and their distances (estimated from period-luminosity relations) are consistent with the distance to the NSD. From the period-age relation [@Bono-2005], these Cepheids were found to be similarly aged ($\sim 25$ Myr). Populations of relatively young stars are also evidenced by the concentration of OH/IR stars and mass-losing red supergiants [@Lindqvist-1992]. @Blommaert-1998 found that some OH/IR stars are very luminous, suggesting ages of $\sim 1$ Gyr or even $\sim 100$ Myr [see also @vanLoon-2003]. However, these estimates have large uncertainties because we lack understanding of the mass-loss phenomena in the stars [@Gallart-2005]. From surface-density distributions, @Matsunaga-2013 suggested that a fraction of old stellar population represented by short-period Miras belongs to the NSD, although NSD membership requires confirmation by kinematical information.

The disk-like structure of the NSD and CMZ suggests that these systems spin around the Galactic Center. In fact, the $l$-$v$ diagram reveals elliptical orbits of the molecular gas in this region [@Binney-1991; @Stark-2004], which are understood as $x_2$ orbits expected near or inside the inner Lindblad resonance [@Contopoulos-1980; @Athanassoula-1992]. On the other hand, $x_1$ orbits are elongated along the bar potential and are exhibited by a portion of gas within the Galaxy. Gas transfer from $x_1$ to $x_2$ orbits is 