arXiv subjects

Kirsten Hall

Publications and source records attributed to Kirsten Hall.

7 recordsLinked to original sources

arXiv2021

Lower-Luminosity Obscured AGN Host Galaxies are Not Predominantly in Major-Merging Systems at Cosmic Noon

For over 60 years, the scientific community has studied actively growing central super-massive black holes (active galactic nuclei -- AGN) but fundamental questions on their genesis remain unanswered. Numerical simulations and theoretical arguments show that black hole growth occurs during short-lived periods ($\sim$ 10$^{7}$ -10$^{8}$ yr) of powerful accretion. Major mergers are commonly invoked as the most likely dissipative process to trigger the rapid fueling of AGN. If the AGN-merger paradigm is true, we expect galaxy mergers to coincide with black hole accretion during a heavily obscured AGN phase (N$_H$ $ > 10^{23}$ cm$^{-2}$). Starting from one of the largest samples of obscured AGN at 0.5 $<$ $z$ $<$ 3.1, we select 40 non-starbursting lower-luminosity obscured AGN. We then construct a one-to-one matched redshift- and near-IR magnitude-matched non-starbursting inactive galaxy control sample. Combining deep color \textit{Hubble Space Telescope} imaging and a novel method of human classification, we test the merger-AGN paradigm prediction that heavily obscured AGN are strongly associated with galaxies undergoing a major merger. On the total sample of 80 galaxies, we estimate each individual classifier's accuracy at identifying merging galaxies/post-merging systems and isolated galaxies. We calculate the probability of each galaxy being in either a major merger or isolated system, given the accuracy of the human classifiers and the individual classifications of each galaxy. We do not find statistically significant evidence that obscured AGN at cosmic noon are predominately found in systems with evidence of significant merging/post-merging features.

astro-ph.GA↗

arXiv2020

The Atacama Cosmology Telescope: DR4 Maps and Cosmological Parameters

We present new arcminute-resolution maps of the Cosmic Microwave Background temperature and polarization anisotropy from the Atacama Cosmology Telescope, using data taken from 2013-2016 at 98 and 150 GHz. The maps cover more than 17,000 deg$^2$, the deepest 600 deg$^2$ with noise levels below 10 $μ$K-arcmin. We use the power spectrum derived from almost 6,000 deg$^2$ of these maps to constrain cosmology. The ACT data enable a measurement of the angular scale of features in both the divergence-like polarization and the temperature anisotropy, tracing both the velocity and density at last-scattering. From these one can derive the distance to the last-scattering surface and thus infer the local expansion rate, $H_0$. By combining ACT data with large-scale information from WMAP we measure $H_0 = 67.6 \pm 1.1$ km/s/Mpc, at 68% confidence, in excellent agreement with the independently-measured Planck satellite estimate (from ACT alone we find $H_0 = 67.9 \pm 1.5$ km/s/Mpc). The $Λ$CDM model provides a good fit to the ACT data, and we find no evidence for deviations: both the spatial curvature, and the departure from the standard lensing signal in the spectrum, are zero to within 1$σ$; the number of relativistic species, the primordial Helium fraction, and the running of the spectral index are consistent with $Λ$CDM predictions to within $1.5 - 2.2σ$. We compare ACT, WMAP, and Planck at the parameter level and find good consistency; we investigate how the constraints on the correlated spectral index and baryon density parameters readjust when adding CMB large-scale information that ACT does not measure. The DR4 products presented here will be publicly released on the NASA Legacy Archive for Microwave Background Data Analysis.

astro-ph.CO↗

arXiv2020

The Atacama Cosmology Telescope: A Measurement of the Cosmic Microwave Background Power Spectra at 98 and 150 GHz

We present the temperature and polarization angular power spectra of the CMB measured by the Atacama Cosmology Telescope (ACT) from 5400 deg$^2$ of the 2013-2016 survey, which covers $>$15000 deg$^2$ at 98 and 150 GHz. For this analysis we adopt a blinding strategy to help avoid confirmation bias and, related to this, show numerous checks for systematic error done before unblinding. Using the likelihood for the cosmological analysis we constrain secondary sources of anisotropy and foreground emission, and derive a "CMB-only" spectrum that extends to $\ell=4000$. At large angular scales, foreground emission at 150 GHz is $\sim$1% of TT and EE within our selected regions and consistent with that found by Planck. Using the same likelihood, we obtain the cosmological parameters for $Λ$CDM for the ACT data alone with a prior on the optical depth of $τ=0.065\pm0.015$. $Λ$CDM is a good fit. The best-fit model has a reduced $χ^2$ of 1.07 (PTE=0.07) with $H_0=67.9\pm1.5$ km/s/Mpc. We show that the lensing BB signal is consistent with $Λ$CDM and limit the celestial EB polarization angle to $ψ_P =-0.07^{\circ}\pm0.09^{\circ}$. We directly cross correlate ACT with Planck and observe generally good agreement but with some discrepancies in TE. All data on which this analysis is based will be publicly released.

astro-ph.CO↗

arXiv2020

Atacama Cosmology Telescope: Dusty star-forming galaxies and active galactic nuclei in the equatorial survey

We present a catalog of 510 radio-loud active galactic nuclei (AGN, primarily blazars) and 287 dusty star-forming galaxies (DSFGs) detected by the Atacama Cosmology Telescope at $>5σ$ significance in bands centered on 148 GHz (2 mm), 218 GHz (1.4 mm) and 277 GHz (1.1 mm), from a 480 square degrees strip on the celestial equator with additional (360 square degrees) shallower fields. Combining the deepest available 218 GHz wide-field imaging, 277 GHz data, and multi-band filtering yields the most sensitive wide-field millimeter-wave DSFG selection to date with rms noise referenced to 218 GHz reaching $<2$ mJy. We developed techniques to remove Galactic contamination from the extragalactic catalog, yielding 321 additional Galactic sources. We employ a new flux debiasing method that handles the heterogeneous sample selection due to Galactic cuts. We present spectral properties and source counts of the AGN and DSFGs. The DSFG spectra depart from an optically thin modified blackbody between 218 GHz and 277 GHz, consistent with optically thick emission or an additional cold dust component. For bright AGN, the inter-year RMS fractional deviation in flux density from source variability is $\sim40\%$. We report 8$-$2870 mJy source counts for AGN and 8$-$90 mJy source counts for DSFGs, the latter probing both the brighter, lensed population and the fainter, unlensed population. At 277 GHz we report the first source counts measurements at these flux densities, finding an excess above most model count predictions. Finally, we select thirty of the brightest DSFGs for multi-frequency study as candidate high-$z$ lensed systems.

astro-ph.GA↗

arXiv2019

Astro2020 APC White Paper: The Early Career Perspective on the Coming Decade, Astrophysics Career Paths, and the Decadal Survey Process

In response to the need for the Astro2020 Decadal Survey to explicitly engage early career astronomers, the National Academies of Sciences, Engineering, and Medicine hosted the Early Career Astronomer and Astrophysicist Focus Session (ECFS) on October 8-9, 2018 under the auspices of Committee of Astronomy and Astrophysics. The meeting was attended by fifty six pre-tenure faculty, research scientists, postdoctoral scholars, and senior graduate students, as well as eight former decadal survey committee members, who acted as facilitators. The event was designed to educate early career astronomers about the decadal survey process, to solicit their feedback on the role that early career astronomers should play in Astro2020, and to provide a forum for the discussion of a wide range of topics regarding the astrophysics career path. This white paper presents highlights and themes that emerged during two days of discussion. In Section 1, we discuss concerns that emerged regarding the coming decade and the astrophysics career path, as well as specific recommendations from participants regarding how to address them. We have organized these concerns and suggestions into five broad themes. These include (sequentially): (1) adequately training astronomers in the statistical and computational techniques necessary in an era of "big data", (2) responses to the growth of collaborations and telescopes, (3) concerns about the adequacy of graduate and postdoctoral training, (4) the need for improvements in equity and inclusion in astronomy, and (5) smoothing and facilitating transitions between early career stages. Section 2 is focused on ideas regarding the decadal survey itself, including: incorporating early career voices, ensuring diverse input from a variety of stakeholders, and successfully and broadly disseminating the results of the survey.

astro-ph.IM↗

arXiv2018

An ngVLA Wide Area AGN Survey

The next generation Very Large Array (ngVLA) will have unprecedented sensitivities and mapping speeds at $1-8$ GHz. We discuss how the active galactic nuclei (AGN) community can benefit from a wide-area, medium depth ngVLA survey. We propose a 10 deg$^2$ survey in the Stripe 82 field using the 8 GHz band with an rms depth of $1\,μ$Jy beam$^{-1}$. We will detect $\sim$130,000 galaxies, including radio-quiet AGN out to $z\sim7$. We can measure the luminosity and space density evolution of radio-quiet and radio-loud AGN. We can also measure AGN evolution through clustering of both populations using cross-correlation functions. A wide area ngVLA survey will benefit from existing multiwavelength AGN populations, particularly in the Stripe 82 field, as well as new information from next-generation optical and infrared survey instruments such as LSST and WFIRST.

astro-ph.GA↗

arXiv2016

Evidence for the Thermal Sunyaev-Zel'dovich Effect Associated with Quasar Feedback

Using a radio-quiet subsample of the Sloan Digital Sky Survey spectroscopic quasar catalogue, spanning redshifts 0.5-3.5, we derive the mean millimetre and far-infrared quasar spectral energy distributions (SEDs) via a stacking analysis of Atacama Cosmology Telescope and Herschel-Spectral and Photometric Imaging REceiver data. We constrain the form of the far-infrared emission and find 3$σ$-4$σ$ evidence for the thermal Sunyaev-Zel'dovich (SZ) effect, characteristic of a hot ionized gas component with thermal energy $(6.2 \pm 1.7)\times 10^{60}$ erg. This amount of thermal energy is greater than expected assuming only hot gas in virial equilibrium with the dark matter haloes of $(1-5)\times 10^{12}h^{-1}$M$_\odot$ that these systems are expected to occupy, though the highest quasar mass estimates found in the literature could explain a large fraction of this energy. Our measurements are consistent with quasars depositing up to $(14.5 \pm 3.3)~τ_8^{-1}$ per cent of their radiative energy into their circumgalactic environment if their typical period of quasar activity is $τ_8\times~10^8$ yr. For high quasar host masses, $\sim10^{13}h^{-1}$M$_\odot$, this percentage will be reduced. Furthermore, the uncertainty on this percentage is only statistical and additional systematic uncertainties enter at the 40 per cent level. The SEDs are dust dominated in all bands and we consider various models for dust emission. While sufficiently complex dust models can obviate the SZ effect, the SZ interpretation remains favoured at the 3$σ$-4$σ$ level for most models.

astro-ph.GA↗