Aparna Venkatesan

Aparna Venkatesan
University of San Francisco | USFCA · Department of Physics and Astronomy

About

70
Publications
2,220
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914
Citations

Publications

Publications (70)
Preprint
Full-text available
Our planet and our species are at an existential crossroads. In the long term, climate change threatens to upend life as we know it, while the ongoing COVID-19 pandemic revealed that the world is unprepared and ill-equipped to handle acute shocks to its many systems. These shocks exacerbate the inequities and challenges already present prior to COV...
Preprint
Active galactic nuclei (AGN) forming in the early universe are thought to be the primary source of hard ionizing photons contributing to the reionization of intergalactic helium. However, the number density and spectral properties of high-redshift AGN remain largely unconstrained. In this work, we make use of physically-informed models calibrated w...
Article
Active galactic nuclei (AGN) forming in the early universe are thought to be the primary source of hard ionizing photons contributing to the reionization of intergalactic helium. However, the number density and spectral properties of high-redshift AGN remain largely unconstrained. In this work, we make use of physically-informed models calibrated w...
Preprint
As asteroid mining moves toward reality, the high bar to entering the business may limit participation and increase inequality, reducing or eliminating any benefit gained by marginalized people or developing nations. Consideration of ethical issues is urgently needed, as well as participation in international, not merely multilateral, solutions.
Article
Near-Earth space is becoming increasingly privatized, with the number of satellites in low-Earth orbits predicted to grow dramatically from about 2,000 at present to over 100,000 in the next decade due to the launch of planned satellite constellations. In addition to their direct impact on astronomy, the manner and pace of ‘occupying’ near-Earth sp...
Preprint
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We recommend that the planetary science and space exploration community engage in a robust reevaluation concerning the ethics of how future crewed and uncrewed missions to the Moon and Mars will interact with those planetary environments. This should occur through a process of community input, with emphasis on how such missions can resist colonial...
Preprint
Astronomy across world cultures is rooted in Indigenous Knowledge. We share models of partnering with indigenous communities involving Collaboration with Integrity to co-create an inclusive scientific enterprise on Earth and in space.
Preprint
Galaxies forming in low-mass halos are thought to be primarily responsible for reionizing the Universe during the first billion years after the Big Bang. Yet, these halos are extremely inefficient at forming stars in the nearby Universe. In this work, we address this apparent tension, and ask whether a physically motivated model of galaxy formation...
Article
Astronomy across world cultures is rooted in indigenous knowledge. We share models of partnering with indigenous communities involving collaboration with integrity to co-create an inclusive scientific enterprise on Earth and in space.
Preprint
As the oldest science common to all human cultures, astronomy has a unique connection to indigenous knowledge (IK) and the long history of indigenous scientific contributions. Many STEM disciplines, agencies and institutions have begun to do the work of recruiting and retaining underrepresented minorities, including indigenous, Native American and...
Preprint
The Committee on the Status of Women in Astronomy (CSWA) is calling on federal science funding agencies, in their role as the largest sources of funding for astronomy in the United States, to take actions that will advance career development and improve workplace conditions for women and minorities in astronomy. Funding agencies can and should lead...
Preprint
The Committee on the Status of Women in Astronomy (CSWA) is calling on federal science funding agencies, in their role as the largest sources of funding for astronomy in the United States, to take actions that will end harassment, particularly sexual harassment, in astronomical workplaces. Funding agencies can and should lead the charge to end hara...
Preprint
Organizations that support science (astronomy) such as federal agencies, research centers, observatories, academic institutions, societies, etc. employ advisory committees and boards as a mechanism for reviewing their activities and giving advice on practices, policies and future directions. As with any scientific endeavor, there is concern over co...
Preprint
The US professional astronomy and astrophysics fields are not representative of the diversity of people in the nation. For example, 2017 AIP reports show that in 2014, women made up only about 20 percent of the faculty in astronomy and physics departments, and the numbers for under-represented minorities (men and women) were, and remain, low. Howev...
Preprint
This White Paper highlights the role Primarily Undergraduate Institutions (PUIs) play within the astronomy profession, addressing issues related to employment, resources and support, research opportunities and productivity, and educational and societal impacts, among others. Astronomers working at PUIs are passionate about teaching and mentoring un...
Preprint
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The NSF-sponsored Undergraduate ALFALFA Team (UAT) promotes long-term collaborative research opportunities for faculty and students from 23 U.S. public and private primarily undergraduate institutions (PUIs) within the context of the extragalactic ALFALFA HI blind legacy survey project. Over twelve project years of partnering with Arecibo and Green...
Preprint
Full-text available
The cosmic dawn and epoch of reionization mark the time period in the universe when stars, galaxies, and blackhole seeds first formed and the intergalactic medium changed from neutral to an ionized one. Despite substantial progress with multi-wavelength observations, astrophysical process during this time period remain some of the least understood...
Article
The chronic underrepresentation of Native and indigenous peoples in STEM fields (Fig. 1) has been a longstanding issue in the United States, despite concentrated efforts by many local and national groups, including the Society for Advancement of Chicanos/Hispanics and Native Americans in Science (SACNAS) and the American Indian Science and Engineer...
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We present H I spectral-line imaging of the extremely metal-poor galaxy DDO 68. This system has a nebular oxygen abundance of only ~3% Z ☉, making it one of the most metal-deficient galaxies known in the local volume. Surprisingly, DDO 68 is a relatively massive and luminous galaxy for its metal content, making it a significant outlier in the mass-...
Article
The first metal-free stars in the universe had hard ionizing photon spectra and unique element yields from their supernovae, leaving signatures in the reionization of the intergalactic medium and in the metal enrichment of gas in the early universe. Here, we examine the metal abundances in a variety of systems in the local universe, from very metal...
Article
Dwarf galaxies are thought to be extremely numerous at early times, and the possible building blocks of many of the galaxies in the modern-day universe. In this all-too-short talk, I will present recent calculations of the escape fraction of ionizing radiation from early low-mass galaxies and their potential impact on cosmic reionization. I will al...
Article
Full-text available
The escape of ionizing radiation from galaxies plays a critical role in the evolution of gas in galaxies, and the heating and ionization history of the intergalactic medium. We present semi-analytic calculations of the escape fraction of ionizing radiation for both hydrogen and helium from galaxies ranging from primordial systems to disk-type galax...
Article
We present calculations of the escape fraction of ionizing radiation from galaxies in the early universe. Such galaxies are the formation sites of the first stars and quasars, which are strong sources of hard ionizing radiation. The escape fraction is a critical input parameter for any cosmological code or simulation that tracks hydrogen or helium...
Article
The first stars and quasars are known sources of hard ionizing radiation in the first billion years of the Universe. We examine the joint effects of X-rays and hard ultraviolet (UV) radiation from such first-light sources on the hydrogen and helium reionization of the intergalactic medium (IGM) at early times, and the associated heating. We study t...
Article
Full-text available
The first stars and quasars are known sources of hard ionizing radiation in the first billion years of the Universe. We examine the joint effects of X-rays and hard UV radiation from such first-light sources on the hydrogen and helium reionization of the intergalactic medium (IGM) at early times, and the associated heating. We study the growth and...
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We perform a detailed and systematic investigation of the possible impacts of dark stars upon the reionization history of the Universe, and its signatures in the cosmic microwave background (CMB). We compute hydrogen reionization histories, CMB optical depths and anisotropy power spectra for a range of stellar populations including dark stars. If d...
Article
The thermal and ionization history of the intergalactic medium (IGM) strongly affects the visibility of distant galaxies, and interactions with the microwave background. We investigate the joint effects of X-rays and helium-ionizing radiation from the first galaxies and quasars on the topology of reionization in the IGM and its heating history, usi...
Article
We investigate the minimum timescales required for the universe to generate the biogenic elements, i.e., the elements that are essential for life. Using recent models of stellar evolution and nucleosynthesis, we calculate the concentrations of the alpha-elements and iron relative to hydrogen that build up after a few generations of early metal-free...
Article
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The cosmic dark ages are the mysterious epoch during which the pristine gas began to condense and ultimately form the first stars. Although these beginnings have long been a topic of theoretical interest, technology has only recently allowed the beginnings of observational insight into this epoch. Many questions surround the formation of stars in m...
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(Brief Summary) What is the total radiative content of the Universe since the epoch of recombination? The extragalactic background light (EBL) spectrum captures the redshifted energy released from the first stellar objects, protogalaxies, and galaxies throughout cosmic history. Yet, we have not determined the brightness of the extragalactic sky fro...
Article
The energetics, spectrum, and composition of cosmic rays with energies below ~1015 eV are fairly well explained by models involving supernova shocks. In contrast, no widely accepted theory exists for the origin of ultra-high-energy cosmic rays (UHECRs), which have energies above 1015 eV. Instead of proposing a specific model, here we place strong c...
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We examine the constraints on high-redshift star formation, ultraviolet and X-ray pre-ionization, and the epoch of reionization at redshift z_r, inferred from the recent WMAP-5 measurement, tau_e = 0.084 +/- 0.016, of the electron scattering optical depth of the cosmic microwave background (CMB). Half of this scattering can be accounted for by the...
Article
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We examine the constraints on the epoch of reionization (redshift z_r) set by recent WMAP-3 observations of tau_e = 0.09 +/- 0.03, the electron-scattering optical depth of the cosmic microwave background (CMB), combined with models of high-redshift galaxy and black hole formation. Standard interpretation begins with the computed optical depth, tau_...
Article
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Theoretical studies and current observations of the high-redshift intergalactic medium (IGM) indicate that at least two cosmic transitions occur by the time the universe reaches gas metallicities of about $10^{-3}$ of solar values. These are the cosmological reionization of the IGM, and the transition from a primordial to present-day mode of star f...
Article
We discuss the cosmological significance of the transition from the Pop III to Pop II mode of star formation in the early universe, and when and how it may occur in primordial galaxies. Observations that could detect this transition include those of element abundances in metal-poor Galactic halo stars, and of the helium reionization and associated...
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We investigate the radiative transport of dust in primordial galaxies in the presence of the UV radiation field from the first metal-free stars. We find that dust created in the first supernova (SN) explosions can be driven through the interior of the SN remnant to accumulate in the SN shells, where second-generation stars may form from compressed...
Article
We present constraints on the masses and ionizing efficiencies of the first stars by considering current data on reionization, the microwave background, and the metal enrichment of the high-redshift intergalactic medium (IGM) and of quasar broad emission line regions. We find, in contrast to some recent studies, that these observations indicate tha...
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We critique the hypothesis that the first stars were very massive stars (VMS; M > 140 Msun). We review the two major lines of evidence for the existence of VMS: (1) that the relative metal abundances of extremely metal-poor Galactic halo stars show evidence of VMS enrichment, and (2) that the high electron-scattering optical depth (tau_e) to the CM...
Article
Studies of the broad emission-line regions (BLRs) in quasars have revealed solar or higher enrichment levels up to the highest redshifts. In combination with the presence of large amounts of dust in QSOs at $z$ about 6, this implies that substantial amounts of star formation and nucleosynthesis took place at significantly earlier epochs. Here, we e...
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We investigate whether a single population of first stars could have influenced both the metal enrichment and reionization of the high-redshift intergalactic medium (IGM), by calculating the generated ionizing radiation per unit metal yield as a function of the metallicity of stellar populations. We examine the relation between the ionizing radiati...
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We investigate the chemo-dynamical effects of multiple supernova explosions in the central region of primordial galaxies using three-dimensional hydrodynamical simulations of the inhomogenous interstellar medium down to parsec-scales. We find that the final protogalactic structure and metal distribution depend strongly on the number of SNe. Specifi...
Article
The first stars have received attention as the leading candidates for the sources that reionized the IGM at redshifts z > 6. These stars are hotter and smaller than their counterparts of finite metallicity. We present recent results from an ongoing study of metal-free stars from a cosmological point of view. We have calculated evolving spectra of P...
Article
Full-text available
The first stars hold intrinsic interest for their uniqueness and for their potential importance to galaxy formation, chemical enrichment, and feedback on the intergalactic medium (IGM). The metal-free composition of the first stars restricts the stellar energy source to proton-proton burning rather than the more efficient CNO cycle. Consequently th...
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We examine the significance of the first metal-free stars (Pop III) for the cosmological reionization of HI and HeII. These stars have unusually hard spectra, with the integrated ionizing photon rates from a Pop III stellar cluster for HI and HeII being 1.6 and $10^5$ times stronger respectively than those from a Pop II cluster. For the currently f...
Article
Spectroscopic studies of high-redshift objects and increasingly precise data on the cosmic microwave background (CMB) are beginning to independently place strong complementary bounds on the epoch of hydrogen reionization. Here, we present two methods of using the reionization epoch to constrain the amplitude and index of the primordial scalar power...
Article
Full-text available
Spectroscopic studies of high-redshift objects and increasingly precise data on the cosmic microwave background (CMB) are beginning to independently place strong complementary bounds on the epoch of hydrogen reionization. Parameter estimation from current CMB data continues, however, to be subject to several degeneracies. Here, we focus on those de...
Article
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Observational studies indicate that the intergalactic medium (IGM) is highly ionized up to redshifts just over 6. A number of models have been developed to describe the process of reionization and the effects of the ionizing photons from the first luminous objects. In this paper, we study the impact of an X-ray background, such as high-energy photo...
Article
Observational studies indicate that the intergalactic medium (IGM) is highly ionized up to redshifts of about 6. Here, we examine the effects of an X-ray background, such as high-energy photons from early quasars, on the temperature and ionization of the IGM before reionization has occurred, i.e., before the fully ionized bubbles associated with in...
Article
Full-text available
At present, there are several feasible observational probes of the first stars in the universe. Here, we examine the constraints on early stellar activity from the metallicity of the high-redshift Ly-$\alpha$ clouds, from the effects of stellar ionizing photons on reionization and the cosmic microwave background (CMB), and from the implications of...
Article
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Current data of high-redshift absorption-line systems imply that hydrogen reionization occurred before redshifts of about 5. Previous works on reionization by the first stars or quasars have shown that such scenarios are described by a large number of cosmological and astrophysical parameters. Here, we adopt a semi-analytic model of stellar reioniz...
Article
Observations of high-redshift quasar spectra have revealed that the intergalactic medium is highly ionized up to redshifts of about 5, implying that universal reionization occurred before then. A number of authors have recently investigated reionization by the first stars or quasars through semi-analytic models or numerical simulations. A large num...
Article
Full-text available
We consider the contribution of neutron stars and black holes to the dynamical mass of galactic halos. In particular, we show that if these compact objects were produced by an early generation of stars with initial metallicity {approx_lt}10{sup {minus}4} {ital Z}{sub {circle_dot}}, they can contribute at most 30{percent}{endash}40{percent} of the G...
Article
After several years of ongoing microlensing experiments, the nature of the lenses in the Galactic halo, or MACHOs, is still unknown. The data, assuming a standard isothermal halo, imply a most likely lens mass in the white dwarf regime. However, the lens mass estimate depends upon the phase space distribution of the lenses within the halo. Recently...
Article
The origin of ultra-high energy cosmic rays (UHECR's) is still unknown. Proposed models involve either extragalactic sources (constrained to originate from within a distance of about 100 Mpc) or sources in our galaxy; galactic models frequently involve processes related to neutron stars. Here, we evaluate the proposal that neutron stars in our gala...
Article
I will share the results of having used some of the exercises and techniques of science integration in astronomy courses that I teach through the University of Colorado Upward Bound (CUUB) Programs. CUUB targets Native American high school students from economically disadvantaged backgrounds from as many as eighteen tribes and ten states, typically...

Projects

Project (1)
Project
In anticipation of the upcoming launch of the James Webb Space Telescope (JWST), we provide detailed predictions for a wide variety of observable and physical properties of high redshift galaxies (z~4-10). Project website here: https://www.simonsfoundation.org/semi-analytic-forecasts-for-jwst/