc long segment of the stream. We have fitted the stellar spectral energy distribution (SED) with a single-burst stellar population synthesis model and we use it to distinguish between the proposed satellite accretion and minor/major merger formation models of the stellar stream around this galaxy. We conclude that a massive minor merger (stellar mass ratio of at least 1:8) can best account for the metallicity of -0.3 inferred along the brightest parts of the stream.

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arXiv2013

A Survey for Planetary Nebulae in M31 Globular Clusters

We report the results of an [O III] 5007 spectroscopic survey for planetary nebulae (PNe) located within the star clusters of M31. By examining R ~ 5000 spectra taken with the WIYN+Hydra spectrograph, we identify 3 PN candidates in a sample of 274 likely globular clusters, 2 candidates in objects which may be globular clusters, and 5 candidates in a set of 85 younger systems. The possible PNe are all faint, between ~2.5 and ~6.8 mag down the PN luminosity function, and, partly as a consequence of our selection criteria, have high excitation, with [O III] 5007 to H-beta ratios ranging from 2 to ~12. We discuss the individual candidates, their likelihood of cluster membership, and the possibility that they were formed via binary interactions within the clusters. Our data are consistent with the suggestion that PN formation within globular clusters correlates with binary encounter frequency, though, due to the small numbers and large uncertainties in the candidate list, this study does not provide sufficient evidence to confirm the hypothesis.

astro-ph.CO↗

arXiv2010

A Detailed Look at Chemical Abundances in Magellanic Cloud Planetary Nebulae. I. The Small Magellanic Cloud

We present an analysis of elemental abundances of He, N, O, Ne, S, and Ar in Magellanic Cloud planetary nebulae (PNe), and focus initially on 14 PNe in the Small Magellanic Cloud (SMC). We derived the abundances from a combination of deep, high dispersion optical spectra, as well as mid-infrared (IR) spectra from the Spitzer Space Telescope. A detailed comparison with prior SMC PN studies shows that significant variations among authors of relative emission line flux determinations lead to systematic discrepancies in derived elemental abundances between studies that are >~0.15 dex, in spite of similar analysis methods. We used ionic abundances derived from IR emission lines, including those from ionization stages not observable in the optical, to examine the accuracy of some commonly used recipes for ionization correction factors (ICFs). These ICFs, which were developed for ions observed in the optical and ultraviolet, relate ionic abundances to total elemental abundances. We find that most of these ICFs work very well even in the limit of substantially sub-Solar metallicities, except for PNe with very high ionization. Our abundance analysis shows enhancements of He and N that are predicted from prior dredge-up processes of the progenitors on the AGB, as well as the well known correlations among O, Ne, S, and Ar that are little affected by nucleosynthesis in this mass range. We identified MG_8 as an interesting limiting case of a PN central star with a ~3.5 M_sun progenitor in which hot-bottom burning did not occur in its prior AGB evolution. We find no evidence for O depletion in the progenitor AGB stars via the O-N cycle, which is consistent with predictions for lower-mass stars. We also find low S/O ratios relative to SMC H_II regions, with a deficit comparable to what has been found for Galactic PNe.

astro-ph.SR↗

arXiv1999

A Survey for Low Surface Brightness Galaxies Around M31. II. The Newly Discovered Dwarf Andromeda VI

We present B-, V-, and I-band images, as well as an H alpha image, of And VI. This is the second newly identified dwarf spheroidal (dSph) companion to M31 found using a digital filtering technique applied to the second Palomar Sky Survey for which 1550 square degrees now have been surveyed. And VI was confirmed to be a nearby dSph galaxy when it resolved into stars easily with a short 4-m V-band exposure. Sub-arcsec images taken at the Kitt Peak WIYN 3.5-m telescope provided (I,V-I) and (V,B-V) color-magnitude diagrams that yield a distance of 775 +/- 35 kpc using the tip of the red giant branch method, and a mean metallicity of [Fe/H] = -1.58 +/- 0.20 with a dispersion of approximately 0.3 dex. And VI has a galactocentric distance of approximately 271 kpc and an absolute V magnitude of -11.3. All observed properties of And VI are consistent with its classification as a dSph companion to M31. Despite the recent identification of the And V, And VI, and Cas dwarfs, the Local Group luminosity function remains highly deficient in faint galaxies relative both to CDM simulations of its formation, and to the luminosity functions for richer clusters of galaxies.

astro-ph↗

arXiv1998

A Survey for Low Surface Brightness Dwarf Galaxies Around M31

By applying a digital filtering technique to 1550 square degrees of the POSS-II in the vicinity of M31, we found two previously unidentified very low surface brightness dwarf galaxies which we designate And V and VI. Follow-up imaging with the KPNO 4-m telescope resolved these into stars easily. The V- and I- band images of And V indicate a distance similar to that of M31, and <[Fe/H]> approximately -1.5. All evidence strongly supports its classification as a dwarf spheroidal companion to M31. Data for And VI are being analyzed, but preliminary indications support a similar conclusion. Our search for more dwarfs, including follow-up observations of numerous candidates found via digital filtering, is incomplete; thus, further identifications may be forthcoming.

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arXiv1998

A Survey for Low-Surface-Brightness Galaxies Around M31. I. The Newly Discovered Dwarf Andromeda V

We present images and a color-magnitude diagram for And V, a new dwarf spheroidal companion to M31 that was found using a digital filtering technique applied to 1550 square degrees of the second Palomar Sky Survey. And V resolves into stars easily in follow-up 4-m V- and I-band images, from which we deduce a distance of 810 +/- 45 kpc using the tip of the red giant branch method. Within the uncertainties, this distance is identical to the Population II distances for M31 and, combined with a projected separation of 112 kpc, provides strong support for a physical association between the two galaxies. There is no emission from And V detected in H alpha, 1.4 GHz radio continuum, or IRAS bandpasses, and there is no young population seen in the color-magnitude diagram that might suggest that And V is an irregular. Thus, the classification as a new dwarf spheroidal member of the Local Group seems secure. With an extinction-corrected central surface brightness of 25.2 V mag per square arcsec, a mean metal abundance of [Fe/H] approximately -1.5, and no evidence for upper AGB stars, And V resembles And I & III.

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