The decrease of the B/D flux ratio along the Hubble sequence from early- to late-type galaxies is well known, but this ratio, among others, depends on the structural component-fitting procedure of the galaxy, on inclination, and on dust-extinction (see Graham & Worley 2008, and references therein). Morphological classification is only available from the largest catalogue so far (Nair & Abraham 2010) for less than 50% of our galaxy sample. Our aim here is to use the B/D flux ratios (Gadotti 2009) and the morphological classification in T-types for our sub-sample of galaxies included in the Nair & Abraham (2010) catalogue, to separate our galaxies into early- and late-types, according to their B/D flux ratios.
Figure A.1 shows the dependence of the logarithm of the g-, r-, and i-band B/D light ratios (from BUDDA, Gadotti 2009), with the corresponding T-type (Nair & Abraham 2010) for barred and unbarred galaxies occurring in both samples simultaneously. The top and bottom left-hand panels show this relation for the three different photometric bands, with small crosses for unbarred galaxies and small, filled circles for barred galaxies. The large crosses and big empty circles indicate the median log (B/D) value for each T-type value for unbarred and barred galaxies, respectively. As can be seen, there are no clear
differences between barred and unbarred galaxies. The bottom right-hand panel shows the median log (B/D) values, for barred and unbarred galaxies together, for each T-type for all photometric bands. The differences in the median B/D light ratio between the different bands are, in general, smaller than the error bars.
Our median values correspond well (within errors) to those from Graham & Worley (2008, their Fig. 6) in the T-type range in common (between T-type 0 and 5), except for T = 0, where we obtain a marginally larger B/D ratio in the r and i bands. These differences could arise from poor statistics at T = 0 in Graham & Worley (2008) (only three galaxies versus 30 in our sub-sample of galaxies are included in the Nair & Abraham (2010) catalogue).
We point out that a number of unbarred galaxies in our sample (~15), supposed to be disc galaxies, are classified with T-type ≤−3 by Nair & Abraham (2010), i.e. as ellipticals (see Figs. A.1 or A.2). These galaxies are very difficult to classify, since a face-on disc without spiral arms or a bar looks just like a round elliptical. Gadotti (2009) classified galaxies after looking at isophotal contours and the profile very carefully, but as stated in the cited paper, the estimated misclassification frequency is ~5−10%. With these uncertainties in mind, we have opted to keep these galaxies in our sample, and to consider them as S0 disc galaxies.
Logarithm of the B/D flux ratio in the g, r, and i-band versus morphological type (expressed as T-type) for the galaxies of the sample contained in the Nair & Abraham (2010) catalogue. Big crosses and big empty circles mark the median log (B/D) for each T-type value for unbarred and barred galaxies, respectively. The bottom right-hand panel shows the median log (B/D) in each band for each T-type value. Error bars represent the 95% confidence level of the median value for the distribution of B/D values at each T-type value. The blue solid straight line marks B/D = 0.5, that we have used to separate early from late-type galaxies.
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Decimal logarithm of the bulge-to-disc flux ratio in the i-band as a function of the morphological T-type, as given in the Nair & Abraham (2010) catalogue for the galaxies of our sample included in this catalogue. The colours represent different values of the predicted T-type Galaxy Zoo 2 parameter Willett et al. (2013).
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The B/D flux ratios decrease towards late-type spirals as expected. We have used this trend to create sub-samples dominated by early- or late-type spirals. In our sub-sample of galaxies included in the Nair & Abraham (2010) catalogue, a value of log (B/D) =−0.30 (or B/D = 0.5) more or less separates galaxies with T-type < 2 (earlier than Sa) from those later or equal to T-type =2 (Sab), but the contamination from late-types (with the previous definition) in the early-type sub-sample and vice versa is approximately 12% and 30%, respectively. The predicted T-type, as given in the Galaxy Zoo 214 catalogue (hereinafter GZ2 Willett et al. 2013), allows us to improve this separation. We then consider early-type galaxies as those galaxies of the sample with either B/D0.5, or those having simultaneously B/D < 0.5 and T-type, as predicted by GZ2, lower than −1. Similarly, we consider as late-type galaxies those with B/D < 0.5 and a T-type as predicted by GZ2, larger than or equal to one. In this way we are able to include ~70% and 90% of the early (T-type < 2) and late types (T-type 2) in the sub-samples of early- and late-type galaxies, respectively, and the contamination from late (early-types) in the early (late-type) sub-sample is ~10% (25%). These numbers are the same in all three bands. However, we have used the i-band, as the B/D flux ratio is available for all sample galaxies in this filter.
© ESO, 2015