• 천문학회보
• /
• 제40권2호
• /
• pp.32.1-32.1
• /
• 2015

The gradual infall of small dark matter halos onto larger ones has become a relatively straightforward aspect of the standard hierarchical formation paradigm. What happens to the baryons they contain, however, is less well understood. Of special relevance are the processes that regulate and ultimately suppress star formation in galaxies in the early universe. The z=1.5-2.5 epoch is then particularly interesting as a transition period when global star-formation in the universe starts peaking but also where the first ostensibly collapsed and virialized galaxy clusters appear, along with segregated galaxy populations. From a theoretical point of view, the mode of gas accretion in massive halos is also expected to change around this time, switching from a cold to a hot phase and affecting the build-up and evolution of the galaxies they host. A lot of effort has thus been devoted to the search for high-redshift structures, in particular galaxy clusters, through a variety of methods. However, as the limited area for which deep datasets are available remains relatively limited, only few massive z>1.5 structures have been found so far. Here I will instead discuss the regulation of star-formation in lower-mass, X-ray detected halos at z~2 and its implication for galaxy quenching at high redshift. As these smaller, group-size halos are vastly more abundant and structurally simpler than massive clusters, they allow for true statistical studies and offer a novel way to probe environmental effects in this transitional epoch.

• 천문학회보
• /
• 제46권1호
• /
• pp.57.2-57.2
• /
• 2021

Variability in Young Stellar Objects (YSOs) can be caused by time-dependent accretion rates, geometric changes in the circumstellar disks, the stochastic hydromagnetic interactions between stellar surfaces and inner disk edges, reconnections within the stellar magnetosphere, and hot/cold spots on stellar surfaces. We uncover ~1400 variables from a sample of ~5300 YSOs in nearby low-mass star-forming regions using mid-IR light curves obtained from the 5.5-years NEOWISE All Sky Survey. The mid-IR variability traces a wide range of dynamical, physical, and geometrical phenomenon. We classify six types of YSO variability based on their light curves: secular variability (Linear, Curved, Periodic) and stochastic variability (Burst, Drop, Irregular). YSOs in earlier evolutionary stages have higher fractions of variables at all types and higher amplitudes for the variability. Along with brightness variability, we also find a diverse range of secular color variations, which can be attributed to a competitive interplay between the variable accretion luminosity of the central source and the variable extinction by material associated with the accretion process. We compare the variability of known FUors/EXors and VeLLOs/LLSs, which represent two extreme ends (burst versus quiescent) of the episodic accretion process; FUors/EXors have a higher fraction of variables (65%) than VeLLOs/LLSs (41%). Short-term (few day) and long-term (decades) variability, as well as possible AGB contamination in the YSO catalogues, are also discussed.molecules become more complex by surface chemistry induced directly by high energy photons or by the thermal energy diffused over heated grain surface. Therefore, the ice composition is an

• 천문학회보
• /
• 제46권1호
• /
• pp.33.3-34
• /
• 2021

In LCDM universe, large, massive structures, like galaxy clusters, grow through the successive accretion/mergers of smaller structures. Therefore, at high redshift, unlike local, it is expected that there would be plenty of galaxy clusters which are still growing. Here, we report the discovery of a high-redshift (z~1) galaxy cluster which is in its active formation stage. This cluster is well connected to the large scale overdense environment and contains high fraction of star-forming galaxies, providing a good example supporting our previously suggested 'Web-feeding model'.

• 천문학회보
• /
• 제40권1호
• /
• pp.46.3-46.3
• /
• 2015

As theoretical and empirical studies have pointed out, galaxy mergers play a pivotal role in galaxy mass assembly histories. Its contribution is considered to be more significant in more massive galaxies. In order to quantitatively understand the origin of stellar components in galaxies, we investigated stellar mass assembly histories as a function of galaxy and halo mass using semi-analytic approaches. In this study, we found that the most massive galaxies (log $M/M_{\odot}$ ~ 11.75 at z = 0), which are mostly the brightest cluster galaxies, obtain roughly 70% of their stellar components via mergers. The role of mergers monotonically declines with galaxy mass: less than 20% for log $M/M_{\odot}$ = 10.75 at z = 0. The contribution of galaxy mergers to stellar mass growth decays more slowly than that of in-situ star formation. Therefore, merger accretion becomes a dominant channel for stellar mass growth of the most massive group since z~2. However, when it comes to central galaxies in haloes less massive than $10^{13}_{\odot}$, star formation is always dominant.

• 천문학회보
• /
• 제37권1호
• /
• pp.43.1-43.1
• /
• 2012

We have performed our first cosmological hydrodynamic simulation using the recently developed SPH+GOPTM code that includes radiative cooling/heating, star formation, and supernova feedback. Here we present preliminary results from the simulation $3.4{\times}10^4M_{\odot}$, thus sub-galactic structures, such as satellite galaxies and globular clusters around a host galaxy, can be resolved with more than hundred particles. We follow formation and evolution of the sub-galactic structures in view of their star formation history, merging/accretion rate, and origins.

• 천문학회보
• /
• 제38권2호
• /
• pp.65.1-65.1
• /
• 2013

Our understanding of how brown dwarfs form is limited by observational evidence. We report identification of a L328-IRS as a proto-brown dwarf embedded in an isolated dense molecular core. This source exhibits typical properties of a protostar, however, its luminosity (~0.05 $L{\odot}$) is far below than expected from the least massive protostar by the standard star formation theory. The most likely mass accretion rate (~2.4 10-7 $M{\odot}$ yr-1) inferred from its small bipolar outflow is an order of magnitude less than the canonical value for a protostar. The mass available in its envelope is less than 0.1 $M{\odot}$. These points suggest that L328-IRS will accrete the mass of a brown dwarf, but not that of a star. L328 is found to be fairly well isolated from other nearby clouds and seems to be forming three sub-cores simultaneously through a gravitational fragmentation process. Altogether with these, our direct detection of inward motions in L328 which harbors this proto-brown dwarf clearly supports the idea that a brown dwarf forms like a normal star.

• 천문학회보
• /
• 제43권1호
• /
• pp.56.1-56.1
• /
• 2018

Tidal disruption events (TDEs) provide evidence for quiescent supermassive black holes (SMBHs) in the centers of inactive galaxies. TDEs occur when a star on a parabolic orbit approaches close enough to a SMBH to be disrupted by the tidal force of the SMBH. The subsequent super-Eddington accretion of stellar debris falling back to the SMBH produces a characteristic flare lasting several months. The theoretically expected bolometric light curve decays with time as proportional to $t^{-5/3}$. However, the light curves observed in most of the optical-UV TDEs deviate from the $t^{-5/3}$ decay rate especially at early time, while the light curves of some soft-X-ray TDEs are overall in good agreement with the $t^{-5/3}$ law. Therefore, it is required to construct the theoretical model for explaining these light curve variations consistently. In this paper, we revisit the mass fallback rates analytically and semi-analytically by taking account of the structure of the star, which is simply modeled by the polytrope. We find the relation between a polytropic index and the power law index of the mass fallback rate. We also discuss whether and how the decay curves, which we derived, fit the observed ones.

• 천문학논총
• /
• 제30권2호
• /
• pp.341-343
• /
• 2015

We present an analysis of the relation between star-formation (SF) and accretion luminosities of local type-2 active galactic nuclei (AGNs) at $0.01{\leq}z<0.22$. We match type-2 AGNs found in the Sloan Digital Sky Survey to current far-infrared (FIR) survey catalogues based on AKARI and Herschel. Estimating AGN luminosities from [$O{\small{III}}$]${\lambda}5007$ and [$O{\small{I}}$]${\lambda}6300$ emission lines, we find a positive linear trend between FIR and AGN luminosities over a wide dynamical range. This result appears to be inconsistent with recent reports that low-luminosity AGNs show no correlation between FIR and X-ray luminosities; this contradiction is likely due to Malmquist and sample selection biases. Moreover, we also find that pure-AGN candidates, for which the FIR radiation is thought to be AGN-dominated, show significant low-SF activities. These AGNs hosted by low-SF galaxies are rare in our sample. However, it is possible that the low fraction of low-SF AGN is caused by observational limitations, as recent FIR surveys are not sufficient to examine the population of high-luminosity AGNs hosted by low-SF galaxies.

• 천문학회보
• /
• 제37권2호
• /
• pp.71.1-71.1
• /
• 2012

We have identified local analogs of strong $H{\alpha}$ Emitters (HAEs) that dominate the z~4 Lyman-break galaxy population using the Sloan Digital Sky Survey (SDSS). At z<0.4, only 0.04% of galaxies are classified as HAEs with $H{\alpha}$ equivalent width larger than $500{\AA}$, comparable to that of z~4 HAEs. The $H{\alpha}$-to-UV luminosity ratio of local HAEs is consistent with that of z~4 HAEs, indicating relatively large specific star formation rate in these galaxies compared to traditionally studied UV-selected Lyman break analogs. Local HAEs are young, less evolved galaxies with low metallicity. It is still difficult to constrain whether the star formation in local HAEs is powered by minor mergers or by cosmological cold gas accretion. However, the stacked optical spectrum of local HAEs shows several strong ionization lines, for example HeII 4686 emission line, which are shown in Wolf-Rayet galaxies. Thus it is highly likely that local HAEs are galaxies with an elevated ionization parameter, either due to a high electron density or large escape fraction of hydrogen ionizing photons.

• 천문학회보
• /
• 제37권1호
• /
• pp.74.2-74.2
• /
• 2012

We present [Fe II] 1.64 ${\mu}m$ imaging observations for jets and outflows from young stellar objects over the northern part (-24'${\times}$45') of the Carina Nebula, a typical evolved massive star forming region. The observations were performed with IRIS2 of Anglo-Autralian Telescope and the seeing was -1.5". Several jets and outflows features are detected at seven different regions, and one new Herbig-Haro Object is identified. The [Fe II] features have knotty and elongated shapes, and distribute around the triangular area formed by the star clusters Tr 14, Tr 15, and Tr 16, which contain many massive stars. The [Fe II] feature shows a highest detection rate (3.2 %) for the earliest stage YSOs, and the rate decreases as the stage evolves. The low detection rate (1.5 %) of [Fe II] features from the numerous YSOs seem to be related with the severe radiation environment of the Carina Nebula. The outflow rate shows reasonable relations with the physical parameters of the corresponding YSOs-derived from the SED fitting-such as the accretion luminosity, the stellar mass, the stellar age, the disk accretion rate, etc.