天文現象及活動

星系團MACSJ0329-0211中央巨大橢圓星系的近期恆星形成史

Public Seminar of RPg Student:- Recent Star Formation History of the Central Giant Elliptical Galaxy of the Cluster MACSJ0329-0211

A number of cluster central giant elliptical galaxies(a.k.a. brightest cluster galaxies, BCGs) exhibit recent or ongoing star formation that is most clearly revealed in the UV continuum. The nature of this star formation activity (i.e., origin of the star-forming gas, star formation rate over time) is poorly understood. The best data currently available for elucidating the star formation history of BCGs come from the Cluster Lensing And Supernova survey with Hubble(CLASH), which spans wavelengths from the near-UV to the near-IR in 16 filters. Previous studies using this data have used only the UV filters to estimate star formation rates(SFR) in this BCG assuming a certain star formation history(i.e., nearly constant SFR over 100 Myr). Here, we derive the spatially-resolved spectral energy distribution (SED) of the relatively young stellar population so as to infer its star-formation history through population synthesis modeling. We anticipate our work to set a benchmark for what can be learnt from this unique and high quality data set having unprecedented wavelength coverage.

To obtain the SED of the young stellar population, we first have to subtract the light contributed by the old stellar population in every filter band. To model the 2-dimensional light distribution of the old stellar population, we used images in the longest-wavelength bands where the old stellar population dominates the overall light. In doing so, we found it necessary to mask out nearby cluster members, as well as both the young stellar population and dust based on color images of the BCG. The model parameters for the 2-D light distribution of the old stellar population thus derived was consistent among the reddest filter bands, allowing us to construct a wavelength-averaged model applicable to all filter bands. The images obtained after subtracting the model for the old stellar population have good consistency with the spatial distribution of the young stellar population that dominates the UV images, as well as color images of the BCG.

To make a preliminary look and to obtain global view of the data, we compared a color-color diagram (constructed from three line-free filters) of the young stellar population with population synthesis model. The results, which are only weakly sensitive to metallicity, implies ages ranging from ~10 Myr to nearly 1Gyr for the young stellar population.

We are now in the process of modeling the entire SED to determine: (i) whether the ages thus inferred are compatible with that those inferred from the color-color diagram; (ii) the degree to which we can break the age-metallicity degeneracy; (iii) whether it is necessary to include dust extinction, and the extra degree of degeneracy in doing so.　
　
時間：2017.05.04(四) 10:00
地點：香港大學莊月明物理樓105室
講者：Junhao LI (香港大學)
語言：英語

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#119

此活動由香港大學物理系主辦。

2017-05-04

上弦

10時47分

2017-05-03

天文嘉年華

觀眾可透過先進星象投影系統，欣賞各有特色的四季星空。此外亦有專業導師將遠在天邊的八大行星帶到太空館內作介紹，讓大家近距離欣賞行星表面的細節。甚至可以透過天文望遠鏡，親眼窺探月球、行星及其他天體的廬山真貌。活動包括「星空巡禮」、「魔幻行星」及「鏡中星空」三節，有興趣的人士可於活動時間內，於太空館各環節地點輪候參加。