Hubble Space Telescope Captures UGC 12591: bulge/disc properties, star formation and 'missing baryons' census in a very massive and fast-spinning hybrid galaxy

We present Hubble Space Telescope (HST) observations of the nearby, massive, highly rotating hybrid galaxy UGC 12591, along with observations in the UV to FIR bands. HST data in V, I, and H bands is used to disentangle the structural components. Surface photometry shows a dominance of the bulge over the disc with an H-band B/D ratio of 69 per cent. The spectral energy distribution (SED) fitting reveals an extremely low global star formation rate (SFR) of similar to 0.1-0.2 M-circle dot yr(-1), exceptionally low for the galaxy's huge stellar mass of 1.6 x10(11)M(circle dot), implying a strong quenching of its SFR with a star formation efficiency of 3-5 per cent. For at least the past 10(8) yr, the galaxy has remained in a quiescent state as a sterile, 'red and dead' galaxy. UGC 12591 hosts a supermassive black hole (SMBH) of 6.18 x10(8) M-circle dot, which is possibly quiescent at present, i.e. we neither see large ( > 1kpc) radio jets nor the SMBH contributing significantly to the mid-IR SED, ruling out strong radiative feedback of AGN. We obtained a detailed census of all observable baryons with a total mass of 6.46 x10(11) M-circle dot within the virial radius, amounting to a baryonic deficiency of similar to 85 per cent relative to the cosmological mean. Only a small fraction of these baryons reside in a warm/hot circumgalactic X-ray halo, while the majority are still unobservable. We discussed various astrophysical scenarios to explain its unusual properties. Our work is a major step forward in understanding the assembly history of such extremely massive, isolated galaxies.