分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present the JWST Resolved Stellar Populations Early Release Science (ERS) science program. We obtained 27.5 hours of NIRCam and NIRISS imaging of three targets in the Local Group (Milky Way globular cluster M92, ultra-faint dwarf galaxy Draco II, star-forming dwarf galaxy WLM), which span factors of $\sim10^5$ in luminosity, $\sim10^4$ in distance, and $\sim10^5$ in surface brightness. We describe the survey strategy, scientific and technical goals, implementation details, present select NIRCam color-magnitude diagrams (CMDs), and validate the NIRCam exposure time calculator (ETC). Our CMDs are among the deepest in existence for each class of target. They touch the theoretical hydrogen burning limit in M92 ($<0.08$ $M_{\odot}$; SNR $\sim5$ at $m_{F090W}\sim28.2$; $M_{F090W}\sim+13.6$), include the lowest-mass stars observed outside the Milky Way in Draco II (0.09 $M_{\odot}$; SNR $=10$ at $m_{F090W}\sim29$; $M_{F090W}\sim+12.1$), and reach $\sim1.5$ magnitudes below the oldest main sequence turnoff in WLM (SNR $=10$ at $m_{F090W}\sim29.5$; $M_{F090W}\sim+4.6$). The PARSEC stellar models provide a good qualitative match to the NIRCam CMDs, though are $\sim0.05$ mag too blue compared to M92 F090W$-$F150W data. The NIRCam ETC (v2.0) matches the SNRs based on photon noise from DOLPHOT stellar photometry in uncrowded fields, but the ETC may not be accurate in more crowded fields, similar to what is known for HST. We release beta versions of DOLPHOT NIRCam and NIRISS modules to the community. Results from this ERS program will establish JWST as the premier instrument for resolved stellar populations studies for decades to come.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We build a statistical framework to infer the global properties of the satellite system of the Andromeda galaxy (M31) from the properties of individual dwarf galaxies located in the Pan-Andromeda Archaelogical Survey (PAndAS) and the previously determined completeness of the survey. Using forward modeling, we infer the slope of the luminosity function of the satellite system, the slope of its spatial density distribution, and the size-luminosity relation followed by the dwarf galaxies. We find that the slope of the luminosity function is $\beta=-1.5\pm0.1$. Combined with the spatial density profile, it implies that, when accounting for survey incompleteness, M31 hosts $92_{-26}^{+19}$ dwarf galaxies with $M_\textrm{V}99.9\%$ confidence and conclude that this anisotropy is an intrinsic feature of the M31 satellite system. The statistical framework we present here is a powerful tool to robustly constrain the properties of a satellite system and compare those across hosts, especially considering the upcoming start of the Euclid or Rubin large photometric surveys that are expected to uncover a large number of dwarf galaxies in the Local Volume.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Once only accessible in nearby galaxies, we can now study individual stars across much of the observable universe aided by galaxy-cluster gravitational lenses. When a star, compact object, or multiple such objects in the foreground galaxy-cluster lens become aligned, they can magnify a background individual star, and the timescale of a magnification peak can limit its size to tens of AU. The number and frequency of microlensing events therefore opens a window into the population of stars and compact objects, as well as high-redshift stars. To assemble the first statistical sample of stars in order to constrain the initial mass function (IMF) of massive stars at redshift z=0.7-1.5, the abundance of primordial black holes in galaxy-cluster dark matter, and the IMF of the stars making up the intracluster light, we are carrying out a 192-orbit program with the Hubble Space Telescope called "Flashlights," which is now two-thirds complete owing to scheduling challenges. We use the ultrawide F200LP and F350LP long-pass WFC3 UVIS filters and conduct two 16-orbit visits separated by one year. Having an identical roll angle during both visits, while difficult to schedule, yields extremely clean subtraction. Here we report the discovery of more than a dozen bright microlensing events, including multiple examples in the famous "Dragon Arc" discovered in the 1980s, as well as the "Spocks" and "Warhol" arcs that have hosted already known supergiants. The ultradeep observer-frame ultraviolet-through-optical imaging is sensitive to hot stars, which will complement deep James Webb Space Telescope infrared imaging. We are also acquiring Large Binocular Telescope LUCI and Keck-I MOSFIRE near-infrared spectra of the highly magnified arcs to constrain their recent star-formation histories.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: UVEX is a proposed medium class Explorer mission designed to provide crucial missing capabilities that will address objectives central to a broad range of modern astrophysics. The UVEX design has two co-aligned wide-field imagers operating in the FUV and NUV and a powerful broadband medium resolution spectrometer. In its two-year baseline mission, UVEX will perform a multi-cadence synoptic all-sky survey 50/100 times deeper than GALEX in the NUV/FUV, cadenced surveys of the Large and Small Magellanic Clouds, rapid target of opportunity followup, as well as spectroscopic followup of samples of stars and galaxies. The science program is built around three pillars. First, UVEX will explore the low-mass, low-metallicity galaxy frontier through imaging and spectroscopic surveys that will probe key aspects of the evolution of galaxies by understanding how star formation and stellar evolution at low metallicities affect the growth and evolution of low-metallicity, low-mass galaxies in the local universe. Such galaxies contain half the mass in the local universe, and are analogs for the first galaxies, but observed at distances that make them accessible to detailed study. Second, UVEX will explore the dynamic universe through time-domain surveys and prompt spectroscopic followup capability will probe the environments, energetics, and emission processes in the early aftermaths of gravitational wave-discovered compact object mergers, discover hot, fast UV transients, and diagnose the early stages of stellar explosions. Finally, UVEX will become a key community resource by leaving a large all-sky legacy data set, enabling a wide range of scientific studies and filling a gap in the new generation of wide-field, sensitive optical and infrared surveys provided by the Rubin, Euclid, and Roman observatories. This paper discusses the scientific potential of UVEX, and the broad scientific program.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present novel constraints on the underlying galaxy formation physics (e.g., mass loading factor, star formation history, metal retention) at $z \gtrsim 7$ for the low-mass ($M_*\sim10^5$ M$_\odot$) Local Group ultra-faint dwarf galaxy (UFD) Eridanus II (Eri II) Using a hierarchical Bayesian framework, we apply a one-zone chemical evolution model to Eri II's CaHK-based photometric metallicity distribution function (MDF; [Fe/H]) and find that the evolution of Eri II is well-characterized by a short, exponentially declining star-formation history ($\tau_\text{SFH} = 0.39\pm_{0.13}^{0.18}$ Gyr), a low star-formation efficiency ($\tau_\text{SFE} = 27.56\pm_{12.92}^{25.14}$ Gyr), and a large mass-loading factor ($\eta = 194.53\pm_{42.67}^{33.37}$). Our results are consistent with Eri II forming the majority of its stars before the end of reionization. The large mass-loading factor is consistent with strong outflows in Eri II and is in good agreement with theoretical predictions for momentum-driven galactic winds. It also results in the ejection of $>$90% of the metals produced in Eri II. We make predictions for the distribution of [Mg/Fe]-[Fe/H] in Eri II as well as the prevalence of ultra metal-poor stars, both of which can be tested by future chemical abundance measurements. Spectroscopic follow-up of the highest metallicity stars in Eri II ($\text{[Fe/H]}> -2$) will greatly improve model constraints. Our new framework can readily be applied to all UFDs throughout the Local Group, providing new insights in to the underlying physics governing the evolution of the faintest galaxies in the reionization-era.