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Register a new userExtreme Outer Galaxy: A Laboratory of Star Formation in an Early Epoch of Galaxy Formation
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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.
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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.
NGC 4203 is a nearby early-type galaxy surrounded by a very large, low-column-density HI disc. In this paper we study the star formation efficiency in the gas disc of NGC 4203 by using the UV, deep optical imaging and infrared data. We confirm that the HI disc consists of two distinct components: an inner star forming ring with radius from $sim$ 1 to $sim$ 3 R$_{eff}$, and an outer disc. The outer HI disc is 9 times more massive than the inner HI ring. At the location of the inner HI ring we detect spiral-like structure both in the deep $g-r$ image and in the 8 $mu$m $Spitzer$-IRAC image, extending in radius up to $sim$ 3 R$_{eff}$. These two gas components have a different star formation efficiency likely due to the different metallicity and dust content. The inner component has a star formation efficiency very similar to the inner regions of late-type galaxies. Although the outer component has a very low star formation efficiency, it is similar to that of the outer regions of spiral galaxies and dwarfs. We suggest that these differences can be explained with different gas origins for the two components such as stellar mass loss for the inner HI ring and accretion from the inter galactic medium (IGM) for the outer HI disc. The low level star formation efficiency in the outer HI disc is not enough to change the morphology of NGC 4203, making the depletion time of the HI gas much too long.
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 discuss on the early stage of galaxy formation based on recent deep surveys for very high-redshift galaxies, mostly beyond redshift of 6. These galaxies are observed to be strong Lyman$alpha$ emitters, indicating bursts of massive star formation in them. The fraction of such star-forming system appears to increase with increasing redshift. On the other hand, the star formation rate density derived from Lyman$alpha$ emitters tends to decrease with increasing redshift. It is thus suggested that the major epoch of initial starbursts may occur around $z sim$ 6 -- 7. In order to understand the early stage of galaxy formation, new surveys for galaxies beyond redshift of 7 will be important in near future.
We have conducted a study of star formation in the outer Galaxy from 65degr$< l <$265degr~in the region observed by the GLIMPSE360 program. This {it Spitzer} warm mission program mapped the plane of the outer Milky Way with IRAC at 3.6 and 4.5~$mu$m. We combine the IRAC, {it WISE}, and 2MASS catalogs and our previous results from another outer Galaxy survey and identify a total of 47,338 Young Stellar Objects (YSOs) across the field spanning $>$180degr in Galactic longitude. Using the $DBSCAN$ method on the combined catalog, we identify 618 clusters or aggregations of YSOs having 5 or more members. We identify 10,476 Class I, 29,604 Class II, and 7,325 anemic Class II/Class III YSOs. The ratio of YSOs identified as members of clusters was 25,528/47,338, or 54%. We found 100 of the clusters identified have previously measured distances in the {it WISE} ion{H}{2} survey. We used these distances in our spectral energy distribution (SED) fitting of the YSOs in these clusters, of which 96 had YSOs with $<3sigma$ fits. We used the derived masses from the SED model fits to estimate the initial mass function (IMF) in the inner and outer Galaxy clusters: dividing the clusters by Galactocentric distances, the slopes were $Gamma = 1.87 pm 0.31$ above 3~M$_{odot}$ for $R_{Gal} < 11.5$~kpc and $Gamma = 1.15 pm 0.24$ above 3~M$_{odot}$ for $R_{Gal} > 11.5$~kpc. The slope of the combined IMF was found to be $Gamma = 1.92 pm 0.42$ above 3~M$_{odot}$. These values are consistent with each other within the uncertainties, and with literature values in the inner Galaxy high-mass star formation regions. The slopes are likely also consistent with a universal Salpeter IMF.
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