Engineering topics

Gruendl, Robert A.

Publications and source records attributed to Gruendl, Robert A..

6 records

DOE OSTI2022

Dwarf AGNs from Optical Variability for the Origins of Seeds (DAVOS): insights from the dark energy survey deep fields

ABSTRACT We present a sample of 706, z < 1.5 active galactic nuclei (AGNs) selected from optical photometric variability in three of the Dark Energy Survey (DES) deep fields (E2, C3, and X3) over an area of 4.64 deg2. We construct light curves using difference imaging aperture photometry for resolved sources and non-difference imaging PSF photometry for unresolved sources, respectively, and characterize the variability significance. Our DES light curves have a mean cadence of 7 d, a 6-yr baseline, and a single-epoch imaging depth of up to g ∼ 24.5. Using spectral energy distribution (SED) fitting, we find 26 out of total 706 variable galaxies are consistent with dwarf galaxies with a reliable stellar mass estimate ($M_{\ast }\lt 10^{9.5}\, {\rm M}_\odot$; median photometric redshift of 0.9). We were able to constrain rapid characteristic variability time-scales (∼ weeks) using the DES light curves in 15 dwarf AGN candidates (a subset of our variable AGN candidates) at a median photometric redshift of 0.4. This rapid variability is consistent with their low black hole (BH) masses. We confirm the low-mass AGN nature of one source with a high S/N optical spectrum. We publish our catalogue, optical light curves, and supplementary data, such as X-ray properties and optical spectra, when available. We measure a variable AGN fraction versus stellar mass and compare to results from a forward model. This work demonstrates the feasibility of optical variability to identify AGNs with lower BH masses in deep fields, which may be more ‘pristine’ analogues of supermassive BH seeds.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

Candidate Periodically Variable Quasars from the Dark Energy Survey and the Sloan Digital Sky Survey

Periodically variable quasars have been suggested as close binary supermassive black holes. We present a systematic search for periodic light curves in 625 spectroscopically confirmed quasars with a median redshift of 1.8 in a 4.6 deg 2 overlapping region of the Dark Energy Survey Supernova (DES-SN) fields and the Sloan Digital Sky Survey Stripe 82 (SDSS-S82). Our sample has a unique 20-year long multi-color ( griz ) light curve enabled by combining DES-SN Y6 observations with archival SDSS-S82 data. The deep imaging allows us to search for periodic light curves in less luminous quasars (down to r ~23.5 mag) powered by less massive black holes (with masses ≳ 10 8.5 M ⊙ ) at high redshift for the first time. We find five candidates with significant (at >99.74% single-frequency significance in at least two bands with a global p-value of ~7 × 10 -4 –3× 10 -3 accounting for the look-elsewhere effect) periodicity with observed periods of ~3–5 years (i.e., 1–2 years in rest frame) having ~4–6 cycles spanned by the observations. If all five candidates are periodically variable quasars, this translates into a detection rate of |${\sim }0.8^{+0.5}_{-0.3}$| % or |${\sim }1.1^{+0.7}_{-0.5}$| quasar per deg 2 . Our detection rate is 4–80 times larger than those found by previous searches using shallower surveys over larger areas. This discrepancy is likely caused by differences in the quasar populations probed and the survey data qualities. Furthermore, we discuss implications on the future direct detection of low-frequency gravitational waves. Additionally, continued photometric monitoring will further assess the robustness and characteristics of these candidate periodic quasars to determine their physical origins.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

Dark Energy Survey Identification of A Low-Mass Active Galactic Nucleus at Redshift 0.823 from Optical Variability

We report the identification of a low-mass active galactic nucleus (AGN), DES J0218−0430, in a redshift z = 0.823 galaxy in the Dark Energy Survey (DES) Supernova field. We select DES J0218−0430 as an AGN candidate by characterizing its long-term optical variability alone based on DES optical broad-band light curves spanning over 6 yr. An archival optical spectrum from the fourth phase of the Sloan Digital Sky Survey shows both broad Mg ii and broad H β lines, confirming its nature as a broad-line AGN. Archival XMM–Newton X-ray observations suggest an intrinsic hard X-ray luminosity of |$L_{{\rm 2-12\, keV}}\approx 7.6\pm 0.4\times 10^{43}$| erg s^−1, which exceeds those of the most X-ray luminous starburst galaxies, in support of an AGN driving the optical variability. Based on the broad H β from SDSS spectrum, we estimate a virial black hole (BH) mass of M_• ≈ 10^6.43–10^6.72 M_⊙ (with the error denoting the systematic uncertainty from different calibrations), consistent with the estimation from OzDES, making it the lowest mass AGN with redshift > 0.4 detected in optical. We estimate the host galaxy stellar mass to be M_* ≈ 10^10.5 ± 0.3 M_⊙ based on modelling the multiwavelength spectral energy distribution. DES J0218−0430 extends the M_•–M_* relation observed in luminous AGNs at z ∼ 1 to masses lower than being probed by previous work. Our work demonstrates the feasibility of using optical variability to identify low-mass AGNs at higher redshift in deeper synoptic surveys with direct implications for the upcoming Legacy Survey of Space and Time at Vera C. Rubin Observatory.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2003

Hot Gas in Planetary Nebulae

It was successfully obtained the FUSE spectra of all targets awarded. The analysis of the spectra has been a complex task due to the superposition of the P-Cygni profile from the wind of the central star and absorption components from low ionization and molecular species in the nebular shell. In six of the eight targets there are narrow O VI absorption components that may arise from the interface layer between the hot (l0(exp 6) K) interior gas and the surrounding warm (l0(exp 4) K) dense nebular shell. To better determine whether these narrow O VI absorption lines arise from the interface region we have obtained ground-based high-dispersion spectroscopic observations of the central star and nebula to pin-point the precise line-of-sight velocity of the nebular emission lines. The comparison between these optical spectra with the far-UV spectra obtained with FUSE is complete. The analysis shows that in most cases the narrow O VI absorption components have velocities slightly redshifted from the emission lines which arise from the approaching side of the nebular shell. Preliminary results have been published in two papers.

Gruendl, Robert A.↗

NASA NTRS2001

Origin of X-ray Emission from Planetary Nebulae

We proposed to use the entire ROSAT archive to search for observations of Galactic planetary nebulae, and analyze these observations, in order to determine the origin of X-ray emission from planetary nebulae. We have cross-correlated the ROSAT archive with a catalog of Galactic planetary nebulae and found useful ROSAT observations for approx. 80 planetary nebulae. These observations have been analyzed. X-ray emission is detected in 13 nebulae. The X-ray spectra and luminosities of these positive detections have been analyzed, and the 3-sigma upper limits are determined for the non-detections. The X-ray properties of planetary nebulae are correlated with the distance and extinction of the nebula, and the temperature and surface gravity of the central star, in order to explain the observed X-ray emission. The results are reported in a conference and a paper that has been published in the Astrophysical Journal Supplement. We conclude that this program has been successfully completed.

Chu, You-Hua↗

NASA NTRS1993

Interaction of NGC 2276 with the NGC 2300 group - Fabry-Perot observations of the H-alpha velocity field

We report kinematic observations of H-alpha emission from the spiral galaxy NGC 2276 obtained with a Fabry-Perot Camera. The 'bow shock' appearance and enhanced star formation in NGC 2276 have been attributed by Mulchaey et al. (1993) to a ram-pressure interaction with the dense IGM detected in ROSAT observations of the NGC 2300 group of galaxies. Along the 'bow shock' limb of the galaxy, we observe strong H-alpha emission and significant kinematic perturbations located immediately interior to an abrupt decrease in the scale length of the optical disk. Although ram-pressure forces may be important in the evolution of the outer gaseous disk, the peculiar kinematics and the truncation in the stellar disk are difficult to explain in a ram-pressure model; a more likely cause is tidal interaction, probably with the elliptical galaxy NGC 2300.

Gruendl, Robert A.↗