Empirical estimates of the Galactic halo contribution to the dispersion measures of extragalactic fast radio bursts using X-ray absorption

ABSTRACT We provide an empirical list of the Galactic dispersion measure (DMGal) contribution to the extragalactic fast radio bursts (FRBs) along 72 sightlines. It is independent of any model of the Galaxy, i.e. we do not assume the density of the disc or the halo, spatial extent of the halo, baryonic mass content, or any such external constraints to measure DMGal. We use 21-cm, UV, EUV, and X-ray data to account for different phases, and find that DMGal is dominated by the hot phase probed by X-ray absorption. We improve upon the measurements of N($\rm{O}\,{\small VII}$) and f$_{\rm O\,{\small VII}}$ compared to previous studies, thus providing a better estimate of the hot phase contribution. The median DMGal = 64$^{+20}_{-23}$ cm−3 pc, with a 68 per cent (90 per cent) confidence interval of 33–172 (23–660) cm−3 pc. The DMGal does not appear to follow any trend with the Galactic longitude or latitude, and there is a large scatter around the values predicted by simple disc + spherical halo models. Our measurements provide more complete and accurate estimates of DMGal independent from the previous studies. We provide a table and a code to retrieve DMGal for any FRB localized in the sky.

Anisotropy of long-period comets explained by their formation process

<p>Long-period comets coming from the Oort cloud are thought to be<span class="Apple-converted-space"> </span>planetesimals formed in the planetary region on the ecliptic plane.<span class="Apple-converted-space"> </span>We have investigated the orbital evolution of these bodies<span class="Apple-converted-space"> </span>due to the Galactic tide.<span class="Apple-converted-space"> </span>We extended Higuchi et al. (2007) and derived the<span class="Apple-converted-space"> </span>analytical solutions to the Galactic longitude and latitude of<span class="Apple-converted-space"> </span>the direction of aphelion, <em>L</em> and <em>B</em>.<span class="Apple-converted-space"> </span>Using the analytical solutions,<span class="Apple-converted-space"> </span>we show that the ratio of the periods of the evolution of <em>L</em> and <em>B</em> is very close to either 2 or ∞ for initial eccentricities <em>e</em><sub>i</sub>∼1,<span class="Apple-converted-space"> </span>as is true for the Oort cloud comets.<span class="Apple-converted-space"> </span>From the relation between <em>L</em> and <em>B</em>, we predict that Oort cloud comets returning to the planetary region concentrate on the ecliptic plane<span class="Apple-converted-space"> </span>and a second plane, which we call the "empty ecliptic".<span class="Apple-converted-space"> </span>This consists in a rotation of 180<sup>°</sup> of the ecliptic around the Galactic pole. Our numerical integrations confirm that the radial component of the Galactic tide, which is neglected in the derivation of the analytical solutions,<span class="Apple-converted-space"> </span>is not strong enough to break the relation between <em>L</em> and <em>B</em> derived analytically. Brief examination of observational data shows<span class="Apple-converted-space"> </span>that there are concentrations near both the ecliptic and the empty ecliptic. We also show that the anomalies of the distribution of <em>B</em> of long-period comets mentioned by several authors are explained by the concentrations on the two planes more consistently than the previous explanation.</p>

The complex large-scale magnetic fields in the first Galactic quadrant as revealed by the Faraday depth profile disparity

ABSTRACT The Milky Way is one of the very few spiral galaxies known to host large-scale magnetic field reversals. The existence of the field reversal in the first Galactic quadrant near the Sagittarius spiral arm has been well established, yet poorly characterized due to the insufficient number of reliable Faraday depths (FDs) from extragalactic radio sources (EGSs) through this reversal region. We have therefore performed broad-band (1–$2\, {\rm GHz}$) spectropolarimetric observations with the Karl G. Jansky Very Large Array (VLA) to determine the FD values of 194 EGSs in the Galactic longitude range of 20°–52° within ±5° from the Galactic mid-plane, covering the Sagittarius arm tangent. This factor of five increase in the EGS FD density has led to the discovery of a disparity in FD values across the Galactic mid-plane in the Galactic longitude range of 40°–52°. Combined with existing pulsar FD measurements, we suggest that the Sagittarius arm can host an odd-parity disc field. We further compared our newly derived EGS FDs with the predictions of three major Galactic magnetic field models, and concluded that none of them can adequately reproduce our observational results. This has led to our development of new, improved models of the Milky Way disc magnetic field that will serve as an important step towards major future improvements in Galactic magnetic field models.

Spatial distribution of exoplanet candidates based on Kepler and Gaia data

Context. Surveying the spatial distribution of exoplanets in the Galaxy is important for improving our understanding of planet formation and evolution. Aims. We aim to determine the spatial gradients of exoplanet occurrence in the Solar neighbourhood and in the vicinity of open clusters. Methods. We combined Kepler and Gaia DR2 data for this purpose, splitting the volume sampled by the Kepler mission into certain spatial bins. We determined an uncorrected and bias-corrected exoplanet frequency and metallicity for each bin. Results. There is a clear drop in the uncorrected exoplanet frequency with distance for F-type stars (mainly for smaller planets), a decline with increasing distance along the Galactic longitude l = 90°, and a drop with height above the Galactic plane. We find that the metallicity behaviour cannot be the reason for the drop of the exoplanet frequency around F stars with increasing distance. This might have only contributed to the drop in uncorrected exoplanet frequency with the height above the Galactic plane. We argue that the above-mentioned gradients of uncorrected exoplanet frequency are a manifestation of a single bias of undetected smaller planets around fainter stars. When we correct for observational biases, most of these gradients in exoplanet frequency become statistically insignificant. Only a slight decline of the planet occurrence with distance for F stars remains significant at the 3σ level. Apart from that, the spatial distribution of exoplanets in the Kepler field of view is compatible with a homogeneous one. At the same time, we do not find a significant change in the exoplanet frequency with increasing distance from open clusters. In terms of byproducts, we identified six exoplanet host star candidates that are members of open clusters. Four of them are in the NGC 6811 (KIC 9655005, KIC 9533489, Kepler-66, Kepler-67) and two belong to NGC 6866 (KIC 8396288, KIC 8331612). Two out of the six had already been known to be cluster members.

Vertical structure and kinematics of the Galactic outer disk

We report on measurements of parallax and proper motion for four 22 GHz water maser sources as part of the VERA Outer Rotation Curve project. All the sources show Galactic latitudes of >2° and Galactocentric distances of >11 kpc at the Galactic longitude range of 95° < l < 126°. The sources trace the Galactic warp reaching to 200–400 pc, and also the signature of the warp to 600 pc toward the north Galactic pole. The new results, along with previous results in the literature, show that the maximum height of the Galactic warp increases with Galactocentric distance. Also, we examined velocities perpendicular to the disk for the sample, and found oscillatory behavior between the vertical velocities and Galactic heights. This behavior suggests the existence of bending (vertical density) waves, possibly induced by a perturbing satellite (e.g., the passage of the Sagittarius dwarf galaxy).

Anomalous peculiar motions of high-mass young stars in the Scutum spiral arm

We present trigonometric parallax and proper motion measurements toward 22 GHz water and 6.7 GHz methanol masers in 16 high-mass star-forming regions. These sources are all located in the Scutum spiral arm of the Milky Way. The observations were conducted as part of the Bar and Spiral Structure Legacy (BeSSeL) survey. A combination of 14 sources from a forthcoming study and 14 sources from the literature, we now have a sample of 44 sources in the Scutum spiral arm, covering a Galactic longitude range from 0° to 33°. A group of 16 sources shows large peculiar motions of which 13 are oriented toward the inner Galaxy. A likely explanation for these high peculiar motions is the combined gravitational potential of the spiral arm and the Galactic bar.

Three-dimensional dust mapping of 12 supernovae remnants in the Galactic anticentre

ABSTRACT We present three-dimensional (3D) dust mapping of 12 supernova remnants (SNRs) in the Galactic anticentre (Galactic longitude l between 150○ and 210○) based on a recent 3D interstellar extinction map. The dust distribution of the regions, which cover the full extents in the radio continuum for the individual SNRs, is discussed. Four SNRs show significant spatial coincidences between molecular clouds (MCs) revealed from the 3D extinction mapping and the corresponding radio features. The results confirm the interactions between these SNRs and their surrounding MCs. Based on these correlations, we provide new distance estimates of the four SNRs, G189.1+3.0 (IC443, $d=1729^{+116}_{-94} \rm \, pc$), G190.9–2.2 ($d=1036^{+17}_{-81} \rm \, pc$), G205.5+0.5 ($d=941^{+96}_{-94}$ or $1257^{+92}_{-101} \rm \, pc$), and G213.0–0.6 ($d=1146^{+79}_{-80} \rm \, pc$). In addition, we find indirect evidences of potential interactions between SNRs and MCs for three other SNRs. New distance constraints are also given for these three SNRs.

New candidate radio supernova remnants detected in the GLEAM survey over 345° < l < 60°, 180° < l < 240°

We have detected 27 new supernova remnants (SNRs) using a new data release of the GLEAM survey from the Murchison Widefield Array telescope, including the lowest surface brightness SNR ever detected, G 0.1 – 9.7. Our method uses spectral fitting to the radio continuum to derive spectral indices for 26/27 candidates, and our low-frequency observations probe a steeper spectrum population than previously discovered. None of the candidates have coincident WISE mid-IR emission, further showing that the emission is non-thermal. Using pulsar associations we derive physical properties for six candidate SNRs, finding G 0.1 – 9.7 may be younger than 10 kyr. Sixty per cent of the candidates subtend areas larger than 0.2 deg2 on the sky, compared to < 25% of previously detected SNRs. We also make the first detection of two SNRs in the Galactic longitude range 220°–240°.

GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey II: Galactic plane 345° < l < 67°, 180° < l < 240°

This work makes available a further $2\,860~\text{deg}^2$ of the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey, covering half of the accessible galactic plane, across 20 frequency bands sampling 72–231 MHz, with resolution $4\,\text{arcmin}-2\,\text{arcmin}$ . Unlike previous GLEAM data releases, we used multi-scale CLEAN to better deconvolve large-scale galactic structure. For the galactic longitude ranges $345^\circ < l < 67^\circ$ , $180^\circ < l < 240^\circ$ , we provide a compact source catalogue of 22 037 components selected from a 60-MHz bandwidth image centred at 200 MHz, with RMS noise $\approx10-20\,\text{mJy}\,\text{beam}^{-1}$ and position accuracy better than 2 arcsec. The catalogue has a completeness of 50% at ${\approx}120\,\text{mJy}$ , and a reliability of 99.86%. It covers galactic latitudes $1^\circ\leq|b|\leq10^\circ$ towards the galactic centre and $|b|\leq10^\circ$ for other regions, and is available from Vizier; images covering $|b|\leq10^\circ$ for all longitudes are made available on the GLEAM Virtual Observatory (VO).server and SkyView.