The stellar mass distribution in early‐type disc galaxies: surface photometry and bulge–disc decompositions

ABSTRACT We present deep B ‐ and R ‐band surface photometry for a sample of 21 early‐type disc galaxies with morphological types between S0 and Sab and absolute B ‐band magnitudes between −17 and −22. Six galaxies were also observed in the I ‐band. We present radial profiles of surface brightness, colour, ellipticity, position angle and deviations of axisymmetry for all galaxies, as well as isophotal and effective radii and total magnitudes. We have decomposed the images into contributions from a spheroidal bulge with a general Sérsic profile and a flat disc with an arbitrary intensity distribution, using an interactive, 2D decomposition technique. We caution against the use of simple 1D decomposition methods and show that they can lead to systematic biases, particularly in the derived bulge parameters. We study in detail the relations between various bulge and disc parameters. In particular, we find that the bulges of our galaxies have surface brightness profiles ranging from exponential to De Vaucouleurs, with the average value of the Sérsic shape parameter n being 2.5. In agreement with previous studies, we find that the shape of the bulge intensity distribution depends on luminosity, with the more luminous bulges having more centrally peaked light profiles (i.e. higher n ). By comparing the ellipticity of the isophotes in the bulges to those in the outer, disc‐dominated regions, we are able to derive the intrinsic axis ratio q b of the bulges. The average axis ratio is 0.55, with a rms spread of 0.12. None of the bulges in our sample is spherical, whereas in some cases, the bulges can be as flat as q b = 0.3–0.4 . The bulge flattening seems to be weakly coupled to luminosity, more luminous bulges being on average slightly more flattened than their lower luminosity counterparts. Our finding that most bulges are significantly flattened and have an intensity profile shallower than R 1/4 suggests that ‘pseudo‐bulges’, formed from disc material by secular processes, do not occur in late‐type spiral galaxies only, but are a common feature in early‐type disc galaxies as well. Most galaxies in our sample have radial colour gradients, becoming bluer towards larger radii. Although this can partly be explained by the radially declining contribution of the red bulges to the observed light, we show that discs must also have intrinsic colour gradients.