分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We use the Galaxy Morphology Posterior Estimation Network (GaMPEN) to estimate morphological parameters and associated uncertainties for $\sim 8$ million galaxies in the Hyper Suprime-Cam (HSC) Wide survey with $z \leq 0.75$ and $m \leq 23$. GaMPEN is a machine learning framework that estimates Bayesian posteriors for a galaxy's bulge-to-total light ratio ($L_B/L_T$), effective radius ($R_e$), and flux ($F$). By first training on simulations of galaxies and then applying transfer learning using real data, we trained GaMPEN with $<1\%$ of our dataset. This two-step process will be critical for applying machine learning algorithms to future large imaging surveys, such as the Rubin-Legacy Survey of Space and Time (LSST), the Nancy Grace Roman Space Telescope (NGRST), and Euclid. By comparing our results to those obtained using light-profile fitting, we demonstrate that GaMPEN's predicted posterior distributions are well-calibrated ($\lesssim 5\%$ deviation) and accurate. This represents a significant improvement over light profile fitting algorithms which underestimate uncertainties by as much as $\sim60\%$. For an overlapping sub-sample, we also compare the derived morphological parameters with values in two external catalogs and find that the results agree within the limits of uncertainties predicted by GaMPEN. This step also permits us to define an empirical relationship between the S\'ersic index and $L_B/L_T$ that can be used to convert between these two parameters. The catalog presented here represents a significant improvement in size ($\sim10 \times $), depth ($\sim4$ magnitudes), and uncertainty quantification over previous state-of-the-art bulge+disk decomposition catalogs. With this work, we also release GaMPEN's source code and trained models, which can be adapted to other datasets.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We propose to trace the dynamical motion of a shearing hot spot near the SgrA* source through a dynamical image reconstruction algorithm, StarWarps. Such a hot spot may form as the exhaust of magnetic reconnection in a current sheet near the black hole horizon. A hot spot that is ejected from the current sheet into an orbit in the accretion disk may shear and diffuse due to instabilities at its boundary during its orbit, resulting in a distinct signature. We subdivide the motion to two distinct phases; the first phase refers to the appearance of the hot spot modelled as a bright blob, followed by a subsequent shearing phase simulated as a stretched ellipse. We employ different observational arrays, including EHT(2017,2022) and the next generation event horizon telescope (ngEHTp1, ngEHT) arrays, in which few new additional sites are added to the observational array. We make dynamical image reconstructions for each of these arrays. Subsequently, we infer the hot spot phase in the first phase followed by the axes ratio and the ellipse area in the second phase. We focus on the direct observability of the orbiting hot spot in the sub-mm wavelength. Our analysis demonstrates that newly added dishes may easily trace the first phase as well as part of the second phase, before the flux is reduced substantially. The algorithm used in this work can be extended to any other types of the dynamical motion. Consequently, we conclude that the ngEHT is a key to directly observe the dynamical motions near variable sources, such as SgrA*.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The discovery of two UV-bright galaxy candidates at $z\sim 13$, HD1 and HD2, laid the foundation for a new race to study the early Universe. Previous investigations suggested that they are either powered by a supermassive black hole or by an extreme, transient burst of star formation. Given their uncertain nature, we investigate whether these sources could be lensed by a hitherto undetected, faint foreground galaxy. We find that at the current limiting magnitude with which HD1 and HD2 were imaged, there is only a $7.39\%$ probability they are strongly lensed by spherical deflectors and that the hypothetical lensing galaxy was too faint to be detected. Meanwhile, with the limiting magnitudes of the Hubble Space Telescope (HST) and James Webb Space Telescope (JWST), the theoretical probability would drop precipitously to $0.058\%$ and $0.0012\%$, respectively. We further find it unlikely that the luminosities of both sources can be accounted for by lensing that produces a single, resolved image with sufficiently high magnification. Alternatively, in the unlikely event that their brightness results from lensing by an elliptical isothermal galaxy, there is a $30.9 \%$ probability that the lensing galaxy is too faint to be observable at the current limiting magnitude. Future HST (JWST) imaging will drop this probability to $0.245 \%$ ($0.0025 \%$). In summary, while deep imaging with HST and JWST is required to discard the lensing hypothesis entirely, it is unlikely that the exceptional luminosity of the two $z \sim 13$ sources can be accounted for by gravitational lensing.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The interaction of a runaway supermassive black hole (SMBH) with the circumgalactic medium (CGM) can lead to the formation of a wake of shocked gas and young stars behind it. Here we report the serendipitous discovery of an extremely narrow linear feature in HST/ACS images that may be an example of such a wake. The feature extends 62 kpc from the nucleus of a compact star-forming galaxy at z=0.964. Keck LRIS spectra show that the [OIII]/H$\beta$ ratio varies from ~1 to ~10 along the feature, indicating a mixture of star formation and fast shocks. The feature terminates in a bright [OIII] knot with a luminosity of 1.9x10$^{41}$ ergs/s. The stellar continuum colors vary along the feature, and are well-fit by a simple model that has a monotonically increasing age with distance from the tip. The line ratios, colors, and the overall morphology are consistent with an ejected SMBH moving through the CGM at high speed while triggering star formation. The best-fit time since ejection is ~39 Myr and the implied velocity is v~1600 km/s. The feature is not perfectly straight in the HST images, and we show that the amplitude of the observed spatial variations is consistent with the runaway SMBH interpretation. Opposite the primary wake is a fainter and shorter feature, marginally detected in [OIII] and the rest-frame far-ultraviolet. This feature may be shocked gas behind a binary SMBH that was ejected at the same time as the SMBH that produced the primary wake.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present the Next Generation Deep Extragalactic Exploratory Public (NGDEEP) Survey, a deep slitless spectroscopic and imaging Cycle 1 JWST treasury survey designed to constrain feedback mechanisms in low-mass galaxies across cosmic time. NGDEEP targets the Hubble Ultra Deep Field (HUDF) with NIRISS slitless spectroscopy (f~1.2e-18 erg/s/cm^2, 5sigma) to measure metallicities and star-formation rates (SFRs) for low-mass galaxies through the peak of the cosmic SFR density (0.512, constraining the slope of the faint-end of the rest-ultraviolet luminosity function. NGDEEP overlaps with the deepest HST ACS optical imaging in the sky: F435W in the HUDF (m=29.6), and F814W in HUDF-Par2 (m=30), making this a premier HST+JWST Deep Field. As a treasury survey, NGDEEP data is public immediately, and we will rapidly release data products and catalogs in the spirit of previous deep field initiatives. In this paper we present the NGDEEP survey design, summarize the science goals, and detail plans for the public release of NGDEEP reduced data products.