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First-authored Papers

The Cosmic Ultraviolet Baryon Survey (CUBS) - III. Physical properties and elemental abundances of Lyman limit systems at z < 1

Fakhri S. Zahedy and the CUBS Collaboration (20 co-authors), Monthly Notices of the Royal Astronomical Society, Volume 506, Issue 1, p.877–902, September 2021

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Abstract:  We present a systematic investigation of physical conditions and elemental abundances in four optically thick Lyman-limit systems (LLSs) at z = 0.36−0.6 discovered within the Cosmic Ultraviolet Baryon Survey (CUBS). Because intervening LLSs at z < 1 suppress FUV light from background QSOs, an unbiased search of these absorbers requires a NUV-selected QSO sample, as achieved by CUBS. CUBS LLSs exhibit multi-component kinematic structure and a complex mix of multiphase gas, with associated metal transitions from multiple ionization states such as C II, C III, N III, Mg II, Si II, Si III, O II, O III, O VI, and Fe II absorption that span several hundred km/s in line-of-sight velocity. Specifically, higher column density components (log N(H I)>~16) in all four absorbers comprise dynamically cool gas with ⟨T⟩ = (2±1)×10^4 K and modest non-thermal broadening of ⟨b_nt⟩ = 5±3 km/s. The high quality of the QSO absorption spectra allows us to infer the physical conditions of the gas, using a detailed ionization modeling that takes into account the resolved component structures of H I and metal transitions. The range of inferred gas densities indicates that these absorbers consist of spatially compact clouds with a median line-of-sight thickness of 160(+140,-50) pc. While obtaining robust metallicity constraints for the low-density, highly ionized phase remains challenging due to the uncertain N(H I), we demonstrate that the cool-phase gas in LLSs has a median metallicity of [α/H] = −0.7±0.1, with a 16-84 percentile range of [α/H] = (−1.3, −0.1). Furthermore, the wide range of inferred elemental abundance ratios ([C/α], [N/α], and [Fe/α]) indicate a diversity of chemical enrichment histories. Combining the absorption data with deep galaxy survey data characterizing the galaxy environment of these absorbers, we discuss the physical connection between star-forming regions in galaxies and diffuse gas associated with optically thick absorption systems in the z< 1 circumgalactic medium.


Evidence for Late-Time Feedback from the Discovery of Multiphase Gas in a Massive Elliptical
at z=0.4

Fakhri S. Zahedy, Hsiao-Wen Chen, Erin Boettcher, Michael Rauch, K. Decker French, and Ann I. Zabludoff, 2020, The Astrophysical Journal Letters, Volume 904, Issue 1, article id. L10, November 2020

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Abstract: We report the first detection of multiphase gas within a quiescent galaxy beyond z ≈ 0. The observations use the brighter image of doubly lensed QSO HE 0047−1756 to probe the ISM of the massive (M_star ≈ 10^11 M_sun) elliptical lens galaxy at z = 0.408. Using Hubble Space Telescope’s Cosmic Origins Spectrograph (COS), we obtain a medium-resolution FUV spectrum of the lensed QSO and identify numerous absorption features from molecular hydrogen (H2) in the lens ISM at projected distance d = 4.6 kpc. The H2 column density is log N(H2)/cm−2 = 17.8(+0.1,-0.3) with a molecular gas fraction of f_H2 = 2−5%, roughly consistent with some local quiescent galaxies. The new COS spectrum also reveals kinematically complex absorption features from highly ionized species OVI and NV with column densities log N(OVI)/cm−2 = 15.2 ± 0.1 and log N(NV)/cm−2 = 14.6 ± 0.1, among the highest known in external galaxies. Assuming the high-ionization absorption features originate in a transient warm (T~10^5 K) phase undergoing radiative cooling from a hot halo surrounding the galaxy, we infer a mass accretion rate of ~0.5 - 1.5 M_sun/yr. The lack of star formation in the lens suggests the bulk of this flow is returned to the hot halo, implying a heating rate of ~10^48 erg/yr. Continuous heating from evolved stellar populations (primarily SNe Ia but also winds from AGB stars) may suffice to prevent a large accumulation of cold gas in the ISM, even in the absence of strong feedback from an active nucleus. [ADS]

Probing IGM Accretion onto Faint Ly-alpha Emitters at z~2.8

Fakhri S. Zahedy, Michael Rauch, Hsiao-Wen Chen, Robert F. Carswell, Brian Stalder, and Antony A. Stark, Monthly Notices of the Royal Astronomical Society, Volume 486, Issue 1, p.1392–1403, June 2019


Abstract: Observing the signature of accretion from the intergalactic medium (IGM) onto galaxies at redshift z∼3 requires the detection of faint (L≪L∗) galaxies embedded in a filamentary matrix of low density, metal poor gas (Z∼10^−2.5 Z⊙) coherent over hundreds of kpc. We study the gaseous environment of three faint Lyα emitters (LAEs) at z = 2.7−2.8, found to be aligned in projection with a background QSO over ∼250 kpc along the slit of a long-slit spectrum. The lack of detection of the LAEs in deep continuum images and the low inferred Lyα luminosities show the LAEs to be intrinsically faint, low-mass galaxies (L < 0.1L*,M_star < 0.1 M*). An echelle spectrum of the QSO reveals strong Lyα absorption within ±200 km/s from the LAEs. Our absorption line analysis leads to H I column densities in the range of log N(H I) = 16−18. Associated absorption from ionic metal species C IV and Si IV constrains the gas metallicities to ∼0.01 solar if the gas is optically thin, and possibly as low as ∼0.001 solar if the gas is optically thick, assuming photo-ionization equilibrium. While the inferred metallicities are at least a factor of ten lower than expected metallicity in the interstellar medium (ISM) of these LAEs, they are consistent with the observed chemical enrichment level in the IGM at the same epoch. Total metal abundances and kinematic arguments suggest that these faint galaxies have not been able to affect the properties of their surrounding gas. The projected spatial alignment of the LAEs, together with the kinematic quiescence and correspondence between the LAEs and absorbing gas in velocity space suggests that these observations probe a possible filamentary structure. Taken together with the blue-dominant Lyα emission line profile of one of the objects, the evidence suggests that the absorbing gas is part of an accretion stream of low-metallicity gas in the IGM. [ADS]

Characterizing Circumgalactic Gas around Massive Ellipticals at z~0.4 - II. Physical Properties and Elemental Abundances

Fakhri S. Zahedy, Hsiao-Wen Chen, Sean D. Johnson, Rebecca M. Pierce, Michael Rauch, Yun-Hsin Huang, Benjamin J. Weiner, and Jean-René Gauthier, Monthly Notices of The Royal Astronomical Society, Volume 484, Issue 2, p.2257-2280, April 2019

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Abstract: We present a systematic investigation of the circumgalactic medium (CGM) within projected distances d<160 kpc of luminous red galaxies (LRGs). The sample comprises 16 intermediate-redshift (z=0.21-0.55) LRGs of stellar mass M_star>1e11 M_sun. Combining far-ultraviolet Cosmic Origin Spectrograph spectra from the Hubble Space Telescope and optical echelle spectra from the ground enables a detailed ionization analysis based on resolved component structures of a suite of absorption transitions, including the full H I Lyman series and various ionic metal transitions. By comparing the relative abundances of different ions in individually-matched components, we show that cool gas (T~1e4 K) density and metallicity can vary by more than a factor of ten in a single halo. Specifically, metal-poor absorbing components with <1/10 solar metallicity are seen in 50% of the LRG halos, while gas with solar and super-solar metallicity is also common. These results indicate a complex multiphase structure and poor chemical mixing in these quiescent halos. We calculate the total surface mass density of cool gas, Σ_cool, by applying the estimated ionization fraction corrections to the observed H I column densities. The radial profile of Σ_cool is best-described by a projected Einasto profile of slope α=1 and scale radius r_s=48 kpc. We find that typical LRGs at z~0.4 contain cool gas mass of M_cool= (1-2) x 1e10 M_sun at d<160kpc (or as much as 4 x 1e10 Msun at d<500 kpc), comparable to the cool CGM mass of star-forming galaxies. Furthermore, we show that high-ionization O VI and low-ionization absorption species exhibit distinct velocity profiles, highlighting their different physical origins. We discuss the implications of our findings for the origin and fate of cool gas in LRG halos. [ADS]

HST Detection of Extended Neutral Hydrogen in a Massive Elliptical at z = 0.4

Fakhri S. Zahedy, Hsiao-Wen Chen, Michael Rauch, and Ann Zabludoff, The Astrophysical Journal Letters, Volume 846, Issue 2, article id. L29, September 2017

Abstract: We report the first detection of extended neutral hydrogen (HI) gas in the interstellar medium (ISM) of a massive elliptical galaxy beyond z~0. The observations utilize the doubly lensed images of QSO HE 0047-1756 at z_QSO = 1.676 as absorption-line probes of the ISM in the massive (M_star ~ 10^11 M⊙) elliptical lens at z = 0.408, detecting gas at projected distances of d = 3.3 and 4.6 kpc on opposite sides of the lens. Using the Space Telescope Imaging Spectrograph (STIS), we obtain UV absorption spectra of the lensed QSO and identify a prominent flux discontinuity and associated absorption features matching the Lyman series transitions at z = 0.408 in both sightlines. The H I column density is log N(H I) = 19.6-19.7 at both locations across the lens, comparable to what is seen in 21 cm images of nearby ellipticals. The H I gas kinematics are well-matched with the kinematics of the Fe II absorption complex revealed in ground-based echelle data, displaying a large velocity shear of 360 km/s across the galaxy. We estimate an ISM Fe abundance of 0.3-0.4 solar at both locations. Including likely dust depletions increases the estimated Fe abundances to solar or supersolar, similar to those of the hot ISM and stars of nearby ellipticals. Assuming 100% covering fraction of this Fe-enriched gas,we infer a total Fe mass of M_cool(Fe)~(5-8)x 10^4 M⊙ in the cool ISM of the massive elliptical lens, which is no more than 5% of the total Fe mass observed in the hot ISM. [ADS]

On the Radial Profile of Gas-phase Fe/α Ratio Around Distant Galaxies

Fakhri S. Zahedy, Hsiao-Wen Chen, Jean-René Gauthier, and Michael Rauch, Monthly Notices of the Royal Astronomical Society, Volume 466, Issue 1, p.1071-1081, April 2017

Abstract: This paper presents a study of the chemical compositions in cool gas around a sample of 27 intermediate-redshift galaxies. The sample comprises 13 massive quiescent galaxies at z = 0.40-0.73 probed by QSO sightlines at projected distances d = 3-400 kpc, and 14 star-forming galaxies at z = 0.10-1.24 probed by QSO sightlines at d = 8-163 kpc. The main goal of this study is to examine the radial profiles of the gas-phase Fe/α ratio in galaxy haloes based on the observed Fe II to Mg II column density ratios. Because Mg+ and Fe+ share similar ionization potentials, the relative ionization correction is small in moderately ionized gas and the observed ionic abundance ratio N(Fe II)/N(Mg II) places a lower limit to the underlying (Fe/Mg) elemental abundance ratio. For quiescent galaxies, a median and dispersion of log <N(Fe II)/N(Mg II)> = -0.06± 0.15 is found at d ≲ 60 kpc, which declines to log <N(Fe II)/N(Mg II)>  <-0.3 at d ≳ 100 kpc. On the other hand, star-forming galaxies exhibit log <N(Fe II)/N(Mg II)> = -0.25± 0.21 at d ≲ 60 kpc and log <N(Fe II)/N(Mg II)> = -0.9± 0.4 at larger distances. Including possible differential dust depletion or ionization correction would only increase the inferred Fe/Mg ratio. The observed N(Fe II)/N(Mg II) implies super-solar Fe/α ratios in the inner halo of quiescent galaxies. An enhanced Fe abundance indicates a substantial contribution by Type Ia supernovae in the chemical enrichment, which is at least comparable to what is observed in the solar neighborhood or in intra-cluster media but differs from young star-forming regions. In the outer haloes of quiescent galaxies and in haloes around star-forming galaxy, however, the observed Fe/Mg is consistent with an α-element enhanced enrichment pattern, suggesting a core-collapse supernovae dominated enrichment history.  [ADS]

Probing the Cool Interstellar & Circumgalactic Gas of Three Massive Lensing Galaxies at z = 0.4-0.7

Fakhri S. Zahedy, Hsiao-Wen Chen, Michael Rauch, Michelle L. Wilson, and Ann Zabludoff, Monthly Notices of the Royal Astronomical Society, Volume 458, Issue 3, p.2423-2442, May 2016

Abstract: We present multi-sightline absorption spectroscopy of cool gas around three lensing galaxies at z = 0.4-0.7. These lenses have half-light radii r_e = 2.6-8 kpc and stellar masses of log M_star/M⊙ = 10.9-11.4, and therefore resemble nearby passive elliptical galaxies. The lensed QSO sightlines presented here occur at projected distances of d = 3-15 kpc (or d ≈ 1-2 r_e) from the lensing galaxies, providing for the first time an opportunity to probe both interstellar gas at r ~r_e and circumgalactic gas at larger radii rr_e of these distant quiescent galaxies. We observe distinct gas absorption properties among different lenses and among sightlines of individual lenses. Specifically, while the quadruple lens for HE 0435-1223 shows no absorption features to very sensitive limits along all four sightlines, strong Mg II, Fe II, Mg I, and Ca II absorption transitions are detected along both sightlines near the double lens for HE 0047-1756, and in one of the two sightlines near the double lens for HE 1104-1805. The absorbers are resolved into 8-15 individual components with a line-of-sight velocity spread of Δv ≈ 300-600 km/s. The large ionic column densities, log N >14, observed in two components suggest that these may be Lyman limit or damped Lyα absorbers with a significant neutral hydrogen fraction. The majority of the absorbing components exhibit a uniform supersolar Fe/Mg ratio with a scatter of <0.1 dex across the full Δv range. Given a predominantly old stellar population in these lensing galaxies, we argue that the observed large velocity width and Fe-rich abundance pattern can be explained by SNe Ia enriched gas at radius rr_e. We show that additional spatial constraints in line-of-sight velocity and relative abundance ratios afforded by a multisightline approach provide a powerful tool to resolve the origin of chemically enriched cool gas in massive haloes. [ADS]

Co-Authored Papers

Nakajima, Kimihiko., et al. (incl Zahedy, Fakhri S.), 2022, EMPRESS. V. Metallicity Diagnostics of Galaxies over 12+log(O/H)=~6.9-8.9 Established by a Local Galaxy Census: Preparing for JWST Spectroscopy, ApJS in press [ADS]

Xu, Y., et al. (incl Zahedy, Fakhri S.), 2022, EMPRESS. VI. Outflows Investigated in Low-mass Galaxies with M∗ = 10^4-10^7 M: Weak Feedback in Low-mass Galaxies? , ApJ, 929, 134 [ADS]

Boettcher, E., et al. (incl Zahedy, Fakhri S.), 2022, Discovery of a Damped Lyα Absorber Originating in a Spectacular Interacting Dwarf Galaxy Pair at z = 0.026, ApJL, 926L33 [ADS]

Cooper, T., et al. (incl Zahedy, Fakhri S.), 2021, The Cosmic Ultraviolet Baryon Survey (CUBS) - IV. The complex multiphase circumgalactic medium as revealed by partial Lyman limit systems, MNRAS, 508, 4359 [ADS]

Boettcher, E., Chen, H.-W., Zahedy, Fakhri S., et al., 2020, The Cosmic Ultraviolet Baryon Survey (CUBS) - II. Discovery of an H2-Bearing DLA in the Vicinity of an Early-Type Galaxy at z = 0.576, 2021, ApJ, 213, 18 [ADS]


Huang, Y.-H., Chen, H.-W., Shectman, S. A., Johnson, S. D., Zahedy, Fakhri S., Helsby, J. E., Gauthier, J.-R., Thompson, I. B., 2021, A Complete Census of Circumgalactic MgII at Redshift z<0.5, MNRAS, 502, 4743 [ADS]

Chen, H.-W., Zahedy, Fakhri. S., et al, 2020, The Cosmic Ultraviolet Baryon Survey (CUBS) I. Overview and the diverse environments of Lyman limit systems at z<1, MNRAS, 497, 498 [ADS]


Gaikwad, P., Rauch, M., Haehnelt, M. G., Puchwein, E., Bolton, J. S., Keating, L. C., Kulkarni, G., Iršič, V., Bañados, E., Becker, G. D., Boera, E., Zahedy, Fakhri. S., Chen, H.-W., Carswell, R. F., Chardin, J., Rorai, A., 2020, Probing the thermal state of the intergalactic medium at z > 5 with the transmission spikes in high-resolution Ly-alpha forest spectra, MNRAS, 494, 5091 [ADS]

Connor, T., Zahedy, Fakhri S., Chen, H.-W., Cooper, T. J., Mulchaey, J. S., & Vikhlinin, A., 2019,  COS Observations of the Cosmic Web: A Search for the Cooler Components of a Hot, X-ray Identified Filament, ApJL, 884, L20. [ADS]

Chen, H.-W., Boettcher, E., Johnson, S. D., Zahedy, Fakhri S., Rudie, G. C., Cooksey, K. L., Rauch, M., & Mulchaey, J. S., 2019,  A Giant Intragroup Nebula Hosting a Damped Lya Absorbers at z=0.313, ApJL, 878, L33. [ADS]

VoitG. M., DonahueM., Zahedy, Fakhri S., ChenH.-W., WerkJ. K.,  BryanG. L., O'SheaB. W., 2019,  Circumgalactic Pressure Profiles Indicate Precipitation-Limited Atmospheres for M_star ~ 10^9 - 10^11.5 M_sun, ApJL, 879, L1. [ADS]

Chen, H.-W., Johnson, S. D., Straka, L. A., Zahedy, Fakhri S., Schaye, J., Muzahid, S., Bouche, N., Cantalupo, S., Marino, R. A., & Wendt, M., 2019, Characterizing Circumgalactic Gas around Massive Ellipticals at z 0.4: III. The Galactic Environment of a Chemically Pristine Lyman Limit Absorber, MNRAS, 484, 431. [ADS]  

Chen, H.-W., Zahedy, Fakhri S., Johnson, S. D., Pierce, R. M., Huang, Y.-H., Weiner, B. J., Gauthier, J.-R, 2018, Characterizing Circumgalactic Gas around Massive Ellipticals at z ~0.4: I. Initial Results. MNRAS, 479, 2547.  [ADS] 

Chen, H.-W., Johnson, S. D., Zahedy, Fakhri S., Rauch, M., Mulchaey, J. S., 2017, Gauging Metallicity of Diffuse Gas under an Uncertain Ionizing Radiation Field, ApJL, 842, L19. [ADS] 

Hunter, D. A., Zahedy, Fakhri S., Bowsher, E. C., Wilcots, E. M., Kepley, A. A., Gaal, V., 2011, Mapping the Extended H I Distribution of Three Dwarf Galaxies, AJ, 142, 173. [ADS] 

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