分类： 天文学 >> 天文学 提交时间： 2023-02-19
G. S. Da Costa
M. S. Bessell
Thomas Nordlander
Arvind C. N. Hughes
Sven Buder
A. D. Mackey
Lee R. Spitler
D. B. Zucker
摘要： The advent of large-scale stellar spectroscopic surveys naturally leads to the implementation of machine learning techniques to isolate, for example, small sub-samples of potentially interesting stars from the full data set. A recent example is the application of the t-SNE statistical method to $\sim$600,000 stellar spectra from the GALAH survey in order to identify a sample of candidate extremely metal-poor (EMP, [Fe/H] $\leq$ -3) stars. We report the outcome of low-resolution spectroscopic follow-up of 83 GALAH EMP candidates that lack any previous metallicity estimates. Overall, the statistical selection is found to be efficient ($\sim$one-third of the candidates have [Fe/H] $\leq$ -2.75) with low contamination ($$ -2), and with a metallicity distribution function that is consistent with previous work. Five stars are found to have [Fe/H] $\leq$ -3.0, one of which is a main sequence turnoff star. Two other stars are revealed as likely carbon-enhanced metal-poor (CEMP) stars of type CEMP-$s$, and a known carbon star is re-identified. The results indicate that the statistical selection approach employed was successful, and therefore it can be applied to forthcoming even larger stellar spectroscopic surveys with the expectation of similar positive outcomes.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Large and less-biased samples of star-forming galaxies are essential to investigate galaxy evolution. H$\rm\alpha$ emission line is one of the most reliable tracers of star-forming galaxies because its strength is directly related to recent star formation. However, it is observationally expensive to construct large samples of H$\rm\alpha$ emitters by spectroscopic or narrow-band imaging survey at high-redshifts. In this work, we demonstrate a method to extract H$\rm\alpha$ fluxes of galaxies at $z=2.1$-$2.5$ from $K_s$ broad-band photometry of ZFOURGE catalog. Combined with 25-39 other filters, we estimate the emission line fluxes by SED fitting with stellar population models that incorporate emission-line strengths. 2005 galaxies are selected as H$\rm\alpha$ emitters by our method and their fluxes show good agreement with previous measurements in the literature. On the other hand, there are more H$\rm\alpha$ luminous galaxies than previously reported. The discrepancy can be explained by extended H$\rm\alpha$ profiles of massive galaxies and a luminosity dependence of dust attenuation, which are not taken into account in the previous work. We also find that there are a large number of low-mass galaxies with much higher specific star formation rate (sSFR) than expected from the extrapolated star formation main sequence. Such low-mass galaxies exhibit larger ratios between H$\rm\alpha$ and UV fluxes compared to more massive high sSFR galaxies. This result implies that a ``starburst'' mode may differ among galaxies: low-mass galaxies appear to assemble their stellar mass via short-duration bursts while more massive galaxies tend to experience longer-duration ($>10\ \mathrm{Myr}$) bursts.
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