arXiv subjects

T. Harvey

Publications and source records attributed to T. Harvey.

6 recordsLinked to original sources

arXiv2025

ALMA Band 3 Selection of Ultra-high Redshift Dropouts: The final challenge to {\Lambda}CDM

The Lyman-break technique has been used to successfully identify high-redshift candidates in broad-band photometric data in the rest-frame optical and NIR using the dropout technique. We pioneer the application of this technique to new wavelength regimes, and search for dropouts in combined ALMA and JWST data. We find a candidate that is undetected in NIRCam imaging including and blueward of the F444W filter, but clearly identified in ALMA band 3. Assuming this is a Lyman-break candidate, we measure a redshift in the range $40 < z < 21\,380$. This is the highest redshift galaxy candidate discovered to date, and is in significant tension with current and future predictions from cosmological simulations, with implications for galaxy evolution in the (very) early Universe.

astro-ph.GA↗

arXiv2025

Little red dots as young supermassive black holes in dense ionized cocoons

The James Webb Space Telescope (JWST) has uncovered many compact galaxies at high redshift with broad hydrogen and helium lines, including the enigmatic population of little red dots (LRDs). The nature of these galaxies is debated and is attributed to supermassive black holes (SMBHs) or intense star formation. They exhibit unusual properties for SMBHs, such as black holes that are overmassive for their host galaxies and extremely weak X-ray and radio emission. Here we show that in most objects studied with the highest-quality JWST spectra, the lines are broadened by electron scattering with a narrow intrinsic core. The data require very high electron column densities and compact sizes (light days), which, when coupled with their high luminosities, can be explained only by SMBH accretion. The narrow intrinsic line cores imply black hole masses of $10^{5-7}$ $M_{\odot}$, two orders of magnitude lower than previous estimates. These are the lowest mass black holes known at high redshift, to our knowledge, and suggest a population of young SMBHs. They are enshrouded in a dense cocoon of ionized gas producing broad lines from which they are accreting close to the Eddington limit, with very mild neutral outflows. Reprocessed nebular emission from this cocoon dominates the optical spectrum, explaining most LRD spectral characteristics, including the weak radio and X-ray emission.

astro-ph.GA↗

arXiv2025

The impact of medium-width bands on the selection, and subsequent luminosity function measurements, of high-z galaxies

New, ultra-deep medium-width photometric coverage with JWST's NIRCam instrument provides the potential for much improved photo-z reliability at high redshifts. In this study, we conduct a systematic analysis of the JADES Origins Field, which contains 14 broad- and medium-width near-infrared bands, to assess the benefits of medium band photometry on high-z completeness and contamination rates. Using imaging reaching AB mag $29.8-30.35$ depth, we test how high-z selections differ when images are artificially degraded or bands are removed. In parallel, the same experiments are conducted on simulated catalogues from the JAGUAR semi-analytic model to examine if observations can be replicated. We find sample completeness is high ($80\%+$) and contamination low ($<4\%$) when in the $10\sigma+$ regime, even without the use of any medium-width bands. The addition of medium-width bands leads to increases in completeness ($\sim10\%$) but multiple bands are required to improve contamination rates due to the small redshift ranges over which they probe strong emission lines. Incidents of Balmer-Lyman degeneracy increase in the $5-7\sigma$ regime and this can be replicated in both simulated catalogues and degraded real data. We measure the faint-end of the UV LF at $8.5<z<13.5$, finding high number densities that agree with previous JWST observations. Overall, medium bands are effective at increasing completeness and reducing contamination, but investment in achieving comparable depths in the blue ($<1.5\mu$m) as achieved in the red is also found to be key to fully reducing contamination from high-z samples.

astro-ph.GA↗

arXiv2023

EPOCHS VI: The Size and Shape Evolution of Galaxies since z ~ 8 with JWST Observations

We present the results of a size and structural analysis of 1395 galaxies at $0.5 \leq z \lesssim 8$ with stellar masses $\log \left(M_* / M_{\odot}\right)$ $>$ 9.5 within the JWST Public CEERS field that overlaps with the HST CANDELS EGS observations. We use GALFIT to fit single S\'ersic models to the rest-frame optical profile of our galaxies, which is a mass-selected sample complete to our redshift and mass limit. Our primary result is that at fixed rest-frame wavelength and stellar mass, galaxies get progressively smaller, evolving as $\sim (1+z)^{-0.71\pm0.19}$ up to $z \sim 8$. We discover that the vast majority of massive galaxies at high redshifts have low S\'ersic indices, thus do not contain steep, concentrated light profiles. Additionally, we explore the evolution of the size-stellar mass relationship, finding a correlation such that more massive systems are larger up to $z \sim 3$. This relationship breaks down at $z > 3$, where we find that galaxies are of similar sizes, regardless of their star formation rates and S\'ersic index, varying little with mass. We show that galaxies are more compact at redder wavelengths, independent of sSFR or stellar mass up to $z \sim 3$. We demonstrate the size evolution of galaxies continues up to $z \sim 8$, showing that the process or causes for this evolution is active at early times. We discuss these results in terms of ideas behind galaxy formation and evolution at early epochs, such as their importance in tracing processes driving size evolution, including minor mergers and AGN activity.

astro-ph.GA↗

arXiv2023

A Large Population of Faint 8<z<16 Galaxies Found in the First JWST NIRCam Observations of the NGDEEP Survey

We present an early analysis on the search for high redshift galaxies using the deepest public JWST imaging to date, the NGDEEP field. This data consists of 6-band NIRCam imaging on the Hubble Ultra Deep Field-Par2, covering a total area of 6.3 arcmin$^{2}$. Based on our initial reduction of the first half of this survey, we reach 5$\sigma$ depths up to mag = 29.5--29.9 between $1-5$ um. Such depths present an unprecedented opportunity to begin exploring the early Universe with JWST. As such, we find high redshift galaxies in this field by examining the spectral energy distribution of these systems and present 18 new $z > 8$ systems identified using two different photometric redshift codes: LePhare and EAZY, combined with other significance criteria. The highest redshift object in our sample is at $z=15.57^{+0.39}_{-0.38}$, which has a blue beta slope of $\beta=-3.25^{+0.41}_{-0.46}$ and a very low inferred stellar mass of $M_{*} = 10^{7.39}$ M_0,. We also discover a series of faint, low-mass dwarf galaxies with $M_{*} < 10^{8.5}$ M_0 at $z \sim 9$ that have blue colors and UV slopes. The structure of these galaxies is such that they all have very flat surface brightness profiles and small sizes $< 1 \ \mathrm{kpc}$. We also compare our results to theory, finding no significant disagreement with some CDM based models.The discovery of these objects, most of which are low luminosity and inferred stellar mass, demonstrates the power of probing continuously deeper into the Universe, pointing the way to deeper, or similar depth but wider area, surveys and demonstrate the critical need for JWST deep fields to explore this aspect of the early Universe.

astro-ph.GA↗

arXiv2022

Detailed accretion history of the supermassive black hole in NGC 5972 over the past $\gtrsim$10$^4$ years through the extended emission line region

We present integral field spectroscopic observations of NGC 5972 obtained with the Multi Unit Spectroscopic Explorer (MUSE) at VLT. NGC 5972 is a nearby galaxy containing both an active galactic nucleus (AGN), and an extended emission line region (EELR) reaching out to $\sim 17$ kpc from the nucleus. We analyze the physical conditions of the EELR using spatially-resolved spectra, focusing on the radial dependence of ionization state together with the light travel time distance to probe the variability of the AGN on $\gtrsim 10^{4}$ yr timescales. The kinematic analysis suggests multiple components: (a) a faint component following the rotation of the large scale disk; (b) a component associated with the EELR suggestive of extraplanar gas connected to tidal tails; (c) a kinematically decoupled nuclear disk. Both the kinematics and the observed tidal tails suggest a major past interaction event. Emission line diagnostics along the EELR arms typically evidence Seyfert-like emission, implying that the EELR was primarily ionized by the AGN. We generate a set of photoionization models and fit these to different regions along the EELR. This allows us to estimate the bolometric luminosity required at different radii to excite the gas to the observed state. Our results suggests that NGC 5972 is a fading quasar, showing a steady gradual decrease in intrinsic AGN luminosity, and hence the accretion rate onto the SMBH, by a factor $\sim 100$ over the past $5 \times 10^{4}$ yr.

astro-ph.GA↗