A Galaxy Merger Scenario for the NGC 1550 Galaxy from Metal Distributions in the X-Ray Emitting Plasma

The elliptical galaxy NGC 1550 at a redshift of z = 0.01239, identified with an extended X-ray source RX J0419+0225, was observed with XMM-Newton for 31 ks. From the X-ray data and archival near-IR data of Two Micron All Sky survey, we derive the profiles of components constituting the NGC 1550 system: the gas mass, total mass, metal mass, and galaxy luminosity. The metals (oxygen, silicon, and iron) are extended to ~200 kpc from the center, wherein ~ 70% of the K-band luminosity is carried by NGC 1550 itself. As first revealed with ASCA, the data reconfirms the presence of a dark halo, of which the mass (1.6 × 10¹³ M _sun) is typical of a galaxy group rather than of a single galaxy. Within 210 kpc, the K-band mass-to-light ratio reaches 75 M _sun/L _sun, which is comparable to those of clusters of galaxies. The iron mass-to-light ratio profile (silicon and oxygen mass-to-light ratio profiles as well) exhibits about 2 orders of magnitude decrease toward the center. Further studies comparing mass densities of metals with those of the other cluster components reveal that the iron (as well as silicon) in the ICM traces very well the total gravitating mass, whereas the stellar component is significantly more concentrated to within several tens of kpc of the NGC 1550 nucleus. Thus, in the central region, the amount of metals is significantly depleted for the luminous galaxy light. Among a few possible explanations of this effect, the most likely scenario is that galaxies in this system were initially much more extended than today, and gradually fell to the center and merged into NGC 1550.