Abstract
A number of cluster central giant elliptical galaxies(a.k.a. brightest cluster galaxies, BCGs) exhibitrecent or ongoing star formation that is most clearly revealed in the UV continuum. The nature ofthis star formationactivity (i.e., origin of the star-forming gas, star formation rate over time) ispoorly understood. The best data currently available for elucidating the star formation history ofBCGs 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 thisdata have used only the UV filters to estimate star formation rates(SFR) in this BCG assuming acertain star formation history(i.e., nearly constant SFR over 100 Myr). Here, we derive thespatially-resolved spectral energy distribution (SED) of the relatively young stellar population so asto infer its star-formation history through population synthesis modeling. We anticipate our work toset a benchmark for what can be learnt from this unique and high quality dataset havingunprecedented wavelength coverage.
To obtain the SED of the young stellar population, we first have to subtract the light contributed bythe old stellar population in every filter band. To model the 2-dimensional light distribution of theold stellar population, we used images in the longest-wavelength bands where the old stellarpopulation dominates the overall light. In doing so, we found it necessary to mask out nearbycluster members, as well as both the young stellar population and dust based on color images of theBCG. The model parameters for the 2-D light distribution of the old stellar population thus derivedwas consistent among the reddest filter bands, allowing us to construct a wavelength-averagedmodel applicable to all filter bands. The images obtained after subtracting the model for the oldstellar population have good consistency with the spatial distribution of the young stellar populationthat 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-colordiagram (constructed from three line-free filters) of the young stellar population with populationsynthesis model. The results, which are only weakly sensitive to metallicity, implies ages rangingfrom ~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 thusinferred are compatible with that those inferred from the color-color diagram; (ii) the degree towhich we can break the age-metallicity degeneracy; (iii) whether it is necessary to include dustextinction, and the extra degree of degeneracy in doing so.
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