按日期筛选项目： 3月 2020

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按日期筛选项目： 3月 2020

Estimating stellar dust attenuation from galactic spectra

The observed spectrum of a galaxy is a combination of several components: a continuum, absorption and emission lines. The continuum and absorption lines are both dominated by starlight, thus usually referred to as the stellar component of the spectrum. The emission-line component is produced in ionized Hydrogen (HII) regions around hot stars, or emission-line regions of active nuclei, or both. All these components, however, are modified by the attenuation of dust grains distributed in the inter-stellar space. Dust attenuation can affect galaxy spectra over a wide range of wavelengths, from ultraviolet (UV), optical to infrared, by absorbing short-wavelength photons in UV/optical and re-emitting photons in the infrared, and the absorption is stronger in shorter wavelength. Consequently, dust attenuation can cause changes in the overall shape of a galaxy spectrum. Such attenuation has to be taken into account before one can measure the different components of an observed spectrum reliably. Traditionally, dust attenuation is treated as a free parameter when fitting the spectrum with a stellar population synthesis model, and so it is hard to measure the dust attenuation given the well-known dust–age–metallicity degeneracy.

Recovering star formation histories of low-mass galaxies

Characterizing the star formation history (SFH) of galaxies is an important step toward a full picture of galaxy formation and evolution. Despite being the most abundant galaxy population in the universe, dwarf (low-mass) galaxies remain elusive as far as their formation and evolution path is concerned. The left panel of Figure 1 shows the optical image of a typical low-mass galaxy observed by the SDSS-IV MaNGA survey. Impressed by their blue color, people have long believed dwarf galaxies are exclusively composed of massive，young stars that are bright, blue, and formed very recently. Recent investigations about low-mass galaxies challenge this stereotype. From ‘archaeological’ age reconstruction of local group dwarf galaxies in which individual stars can be resolved, most stars in these galaxies are found to be older than 5 Gyr, with an age similar to or even older than the Sun. However, the number of dwarf galaxies for which stars can be resolved is very limited, and so it is difficult to draw reliable statistical conclusions.

Searching for Wolf-Rayet Regions of Massive Stars in Galaxies

Wolf-Rayet (WR) galaxies are a rare population of galaxies that host living high-mass stars during their WR phase (called WR stars). These galaxies can be used to study a variety of important astrophysical questions including constraints on the stellar initial mass function, stellar evolution models, the relation between supernovae and gamma-ray bursts, etc. The first WR galaxy was identified in 1976 and a total of about 130 WR galaxies had been reported by the end of last century. Thanks to the SDSS I & II survey of nearby galaxies (targeting galaxy centers), many more WR galaxies were found, but the number was still limited to a few hundred [1]. Integral field spectroscopy (IFS) surveys have become available recently and provides a more efficient way of identifying WR galaxies, as WR stars are expected to be more preferentially found in discs than central regions that earlier phases of the SDSS probed.

LHS 1815b: The First Thick-Disk Planet Detected By TESS

To date, more than 4000 exoplanets have been detected, but few of them have been claimed to be in the thick disk of the Milky Way. A common way to separate different components of the Milky Way (for example, thin and thick disks) relies on the three-dimensinal (3D) spatial motions of stars. This has become possible with GAIA, a space telescope which gives distance, proper motions for relatively bright stars. Furthermore, the recent launched Transiting Exoplanet Survey Satellite (TESS) aims to discover a large sample (10,000) of planets around bright stars in the solar neighborhood across the whole sky. Combined with GAIA, TESS offers an exciting opportunity to study the difference in the planet formation efficiency between stars in the thin and thick disks, which have different age and metallicity distributions.

Relating structure of dark halos to their assembly and environment

Dark matter halos are the building blocks of the cosmic large-scale structures and the bridges between the dark and luminous sector of the Universe. They are diverse in internal structure, mass assembly history and interactions with environment. The figure at the top illustrates various quantities that are commonly adopted in literature to describe the structural properties of a typical dark matter halo, as well as its formation history and environment. A crucial step toward a full picture of dark matter halo formation is to understand the intrinsic relationship between the structure and the assembly history and environment of dark halos.

Tsinghua Vice President Xue Qikun visits DoA

In the morning of March 6, 2020, Tsinghua University Vice President XUE Qikun visited the Department of Astronomy (DoA) to inspect the work under Coronavirus epidemic since its outbreak in early 2020. Chair of DoA, MAO Shude, first introduced general information of DoA students, then briefed in details regarding recent work of epidemic prevention and control, and employment promotion plan for graduates. Prof. LI Cheng who have two graduate students graduating this year, added with more details regarding their career interests and future employment planning. DoA colloquium organizer, Prof. CAI Zheng, also briefed the Vice President about the change from traditional colloquium to online colloquium; it turns out attendee numbers increased by 20%.

Machine learning recovers H II morphology during reionization

The epoch of reionization (EoR) is a unique period of time in cosmic evolution, during which ultraviolet (UV) and X-ray photons emitted from the first luminous objects (e.g. first stars and galaxies) ionize hydrogen atoms first in the surrounding intergalactic medium (IGM) and form bubbles of H II regions, and eventually these H II bubbles fill the whole Universe. The bubble size distribution of ionized hydrogen regions probes the information about the morphology of H II bubbles during the reionization, and it can be derived from the tomographic imaging data of the redshifted 21cm signal.

按日期筛选项目： 3月 2020