A gas and dust rich giant elliptical galaxy in the ISOPHOT Serendipity Survey

We present a multi-wavelength study of the unusual galaxy ISOSS J 15079+7247. With a bolometric luminosity of <formula notation="TeX">$L_{\rm FIR} \sim 2 \times 10^{12}~{L_{\odot}}$</formula> this object is one of the most luminous galaxies detected by the 170 <formula notation="TeX">${\rm \mu m}$</formula> ISOPHOT Serendipity Survey (ISOSS). We have obtained 1.2 mm continuum and CO (1–0) line observations, optical spectra and deep <formula notation="TeX">$R$</formula>-band images of the system. The CO emission originates from a giant elliptical galaxy at redshift <formula notation="TeX">$z=0.2136$</formula>, yielding a molecular gas mass of <formula notation="TeX">$2.9 \times 10^{10}~{M_{\odot}}$</formula>. The high gas mass favors the picture that the dust emission is associated with the elliptical galaxy. The nature of the ultraluminous IR emission can be explained by an opaque, hidden starburst in the center of the elliptical. The huge dust mass of <formula notation="TeX">$5 \times 10^{8}~{M_{\odot}}$</formula> corresponds to a visual extinction of <formula notation="TeX">$A_{\rm V} \sim 1000~{\rm mag}$</formula>. This is consistent with the non-detection of any optical signatures of a strong starburst in ISOSS J 15079+7247 and the strength of non-thermal radio continuum emission.