FIGURE 2.4 - Comparison of photographic Zwicky magnitudes and (a) B₂₆ isophotal magnitudes and (b) total B magnitudes. For m_Z brighter than 14, the observed rms is 0.30 in both cases; for m_Z    14 the rms is 0.42 and 0.45 mag for isophotal and total magnitudes, respectively. (c) Difference m_Z-B₂₆ as a function of B₂₆. For m_Z > 14 this difference is correlated with isophotal magnitude. (d) m_Z-B₂₆ as a function of galaxy type. A weak trend of larger differences toward later galaxy type may be discerned, but the scatter on this trend is large.  

irregular galaxies. A similar conclusion is reached if the average B surface brightness within the effective radius is used, but the trend is somewhat less pronounced.  
        We now explore the radial color gradient in our sample galaxies. We measure (B-R) in the central region of the galaxy that contains 25% of the light, (B-R)₂₅, and in the region that contains the next 50% of the light, (B-R)_25-75. We define (B-R)_25-75 as the difference between these two measurements: (B-R)_25-75 - (B-R)₂₅. (U-B)_25-75 is defined similarly. If (B-R)_25-75 is negative, the inner parts of the galaxy are redder than the outer parts.  
        Figure 2.6a is a plot of (B-R)_25-75 against galaxy type for the 168 ``normal'' galaxies. The median color difference values for each type have been connected by solid lines in this plot. The color difference between the inner and outer parts of a galaxy remains fairly constant for most of the early-type galaxies (T 0): the inner parts are on average 0.09 mag (with a range of ~0.15 mag) redder than the outer parts. Moving from S0/a to Sa, this difference becomes larger. The centers of spiral galaxies are redder by 0.18 mag, on average (with a range of ~0.30 mag), than their outer parts. The color differences decrease for late-type spiral galaxies, reflecting the decreasing prominence of bulges and the more extended star formation. The inner parts of irregular galaxies can be either bluer or redder than the outer parts. We will return to this issue below.  
        The bluing of galaxies at larger radii is not a new result (Sandage 1972; Persson, Frogel & Aaronson 1979; de Jong 1996). Recently, however, TVPHW96 describe an absolute magnitude-dependent behaviour for galaxies in the Ursa Major cluster. TVPHW96 find