arXiv subjects

Anthony J. Pahl

Publications and source records attributed to Anthony J. Pahl.

23 records · Page 2Linked to original sources

arXiv2022

An uncontaminated measurement of the escaping Lyman continuum at $z\sim3$

Observations of reionization-era analogs at $z\sim3$ are a powerful tool for constraining reionization. Rest-ultraviolet observations are particularly useful, in which both direct and indirect tracers of ionizing-photon production and escape can be observed. We analyse a sample of 124 $z\sim3$ galaxies from the Keck Lyman Continuum Spectroscopic Survey, with sensitive spectroscopic measurements of the Lyman continuum (LyC) region. We present a method of removing foreground contamination from our sample using high-resolution, multiband Hubble Space Telescope imaging. We re-measure the global properties of the cleaned sample of 13 individually-detected LyC sources and 107 individually-undetected sources, including a sample-averaged absolute escape fraction of $f_{esc,abs}=0.06\pm0.01$ and a sample-averaged ratio of ionizing to non-ionizing ultraviolet flux density of $ _{out}=0.040\pm0.006$, corrected for attenuation from the intergalactic and circumgalactic media. Based on composite spectra, we also recover a strong positive correlation between $ _{out}$ and Ly$α$ equivalent width (W$_λ$(Ly$α$)) and a negative correlation between $ _{out}$ and UV luminosity. As in previous work, we interpret the relationship between $ _{out}$ and W$_λ$(Ly$α$) in terms of the modulation of the escape of ionizing radiation from star-forming galaxies based on the covering fraction of neutral gas. We also use a W$_λ$(Ly$α$)-weighted $ _{out}$ to estimate an ionizing emissivity from star-forming galaxies at $z\sim3$ as $ε_{\rm LyC}\simeq5.5\times10^{24}$erg s$^{-1}$Hz$^{-1}$Mpc$^{-3}$. This estimate, evaluated using the uncontaminated sample of this work, affirms that the contribution of galaxies to the ionizing background at $z\sim3$ is comparable to that of active galactic nuclei.

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arXiv2022

Searching for the connection between ionizing-photon escape and the surface density of star formation at z~3

The connection between the escape fraction of ionizing photons ($f_{\rm esc}$) and star-formation rate surface density ($Σ_{\rm SFR}$) is a key input for reionization models, but remains untested at high redshift. We analyse 35 z~3 galaxies from the Keck Lyman Continuum Survey (KLCS) covered by deep, rest far-UV spectra of the Lyman continuum (LyC) and high-resolution HST V$_{606}$ imaging, enabling estimates of both $f_{\rm esc}$ and rest-UV sizes. Using Sérsic profile fits to HST images and spectral-energy distribution fits to multi-band photometry, we measure effective sizes and star-formation rates for the galaxies in our sample, and separate the sample into two bins of $Σ_{\rm SFR}$. Based on composite spectra, we estimate <$f_{\rm esc}$> for both $Σ_{\rm SFR}$ subsamples, finding no significant difference in <$f_{\rm esc}$> between the two. To test the representativeness of the KLCS HST sample and the robustness of this result, we attempt to recover the well-established correlation between $f_{\rm esc}$ and Ly$α$ equivalent width. This correlation is not significant within the KLCS HST sample, indicating that the sample is not sufficient for correlating $f_{\rm esc}$ and galaxy properties such as $Σ_{\rm SFR}$. We perform stacking simulations using the KLCS parent sample to determine the optimal sample size for robust probes of the $f_{\rm esc}$-$Σ_{\rm SFR}$ connection to inform future HST LyC observing programs. For a program with a selection independent of ionizing properties, >= 90 objects are required; for one preferentially observing strongly-leaking LyC sources, >= 58 objects are required. More generally, measuring the connection between $f_{\rm esc}$ and $Σ_{\rm SFR}$ requires a larger, representative sample spanning a wide dynamic range in galaxies properties such as $Σ_{\rm SFR}$.

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arXiv2021

The KBSS-KCWI Survey: The connection between extended Ly$α$ halos and galaxy azimuthal angle at $z\sim 2-3$

We present the first statistical analysis of kinematically-resolved, spatially-extended Ly$α$ emission around $z = 2-3$ galaxies in the Keck Baryonic Structure Survey (KBSS) using the Keck Cosmic Web Imager (KCWI). Our sample of 59 star-forming galaxies ($z_\mathrm{med} = 2.29$) comprises the subset with typical KCWI integration times of ~5 hours and with existing imaging data from the Hubble Space Telescope and/or adaptive optics-assisted integral field spectroscopy. The high resolution images were used to evaluate the azimuthal dependence of the diffuse Ly$α$ emission with respect to the stellar continuum within projected galactocentric distances of $\lesssim 30$ proper kpc. We introduce cylindrically-projected 2D spectra (CP2D) that map the averaged Ly$α$ spectral profile over a specified range of azimuthal angle, as a function of impact parameter around galaxies. The averaged CP2D spectrum of all galaxies shows clear signatures of Ly$α$ resonant scattering by outflowing gas. We stacked the CP2D spectra of individual galaxies over ranges of azimuthal angle with respect to their major axes. The extended Ly$α$ emission along the galaxy principal axes are statistically indistinguishable, with residual asymmetry of $\le$ 2% ($\sim 2 σ$) of the integrated Ly$α$ emission. The symmetry implies that the Ly$α$ scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of Ly$α$ emission is marginally stronger along galaxy minor axes for galaxies with relatively weak Ly$α$ emission. We speculate that this weak directional dependence of Ly$α$ emission becomes discernible only when the Ly$α$ escape fraction is low. These discoveries highlight the need for similar analyses in simulations with Ly$α$ radiative transfer modeling.

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arXiv2020

The Redshift Evolution of Rest-UV Spectroscopic Properties to z~5

We perform a comprehensive analysis of the redshift evolution of the rest-UV spectra of star-forming galaxies out to z~5. We combine new z~5 measurements of HI Ly$α$ and low- and high-ionization interstellar metal absorption features with comparable measurements at z~2-4. We measure the equivalent widths of interstellar absorption features using stacked spectra in bins of Ly$α$ equivalent width, performing corrections to Ly$α$ strengths based on a model for the transmission of the intergalactic medium. We find a strong correlation between decreasing low-ionization absorption strength and increasing Ly$α$ emission strength over the redshift range z~2-5, suggesting that both of these quantities are fundamentally linked to neutral gas covering fraction. At the highest Ly$α$ equivalent widths, we observe evolution at $z\sim5$ towards greater Ly$α$ emission strength at fixed low-ionization absorption strength. If we interpret the non-evolving relationship of Ly$α$ emission strength and low-ionization line strength at z~2-4 as primarily reflecting the radiative transfer of Ly$α$ photons, this evolution at z~5 suggests a higher intrinsic production rate of Ly$α$ photons than at lower redshift. Our conclusion is supported by the joint evolution of the relationships among Ly$α$ emission strength, interstellar absorption strength, and dust reddening. We perform additional analysis in bins of stellar mass, star-formation rate, UV luminosity, and age, examining how the relationships between galaxy properties and Ly$α$ emission evolve towards higher redshift. We conclude that increasing intrinsic Ly$α$ photon production and strong detection of nebular CIV emission (signaling lower metallicity) at z~5 indicate an elevated ionized photon production efficiency ($ξ_{\rm ion}$).

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arXiv2015

The Lyman Continuum Escape Fraction of Low-Mass Star-Forming Galaxies at z~1

To date no direct detection of Lyman continuum emission has been measured for intermediate--redshift z~1 star-forming galaxies . We combine HST grism spectroscopy with GALEX UV and ground--based optical imaging to extend the search for escaping Lyman continuum to a large (~600) sample of z~1 low-mass, moderately star-forming galaxies selected initially on H$α$ emission. The characteristic escape fraction of LyC from SFGs that populate this parameter space remains weakly constrained by previous surveys, but these faint SFGs are assumed to play a significant role in the reionization of neutral hydrogen in the intergalactic medium (IGM) at high redshift (z>6). We do not make an unambiguous detection of escaping LyC radiation from this $z\sim1$ sample, individual non--detections to constrain the absolute Lyman continuum escape fraction, $f_{esc}$<2.1% (3$σ$). We measure upper limits of $f_{esc}$<9.6% from a sample of SFGs selected on high H$α$ equivalent width (EW>200Å), which are thought to be close analogs of high redshift sources of reionization. For reference, we also present an emissivity--weighted escape fraction which is useful as a measurement of the general contribution of the SFGs to the z~1 ionizing UV background. In the discussion, we consider the implications of these intermediate redshift constraints for the reionization of hydrogen in the intergalactic medium at high (z > 6) redshift. If we assume our $z\sim1$ SFGs, for which we measure this emissivity-weighted $f_{esc}$, are analogs to the high redshift sources of reionization, we find is difficult reconcile reionization by faint (M$_{UV}<-13$) SFGs with a low escape fraction ($f_{esc}<$3%), with constraints from independent high redshift observations. If $f_{esc}$ evolves with redshift, reionization by SFGs may be consistent with observations from Planck.

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