The unexpected rising flux of early-type galaxies at decreasing ultraviolet (UV) wavelengths is a long-standing mystery. One important observational constraint is the correlation between UV-optical colours and Mg 2 line strengths found by Burstein et al. The simplest interpretation of this phenomenon is that the UV strength is related to the Mg line strength. Under this assumption, we expect galaxies with larger Mg gradients to have larger UV colour gradients. By combining UV imaging from GALEX, optical imaging from MDM and SAURON integral-field spectroscopy, we investigate the spatially resolved relationships between UV colours and stellar population properties of 34 early-type galaxies from the SAURON survey sample. We find that galaxies with old stellar populations show tight correlations between the far-UV (FUV) colours (FUV -V and FUV - NUV) and the Mgb index, Hβ index and metallicity [Z/H]. The equivalent correlations for the Fe5015 index, α-enhancement [α/Fe] and age are present but weaker. We have also derived logarithmic internal radial colour, measured line strength and derived stellar population gradients for each galaxy and again found a strong dependence of the FUV -V and FUV - NUV colour gradients on both the Mgb line strength and the metallicity gradients for galaxies with old stellar populations. In particular, global gradients of Mgb and [Z/H] with respect to the UV colour [e.g. Δ(Mgb)/Δ(FUV - NUV) and Δ[Z/H]/Δ(FUV - NUV)] across galaxies are consistent with their local gradients within galaxies, suggesting that the global correlations also hold locally. From a simple model based on multiband colour fits of UV upturn and UV-weak galaxies, we have identified a plausible range of parameters that reproduces the observed radial colour profiles. In these models, the centres of elliptical galaxies, where the UV flux is strong, are enhanced in metals by roughly 60 per cent compared to UV-weak regions.
Bibliographical noteFunding Information:
Funding for this study was provided by Sakhalin Energy Investment Company Ltd. and Exxon Neftegas Ltd. The funders were involved in study design, logistics and safety management, and agreed with this publication but were not directly involved in data collection and analysis or preparation of the manuscript. We thank all scientists working for or on behalf of the A.V. Zhirmunsky National Scientific Center of Marine Biology of the Far Eastern Branch of the Russian Academy of Sciences (NSCMB FEB RAS) in Vladivostok, for the data collection and processing in the field, and Igor Zhmaev (LGL Eco) for coordination and logistics. We also thank Peter van der Wolf for shore-based images. Rodger Melton (ExxonMobil-deceased), Michael Macrander (Integral), Per Palsb?ll (University of Groningen) and Kees Camphuysen (Royal Netherlands Institute for Sea Research) and an anonymous reviewer provided valuable comments on the draft of this manuscript. We dedicate this publication to Rodger Melton (deceased) for establishing this program and dedicating a large part of his career to studying and minimizing impacts of exploration activities on marine mammals. We are thankful for his friendship, guidance, and novel thinking.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science