arXiv subjects

L. G. Hou

Publications and source records attributed to L. G. Hou.

25 records · Page 2Linked to original sources

arXiv2013

A statistical study of gaseous environment of Spitzer interstellar bubbles

The expansion of interstellar bubbles is suggested to be an important mechanism of triggering material accumulation and star formation. In this work, we study the gaseous environment of a large sample of bubbles, aiming to explore the possible evidence of triggered gas accumulation and star formation in a statistical sense. By cross-matching 6124 Spitzer bubbles from the Milky Way Project (MWP) and more than 2500 Galactic HII regions, we obtain the velocity information for 818 MWP bubbles. To study the gaseous environment of the interstellar bubbles and get rid of the projection effect as much as possible, we constrain the velocity difference between the bubbles and the 13CO(1-0) emission extracted from the Galactic Ring Survey (GRS). Three methods: the mean azimuthally averaged radial profile of 13CO emission, the surface number density of molecular clumps, and the angular cross-correlation function of MWP bubbles and the GRS molecular clumps are adopted. Significant over density of molecular gas is found to be close to the bubble rims. 60 percent of the studied bubbles were found to have associated molecular clumps. By comparing the clump-associated and the clump-unassociated MWP bubbles, we reveal that the bubbles in associations tend to be larger and thicker in physical sizes. We speculate that some of the bubble-associated clumps result from the expansion of bubbles. The fraction of the molecular clumps associated with the MWP bubbles is estimated to be about 20 percent after considering the projection effect. For the bubble-clump complexes, we found that the bubbles in the complexes with associated massive young stellar object(s) (MYSO(s)) have larger physical sizes, hence the complexes tend to be older. We propose that an evolutionary sequence might exist between the relative younger MYSO-unassociated bubble-clump complexes and the MYSO-associated complexes.

astro-ph.GA↗

arXiv2013

The spiral structure of our Milky Way

The spiral structure of our Milky Way has not yet been well outlined. HII regions, giant molecular clouds (GMCs) and 6.7-GHz methanol masers are primary tracers for spiral arms. We collect and update the database of these tracers which has been used in Hou, Han & Shi (2009) for mapping the spiral structure.

astro-ph.GA↗

arXiv2012

Influence of major mergers on the radio emission of elliptical galaxies

We investigate the influence of major mergers on the radio emission of elliptical galaxies. We use a complete sample of close pairs, which contains 475 merging and 1828 non-merging paired elliptical galaxies of M_r<-21.5 selected from the Sloan Digital Sky Survey. In addition, a control sample of 2000 isolated field galaxies is used for comparison. We cross-identify the optical galaxies with the radio surveys of FIRST and NVSS. We find that the radio fraction of merging paired galaxies is about 6%, which is slightly higher than the 5% obtained for non-merging paired galaxies, although these values are consistent with each other owing to the large uncertainty caused by the limited sample. The radio fraction is twice as that of isolated galaxies, which is less than 3%. Radio emission of elliptical galaxies is only slightly affected by major mergers, but predominantly depends on their optical luminosities. Therefore, merging is not important in triggering the radio emission of elliptical galaxies.

astro-ph.CO↗

arXiv2011

Stellar populations of ultraluminous infrared galaxies

Ultraluminous infrared galaxies (ULIRGs) have several types according to dominance of starburst or AGN component. We made stellar population analysis for a sample of 160 ULIRGs to study the evolution of ULIRGs. We found that the dominance of intermediate-age and old stellar populations increases along the sequence of HII-like ULIRGs, Seyfert-HII composite ULIRGs, and Seyfert 2 ULIRGs. Consequently the typical mean stellar age and the stellar mass increase along the sequence. Comparing the gas mass estimated from the CO measurements with the stellar mass estimated from the optical spectra, we found that gas fraction is anti-correlated with the stellar mass. HII-like ULIRGs with small stellar masses do not possess enough gas and the total mass, and therefore have no evolution connections with massive Seyfert 2 ULIRGs. Only massive ULIRGs may follow the evolution sequence toward AGNs, and massive HII-like ULIRGs are probably in an earlier stage of the sequence.

astro-ph.CO↗

arXiv2010

Do Ultrahigh Energy Cosmic Rays Come from Active Galactic Nuclei and Fermi $γ$-ray Sources?

We study possible correlations between ultrahigh energy cosmic rays (UHECRs), observed by Auger, AGASA and Yakutsk, and nearby active galactic nuclei (AGNs) and $Fermi$ sources. We consider the deflection effects by a Galactic magnetic field (GMF) model constrained by the most updated measurements. We found that the average deflection angles of UHECRs by the Galactic magnetic fields are less than $4^\circ$. A correlation between the Auger cosmic-ray events and nearby AGNs with a significance level of $\sim 4σ$ was found for the Auger UHECR data sets with or without deflection correction. No correlation was found between the AGASA/Yakutsk events with nearby AGNs. Marginal correlations between the Auger events and the $Fermi$ sources, and between AGASA events and $Fermi$ AGNs were found when the deflections calculated by the GMF model were considered. However, no correlation was found between the Yakutsk data and $Fermi$ sources. Some $Fermi$ sources are close to the arrival directions of UHECR events detected by Auger, AGASA, and Yakutsk, most of which are probably chance coincidence rather than objects producing UHECRs in the nearby Universe. Four $Fermi$ sources, NGC 4945, ESO 323-G77, NGC 6951, and Cen A, within 100~Mpc have UHECR events within $3.1^{\circ}$ from their positions, which could potentially be cosmic ray accelerators. However, the association can only be confirmed if more UHECRs are preferably detected in these directions.

astro-ph.HE↗

arXiv2009

Ultra-luminous Infrared Galaxies in Sloan Digital Sky Survey Data Release 6

Ultra-luminous infrared galaxies (ULIRGs) are interesting objects with dramatic properties. Many efforts have been made to understand the physics of their luminous infrared emission and evolutionary stages. However, a large ULIRG sample is still needed to study the properties of their central black holes (BHs), the BH-host galaxy relation, and their evolution. We identified 308 ULIRGs from the Sloan Digital Sky Survey Data Release 6, and classified them into the NL ULIRGs (with only narrow emission lines) and the Type I ULIRGs (with broad emission lines). About 56% of ULIRGs in our total sample show interaction features, and this percentage is 79% for redshift z < 0.2. Optical identifications of these ULIRGs show that the AGN percentage is at least 49%, and the percentage increases with the infrared luminosity. We found 62 Type I ULIRGs, and estimated their BH masses and velocity dispersions from their optical spectra. Together with known Type I ULIRGs in the literature, a sample of 90 Type I ULIRGs enables us to make a statistical study. We found that the BH masses of Type I ULIRGs are typically smaller than those of PG QSOs, and most Type I ULIRGs follow the M_{BH}-sigma relation. However, some ULIRGs with larger Eddington ratio deviate from this relation, even the line width of the [OIII] narrow line (NL) core or the [SII] line was used as the surrogate of velocity dispersion. This implies that at least some ULIRGs are probably still in the early evolution stage toward QSOs. The anti-correlation between the mass deviation from the M_{BH}-sigma relation and the Eddington ratio supports that the evolution of Type I ULIRGs is probably followed by the building up of the M_{BH}-sigma relation and the evolution to the QSO phase.

astro-ph.CO↗

arXiv2009

The spiral structure of our Milky Way Galaxy

The spiral structure of our Milky Way Galaxy is not yet known. HII regions and giant molecular clouds are the most prominent spiral tracers. We collected the spiral tracer data of our Milky Way from the literature, namely, HII regions and giant molecular clouds (GMCs). With weighting factors based on the excitation parameters of HII regions or the masses of GMCs, we fitted the distribution of these tracers with models of two, three, four spiral-arms or polynomial spiral arms. The distances of tracers, if not available from stellar or direct measurements, were estimated kinetically from the standard rotation curve of Brand & Blitz (1993) with $R_0$=8.5 kpc, and $Θ_0$=220 km s$^{-1}$ or the newly fitted rotation curves with $R_0$=8.0 kpc and $Θ_0$=220 km s$^{-1}$ or $R_0$=8.4 kpc and $Θ_0$=254 km s$^{-1}$. We found that the two-arm logarithmic model cannot fit the data in many regions. The three- and the four-arm logarithmic models are able to connect most tracers. However, at least two observed tangential directions cannot be matched by the three- or four-arm model. We composed a polynomial spiral arm model, which can not only fit the tracer distribution but also match observed tangential directions. Using new rotation curves with $R_0$=8.0 kpc and $Θ_0$=220 km s$^{-1}$ and $R_0$=8.4 kpc and $Θ_0$=254 km s$^{-1}$ for the estimation of kinematic distances, we found that the distribution of HII regions and GMCs can fit the models well, although the results do not change significantly compared to the parameters with the standard $R_0$ and $Θ_0$.

astro-ph.GA↗