We report the discovery of star formation activity in perhaps the most distant molecular cloud in the extreme outer galaxy. We performed deep near infrared imaging with the Subaru 8.2 m telescope, and found two young embedded clusters at two CO peaks
of Digel Cloud 1 at the kinematic distance of D = 16 kpc (Galactocentric radius RG = 22 kpc). We identified 18 and 45 cluster members in the two peaks, and the estimated stellar density are ~ 5 and ~ 3 pc^-2, respectively. The observed K-band luminosity function suggests that the age of the clusters is less than 1 Myr and also the distance to the clusters is consistent with the kinematic distance. On the sky, Cloud 1 is located very close to the H I peak of high-velocity cloud (HVC) Complex H, and there are some H I intermediate velocity structures between the Complex H and the Galactic disk, which could indicate an interaction between them. We suggest possibility that Complex H impacting on the Galactic disk has triggered star formation in Cloud 1 as well as the formation of Cloud 1 molecular cloud.
We performed multi-band deep imaging of the field around GRB 050730 to identify the host galaxies of intervening absorbers, which consist of a damped Ly{alpha} absorption (DLA) system at zabs=3.564, a sub-DLA system at zabs=3.022, and strong MgII abs
orption systems at zabs=1.773 and 2.253. Our observations were performed after the gamma-ray burst afterglow had disappeared. Thus, our imaging survey has a higher sensitivity to the host galaxies of the intervening absorbers than the normal imaging surveys in the direction of QSOs, for which the QSO glare tends to hide the foreground galaxies. In this deep imaging survey, we could not detect any unambiguous candidates for the host galaxies of the intervening absorbers. Using the 3sigma upper limit of the flux in the optical to mid-infrared observing bands, which corresponds to the UV to optical bands in the rest-frame of the intervening absorbers, we constrained the star-formation rates and stellar masses of the hosts. We estimated the star-formation rates for the intervening absorbers as < 2.5 Msun/yr for z>3 DLAs and < 1.0 Msun/yr for z~2 MgII systems. Their stellar masses are estimated to be several times 10^9 Msun or smaller for all intervening galaxies. These properties are comparable to dwarf galaxies, rather than the massive star-forming galaxies commonly seen in the z>2 galaxy surveys based on emission-line selection or color selection.
We have searched for pulsation of the anomalous X-ray pulsar (AXP) 4U 0142+61 in the K band ($lambda_{rm eff} = 2.11$ $mu$m) using the fast-readout mode of IRCS at the Subaru 8.2-m telescope. We found no significant signal at the pulse frequency expe
cted by the precise ephemeris obtained by the X-ray monitoring observation with RXTE. Nonetheless, we obtained a best upper limit of 17% (90% C.L.) for the root-mean-square pulse fraction in the K band. Combined with i band pulsation (Dhillon et al. 2005), the slope of the pulsed component ($F_ u propto u^alpha$) was constrained to $alpha > -0.87$ (90% C.L.) for an interstellar extinction of $A_{V} = 3.5$.
We report the discovery of active star formation in Digels Cloud 2, which is one of the most distant giant molecular clouds known in the extreme outer Galaxy (EOG). At the probable Galactic radius of ~20 kpc, Cloud 2 has a quite different environment
from that in the solar neighborhood, including lower metallicity, much lower gas density, and small or no perturbation from spiral arms. With new wide-field near-infrared (NIR) imaging that covers the entire Cloud 2, we discovered two young embedded star clusters located in the two dense cores of the cloud. Using our NIR and 12CO data as well as HI, radio continuum, and IRAS data in the archives, we discuss the detailed star formation processes in this unique environment. We show clear evidences of a sequential star formation triggered by the nearby huge supernova remnant, GSH 138-01-94. The two embedded clusters show a distinct morphology difference: the one in the northern molecular cloud core is a loose association with isolated-mode star formation, while the other in the southern molecular cloud core is a dense cluster with cluster-mode star formation. We propose that high compression by the combination of the SNR shell and an adjacent shell caused the dense cluster formation in the southern core. Along with the low metallicity range of the EOG, we suggest that EOG could be an excellent laboratory for the study of star formation processes, such as those triggered by supernovae, that occured during an early epoch of the Galaxys formation. In particular, the study of the EOG may shed light on the origin and role of the thick disk, whose metallicity range matches with that of the EOG well.
The extreme outer Galaxy (EOG) has a very different environment from that in the solar neighborhood, with low metallicity (less than -0.5 dex), much lower gas density, and small or no perturbation from spiral arms. The EOG is an excellent laboratory
for the study of the star formation processes that happened during the formation period of the Galaxy. In particular, the study of the EOG may shed light on the origin and role of the thick disk, whose metallicity range matches well with that of the EOG. We show an example of a molecular cloud in the EOG (Digels Cloud 2), which is located at R_g ~ 20 kpc beyond the Outer arm. Based on our NIR and 12CO data as well as HI, radio continuum, and IRAS data in the archives, we examined the detailed star formation processes in this unique environment, especially the supernova triggered star formation, which should have been the major star formation mode during the halo and thick disk formation.
