span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">['Halpha']</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">vel_rel</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">[1]</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">sigma_rel</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">[1]</span><