Fakhri S. Zahedy
Carnegie Fellow, Observatories of the Carnegie Institution for Science, Pasadena, CA
e-mail: fzahedy 'at' carnegiescience.edu
Galaxies are the major building blocks of our Universe. Theoretically, they are thought to grow inside dark matter halos by accreting gas from the intergalactic and circumgalactic media (IGM & CGM). As the accreted gas falls on to the galaxy, it cools and condenses, eventually forming stars. Star formation and other physical mechanisms within galaxies can in turn regulate future gas accretion and impact future star formation (including stopping it altogether) through various energetic processes such as feedback and galaxy-galaxy interactions. While much progress has been made in the past several decades, we still lack a complete understanding of how these different baryonic processes impact the growth of galaxies over cosmic time.
As an observational astronomer, I am primarily interested in understanding how the gas reservoir of a galaxy determines the course of its evolution. In my work I perform precision analysis of quasar absorption-line spectroscopy, by employing novel techniques to examine the data and maximize the useful information we learn from them, in order to better understand the physical properties of diffuse gas within and surrounding distant galaxies.
I describe the highlights of my research activities below. Click on the citations to check out my main publications in each topic.
What do the extended gaseous reservoirs of "red-and-dead" galaxies tell us about massive galaxy evolution?
One of the biggest unanswered questions in the study of galaxy evolution is how/why do massive galaxies become and remain passive? It is natural to attribute their “red and dead” nature to a lack of cool gas that fuels star formation. Theory predicts that these massive galaxies are surrounded by reservoirs of predominantly hot, multi-million K gas. However, various observational studies conducted during the past decade have discovered that a significant fraction of these retired galaxies are surrounded by chemically enriched cool gas on ~100 kpc scales, challenging our understanding of massive galaxy evolution.
Between 2018 and 2019, I led the first systematic study of the physical properties and elemental abundances in the ~100 kpc-scale gaseous halos (circumgalactic medium; CGM) of massive elliptical galaxies, using the Cosmic Origins Spectrograph onboard the Hubble Space Telescope. We found that the average massive elliptical galaxy is surrounded by a CGM that is comparably gas-rich to that surrounding star-forming galaxies. We also found a significant incidence of very low-metallicity (less than ~0.01 solar) gas, which indicates that accretion of chemically primitive gas from intergalactic space is an important physical process in these massive halos. Furthermore, we found that the observed low velocities of the gas suggest that while cool gas may be abundant in these massive halos, disruptive interactions with the >10^6 K hot gas will prevent much of these cool clouds from successfully accreting on to the galaxy.
see also: Chen et al. (2019); Huang et al. (2021)
Unraveling the role of late-time feedback in distant massive galaxies using gravitational lenses
The existence of large reservoirs of cool gas around quiescent galaxies challenges our understanding of galaxy formation. What prevents so much gas from forming stars? One possibility is most in-falling gas clouds are destroyed at large distances by hydrodynamical interactions with the hot halo, as suggested in Zahedy et al. (2019a), hence depriving these galaxies of cold gas. Alternatively, energetic feedback processes within these galaxies can play important roles in heating their gas, ensuring that even if significant interstellar medium (ISM) gas is present, such gas would be unable to cool enough to trigger new star formation. In 2016 and 2017, I conducted two pilot studies to characterize the physical properties in the ISM of several distant massive elliptical galaxies using multiply lensed background quasars. We found evidence of a high degree of chemical enrichment from Type Ia supernovae (SNe Ia) in these massive elliptical lens galaxies, which was very exciting because it hints at SNe Ia's possible role in heating the gas within these giants.
Following two successful Hubble Space Telescope observing programs (PIDs 14751; 15250) to obtain UV spectroscopic observations on one of these lens systems, we reported the detection of multiphase gas in a massive elliptical galaxy, including highly ionized OVI (quintuply ionized oxygen). The detection of a significant amount of short-lived OVI gas in the multiphase ISM of a massive “red and dead” galaxy indicates the presence of an effective heat source in the galaxy. As discussed in Zahedy et al. (2020), this exciting discovery enabled us to quantitatively constrain feedback energy for the first time in a distant massive elliptical galaxy. Continuous heating from SNe Ia may suffice to provide the power needed to suppress star formation in this galaxy, even in the absence of an active galactic nucleus.
The big question is: is that a common feature of typical massive quiescent galaxies? Using the twin 6.5m Magellan Telescopes at the Las Campanas Observatory in Northern Chile, I am leading an ambitious survey to characterize the cool gas content and identify the dominant late-time feedback mechanism(s) within distant massive "red and dead" galaxies. This survey benefits from the substantial number of recently discovered gravitationally lensed quasars, which has allowed us to assemble the largest sample (to date) of ~30 massive quiescent lensing galaxies at z~0.5.
Fossil record of chemical evolution in the CGM
Gas metallicity is commonly used as a diagnostic for the origins of CGM gas. For instance, we can expect gas ejected from massive galaxies to be more metal-enriched than gas stripped from low-mass satellite galaxies. Unfortunately, these simple expectations are complicated by our lack of understanding of the physical mechanisms responsible for transporting heavy elements out of/on to galaxies. As my work demonstrated in Zahedy et al. (2019a), gas metallicity can vary by >a factor of 10 within an individual galaxy's CGM. Such large variations indicate that the CGM has multiple different physical origins and also suggest that chemical mixing is inefficient. For that reason, gas metallicity alone is an incomplete diagnostic for the origins of chemically enriched gas in the CGM.
In contrast, the elemental abundance ratios (i.e. the relative amounts of one metal to another) of the gas represent an archaeological record of different sources of heavy element production. In two studies I led in 2016 and 2017, we found that compared to star-forming galaxies, the CGM of quiescent galaxies are preferentially enriched in iron (Fe) compared to magnesium (Mg). This is understood to be due to an increased contribution from Type Ia supernovae of evolved, lower-mass stars to chemical enrichment in the CGM of quiescent galaxies. We constrained the relative contributions of Type Ia and core-collapse supernovae in the CGM and showed that contribution from Type Ia supernovae generally decreases with increasing distance from galaxies. Our discovery provides a powerful and novel technique to constrain the origins of CGM gas using the observed abundance ratios of the gas.
I have continued to refine this technique for CGM studies, including expanding it to incorporate additional element groups such as carbon and nitrogen. In 2021, I applied this detailed chemical analysis to probe the physical origins of new, optically thick absorbers at z<1 discovered in the Cosmic Ultraviolet Baryon Survey (CUBS). Using deep galaxy survey data characterizing the galaxy environments of these absorbers, I led the first systematic investigation into the physical connection between star-forming regions in galaxies and diffuse gas associated with these optically thick CGM absorbers. Our findings demonstrate that combining knowledge of the large-scale galaxy environment, the absolute gas metallicities, and relative metal abundance ratios provide a potent tool to fully resolve the nature of chemically enriched gaseous halos around galaxies.
see also: Boettcher, Chen, Zahedy et al. (2021)
The complex relationship between galaxies and the IGM
Much has been written about the ubiquity of outflows in high-redshift (z>2) galaxies. These vigorous outflows are thought to play a significant role in distributing metals to the IGM and enriching the Universe with heavy elements. In contrast, direct detections of gas accretion onto individual high-redshift galaxies remain elusive as they must rely on multiple strands of evidence —essentially requiring two/three-dimensional information— that can only be provided by simultaneous observations of gas and stars in emission. Observing more typical forms of galactic accretion requires the ability to simultaneously detect fainter galaxies and fainter nebular emission features.
As a Brinson Predoctoral Fellow at Carnegie Observatories, I studied one such environment : a grouping of low-mass, Lyman-alpha emitting galaxies at z~2.8 previously discovered through a blind spectroscopic search. These Ly-alpha emitters (LAEs) are kinematically coincident with HI and metal absorbers discovered along a nearby background sightline. Careful photoionization modeling constrains the gas metallicity to between ~0.001 and ~0.01 solar, which is at least 10x lower than the expected metal enrichment level in the ISM of these faint galaxies but is consistent with the IGM metallicity at the same epoch. The projected spatial alignment of these galaxies, together with the observed gas kinematics and the detection of a rare, blue-dominant Ly-alpha emission line profile in one of the galaxies, indicates that the absorbing gas originates in an IGM accretion filament that is feeding these low-mass galaxies.
see also: Connor, Zahedy et al. (2019)