arXiv subjects

Michael Topping

Publications and source records attributed to Michael Topping.

11 recordsLinked to original sources

arXiv2025

Constraints on the $z\sim6-13$ intergalactic medium from JWST spectroscopy of Lyman-alpha damping wings in galaxies

JWST provides a unique dataset to study reionization's earliest stages, promising insights into the first galaxies. Many JWST/NIRSpec prism spectra reveal smooth Lyman-alpha breaks in z>5 galaxies, implying damping wing scattering by neutral hydrogen. We investigate what current prism spectra imply about the intergalactic medium (IGM), and how best to use NIRSpec spectra to recover IGM properties. We use a sample of 99 z~5.5-13 galaxies with high S/N prism spectra in the public archive, including 12 at z>10. We analyse these spectra using damping wing sightlines from inhomogeneous reionizing IGM simulations, mapping between the distance of a source from neutral IGM and the mean IGM neutral fraction. We marginalise over absorption by local HI around galaxies, and Lyman-alpha emission. We observe a decline in the median and variance of flux around the Lyman-alpha break with increasing redshift, consistent with an increasingly neutral IGM, as ionized regions become smaller and rarer. At $z\gtrsim9$ the spectra become consistent with an almost fully neutral IGM. We find S/N>15 per pixel is required to robustly estimate IGM properties from prism spectra. We fit a sub-sample of high S/N spectra and infer mean neutral fractions $\overline{x}_\mathrm{HI}=0.33^{+0.18}_{-0.27}, 0.64^{+0.17}_{-0.23}$ ($>0.70$ excluding GNz11) at $z \approx 6.5, 9.3$. We also investigate local HI absorption, finding median column density $\log_{10}N_\mathrm{HI}\approx10^{20.8}$ cm$^{-2}$, comparable to $z\sim3$ Lyman-break galaxies, with no significant redshift evolution. We find galaxies showing the strongest absorption are more likely to be in close associations (<500 pkpc), implying enhanced absorption in massive dark matter halos. Future deep prism and grating spectroscopy of z>9 sources will provide tighter constraints on the earliest stages of reionization, key for understanding the onset of star formation.

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arXiv2024

Ly$\alpha$ emission in galaxies at $z\simeq5-6$: new insight from JWST into the statistical distributions of Ly$\alpha$ properties at the end of reionization

JWST has recently sparked a new era of Ly$\alpha$ spectroscopy, delivering the first measurements of the Ly$\alpha$ escape fraction and velocity profile in typical galaxies at $z\simeq6-10$. These observations offer new prospects for insight into the earliest stages of reionization. But to realize this potential, we need robust models of Ly$\alpha$ properties in galaxies at $z\simeq5-6$ when the IGM is mostly ionized. Here we use new JWST observations from the JADES and FRESCO surveys combined with VLT/MUSE and Keck/DEIMOS data to characterize statistical distributions of Ly$\alpha$ velocity offsets, escape fractions, and EWs in $z\simeq5-6$ galaxies. We find that galaxies with large Ly$\alpha$ escape fractions (> 0.2) are common at $z\simeq5-6$, comprising 30 per cent of Lyman break selected samples. Comparing to literature studies, our census suggests that Ly$\alpha$ becomes more prevalent in the galaxy population toward higher redshift from $z\sim3$ to $z\sim6$, although we find that this evolution slows considerably between $z\sim5$ and $z\sim6$, consistent with modest attenuation from residual HI in the mostly ionized IGM at $z\simeq5-6$. We find significant evolution in Ly$\alpha$ velocity profiles between $z\simeq2-3$ and $z\simeq5-6$, likely reflecting the influence of resonant scattering from residual intergalactic HI on the escape of Ly$\alpha$ emission near line center. This effect will make it challenging to use Ly$\alpha$ peak offsets as a probe of Lyman continuum leakage at $z\simeq5-6$. We use our $z\simeq5-6$ Ly$\alpha$ distributions to make predictions for typical Ly$\alpha$ properties at $z\gtrsim8$ and discuss implications of a recently-discovered Ly$\alpha$ emitter at $z\simeq8.5$ with a small peak velocity offset (156 km s$^{-1}$).

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arXiv2023

JADES Initial Data Release for the Hubble Ultra Deep Field: Revealing the Faint Infrared Sky with Deep JWST NIRCam Imaging

JWST has revolutionized the field of extragalactic astronomy with its sensitive and high-resolution infrared view of the distant universe. Adding to the new legacy of JWST observations, we present the first NIRCam imaging data release from the JWST Advanced Deep Extragalactic Survey (JADES) providing 9 filters of infrared imaging of $\sim$25 arcmin$^2$ covering the Hubble Ultra Deep Field and portions of Great Observatories Origins Deep Survey (GOODS) South. Utilizing 87 on-sky dual-filter hours of exposure time, these images reveal the deepest ever near-infrared view of this iconic field. We supply carefully constructed 9-band mosaics of the JADES bands, as well as matching reductions of 5 additional bands from the JWST Extragalactic Medium-band Survey (JEMS). Combining with existing HST imaging, we provide 23-band space-based photometric catalogs and photometric redshifts for $\approx47,500$ sources. To promote broad engagement with the JADES survey, we have created an interactive {\tt FitsMap} website to provide an interface for professional researchers and the public to experience these JWST datasets. Combined with the first JADES NIRSpec data release, these public JADES imaging and spectroscopic datasets provide a new foundation for discoveries of the infrared universe by the worldwide scientific community.

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arXiv2023

Cold Dust and Low [OIII]/[CII] Ratios: an Evolved Star-forming Population at Redshift 7

We present new ALMA Band 8 (rest-frame $90\,\mu$m) observations of three massive ($M_\star \approx 10^{10}\,M_\odot$) galaxies at $z\approx7$ previously detected in [CII]$158\,\mu$m and underlying dust continuum emission in the Reionization Era Bright Emission Line Survey (REBELS). We detect the dust continuum emission of two of our targets in Band 8 (REBELS-25 and REBELS-38), while REBELS-12 remains undetected. Through modified blackbody fitting we determine cold dust temperatures ($T_\mathrm{dust} \approx 30 - 35\,$K) in both of the dual-band detected targets, given a fiducial model of optically thin emission with $\beta = 2.0$. Their dust temperatures are lower than most $z\sim7$ galaxies in the literature, and consequently their dust masses are higher ($M_\mathrm{dust} \approx 10^{8}\,M_\odot$). Nevertheless, these large dust masses are still consistent with predictions from models of dust production in the early Universe. In addition, we target and detect [OIII]$88\,\mu$m emission in both REBELS-12 and REBELS-25, and find $L_\mathrm{[OIII]} / L_\mathrm{[CII]}$ ratios of approximately unity, low compared to the $L_\mathrm{[OIII]} / L_\mathrm{[CII]} \gtrsim 2 - 10$ observed in the known $z\gtrsim6$ population thus far. We argue the lower line ratios are due to a comparatively weaker ionizing radiation field resulting from the less starbursty nature of our targets. This low burstiness supports the cold dust temperatures and below average $\mathrm{[OIII]}\lambda\lambda4959,5007 + \mathrm{H}\beta$ equivalent widths of REBELS-25 and REBELS-38, compared to the known high-redshift population. Overall, this provides evidence for the existence of a massive, dust-rich galaxy population at $z\approx7$ which has previously experienced vigorous star formation, but is currently forming stars in a steady, as opposed to bursty, manner.

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arXiv2023

JWST/NIRSpec Spectroscopy of $z=7-9$ Star Forming Galaxies with CEERS: New Insight into Bright Ly$\alpha$ Emitters in Ionized Bubbles

We describe new JWST/NIRSpec observations of galaxies at $z\gtrsim7$ taken from the CEERS survey. Previous observations of this area have revealed associations of Ly$\alpha$ emitters at redshifts ($z=7.5$, $7.7$, $8.7$) where the intergalactic medium (IGM) is thought to be mostly neutral, leading to suggestions that these systems are situated in large ionized bubbles. We identify 21 $z\gtrsim7$ galaxies with robust redshifts in the CEERS dataset, including 10 in the Ly$\alpha$ associations. Their spectra are indicative of very highly ionized and metal poor gas, with line ratios (O32 $=17.84$ and Ne3O2 $=0.89$, linear scale) and metallicity ($12+\log{(\rm{O/H})}=7.84$) that are rarely seen at lower redshifts. We find that the most extreme spectral properties are found in the six $z\gtrsim7$ Ly$\alpha$ emitters in the sample. Each has a hard ionizing spectrum indicating that their visibility is likely enhanced by efficient ionizing photon production. Ly$\alpha$ velocity offsets are found to be very large ($\gtrsim300$ km s$^{-1}$), likely also contributing to their detectability. We find that Ly$\alpha$ in $z\gtrsim7$ galaxies is $6-12\times$ weaker than in lower redshift samples with matched rest-optical spectral properties. If the bubbles around the Ly$\alpha$ emitters are relatively small ($\lesssim0.5-1$ pMpc), we may expect such significant attenuation of Ly$\alpha$ in these ionized regions. We discuss several other effects that may contribute to weaker Ly$\alpha$ emission at $z\gtrsim7$. Deep spectroscopy of fainter galaxies in the vicinity of the Ly$\alpha$ emitters will better characterize the physical scale of the ionized bubbles in this field.

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arXiv2022

The ALMA REBELS Survey: The Dust-obscured Cosmic Star Formation Rate Density at Redshift 7

Cosmic dust is an essential component shaping both the evolution of galaxies and their observational signatures. How quickly dust builds up in the early Universe remains an open question that requires deep observations at (sub-)millimeter wavelengths to resolve. Here we use Atacama Large Millimeter Array observations of 45 galaxies from the Reionization Era Bright Emission Line Survey (REBELS) and its pilot programs, designed to target [CII] and dust emission in UV-selected galaxies at $z\sim7$, to investigate the dust content of high-redshift galaxies through a stacking analysis. We find that the typical fraction of obscured star formation $f_\mathrm{obs} = \mathrm{SFR}_\mathrm{IR} / \mathrm{SFR}_\mathrm{UV + IR}$ depends on stellar mass, similar to what is observed at lower redshift, and ranges from $f_\mathrm{obs} \approx 0.3 - 0.6$ for galaxies with $\log_{10}\left(M_\star / M_\odot\right) = 9.4 - 10.4$. We further adopt the $z\sim7$ stellar mass function from the literature to extract the obscured cosmic star formation rate density (SFRD) from the REBELS survey. Our results suggest only a modest decrease in the SFRD between $3\lesssim z \lesssim 7$, with dust-obscured star formation still contributing $\sim30\%$ at $z\sim7$. While we extensively discuss potential caveats, our analysis highlights the continued importance of dust-obscured star formation even well into the epoch of reionization.

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arXiv2022

On the ages of bright galaxies $\sim 500$ Myr after the Big Bang: insights into star formation activity at $z \gtrsim 15$ with JWST

With JWST, new opportunities to study the evolution of galaxies in the early Universe are emerging. Spitzer constraints on rest-optical properties of $z\gtrsim7$ galaxies demonstrated the power of using galaxy stellar masses and star formation histories (SFHs) to indirectly infer the cosmic star formation history. However, only the brightest individual $z\gtrsim8$ objects could be detected with Spitzer, making it difficult to robustly constrain activity at $z\gtrsim10$. Here, we leverage the greatly improved rest-optical sensitivity of JWST at $z\gtrsim8$ to constrain the ages of seven UV-bright ($M_{UV}\lesssim-19.5$) galaxies selected to lie at $z\sim8.5-11$, then investigate implications for $z\gtrsim15$ star formation. We infer the properties of individual objects with two spectral energy distribution modelling codes, then infer a distribution of ages for bright $z\sim8.5-11$ galaxies. We find a median age of $\sim20$ Myr, younger than that inferred at $z\sim7$ with a similar analysis, consistent with an evolution towards larger specific star formation rates at early times. The age distribution suggests that only $\sim3$ percent of bright $z\sim8.5-11$ galaxies would be similarly luminous at $z\gtrsim15$, implying that the number density of bright galaxies declines by at least an order of magnitude between $z\sim8.5-11$ and $z\sim15$. This evolution is challenging to reconcile with some early JWST results suggesting the abundance of bright galaxies does not significantly decrease towards very early times, but we suggest this tension may be eased if young stellar populations form on top of older stellar components, or if bright $z\sim15$ galaxies are observed during a burst of star formation.

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arXiv2022

JWST/NIRCam Observations of Stars and HII Regions in $z\simeq 6-8$ Galaxies: Properties of Star Forming Complexes on 150 pc Scales

The onset of the {\it JWST}-era provides a much-improved opportunity to characterize the resolved structure of early star forming systems. Previous {\it Spitzer} observations of $z\gtrsim 6$ galaxies revealed the presence of old stars and luminous HII regions (via [OIII]+H$\beta$ emission), but the poor resolution stunted our ability to map their locations with respect to the star forming regions identified in the rest-UV. In this paper, we investigate the internal structure of 12 of the most luminous $z\simeq 6-8$ galaxies in the EGS field observed with recent {\it JWST}/NIRCam imaging. The systems appear clumpy in the rest-UV, with more than half of the light coming from $\simeq 10^7$ to 10$^{9}$ M$_\odot$ star forming complexes that are $\simeq 150$ - 480 pc in size. The clumps tend to be dominated by young stars (median = 36 Myr), but we also find large variations in clump ages within individual galaxies. The [OIII]+H$\beta$ EW varies significantly across individual galaxies (reflecting differences in stellar and gas properties), but the HII regions largely track the UV-bright complexes. Perhaps surprisingly, the rest-optical continuum is just as clumpy as the UV, and we do not find older (and redder) nuclear stellar components that were previously undetected or faint in the UV. The majority of the stellar mass in bright $6<z<8$ galaxies appears to be contained in the $\gtrsim 150$ pc-scale clumpy star forming complexes, reflecting the very active phase of assembly that is common in reionization-era galaxies.

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arXiv2022

The ALMA REBELS Survey: Dust Continuum Detections at z > 6.5

We report 18 dust continuum detections ($\geq 3.3\sigma$) at $\sim88{\rm \mu m}$ and $158{\rm \mu m}$ out of 49 ultraviolet(UV)-bright galaxies ($M_{\rm UV} < -21.3$ mag) at $z>6.5$, observed by the Cycle-7 ALMA Large Program, REBELS and its pilot programs. This has more than tripled the number of dust continuum detections known at $z>6.5$. Out of these 18 detections, 12 are reported for the first time as part of REBELS. In addition, 15 of the dust continuum detected galaxies also show a [CII]$_{\rm 158{\rm \mu m}}$ emission line, providing us with accurate redshifts. We anticipate more line emission detections from six targets (including three continuum detected targets) where observations are still ongoing. The dust continuum detected sources in our sample tend to have a redder UV spectral slope than the ones without a dust continuum detection. We estimate that all of the sources have an infrared (IR) luminosity ($L_{\rm IR}$) in a range of $3-8 \times 10^{11} L_\odot$, except for one with $L_{\rm IR} = 1.5^{+0.8}_{-0.5} \times 10^{12}\,L_{\odot}$. Their fraction of obscured star formation is significant at $\gtrsim 50\%$. Some of the dust continuum detected galaxies show spatial offsets ($\sim 0.5-1.5''$) between the rest-UV and far-IR emission peaks. These separations appear to have an increasing trend against an indicator that suggests spatially decoupled phases of obscured and unobscured star formation. REBELS offers the best available statistical constraints on obscured star formation in UV-bright, massive galaxies at $z > 6.5$.

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arXiv2022

The MOSDEF-LRIS Survey: Connection between Galactic-scale Outflows and the Properties of $z$~2 star-forming Galaxies

We investigate the conditions that facilitate galactic-scale outflows using a sample of 155 typical star-forming galaxies at $z$~2 drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey. The sample includes deep rest-frame UV spectroscopy from the Keck Low-Resolution Imaging Spectrometer (LRIS), which provides spectral coverage of several low-ionisation interstellar (LIS) metal absorption lines and Ly$\alpha$ emission. Outflow velocities are calculated from the centroids of the LIS absorption and/or Ly$\alpha$ emission, as well as the highest-velocity component of the outflow from the blue wings of the LIS absorption lines. Outflow velocities are found to be marginally correlated or independent of galaxy properties, such as star-formation rate (SFR) and star-formation rate surface density ($\Sigma_{\rm SFR}$). Outflow velocity scales with SFR as a power-law with index 0.24, which suggests that the outflows may be primarily driven by mechanical energy generated by supernovae explosions, as opposed to radiation pressure acting on dusty material. On the other hand, outflow velocity and $\Sigma_{\rm SFR}$ are not significantly correlated, which may be due to the limited dynamic range of $\Sigma_{\rm SFR}$ probed by our sample. The relationship between outflow velocity and $\Sigma_{\rm SFR}$ normalised by stellar mass ($\Sigma_{\rm sSFR}$), as a proxy for gravitational potential, suggests that strong outflows (e.g., > 200 km s$^{-1}$) appear ubiquitous above a threshold of log($\Sigma_{\rm sSFR}/\rm{yr}^{-1}\ \rm{kpc}^{-2}$) ~ -11.3, and that above this threshold, outflow velocity uncouples from $\Sigma_{\rm sSFR}$. These results highlight the need for higher resolution spectroscopic data and spatially resolved imaging to test the driving mechanisms of outflows predicted by theory.

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arXiv2022

The ALMA REBELS Survey: Efficient Ly$\alpha$ Transmission of UV-Bright z$\simeq$7 Galaxies from Large Velocity Offsets and Broad Line Widths

Recent work has shown that UV-luminous reionization-era galaxies often exhibit strong Lyman-alpha emission despite being situated at redshifts where the IGM is thought to be substantially neutral. It has been argued that this enhanced Ly$\alpha$ transmission reflects the presence of massive galaxies in overdense regions which power large ionized bubbles. An alternative explanation is that massive galaxies shift more of their Ly$\alpha$ profile to large velocities (relative to the systemic redshift) where the IGM damping wing absorption is reduced. Such a mass-dependent trend is seen at lower redshifts, but whether one exists at $z\sim7$ remains unclear owing to the small number of existing systemic redshift measurements in the reionization era. This is now changing with the emergence of [CII]-based redshifts from ALMA. Here we report MMT/Binospec Ly$\alpha$ spectroscopy of eight UV-bright ($\mathrm{M_{UV}}^{}\sim-22$) galaxies at $z\simeq7$ selected from the ALMA REBELS survey. We detect Ly$\alpha$ in 4 of 8 galaxies and use the [CII] systemic redshifts to investigate the Ly$\alpha$ velocity profiles. The Ly$\alpha$ lines are significantly redshifted from systemic (average velocity offset=223 km/s) and broad (FWHM$\approx$300$-$650 km/s), with two sources showing emission extending to $\approx$750 km/s. We find that the broadest Ly$\alpha$ profiles are associated with the largest [CII] line widths, suggesting a potential link between the Ly$\alpha$ FWHM and the dynamical mass. Since Ly$\alpha$ photons at high velocities transmit efficiently through the $z=7$ IGM, our data suggest that velocity profiles play a significant role in boosting the Ly$\alpha$ visibility of the most UV-luminous reionization-era galaxies.

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