THE LOPSIDEDNESS OF PRESENT-DAY GALAXIES: CONNECTIONS TO THE FORMATION OF STARS, THE CHEMICAL EVOLUTION OF GALAXIES, AND THE GROWTH OF BLACK HOLES

A global lopsidedness in the distribution of the stars and gas is common in galaxies. It is believed to trace a nonequilibrium dynamical state caused by mergers, tidal interactions, asymmetric accretion of gas, or asymmetries related to the dark matter halo. We have used the Sloan Digital Sky Survey (SDSS) to undertake an investigation of lopsidedness in a sample of similar to 25,000 nearby galaxies (z < 0.06). We use the m = 1 azimuthal Fourier mode between the 50% and 90% light radii as our measure of lopsidedness. The SDSS spectra are used to measure the properties of the stars, gas, and black hole in the central-most few-kpc-scale region. We show that there is a strong link between lopsidedness in the outer parts of the galactic disk and the youth of the stellar population in the central region. This link is independent of the other structural properties of the galaxy. These results provide a robust statistical characterization of the connections between accretion/interactions/ mergers and the resulting star formation. We also show that residuals in the galaxy mass-metallicity relation correlate with lopsidedness (at fixed mass, the more metal-poor galaxies are more lopsided). This suggests that the events causing lopsidedness and enhanced star formation deliver lower metallicity gas into the galaxy's central region. Finally, we find that there is a trend for the more powerful active galactic nuclei (the more rapidly growing black holes) to be hosted by more lopsided galaxies (at fixed galaxy mass, density, or concentration). However, if we compare samples matched to have both the same structures and central stellar populations, we then find no difference in lopsidedness between active and non-active galaxies. Indeed, the correlation between the youth of the stellar population and the rate of black hole growth is stronger than the correlation between lopsidedness and either of these other two properties. This leads to the following picture. The presence of cold gas in the central region of a galaxy (irrespective of its origin) is essential for both star formation and black hole growth. The delivery of cold gas is aided by the processes that produce lopsidedness. Other processes on scales smaller than we can probe with our data are required to transport the gas to the black hole.