2017MNRAS.465.1950P

We report a new compact galaxy, SDSS J122958.84+138.0 (SDSS J1229+0001), which has unique morphological and stellar population properties that are rare in observations of the nearby Universe. SDSS J1229+0001 has an r-band absolute magnitude (M_r) and half-light radius (R_h) of -17.75 mag and 520 pc, respectively. Located in a fairly low-density environment, morphologically it is akin to a typical early-type galaxy as it has a smooth appearance and red colour. However, interestingly, it possesses centrally concentrated star-forming activity with a significant amount of dust. We present an analysis of its structural and stellar population properties using archival images and VLT/FORS2 spectroscopy. An analysis of the UKIDSS H-band image shows that the observed light distribution is better fitted with a two-component Sersic function with inner and outer component effective radii of 190 and 330 pc, respectively. The overall half-light radius measured in the H band is much smaller compared to optical, i.e. 290 pc. We prepared a spectral energy distribution from optical to far-infrared and interpret it to derive the star-formation rate, dust mass and stellar mass. We find that SDSS J1229+0001 has dust mass M_dust = 5.1 x 10⁵ M_☉ with a dust-to-stellar mass ratio log (M_dust/M_*) = -3.5. While the observed stellar population properties are - to some extent - like those of a typical S0 galaxy, a unified view from stellar population and structural properties may suggest that SDSS J1229+0001 is a smoking gun example of a compact early-type galaxy in formation.