Scientific Results

Asymmetries in isolated galaxies (2011)

We characterize the HI profile asymmetries in a sample of isolated galaxies and search for correlations between H I asymmetry and their environments, as well as optical and far infrared properties. We use high signal-to-noise global H I profiles for galaxies in the AMIGA project. We restrict our study to N = 166 galaxies (out of 312) with accurate measures of the H I shape properties. We quantify asymmetries using a flux ratio parameter.


The asymmetry parameter distribution of our isolated sample is well described by a Gaussian model. The width of the distribution is σ = 0.13, and could be even smaller (σ = 0.11) if instrumental errors are reduced. Only 2% of our carefully vetted isolated galaxies sample show an asymmetry in excess of 3σ. By using this sample we minimize environmental effects as confirmed by the lack of correlation between H I asymmetry and tidal force (one-on-one interactions) and neighbor galaxy number density. On the other hand, field galaxy samples show wider distributions and deviate from a Gaussian curve. As a result we find larger asymmetry rates (∼ 10–20%) in such samples. We find evidence that spiral arm strength is inversely correlated with HI asymmetry. We also find an excess of FIR luminous galaxies with larger HI asymmetries which may be spirals associated with hidden accretion events.


Our sample presents the smallest fraction of asymmetric H I profiles compared with any other yet studied. The width of the associated asymmetry parameter distribution can help disentangling the frequency and processes of self-induced HI asymmetries, and serve as a baseline to study asymmetry rates in other environments.


PDF File Espada et al. 2011

Fig. 1 A_flux~ratio distribution of the HI refined subsample (N = 166, blue filled histogram), in comparison with that of the HI sample (N = 312, solid line histogram). A half-Gaussian fit (red dashed line) to the HI refined subsample is presented. The half-Gaussian curve is characterized by a standard deviation sigma=0.13. In the lower panel we show the residual of the half-Gaussian fit to the observed A_flux~ratio distribution for the HI refined subsample.