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

Abstract

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 {\it XMM-Newton} for 31 ks. From the X-ray data and archival near infra-red data of Two Micron All Sky survay, 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 $\sim 200$ kpc from the center, wherein $\sim$ 70% of the $K$-band luminosity is carried by NGC 1550 itself. As first revealed with {\it ASCA}, the data reconfirms the presence of a dark halo, of which the mass ($1.6\times 10^{13}{M}_{\odot }$) is typical of a galaxy group rather than of a single galaxy. Within 210 kpc, the $K$-band mass-to-light ratio reaches $75M_{\odot }/L_{\odot }$, 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 two 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 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.

31 pages, 10 figures, 2 tables

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