arXiv subjects

H. B. Ann

Publications and source records attributed to H. B. Ann.

18 recordsLinked to original sources

arXiv2023

Star Formation Histories of Dwarf Spheroidal and Dwarf Elliptical Galaxies in the Local Universe

We present the star formation histories (SFHs) of early-type dwarf galaxies, dSphs and dEs, in the local universe within z=0.01. The SFHs of early-type dwarf galaxies are characterized by pre-enriched, metal-poor old stellar populations, absence of moderately old stars that have ages of a few Gyr. There are some differences in the SFHs of dSphs and dEs. In particular, dSphs formed old ($\gtrsim10$ Gyr old) metal-poor stars $\sim2$ times more than dEs. The effects of reionization and feedback from supernova explosions are thought to be strong enough to remove the gas left, which prevent moderately old stellar populations in dSphs. In contrast, the ejected gas are not completely removed from dEs and fall back to ignite burst of star formation at a few Gyr after the first period of violent bursts of star formation, showing a suppression of star formation at lookback time $\approx 9.6$ Gyr. The second peak of star formation at lookback time $\approx 4.5$ Gyr in dEs produce moderately old stellar populations. Distinction between dSphs and dEs is useful to examine the SFHS of the early-type dwarfs since the cumulative SFHs are most closely related to their morphology. The stellar mass plays an important role in the SFHs of the early-type dwarfs as a driver of star formation, especially in galaxies with primordial origin.

astro-ph.GA↗

arXiv2015

Catalog of Visually Classified Galaxies in the Local ($z\sim0.01$) Universe

The morphological types of 5840 galaxies were classified by a visual inspection of color images using the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) to produce a morphology catalog of a representative sample of local galaxies with $z<0.01$. The sample galaxies are almost complete for galaxies brighter than $r_{pet}=17.77$. Our classification system is basically the same as that of the Third Reference Catalog of Bright Galaxies with some simplification for giant galaxies. On the other hand, we distinguish fine features of dwarf elliptical-like galaxies to classify 5 subtypes: dwarf ellipticals (dE), blue-cored dwarf ellipticals (dE$_{bc}$), dwarf spheroidals (dSph), blue dwarf ellipticals (dE$_{blue}$), and dwarf lenticulars (dS0). In addition, we denote the presence of nucleation in dE, dSph, and dS0. Elliptical galaxies and lenticular galaxies contribute only $\sim1.5\%$ and $\sim4.9\%$ of the local galaxies, respectively, whereas spirals and irregulars contribute $\sim32.1\%$ and $\sim42.8\%$, respectively. The dE$_{blue}$ galaxies, which are recently found populations of galaxies, contribute a significant fraction to the dwarf galaxies. There seems to be structural difference between dSph and dE galaxies. The dSph galaxies are fainter and bluer with shallower surface brightness gradient than dE galaxies. They also have lower fraction of galaxies with small axis ratios ($b/a \lesssim 0.4$) than dE galaxies. The mean projected distance to the nearest neighbor galaxy is $\sim260$kpc. About $1\%$ of local galaxies have no neighbors with comparable luminosity within a projected distance of 2Mpc.

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arXiv2014

Environment Dependence of Disk Morphology of Spiral Galaxies

We analyze the dependence of disk morphology (arm class, Hubble type, bar type) of nearby spiral galaxies on the galaxy environment by using local background density ($Σ_{n}$), projected distance ($r_{p}$), and tidal index ($TI$) as measures of the environment. There is a strong dependence of arm class and Hubble type on the galaxy environment, while the bar type exhibits a weak dependence with a high frequency of SB galaxies in high density regions. Grand design fractions and early-type fractions increase with increasing $Σ_{n}$, $1/r_{p}$, and $TI$, while fractions of flocculent spirals and late-type spirals decrease. Multiple-arm and intermediate-type spirals exhibit nearly constant fractions with weak trends similar to grand design and early-type spirals. While bar types show only a marginal dependence on $Σ_{n}$, they show a fairly clear dependence on $r_{p}$ with a high frequency of SB galaxies at small $r_{p}$. The arm class also exhibits a stronger correlation with $r_{p}$ than $Σ_{n}$ and $TI$, whereas the Hubble type exhibits similar correlations with $Σ_{n}$ and $r_{p}$. This suggests that the arm class is mostly affected by the nearest neighbor while the Hubble type is affected by the local densities contributed by neighboring galaxies as well as the nearest neighbor.

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arXiv2013

Deep and Wide Photometry of Two Open Clusters NGC 1245 and NGC 2506: Dynamical Evolution and Halo

We studied the structure of two old open clusters, NGC 1245 and NGC 2506, from a wide and deep VI photometry data acquired using the CFH12K CCD camera at CFHT. We devised a new method for assigning cluster membership probability to individual stars using both spatial positions and positions in the colour-magnitude diagram. From analyses of the luminosity functions at several cluster-centric radii and the radial surface density profiles derived from stars with different luminosity ranges, we found that the two clusters are dynamically relaxed to drive significant mass segregation and evaporation of some fraction of low-mass stars. There seems to be a signature of tidal tail in NGC 1245 but the signal is too low to be confirmed.

astro-ph.GA↗

arXiv2012

Deep and Wide Photometry of Two Open Clusters NGC 1245 and NGC 2506: CCD Observation and Physical Properties

We have conducted VI CCD photometry of the two open clusters NGC 1245 and NGC 2506 using the CFH12K CCD camera. Our photometry covers a sky area of 84'X82' and 42'X81' for the two clusters, respectively, and reaches down to V = 23. We derived the physical parameters using detailed theoretical isochrone fittings using 2 minimization. The derived cluster parameters are E(B-V) = 0.24+/-0.05 and 0.03+/-0.04, (V-M_V)_0 = 12.25+/-0.12 and 12.47+/-0.08, age(Gyr) = 1.08+/-0.09 and 2.31+/-0.16, and [Fe/H]= -0.08+/-0.06 and -0.24+/-0.06, respectively for NGC 1245 and NGC 2506, We present the luminosity functions (LFs) of the two clusters, which reach down to M_V = 10, and derive mass functions (MFs) with slopes of Gamma = -1.29 for NGC 1245 and Gamma = -1.26 for NGC 2506. The slopes are slightly shallower than that of the solar neighbourhood, implying the existence of dynamical evolution that drives the evaporation of the low-mass stars in the clusters.

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arXiv2009

Isolated Galaxies and Isolated Satellite Systems

We search for isolated galaxies using a volume-limited sample of galaxies with 0.02< z < 0.04742 from SDSS DR7 supplemented by bright galaxies. We devise a diagnostic tool to select isolated galaxies in different environments using the projected separation (r_p) normalized by the virial radius of the nearest neighbor (r_{vir,nei}) and the local background density. We find that the isolation condition of r_p>r_{vir,nei} and ρ<\barρ well segregates the CIG galaxies. We confirm the morphology conformity between the host and their satellites, which suggests importance of hydrodynamic interaction among galaxies within their virial radii in galaxy evolution.

astro-ph.CO↗

arXiv2008

Effect of Central Mass Concentration on the Formation of Nuclear Spirals in Barred Galaxies

We have performed smoothed particle hydrodynamics (SPH) simulations to study the response of the central kiloparsec region of a gaseous disk to the imposition of nonaxisymmetric bar potentials. The model galaxies are composed of the three axisymmetric components (halo, disk, and bulge) and a non-axisymmetric bar. These components are assumed to be invariant in time in the frame corotating with the bar. The potential of spherical $γ$-models of Dehnen is adopted for the bulge component whose density varies as $r^{-γ}$ near the center and $r^{-4}$ at larger radiiand hence, possesses a central density core for $γ= 0$ and cusps for $γ> 0$. Since the central mass concentration of the model galaxies increases with the cusp parameter $γ$, we have examined here the effect of the central mass concentration by varying the cusp parameter $γ$ on the mechanism responsible for the formation of the symmetric two-armed nuclear spirals in barred galaxies. Our simulations show that the symmetric two-armed nuclear spirals are formed by hydrodynamic spiral shocks driven by the gravitational torque of the bar for the models with $γ= 0$ and 0.5. On the other hand, the symmetric two-armed nuclear spirals in the models with $γ=1$ and 1.5 are explained by gas density waves. Thus, we conclude that the mechanism responsible for the formation of the symmetric two-armed nuclear spirals in barred galaxies changes from the hydrodynamic shocks to the gas density waves when the central mass concentration increases from $γ= 0$ to 1.5.

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arXiv2008

Galactic Satellite Systems: Radial Distribution and Environment Dependence of Galaxy Morphology

We have studied the radial distribution of the early (E/S0) and late (S/Irr) types of satellites around bright host galaxies. We made a volume-limited sample of 4,986 satellites brighter than M_r = -18.0 associated with 2,254 hosts brighter than M_r =-19.0 from the SDSS DR5 sample. The morphology of satellites is determined by an automated morphology classifier, but the host galaxies are visually classified. We found segregation of satellite morphology as a function of the projected distance from the host galaxy. The amplitude and shape of the early-type satellite fraction profile are found to depend on the host luminosity. This is the morphology-radius/density relation at the galactic scale. There is a strong tendency for morphology conformity between the host galaxy and its satellites. The early-type fraction of satellites hosted by early-type galaxies is systematically larger than that of late-type hosts, and is a strong function of the distance from the host galaxies. Fainter satellites are more vulnerable to the morphology transformation effects of hosts. Dependence of satellite morphology on the large-scale background density was detected. The fraction of early-type satellites increases in high density regions for both early and late-type hosts. It is argued that the conformity in morphology of galactic satellite system is mainly originated by the hydrodynamical and radiative effects of hosts on satellites.

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arXiv2007

Self-consistent triaxial de Zeeuw-Carollo Models

We use the usual method of Schwarzschild to construct self-consistent solutions for the triaxial de Zeeuw & Carollo (1996) models with central density cusps. ZC96 models are triaxial generalisations of spherical $γ$-models of Dehnen whose densities vary as $r^{-γ}$ near the center and $r^{-4}$ at large radii and hence, possess a central density core for $γ=0$ and cusps for $γ> 0$. We consider four triaxial models from ZC96, two prolate triaxials: $(p, q) = (0.65, 0.60)$ with $γ= 1.0$ and 1.5, and two oblate triaxials: $(p, q) = (0.95, 0.60)$ with $γ= 1.0$ and 1.5. We compute 4500 orbits in each model for time periods of $10^{5} T_{D}$. We find that a large fraction of the orbits in each model are stochastic by means of their nonzero Liapunov exponents. The stochastic orbits in each model can sustain regular shapes for $\sim 10^{3} T_{D}$ or longer, which suggests that they diffuse slowly through their allowed phase-space. Except for the oblate triaxial models with $γ=1.0$, our attempts to construct self-consistent solutions employing only the regular orbits fail for the remaining three models. However, the self-consistent solutions are found to exist for all models when the stochastic and regular orbits are treated in the same way because the mixing-time, $\sim10^{4} T_{D}$, is shorter than the integration time, $10^{5} T_{D}$. Moreover, the ``fully-mixed'' solutions can also be constructed for all models when the stochastic orbits are fully mixed at 15 lowest energy shells. Thus, we conclude that the self-consistent solutions exist for our selected prolate and oblate triaxial models with $γ= 1.0$ and 1.5.

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arXiv2004

A family of triaxial modified Hubble mass models: effects of the additional radial functions

The projected properties of triaxial generalization of the modified Hubble mass models are studied. These models are constructed by adding the additional radial functions, each multiplied by a low-order spherical harmonic, to the models of \citet{ct00}. The projected surface density of mass models can be calculated analytically which allows us to derive the analytic expressions of axial ratio and position angles of major axis of constant density elliptical contours at asymptotic radii. The models are more general than those studied earlier in the sense that the inclusions of additional terms in density distribution, allows one to produce varieties of the radial profile of axial ratio and position angle, in particular, their small scale variations at inner radii. Strong correlations are found to exist between the observed axial ratio evaluated at $0.25 R_{e}$ and at $4 R_{e}$ which occupy well-separated regions in the parameter space for different choices of the intrinsic axial ratios. These correlations can be exploited to predict the intrinsic shape of the mass model, independent of the viewing angles. Using Bayesian statistics, the result of a test case launched for an estimation of the shape of a model galaxy is found to be satisfactory.

astro-ph↗

arXiv2004

Formation of Nuclear Spirals in Barred Galaxies

We have performed smoothed particle hydrodynamics (SPH) simulations for the response of the gaseous disk to the imposed moderately strong non-axisymmetric potentials. The model galaxies are composed of the three stellar components (disk, bulge and bar) and two dark ones (supermassive black hole and halo) whose gravitational potentials are assumed to be invariant in time in the frame corotating with the bar. We found that the torques alone generated by the moderately strong bar that gives the maximum of tangential-to-radial force ratio as $(F_{Tan}/F_{Rad})_{max}= 0.3$ are not sufficient to drive the gas particles close to the center due to the barrier imposed by the inner Lindblad resonances (ILRs). In order to transport the gas particles towards the nucleus ($r<100$ pc), a central supermassive black hole (SMBH) and high sound speed of the gas are required to be present. The former is required to remove the inner inner Lindblad resonance (IILR) that prevents gas inflow close to the nucleus, while the latter provides favourable conditions for the gas particles to lose their angular momentum and to spiral in. Our models that have no IILR show the trailing nuclear spirals whose innermost parts reach close to the center in a curling way when the gas sound speed is $ c_{s} \gtrsim 15$ km s$^{-1}$. They resemble the symmetric two-armed nuclear spirals observed in the central kiloparsec of spiral galaxies. We found that the symmetric two-armed nuclear spirals are formed by the hydrodynamic spiral shocks caused by the gravitational torque of the bar in the presence of a central SMBH that can remove IILR when the sound speed of gas is high enough to drive a large amount of gas inflow deep inside the ILR. However, the detailed morphology of nuclear spirals depends on the sound speed of gas.

astro-ph↗

arXiv2004

Nuclear Spirals and Supermassive Black Holes

We have performed SPH simulations for the response of the gaseous disks to the imposed potentials including those from bars and SMBHs. Evolution of the nuclear regions of gaseous disks depends critically on the masses of SMBHs as well as the sound speeds in the gas.

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arXiv2002

Young Populations in the Nuclei of Barred Galaxies

We have conducted a $UBVRI$ and $H_α$ CCD photometry of 5 barred galaxies, NGC 2523, NGC 2950, NGC 3412, NGC 3945 and NGC 5383, along with SPH simulations, in order to understand the origin of young stellar populations in the nuclei of barred galaxies. The $H_α$ emissions which are thought to be emitted by young stellar populations are either absent or strongly concentrated on the nuclei of early type galaxies (NGC 2950, NGC 3412, NGC 3945), while they are observed in the nuclei and circumnuclear regions of intermediate type galaxies with strong bars (NGC 2523, NGC 5383). SPH simulations of realistic mass models for these galaxies show that some disk material can be driven into the nuclear regions by strong bar potentials. This implies that the young stellar populations in the circumnuclear regions of barred galaxies can be formed out of such gas. The existence of the nuclear dust lanes is an indication of an on-going gas inflow and extremely young stellar populations in these galaxies, because the nuclear dust lanes such as those in NGC 5383 are not long-lasting features according to our simulations.

astro-ph↗

arXiv2002

Hydrodynamic Simulations for the Nuclear Morphology of NGC 4314

We performed SPH simulations to study the nuclear morphology of a barred galaxy NGC 4314. We have constructed the mass models based on the results of a profile decomposition into disk, bulge, and bar components. Our models have three different nuclear structures according to the assumption about the nuclear bar: no nuclear bar, a synchronous nuclear bar and a fast nuclear bar. Our SPH simulations show that the morphology of the nuclear region of NGC 4314 which is characterized by an elongated ring/spiral of newly formed stars and HII regions, aligned nearly parallel to the primary bar can be understood in terms of the secular evolution driven by the non-axisymmetric potential. The slightly elongated and aligned nuclear ring of NGC 4314 can be formed by the strong barred potential and the moderate central concentration of the bulge mass with and without a nuclear bar. However, the nuclear spiral pattern can not be developed without a nuclear bar. The nuclear bar of NGC 4314 seems to rotate faster than the primary bar since the nuclear morphology induced by the synchronous nuclear bar is much different from the observed one.

astro-ph↗

arXiv2002

{BOAO Photometric Survey of Galactic Open Clusters. II. Physical Parameters of 12 Open Clusters

We have initiated a long-term project, the BOAO photometric survey of open clusters, to enlarge our understanding of galactic structure using UBVI CCD photometry of open clusters which have been little studied before. This is the second paper of the project in which we present the photometry of 12 open clusters. We have determined the cluster parameters by fitting the Padova isochrones to the color-magnitude diagrams of the clusters. All the clusters except for Be 0 and NGC 1348 are found to be intermediate-age to old (0.2 - 4.0 Gyrs) open clusters with a mean metallicity of [Fe/H] = 0.0.

astro-ph↗

arXiv1999

Bulges of Two Barred Galaxies: NGC 3412 and NGC 3941

We have conducted near-infrared (J- and H-band) surface photometry for two early type barred galaxies, NGC 3412 and NGC 3941. The bulges of NGC 3412 and NGC 3941 show isophotal twists which indicate that they are triaxial. NGC 3412 has a very short bar and its bulge is more centrally concentrated than that of NGC 3941. The unusually short bar and the centrally concentrated triaxial bulge of NGC 3412 might be the result of bar dissolution. The colors of the nuclear region of NGC 3941 resemble those of the blue nuclei, implying the presence of young stellar populations.

astro-ph↗

arXiv1999

BOAO Photometric Survey of Galactic Open Clusters. I. Berkeley 14, Collinder 74, Biurakan 9, and NGC 2355

Open clusters are useful tools to investigate the structure and evolution of the Galactic disk. We have started a long-term project to obtain UBVI CCD Photometry of open clusters which were little studied before, using the Doyak 1.8 m telescope of Bohyunsan Optical Astronomy Observatory in Korea. The primary goals of this project are (1) to make a catalog of UBVI photometry of open clusters, (2) to make an atlas of open clusters, and (3) to survey and monitor variable stars in open clusters. Here we describe this project and report the first results based on preliminary analysis of the data on four open clusters in the survey sample: Be 14, Cr 74, Biu 9, and NGC 2355. Isochrone fitting of the color-magnitude diagrams of the clusters shows that all of them are intermediate age to old (0.3 - 1.6 Gyrs) open clusters with moderate metallicity.

astro-ph↗

arXiv1998

SPH Simulations of Galactic Gaseous Disk with Bar: Distribution and Kinematic Structure of Molecular Clouds toward the Galactic Center

We have performed Smoothed Particle Hydrodynamic (SPH) simulations to study the response of molecular clouds in the Galactic disk to a rotating bar and their subsequent evolution in the Galactic Center (GC) region. The Galactic potential in our models is contributed by three axisymmetric components (massive halo, exponential disk, compact bulge) and a non-axisymmetric bar. These components are assumed to be invariant in time in the frame corotating with the bar. Some noticeable features such as an elliptical outer ring, spiral arms, a gas-depletion region, and a central concentration have been developed due to the influence of the bar. The rotating bar induces non-circular motions of the SPH particles, but hydrodynamic collisions tend to suppress the random components of the velocity. The velocity field of the SPH particles is consistent with the kinematics of molecular clouds observed in HCN (1-0) transition; these clouds are thought to be very dense clouds. However, the l-v diagram of the clouds traced by CO is quite different from that of our SPH simulation, being more similar to that obtained from simulations using collisionless particles. The $l-v$ diagram of a mixture of collisional and collisionless particles gives better reproduction of the kinematic structures of the GC clouds observed in the CO line. The fact that the kinematics of HCN clouds can be reproduced by the SPH particles suggests that the dense clouds in the GC are formed via cloud collisions induced by rotating bar.

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