The SAURON project-XXI. The spatially-resolved UV-line strength relations of early-type galaxies

Abstract

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 Mg2 line strengths found by Burstein et al. (1988). 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 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 Mg b line strength and the metallicity gradients for galaxies with old stellar populations. In particular, global gradients of Mg b and [Z/H] with respect to the UV colour across galaxies are consistent with their local gradients within galaxies, suggesting that the global correlations also hold locally. From a simple model based on multi-band 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 centers of elliptical galaxies, where the UV flux is strong, are enhanced in metals by roughly 60% compared to UV-weak regions.

22 pages, 20 figures and 4 tables; accepted for publication in MNRAS

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Cited by in corpus (12)

• UV-bright nearby early type galaxies observed in the mid-infrared: evidence for a multi-stage formation history by way of WISE and GALEX imaging
• The ATLAS 3D project - XVI. Physical parameters and spectral line energy distributions of the molecular gas in gas-rich early-type galaxies
• Dissolved Massive Metal-Rich Globular Clusters Can Cause the Range of UV Upturn Strengths Found among Early-type Galaxies
• The Planetary Nebulae Population in the Nuclear Regions of M31: the SAURON view
• Revealing the formation mechanism of the shell galaxy NGC474 with MUSE
• An Empirical Connection between the UV Color of Early Type Galaxies and the Stellar Initial Mass Function
• UV upturn versus UV weak galaxies: differences and similarities of their stellar populations unveiled by a de-biased sample
• Far-UV to mid-IR properties of nearby radio galaxies
• Evolution of the Ultraviolet Upturn at $0.3<z<1$: exploring helium rich stellar populations
• The Ultraviolet Upturn in field Luminous Red Galaxies at $0.3<z<0.7$
• Two categories of UV-upturn galaxies revealed by semi-analytic models
• UV bright red-sequence galaxies: a comparative study between UV upturn and UV weak systems