The SAURON project - XX. The Spitzer [3.6] − [4.5] colour in early-type galaxies: colours, colour gradients and inverted scaling relations

Abstract

We investigate the [3.6]−[4.5] Spitzer‐IRAC colour behaviour of the early‐type galaxies of the SAURON survey, a representative sample of 48 nearby ellipticals and lenticulars. We investigate how this colour, which is unaffected by dust extinction, can be used to constrain the stellar populations in these galaxies. We find a tight relation between the [3.6]−[4.5] colour and effective velocity dispersion, a good mass indicator in early‐type galaxies: ([3.6]−[4.5])e = (−0.109 0.007)+ (0.154 0.016). Contrary to other colours in the optical and near‐infrared, we find that the colours become bluer for larger galaxies. The relations are tighter when using the colour inside re (scatter 0.013 mag), rather than the much smaller re/8 aperture (scatter 0.023 mag), due to the presence of young populations in the central regions. We also obtain strong correlations between the [3.6]−[4.5] colour and three strong absorption lines (H, Mgb and Fe 5015). Comparing our data with the models of Marigo et al., which show that more metal rich galaxies are bluer, we can explain our results in a way consistent with results from the optical, by stating that larger galaxies are more metal rich. The blueing is caused by a strong CO absorption band, whose line strength increases strongly with decreasing temperature and which covers a considerable fraction of the 4.5‐m filter. In galaxies that contain a compact radio source, the [3.6]−[4.5] colour is generally slightly redder (by 0.015 0.007 mag using the re/8 aperture) than in the other galaxies, indicating small amounts of either hot dust, non‐thermal emission, or young stars near the centre. We find that the large majority of the galaxies show redder colours with increasing radius. Removing the regions with evidence for young stellar populations (from the H absorption line) and interpreting the colour gradients as metallicity gradients, we find that our galaxies are more metal poor going outwards. The radial [3.6]−[4.5] gradients correlate very well with the metallicity gradients derived from optical line indices. We do not find any correlation between the gradients and galaxy mass; at every mass, galaxies display a real range in metallicity gradients. Consistent with our previous work on line indices, we find a tight relation between local [3.6]−[4.5] colour and local escape velocity. The small scatter from galaxy to galaxy, although not negligible, shows that the amount and distribution of the dark matter relative to the visible light cannot be too different from galaxy to galaxy. Due to the lower sensitivity of the [3.6]−[4.5] colour to young stellar populations, this relation is more useful to infer the galaxy potential than the Mgb–vesc relation.

SUPERMASSIVE BLACK HOLES AND THEIR HOST SPHEROIDS. II. THE RED AND BLUE SEQUENCE IN THE MBH–M*,SPH DIAGRAM

“…We additionally explored a more sophisticated way to compute mass-to-light ratios, using the color-3.6 G relation published by Meidt et al (2014), their Equation (4)), which allows one to estimate 3.6 G of a galaxy from its Table 1) when available for our galaxies, or were estimated from the bulge stellar velocity dispersion, σ, using the color-σ relation presented by Peletier et al (2012, their Figure 6). We found that the range in [3.6]-[4.5] color is small (0.06 mag), and thus the range in 3.6 G is also small (0.04).…”

Galaxy and mass assembly (GAMA): the consistency of GAMA and WISE derived mass-to-light ratios

“…They argue that the model predictions at these wavelengths are much less susceptible to the uncertainties due to the details of the SPS treatment, with the 3.6µm emission dominated by old stars on the red giant branch (e.g. Spitler et al 2008, da Cunha et al 2008, Peletier et al 2012, Röck et al 2015 and that there is much less variation in mass-to-light ratio due to age/SFH and metallicity than seen at shorter wavelengths. They also argue that the treatment of the contribution of AGB stars in the original BC03 models better matches observations than do those in either the more recent 2007 version of BC03, or the alternative prescription in Maraston (2005).…”

The void galaxy survey: photometry, structure and identity of void galaxies

“…In particular the 3.6µm band data represents a major asset for our understanding of the stellar population of these galaxies, given the insensitivity of the 3.6µm band images to dust extinction and the fact that they provide a good reflection of the old stellar population in these galaxies (see e.g. Peletier et al (2012) and Meidt et al (2014) for a discussion). This also means that the 3.6µm flux, together with the [3.6]-[4.5] colour provide us with a robust estimate of the stellar mass of a galaxy, since the old stellar population constitutes its major share.…”