arXiv subjects

Andrew Fox

Publications and source records attributed to Andrew Fox.

12 recordsLinked to original sources

arXiv2026

The future of high-resolution UV spectroscopy: Science with a UV \'Echelle spectrograph on the Habitable Worlds Observatory, or a dedicated mission

High-resolution UV spectroscopy serves a diversity of science cases, from small bodies to planets, stars, and galaxies, but is currently limited to the Hubble Space Telescope and bright targets. Major advances require increasing sensitivity by at least one order of magnitude. Here we present the UV science cases for PEGASUS (Planets, Earths, Galaxies, And Stars UV Spectrograph), a UV \'Echelle high-resolution spectrograph concept, with $R = \lambda/\delta\lambda \sim 100\,000$ (full range 10 000-140 000) and covering 90--400 nm, with a foreseen extension to at least 800 nm. PEGASUS is ideally suited for the Habitable Worlds Observatory (HWO), enabling transformative science across the UV/optical wavelength ranges. PEGASUS will be unique in high sensitivity (effective area) and high spectral resolution -- an uncharted territory -- as well as robustness, thanks to the simplicity of its design. Its UV science cases include: I) Formation and evolution of planets and their habitability: properties of exoplanets and atmospheres, protoplanetary disks, Solar System bodies; II) Stellar lives and deaths at their extremes: the first stars and the origin of the elements, compact and massive stars, Supernovae; III) Gas and metals in the baryon cycle of galaxies: the interstellar, circumgalactic, and intergalactic medium and their roles in galaxy growth. These are essential for the Astro Decadal 2020 Survey, Voyage 2050, and HWO. While this paper focuses on high-impact science enabled by UV high-resolution spectroscopy, PEGASUS will extend into the optical regime and lower spectral resolution, making it a multi-purpose, widely used, workhorse spectrograph for HWO.

astro-ph.IM↗

arXiv2026

The GOLIATH Survey: OVI Absorption Reveals CGM Evolution through the Starburst-to-Quiescent Transition in Massive Galaxies

We present the GOLIATH survey (Galaxies, Outflows, and the Lifecycle of Immense, Active, Transforming Halos), a study of the multiphase circumgalactic medium (CGM) of massive ($\langle\log M_\star/M_\odot\rangle \approx 11$), blue ($u-r < 1.65$) starburst and post-starburst galaxies at $\langle z\rangle \approx$ 0.43. This work characterizes the warm-hot CGM through OVI absorption in the inner halo ($R/R_{\rm vir} \leq 0.6$) of these rare systems. Across the star-forming population, OVI column density rises by nearly 1~dex from $\log M_\star/M_{\odot} \sim 8$ to $\sim 11.5$ and increases with specific star-formation rate (sSFR). Two GOLIATH galaxies with the highest sSFR show the strongest CGM OVI absorption ($\log N_{\rm O\,VI}[\rm cm^{-2}] \gtrsim 15$). In the $\log M_\star/M_{\odot} = [11,12)$ inner-CGM region, massive star-forming galaxies exceed quiescent galaxies on average by a factor of $\sim 3$ in OVI column density and $\sim 1.5$~dex in CGM OVI mass ($\log(M_{\rm O\, VI}/M_\odot) \approx 7.3$ versus $\approx 5.8$), with covering fractions roughly three times higher (62.5% versus 24% at $\log N_{\rm O\,VI}[\rm cm^{-2}] \geq 14$). The OVI line widths and column densities are consistent with feedback-driven radiative cooling, in which outflow shocks heat the CGM and the gas cools back through the OVI window; the short cooling time, $t_{\rm cool} \sim 10$-$100$~Myr, requires continuous replenishment by active feedback to sustain this reservoir. The residual OVI in quiescent systems may arise from ambient gas at the high-temperature end of the cooling curve. OVI thus traces feedback on short timescales and probes the star-forming--quiescent transition at $\log M_\star/M_{\odot} \gtrsim 11$.

astro-ph.GA↗

arXiv2026

A Decade to Map the Diffuse Universe: FRB-QSO Pairs with HST/COS Spectroscopy

Jointly analyzing the sightlines of arcsecond-localized fast radio bursts (FRBs) and UV-bright quasars (QSOs) nearby in projection has the potential to provide strong constraints on the phases, mass distributions, and magnetic structure of the diffuse universe. Each probe supplies what the other cannot: FRBs provide integrated electron columns (DM), line-of-sight magnetic field estimates (RM), and scattering constraints ($\tau_{\rm scatt}$) that are independent of gas phase; QSOs provide the redshift- and phase-resolved column densities needed to interpret them. Today, there are only $\sim100$ arcsecond-localized FRBs at $z < 1$, making statistical FRB-QSO pair surveys impossible. By 2035, there will be $\sim10^{5}$. Using the most recent FRB localization forecasts and UV-bright QSO catalogs, we estimate that next-generation interferometers will yield thousands of FRB--QSO pairs at angular separations $\theta < 10'$, including $\sim100$ pairs at $\theta < 1'$, over a common 20,000\,deg$^2$ footprint by 2035. We outline the science enabled by this sample: constraints on CGM ionization fractions and baryon masses; observational constraints on the role of magnetic fields and turbulence in the CGM and cosmic web; sightline-by-sightline partitioning of the cosmic DM budget; and three-dimensional mapping of the multiphase Milky Way and M31 halos. Together, these measurements directly address the physics of feedback, non-thermal pressure support, and energy balance in the diffuse gas that regulates galaxy growth. HST/COS is the only instrument that can carry out this program, and the 2030s are the only decade in which to do it before Habitable Worlds Observatory (HWO) defines the next era of diffuse universe science.

astro-ph.GA↗

arXiv2025

Towards Cumulative Abstract Semantics via Handlers

We consider the problem of modularizing control flow in a generic abstract interpretation framework. A generic abstract interpretation framework is not truly flexible if it does not allow interpreting with different path- and flow-sensitivities, by going forwards or backwards, and over- or under-approximately. Most interpreters inherently intertwine syntax and semantics, making the implementation antagonistic to modularity. Current approaches to modular designs require the use of complex data structures (e.g., monad transformers), providing modularity but often proving unwieldy (e.g., lifts). We observe that leveraging scoped effects within an interpreter facilitates the accumulation of semantic fragments against a fixed syntax. In this paper, we define cumulative abstract semantics, illustrating the potential for creating multiple dynamic evaluators and static analyses from one interpreter. This modularity is achieved by grouping effects into two categories: syntax elimination and domain-semantic introduction handlers. Our contribution shows the benefits of using effects as an instrument for designing a clean, elegant, and modular abstract interpretation framework.

cs.PL↗

arXiv2025

Dust in the Smith Cloud? A UV Investigation into the Smith Cloud's Gas-Phase Abundance Patterns

The Smith Cloud is a high-velocity cloud (HVC) on its final approach to the Milky Way that shows evidence of interaction with the Galaxy's disk. We investigate the metallicity and gas-phase chemical depletion patterns in this HVC using UV absorption-line observations toward two background QSOs taken with the Hubble Space Telescope (HST)/Cosmic Origin Spectrograph (COS) and H I 21-cm emission-line observations taken with Green Bank Telescope (GBT). We find evidence of silicon gas-phase depletion with [Si/S]$=\,-0.72^{+0.26}_{-0.24}$ and [Si/O]$_{3\sigma}\,\lesssim\,-0.05$, implying the presence of dust within the Smith Cloud. Because dust is galactic in origin, this HVC could trace the return leg of a Galactic fountain or a dwarf galaxy that passed through the Galactic plane.

astro-ph.GA↗

arXiv2019

The large-scale ionization cones in the Galaxy

There is compelling evidence for a highly energetic Seyfert explosion (10^{56-57} erg) that occurred in the Galactic Centre a few million years ago. The clearest indications are the x-ray/gamma-ray "10 kpc bubbles" identified by the Rosat and Fermi satellites. In an earlier paper, we suggested another manifestation of this nuclear activity, i.e. elevated H-alpha emission along a section of the Magellanic Stream due to a burst (or flare) of ionizing radiation from Sgr A*. We now provide further evidence for a powerful flare event: UV absorption line ratios (in particular CIV/CII, SiIV/SiII) observed by the Hubble Space Telescope reveal that some Stream clouds towards both galactic poles are highly ionized by a source capable of producing ionization energies up to at least 50 eV. We show how these are clouds caught in a beam of bipolar, radiative "ionization cones" from a Seyfert nucleus associated with Sgr A*. In our model, the biconic axis is tilted by about 15 deg from the South Galactic Pole with an opening angle of roughly 60 deg. For the Stream at such large Galactic distances (D > 75 kpc), nuclear activity is a plausible explanation for all of the observed signatures: elevated H-alpha emission and H ionization fraction (X_e > 0.5), enhanced CIV/CII and SiIV/SiII ratios, and high CIV and SiIV column densities. Wind-driven "shock cones" are ruled out because the Fermi bubbles lose their momentum and energy to the Galactic corona long before reaching the Stream. The nuclear flare event must have had a radiative UV luminosity close to the Eddington limit (f_E ~ 0.1-1). Our time-dependent Seyfert flare models adequately explain the observations and indicate the Seyfert flare event took place T_o = 3.5 +/- 1 Myr ago. The timing estimates are consistent with the mechanical timescales needed to explain the x-ray/gamma-ray bubbles in leptonic jet/wind models (2-8 Myr).

astro-ph.GA↗

arXiv2015

Elemental depletions in the Magellanic Clouds and the evolution of depletions with metallicity

We present a study of the composition of gas and dust in the Large and Small Magellanic Clouds (LMC and SMC, together -- the MCs) as measured by UV absorption spectroscopy. We have measured P II and Fe II along 85 sightlines toward the MCs using archival FUSE observations. For 16 of those sightlines, we have measured Si II, Cr II, and Zn II from new HST COS observations. We have combined these measurements with H I and H$_2$ column densities and reference stellar abundances from the literature to derive gas-phase abundances, depletions, and gas-to-dust ratios (GDRs). 80 of our 84 P measurements and 13 of our 16 Zn measurements are depleted by more than 0.1 decades, suggesting that P and Zn abundances are not accurate metallicity indicators at and above the metallicity of the SMC. The maximum P and Zn depletions are the same in the MW, LMC, and SMC. Si, Cr, and Fe are systematically less depleted in the SMC than in the MW or LMC. The minimum Si depletion in the SMC is consistent with zero. Our depletion-derived GDRs broadly agree with GDRs from the literature. The GDR varies from location to location within a galaxy by a factor of up to 2 in the LMC and up to 5 in the SMC. This variation is evidence of dust destruction and/or growth in the diffuse neutral phase of the interstellar medium.

astro-ph.GA↗

arXiv2014

The COS-Dwarfs Survey: The Carbon Reservoir Around sub-L* Galaxies

We report new observations of circumgalactic gas from the COS-Dwarfs survey, a systematic investigation of the gaseous halos around 43 low-mass z $\leq$ 0.1 galaxies using background QSOs observed with the Cosmic Origins Spectrograph. From the projected 1D and 2D distribution of C IV absorption, we find that C IV absorption is detected out to ~ 0.5 R$_{vir}$ of the host galaxies. The C IV absorption strength falls off radially as a power law and beyond 0.5 R$_{vir}$, no C IV absorption is detected above our sensitivity limit of ~ 50-100 m$\AA$. We find a tentative correlation between detected C IV absorption strength and star formation, paralleling the strong correlation seen in highly ionized oxygen for L~L* galaxies by the COS-Halos survey. The data imply a large carbon reservoir in the CGM of these galaxies, corresponding to a minimum carbon mass of $\gtrsim$ 1.2$\times 10^6$ $M_\odot$ out to ~ 110 kpc. This mass is comparable to the carbon mass in the ISM and more than the carbon mass currently in stars of these galaxies. The C IV absorption seen around these sub-L* galaxies can account for almost two-thirds of all $W_r$> 100 m$\AA$ C IV absorption detected at low z. Comparing the C IV covering fraction with hydrodynamical simulations, we find that an energy-driven wind model is consistent with the observations whereas a wind model of constant velocity fails to reproduce the CGM or the galaxy properties.

astro-ph.GA↗

arXiv2012

Unique Astrophysics in the Lyman Ultraviolet

There is unique and groundbreaking science to be done with a new generation of UV spectrographs that cover wavelengths in the "Lyman Ultraviolet" (LUV; 912 - 1216 Ang). There is no astrophysical basis for truncating spectroscopic wavelength coverage anywhere between the atmospheric cutoff (3100 Ang) and the Lyman limit (912 Ang); the usual reasons this happens are all technical. The unique science available in the LUV includes critical problems in astrophysics ranging from the habitability of exoplanets to the reionization of the IGM. Crucially, the local Universe (z <= 0.1) is entirely closed to many key physical diagnostics without access to the LUV. These compelling scientific problems require overcoming these technical barriers so that future UV spectrographs can extend coverage to the Lyman limit at 912 Ang.

astro-ph.IM↗

arXiv2010

X-shooter, NACO, and AMBER observations of the LBV Pistol Star \footnote{Based on ESO runs 85.D-0182A, 085.D-0625AC}

We present multi-instruments and multi-wavelengths observations of the famous LBV star Pistol Star. These observations are part of a larger program about early O stars at different metallicities. The Pistol star has been claimed as the most massive star known, with 250 solar masses. We present the preliminary results based on X-Shooter spectra, as well as the observations performed with the VLTI-AMBER and the VLT-NACO adaptive optics. The X-shooter spectrograph allows to obtain simultaneously a spectrum from the UV to the K-band with a resolving power of $\sim$15000. The preliminary results obtained indicate that Pistol Star has similar properties of Eta Car, including shells of matter, but also the binarity. Other objects of the program, here briefly presented, were selected for their particular nature: early O stars with mass discrepancies between stellar evolution models and observations, discrepancies with the wind momentum luminosity relation.

astro-ph.SR↗

arXiv2010

High-angular resolution observations of the Pistol Star

First results of near-IR adaptive optics (AO)-assisted imaging, interferometry, and spectroscopy of this Luminous Blue Variable (LBV) are presented. They suggest that the Pistol Star is at least double. If the association is physical, it would reinforce questions concerning the importance of multiplicity for the formation and evolution of extremely massive stars.

astro-ph.SR↗

arXiv2007

The Abundance of Deuterium in the Warm Neutral Medium of the Lower Galactic Halo

We use high resolution spectra obtained with FUSE and STIS to obtain interstellar column densities of H I, D I, O I, S II, Fe II and P II toward the QSO HE 0226-4110 ( l = 253.4 deg , b = -65.77 deg). We obtain D/H = 21(+8, -6) ppm. Correcting for a small amount of contamination from D and H in the Local Bubble, we obtain D/H = 22 (+8, -6) for the warm neutral medium of the lower Galactic halo. The medium has [O/H] = 0.12(+0.41, -0.20) and [Fe/H] = -1.01 (+0.10, -0.09). This suggests the abundances in the halo toward HE 0226-4110 are not affected by the infall of low metallicity gas and that the gas originates in the disk and is elevated into the halo by energetic processes which erode but do not totally destroy the dust grains. We compare our result to measured values of D/H in other astrophysical sites. The value we measure in the halo gas is consistent with the hypothesis that for many Galactic disk lines of sight D is incorporated into dust. The high average value of D/H = 23.1+/-2.4 ppm measured along 5 sight lines through disk gas in the Solar neighborhood is similar to D/H in the lower Galactic halo. These observations imply the abundance of deuterium in the Galaxy has only been reduced by a factor of 1.12+/-0.13 since its formation.

astro-ph↗