4. The color-asymmetry diagram (CAD)
The CAD for a sample of 113 nearby galaxies (Frei et al. 1996; Conselice et al. 2000b) and the starburst sample is shown in Fig. 1. A correlation between asymmetry and color can be seen for most Hubble types: Early type galaxies (E, S0) populate the symmetric, red corner of the diagram; later type galaxies become progressively bluer and more asymmetric. This `fiducial' trend with Hubble type is indicated by the dashed line in Fig. 1. Some objects do not lie on the fiducial color-asymmetry sequence. Most of these are highly-inclined systems, as discussed by Conselice et al. 2000b. However, visual inspection of the most extreme outliers at blue colors of B-V0.7 (i.e., NGC 3079, Arp 18, and NGC 4254) indicates they are are too asymmetric for their colors because they are dynamically disturbed. It appears to be possible to distinguish if a galaxy is interacting or merely undergoing normal star formation (such as irregular galaxies) based simply on its position in the CAD.
Where do the starbursts lie in the CAD? Their CAD positions are mostly consistent with a strong interaction, or merger origin (Fig. 1, ). Moreover, the amount of deviation from the fiducial galaxy sequence appears to be correlated roughly with the degree of interaction/merger. For example, NGC 3690 and Markarian 8 are major mergers between galaxies of similar sizes, and have two of the most deviant positions. NGC 3310, on the other hand, almost fits along the fiducial galaxy sequence; this starburst is believed to be produced by a bar instability or minor interaction (Conselice et al. 2000a). The fact that NGC 3310 does not fit exactly along this sequence is probably due to the fact that it was involved with a minor merger in the past (Balik & Heckman 1981).
We can quantify and test our interpretation of the CAD by comparing a starbursts deviance in the CAD with a kinematic indicator of deviance. A variety of HI studies show that interacting galaxies often have unusual and asymmetric global HI emission line profiles, often with extended velocity tails (e.g. Gallagher, Knapp & Faber 1981; Bushouse 1987; Mirabel & Sanders 1988; Sancisi 1997). Here we introduce the use of the ratio of HI line-width at 20% and 50% maximum, as extracted from the Lyon-Meudon Extragalactic Database (LEDA) as a new dynamical indicator. High values of W20 / W50 imply shallower rising HI profiles or wings, and hence should be an indicator of a recent dynamical disturbance.
As Fig. 2 shows, this line-width ratio is large for the starburst galaxies with asymmetries that deviate from the fiducial sequence. Galaxies shown by the color-asymmetry digram to be strongly interacting or merging, namely NGC 7673, NGC 3690 and Markarian 8, all have the largest HI line width ratios. NGC 3310 and NGC 7678, which have symmetric inner spiral structures and are probably older starbursts perhaps triggered by bar instabilities, have smaller line-width ratios and are less deviant from the fiducial sequence in color and asymmetry. Hence our interpretation of starburst origin based on the color-asymmetry diagram is corroborated.
Finally, we note that the starburst galaxies with high asymmetries have blue UBV colors (Fig. 3), but do not lie outside the range for normal galaxies (rectangle, Fig. 3, adopted from Huchra 1977). Larson and Tinsley (1978) showed that galaxies with tidal features, such as tidal tails, had a large scatter on a UBV color-color plots. This is not evident for our small sample, which indicates that the color-asymmetry method is more sensitive to identifying merger-induced starbursts than colors alone.
© European Southern Observatory (ESO) 2000
Online publication: January 31, 2000