[en] We present a sample of 17 newly discovered ultracool dwarf candidates later than ∼M8, drawn from 231.90 arcmin² of Hubble Space Telescope Wide Field Camera 3 infrared imaging. By comparing the observed number counts for 17.5 ≤ J₁₂₅ ≤ 25.5 AB mag to an exponential disk model, we estimate a vertical scale height of z_scl = 290 ± 25 (random) ± 31 (systematic) pc for a binarity fraction of f_b = 0. While our estimate is roughly consistent with published results, we suggest that the differences can be attributed to sample properties, with the present sample containing far more substellar objects than previous work. We predict the object counts should peak at J₁₂₅ ∼ 24 AB mag due to the exponentially declining number density at the edge of the disk. We conclude by arguing that trend in scale height with spectral type may breakdown for brown dwarfs since they do not settle onto the main sequence.

[en] Dust attenuation of an inclined galaxy can cause additional asymmetries in observations, even if the galaxy has a perfectly symmetric structure. Taking advantage of the integral field spectroscopic data observed by the Sloan Digital Sky Survey-IV Mapping Nearby Galaxies at the Apache Point Observatory survey, we investigate the asymmetries of the emission-line and continuum maps of star-forming disk galaxies. We define new parameters, A _a and A _b, to estimate the asymmetries of a galaxy about its major and minor axes, respectively. Comparing A _a and A _b in different inclination bins, we attempt to detect the asymmetries caused by dust. For the continuum images, we find that A _a increases with the inclination, while A _b is a constant as inclination changes. Similar trends are found for g − r, g − i, and r − i color images. The dependence of the asymmetry on inclination suggests a thin dust layer with a scale height smaller than the stellar populations. For the Hα and Hβ images, neither A _a nor A _b shows a significant correlation with inclination. Also, we do not find any significant dependence of the asymmetry of E(B − V)_g on inclination, implying that the dust in the thick disk component is not significant. Compared to the SKIRT simulation, the results suggest that the thin dust disk has an optical depth of τ _V ∼ 0.2. This is the first time that the asymmetries caused by the dust attenuation and the inclination are probed statistically with a large sample. Our results indicate that the combination of the dust attenuation and the inclination effects is a potential indicator of the 3D disk orientation.

[en] Deep ACS slitless grism observations and identification of stellar sources are presented within the Great Observatories Origins Deep Survey North and South fields which were obtained in the Probing Evolution And Reionization Spectroscopically (PEARS) program. It is demonstrated that even low-resolution spectra can be a very powerful means of identifying stars in the field, especially low-mass stars with stellar types M0 and later. The PEARS fields lay within the larger GOODS fields, and we used new, deeper images to further refine the selection of stars in the PEARS field, down to a magnitude of z ₈₅₀ = 25 using a newly developed stellarity parameter. The total number of stars with reliable spectroscopic and morphological identification was 95 and 108 in the north and south fields, respectively. The sample of spectroscopically identified stars allows constraints to be set on the thickness of the Galactic thin disk as well as contributions from a thick disk and a halo component. We derive a thin disk scale height, as traced by the population of M4-M9 dwarfs along two independent lines of sight, of h _thin = 370⁺⁶⁰_-65 pc. When including the more massive M0-M4 dwarf population, we derive h _thin = 300 ± 70 pc. In both cases, we observe that we must include a combination of thick and halo components in our models in order to account for the observed numbers of faint dwarfs. The required thick disk scale height is typically h _thick = 1000 pc and the acceptable relative stellar densities of the thin disk to thick disk and the thin disk to halo components are in the range of 0.00025 < f _halo < 0.0005 and 0.05 < f _thick < 0.08 and are somewhat dependent on whether the more massive M0-M4 dwarfs are included in our sample.

[en] On the basis of a recently derived color-metallicity relation and stellar parameters from the Sloan Digital Sky Survey Data Release 7 spectroscopic survey, a large sample of red horizontal-branch (RHB) candidates have been selected to serve as standard candles. The metallicity and kinematic distributions of these stars indicate that they mainly originate from the thick-disk and the halo populations. The typical thick disk is characterized by the first group peaking at [Fe/H] ∼ -0.6, V_rot ∼ 170 km s^-1 with a vertical scale height around |Z| ∼ 1.2 kpc, while stars with [Fe/H] < -0.9 are dominated by the halo population. Two sub-populations of the halo are suggested by the RHB stars peaking at [Fe/H] ∼ -1.3: one component with V_rot > 0 km s^-1 (Halo I) shows a sign of metallicity gradient in the [Fe/H] versus |Z| diagram, while the other with V_rot < 0 km s^-1 (Halo II) does not. The Halo I mainly clumps at the inner halo with R < 10 kpc and the Halo II comes both from the inner halo with R < 10 kpc and the outer halo with R > 10 kpc based on the star distribution in the R versus |Z| diagram.

[en] We aim at estimating the dust scale height of protoplanetary disks from millimeter continuum observations. First, we present a general expression of intensity of a ring in a protoplanetary disk and show that we can constrain the dust scale height by the azimuthal intensity variation. Then, we apply the presented methodology to the two distinct rings at 68 au and at 100 au of the protoplanetary disk around HD 163296. We constrain the dust scale height by comparing the high-resolution millimeter dust continuum image obtained in the Disk Substructures at High Angular Resolution Project (DSHARP) with radiative transfer simulations using RADMC-3D. We find that h _d/h _g > 0.84 at the inner ring and h _d/h _g < 0.11 at the outer ring with 3σ uncertainties, where h _d is the dust scale height and h _g is the gas scale height. This indicates that the dust is flared at the inner ring and settled at the outer ring. We further constrain the ratio of the turbulence parameter α to the gas-to-dust-coupling parameter St from the derived dust scale height; α/St > 2.4 at the inner ring, and α/St < $1.1\times <!--\; ??\; -->{10}^{-<!--\; ???\; -->2}$ at the outer ring. This result shows that the turbulence is stronger or the dust is smaller at the inner ring than at the outer ring.

[en] We present an analysis of the properties of H I holes detected in 20 galaxies that are part of 'The H I Nearby Galaxy Survey'. We detected more than 1000 holes in total in the sampled galaxies. Where they can be measured, their sizes range from about 100 pc (our resolution limit) to about 2 kpc, their expansion velocities range from 4 to 36 km s^-1, and their ages are estimated to range between 3 and 150 Myr. The holes are found throughout the disks of the galaxies, out to the edge of the H I disk; 23% of the holes fall outside R₂₅. We find that shear limits the age of holes in spirals (shear is less important in dwarf galaxies) which explains why H I holes in dwarfs are rounder, on average than in spirals. Shear, which is particularly strong in the inner part of spiral galaxies, also explains why we find that holes outside R₂₅ are larger and older. We derive the scale height of the H I disk as a function of galactocentric radius and find that the disk flares up in all galaxies. We proceed to derive the surface and volume porosity (Q_2D and Q_3D) and find that this correlates with the type of the host galaxy: later Hubble types tend to be more porous. The size distribution of the holes in our sample follows a power law with a slope of a_ν ∼ -2.9. Assuming that the holes are the result of massive star formation (SF), we derive values for the supernova rate and star formation rate (SFR) which scales with the SFR derived based on other tracers. If we extrapolate the observed number of holes to include those that fall below our resolution limit, down to holes created by a single supernova, we find that our results are compatible with the hypothesis that H I holes result from SF.

[en] We present results of an analysis of the J, H, and K_s Two Micron All Sky Survey (2MASS) images of 139 spiral edge-on galaxies selected from the Revised Flat Galaxies Catalog. The basic structural parameters scale length (h), scale height (z ₀), and central surface brightness of the stellar disks (μ₀) are determined for all selected galaxies in the near-infrared (NIR) bands. The mean relative ratios of the scale heights of the thin stellar disks in the J:H:K_s bands are 1.16:1.08:1.00, respectively. Comparing the scale heights obtained from the NIR bands for the same objects, we estimate the scale heights of the thin stellar disks corrected for the internal extinction. We find that the extinction-corrected scale height is, on average, 11% smaller than that in the K band. Using the extinction-corrected structural parameters, we find that the dark-to-luminous mass ratio is, on average, 1.3 for the galaxies in our sample within the framework of a simplified galactic model. The relative thicknesses of the stellar disks z ₀/h correlates with their face-on central surface brightnesses obtained from the 2MASS images. We also find that the scale height of the stellar disks shows no systematic growth with radius in most of our galaxies.

[en] We use the recent measurement of the velocity dispersion of star-forming, outer-disk knots by Herbert-Fort et al. in the nearly face-on galaxy NGC 628, in combination with other data from the literature, to execute a straightforward test of gravity at low accelerations. Specifically, the rotation curve at large radius sets the degree of non-standard acceleration and then the predicted scale height of the knots at that radius provides the test of the scenario. For our demonstration, we presume that the Hα knots, which are young (age < 10 Myr), are distributed like the gas from which they have recently formed and find a marginal (>97% confidence) discrepancy with a modified gravity scenario given the current data. More interestingly, we demonstrate that there is no inherent limitation that prevents such a test from reaching possible discrimination at the >4σ level with a reasonable investment of observational resources.

[en] We present the discovery and timing of the young (age ∼28.6 kyr) pulsar PSR J0837–2454. Based on its high latitude (b = 98) and dispersion measure (DM = 143 pc cm⁻³), the pulsar appears to be at a z-height of >1 kpc above the Galactic plane, but near the edge of our Galaxy. This is many times the observed scale height of the canonical pulsar population, which suggests this pulsar may have been born far out of the plane. If accurate, the young age and high z-height imply that this is the first pulsar known to be born from a runaway O/B star. In follow-up imaging with the Australia Telescope Compact Array (ATCA), we detect the pulsar with a flux density S ₁₄₀₀ = 0.18 ± 0.05 mJy. We do not detect an obvious supernova remnant around the pulsar in our ATCA data, but we detect a colocated, low-surface-brightness region of ∼15 extent in archival Galactic and Extragalactic All-sky MWA Survey data. We also detect colocated Hα emission from the Southern Hα Sky Survey Atlas. Distance estimates based on these two detections come out to ∼0.9 kpc and ∼0.2 kpc, respectively, both of which are much smaller than the distance predicted by the NE2001 model (6.3 kpc) and YMW model (>25 kpc) and place the pulsar much closer to the plane of the Galaxy. If the pulsar/remnant association holds, this result also highlights the inherent difficulty in the classification of transients as “Galactic” (pulsar) or “extragalactic” (fast radio burst) toward the Galactic anticenter based solely on the modeled Galactic electron contribution to a detection.

[en] NGC 4013 is a distinctly warped galaxy with evidence of disk–halo activity. Through deep H i observations and modeling we confirm that the H i disk is thin (central exponential scale height with an upper limit of 4″ or 280 pc), but flaring. We detect a vertical gradient in rotation velocity (lag), which shallows radially from a value of −35${}_{-<!--\; ???\; -->28}^{+7}$ km s⁻¹ kpc⁻¹ at 1.′4 (5.8 kpc), to a value of zero near R₂₅ (11.2 kpc). Over much of this radial range, the lag is relatively steep. Both the steepness and the radial shallowing are consistent with recent determinations for a number of edge-ons, which have been difficult to explain. We briefly consider the lag measured in NGC 4013 in the context of this larger sample and theoretical models, further illuminating disk–halo flows.