Galaxies: Age and metallicity

Massive early-type galaxies are now widely believed to have been assembled hierarchically through repeated mergers which provide a relatively constant flow of accreted mass over a long time. The innermost regions of massive galaxies appear to have formed the majority of their stars at high redshift and on short time-scales (e.g. Thomas et al. 2005) whereas their outer parts are likely assembled as a consequence of multiple major and minor merging (e.g. Trujillo, Ferreras & de La Rosa 2011). This two-phase formation picture (e.g. Naab, Johansson & Ostriker 2009) agrees with the observed size evolution of the massive galaxies. Atz ∼ 1 (2), massive early-type galaxies (M* ∼ 1011 M) were a factor of 2 (4) smaller than present-day equal mass objects, having an average effective radii of only ∼1 kpc at z ∼ 2 (e.g. Daddi et al. 2005; Trujillo et al. 2006; Buitrago et al. 2008).

Scheme of the contribution of mergers to the total stellar mass at z = 0. The circles illustrate the mean effective radius of galaxies at z=2 (gray), 1:4<z<2 (blue, star formation dominates), and 0 < z < 1:4 (red, mergers dominate). Taken from van Dokkum et al. (2010).

Scheme of the contribution of mergers to the total stellar mass at z = 0. The circles illustrate the mean effective radius of galaxies at z=2 (gray), 1:4<zand 0 < z < 1:4 (red, mergers dominate). Taken from van Dokkum et al. (2010).

Surface brightness profiles of the multiwavelength data set for M87 up to Re. The vertical dashed line marks the position of the Re. Different vertical shifts to the profiles were applied for visualization. Filled circles correspond to HR photometry while open circles depict wide-field.

Surface brightness profiles of the multiwavelength data set for M87 up to Re. The vertical dashed line marks the position of the Re. Different vertical shifts to the profiles were applied for visualization. Filled circles correspond to HR photometry while open circles depict wide-field.

In order to probe this inside-out formation of the most massive galaxies in the Universe, we have explored the radial (0.1 ≲ R ≲ 8 kpc) variation of the spectral energy distribution of M87 from UV to IR. For this purpose, we have combined high-resolution data in 16 different bands.

Gradients of age and metallicity with radial distance for M87. The grey polygon is the 68 per cent confidence region resulting from the BC03 SSP model fit to our data and the dark grey diamonds represent the model with the minimum χ̃ 2. Red squares represent the data obtained with SAURON for the central region of M87 (Kuntschner et al. 2010). Green circles show the values of age and metallicity derived by Liu et al. (2005). Purple stars are data from Davies et al. (1993) using absorption-line strength of Mg2. We have shifted the age estimates of Kuntschner et al. (2010) from 17.7 to 14 Gyr. The orange dashed line indicates the position of M87's Re. The blue polygon defines the range of possible values for the size of M87 at z = 2.

Gradients of age and metallicity with radial distance for M87. The grey polygon is the 68 per cent confidence region resulting from the BC03 SSP model fit to our data and the dark grey diamonds represent the model with the minimum χ̃ 2. Red squares represent the data obtained with SAURON for the central region of M87 (Kuntschner et al. 2010). Green circles show the values of age and metallicity derived by Liu et al. (2005). Purple stars are data from Davies et al. (1993) using absorption-line strength of Mg2. We have shifted the age estimates of Kuntschner et al. (2010) from 17.7 to 14 Gyr. The orange dashed line indicates the position of M87’s Re. The blue polygon defines the range of possible values for the size of M87 at z = 2.

Our analysis indicate that the age of the stellar population of M87 remains almost unchanged with radius. However, the metallicity ([Z/H]) profile presents three different zones: the innermost kpc shows a plateau with supersolar metallicity, followed by a decline in metallicity down to 5 kpc and anotherplateau afterwards. The size of the inner plateau is similar to the expected size (Re) of an object with the predicted mass of M87 at z = 2. The global [Z/H] gradient is −0.26 ± 0.10, similar to those found in other nearby massive ellipticals. The observed change in the stellar population of M87 is consistent with a rapid formation of the central part (R ≲ 5 kpc) of this galaxy followed by the accretion of the outer regions through the infall of more metal-poor material.

 

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