Resolving The Moth at Millimeter Wavelengths

HD 61005, also known as "The Moth," is one of only a handful of debris disks that exhibit swept-back "wings" thought to be caused by interaction with the ambient interstellar medium (ISM). We present 1.3 mm Submillimeter Array (SMA) observations of the debris disk around HD 61005 at a spatial resolution of 1.9 arcsec that resolve the emission from large grains for the first time. The disk exhibits a double-peaked morphology at millimeter wavelengths, consistent with an optically thin ring viewed close to edge-on. To investigate the disk structure and the properties of the dust grains we simultaneously model the spatially resolved 1.3 mm visibilities and the unresolved spectral energy distribution. The temperatures indicated by the SED are consistent with expected temperatures for grains close to the blowout size located at radii commensurate with the millimeter and scattered light data. We also perform a visibility-domain analysis of the spatial distribution of millimeter-wavelength flux, incorporating constraints on the disk geometry from scattered light imaging, and find suggestive evidence of wavelength-dependent structure. The millimeter-wavelength emission apparently originates predominantly from the thin ring component rather than tracing the "wings" observed in scattered light. The implied segregation of large dust grains in the ring is consistent with an ISM-driven origin for the scattered light wings.Comment: 10 pages, 6 figure

The Event Horizon Telescope: exploring strong gravity and accretion physics

The Event Horizon Telescope (EHT), a global sub-millimeter wavelength very long baseline interferometry array, is now resolving the innermost regions around the supermassive black holes Sgr A* and M87. Using black hole images from both simple geometric models and relativistic magnetohydrodynamical accretion flow simulations, we perform a variety of experiments to assess the promise of the EHT for studying strong gravity and accretion physics during the stages of its development. We find that (1) the addition of the LMT and ALMA along with upgraded instrumentation in the "Complete" stage of the EHT allow detection of the photon ring, a signature of Kerr strong gravity, for predicted values of its total flux; (2) the inclusion of coherently averaged closure phases in our analysis dramatically improves the precision of even the current array, allowing (3) significantly tighter constraints on plausible accretion models and (4) detections of structural variability at the levels predicted by the models. While observations at 345 GHz circumvent problems due to interstellar electron scattering in line-of-sight to the galactic center, short baselines provided by CARMA and/or the LMT could be required in order to constrain the overall shape of the accretion flow. Given the systematic uncertainties in the underlying models, using the full complement of two observing frequencies (230 and 345 GHz) and sources (Sgr A* and M87) may be critical for achieving transformative science with the EHT experiment.Comment: 17 pages, 7 figures, 10 tables. Accepted by MNRAS 10 October 201

Resolved Millimeter-wavelength Observations of Debris Disks around Solar-type Stars

The presence of debris disks around young main-sequence stars hints at the existence and structure of planetary systems. Millimeter-wavelength observations probe large grains that trace the location of planetesimal belts. The Formation and Evolution of Planetary Systems Spitzer Legacy survey of nearby young solar analogues yielded a sample of five debris disk-hosting stars with millimeter flux suitable for interferometric follow-up. We present observations with the Submillimeter Array (SMA) and the Combined Array for Research in Millimeter-wave Astronomy at ~2'' resolution that spatially resolve the debris disks around these nearby (d ~ 50 pc) stars. Two of the five disks (HD 377, HD 8907) are spatially resolved for the first time and one (HD 104860) is resolved at millimeter wavelengths for the first time. We combine our new observations with archival SMA and Atacama Large Millimeter/Submillimeter Array data to enable a uniform analysis of the full five-object sample. We simultaneously model the broadband photometric data and resolved millimeter visibilities to constrain the dust temperatures and disk morphologies, and perform a Markov Chain Monte Carlo analysis to fit for basic structural parameters. We find that the radii and widths of the cold outer belts exhibit properties consistent with scaled-up versions of the Solar System's Kuiper Belt. All the disks exhibit characteristic grain sizes comparable to the blowout size, and all the resolved observations of emission from large dust grains are consistent with an axisymmetric dust distribution to within the uncertainties. These results are consistent with comparable studies carried out at infrared wavelengths

Constraints from dwarf galaxies on black hole seeding and growth models with current and future surveys

Dwarf galaxies are considered to be potential ideal test-beds for constraining models of the seeding and tracing of the growth of supermassive and intermediate mass black holes (MBH) via their black hole occupation fraction (BHOF). Disentangling seeding from the confounding effects of mass assembly is, however, challenging. In this work, we use semi-analytical models (SAMs) to probe how various surveys perform at teasing apart different seed and growth scenarios. We check for differences in the measured BHOF given various cuts to black hole mass and AGN luminosity and develop a scheme to robustly compare SAMs, with their intrinsic uncertainties, to X-ray observations. We demonstrate that to tell seeding models apart, we need to detect or model all AGN brighter than $10^{37}\ \rm{erg \ s^{-1}}$ in galaxies of $M_* \sim 10^{8-10} \ \rm{M_{\odot}}$ Shallower surveys, like eRASS, cannot distinguish between seed models even with the compensation of a much larger survey volume. We show that the AMUSE survey strongly favours heavy seed models, growing with empirically motivated growth models either a power-law Eddington Ratio Distribution Function (ERDF) or one in which black hole accretion is tagged to the star-formation rate (AGN-MS). These two growth channels in turn can then be distinguished by the AGN luminosity function at $< 10^{44}\ \rm{erg \ s^{-1}}$. The different models also predict different radio scaling relations, which we quantify using the fundamental plane of black hole activity. We close with recommendations for the design of upcoming multi-wavelength campaigns that can optimally detect MBHs in dwarf galaxies.Comment: Submitted to AAS Journal

X-ray bright AGN in local dwarf galaxies: insights from eROSITA

Although supermassive black holes (SMBHs) reside in the heart of virtually every massive galaxy, it remains debated whether dwarf galaxies also commonly host SMBHs. Because low-mass galaxies may retain a memory of the assembly history of their black holes, probing the black hole occupation fraction of local dwarf galaxies might offer insights into the growth and seeding mechanisms of the first black holes. In this work, we exploit the Western half of the eROSITA all-sky survey (covering $20,000~\rm{deg^2}$) and compile a catalog of accreting SMBHs in local ($D<200$~Mpc) dwarf galaxies. After cleaning our sample from cosmic X-ray background sources, X-ray binaries, and ultraluminous X-ray sources, we identify 74 AGN-dwarf galaxy pairs. Using this large and uniform sample, we derive a luminosity function of dwarf galaxy AGN, fitting it with a power law function and obtaining ${\rm d}N/{\rm d}L_{\rm X} = (15.9\pm2.2)\times L_{\rm X}^{-1.63\pm0.05}$. Measuring the offset between the centroid of dwarf galaxies and the X-ray sources, we find that about $50\%$ of the AGN are likely off-nuclear, in agreement with theoretical predictions. We also compare the black hole-to-stellar mass relation of the AGN in our sample with the local and high-redshift relations, finding that our sources better adhere to the former. This suggests that local AGN across different mass scales underwent a similar growth history. Finally, we compare our sources with semi-analytical models: while our sample is too shallow to distinguish between different seeding models, it favors a growth mechanism linked to the star-formation rate of the host galaxy.Comment: 17 pages, 12 figures, 1 table. Submitted to ApJ. Comments are welcom