The ACS Fornax Cluster Survey: the nuclei of early-type galaxies.

Date

2011-07-27

Authors

Turner, Monica

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Abstract

The Advanced Camera for Surveys (ACS) Fornax Cluster Survey is a Hubble Space Telescope programme that imaged 43 early-type galaxies in the Fornax cluster, using the ACS F475W (~g) and F850LP (~z) bandpasses. We use this data set, which spans a range of ~600 in blue luminosity down to M_B ~ -16 mag, to study and characterize the properties of central galactic nuclei by fitting ellipses to the galaxy isophotes and examining their 1-dimensional surface brightness profiles. To test the robustness of this method, we perform a similar analysis with 2-dimensional surface brightness profile fitting using GALFIT, and find acceptable agreement between the derived structural parameters from the two techniques. We determine 72% of our sample (31 galaxies) to be nucleated, a significant increase from 28% found in the ground-based study of Ferguson et al. (1989). This high frequency of nucleation suggests that the creation of a compact nuclear component is a common outcome in early-type galaxy formation. Only three of the nuclei (10%) are observed to be significantly offset (by > 0.5") from their host galaxy photocentres, and a trend of increasing offset in fainter galaxies is observed, which indicates that nucleus formation timescales and/or pathways may vary with host luminosity. The nuclei are found to be larger and approximately 50 times brighter than typical globular clusters from our Fornax sample, and to follow different half-light surface brightness versus magnitude scaling relations. The colours of the nuclei are characteristic of intermediate to old stellar populations, and those residing in galaxies with B_T > 13 mag are observed to correlate with the nucleus luminosities. Both nucleus and galaxy colours become redder with increasing host luminosity, although the trend with the nuclei is steeper, and the nuclei in the brightest galaxies are found to be redder than their hosts. However, the majority of nuclei are bluer than their hosts, having an average colour difference of 0.27 +/- 0.25 mag. Comparison of our results to the complementary ACS Virgo Cluster Survey (ACSVCS) study of nuclei Côté et al. (2006), which examined 100 early-type galaxies in the Virgo cluster, yields strikingly similar results. Both samples show similar frequency of nucleation (68% in the ACSVCS), a constant nucleus-to-galaxy luminosity ratio (with a mean value of 0.41% +/- 0.04% derived from the combined samples), as well as excellent agreement in the nucleus luminosity functions and sizes (with median values of 6 pc in g and 7 pc in z in both studies). Since the Fornax cluster presents a much denser environment than Virgo, such consistency between the properties of the nuclei indicates that their formation and evolution may be influenced by local factors more than environmental ones. In particular, a constant nucleus-to-galaxy luminosity ratio suggests that a host galaxy's luminosity (or, more likely, mass) may be a key element in determining the properties of its nucleus. Since simulations have found the two main theorized nucleus formation pathways to be effective on different mass scales (with dissipationless infall of star clusters being more efficient in lower-mass galaxies, and in-situ gas accretion in higher-mass hosts), we propose that both processes may both in fact be responsible for nucleus formation, but varying in relative importance along the galaxy luminosity function.

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Keywords

Advanced Camera for Surveys, galaxies, nuclei

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