Yinghe Zhao

Major-merger galaxy pairs in the COSMOS field—mass-dependent merger rate evolution since z = 1

We present results of a statistical study of the cosmic evolution of the mass-dependent major-merger rate since z = 1. A stellar mass limited sample of close major-merger pairs (the CPAIR sample) was selected from the archive of the COSMOS survey. Pair fractions at different redshifts derived using the CPAIR sample and a local K-band-selected pair sample show no significant variations with stellar mass. The pair fraction exhibits moderately strong cosmic evolution, with the best-fitting function of f_(pair) = 10^(–1.88(± 0.03))(1 + z)^(2.2(± 0.2)). The best-fitting function for the merger rate is R_(mg) (Gyr^(–1)) = 0.053 × (M_(star)/10^(10.7) M_☉ )^(0.3)(1 + z)^(2.2)/(1 + z/8). This rate implies that galaxies of M_(star) ~ 10^(10)-10^(11.5) M_☉ have undergone ~0.5-1.5 major mergers since z = 1. Our results show that, for massive galaxies (M_(star) ≥ 10^(10.5) M_☉) at z ≤ 1, major mergers involving star-forming galaxies (i.e., wet and mixed mergers) can account for the formation of both ellipticals and red quiescent galaxies (RQGs). On the other hand, major mergers cannot be responsible for the formation of most low mass ellipticals and RQGs of M_(star) ≾ 10^(10.3) M_☉. Our quantitative estimates indicate that major mergers have significant impact on the stellar mass assembly of the most massive galaxies (M_(star) ≥ 10^(11.3) M_☉), but for less massive galaxies the stellar mass assembly is dominated by the star formation. Comparison with the mass-dependent (ultra)luminous infrared galaxies ((U)LIRG) rates suggests that the frequency of major-merger events is comparable to or higher than that of (U)LIRGs.We present results of a statistical study of the cosmic evolution of the mass dependent major-merger rate since z=1. A stellar mass limited sample of close major-merger pairs (the CPAIR sample) was selected from the archive of the COSMOS survey. Pair fractions at different redshifts derived using the CPAIR sample and a local K-band selected pair sample show no significant variations with stellar mass. The pair fraction exhibits moderately strong cosmic evolution, with the best-fitting evolutionary index m=2.2+-0.2. The best-fitting function for the merger rate implies that galaxies with stellar mass between 1E+10 -- 3E+11 M_sun have undergone 0.5 -- 1.5 major-mergers since z=1. Our results show that, for massive galaxies at z<1, major mergers involving star forming galaxies (i.e. wet and mixed mergers) can account for the formation of both ellipticals and red quiescent galaxies (RQGs). On the other hand, major mergers cannot be responsible for the formation of most low mass ellipticals and RQGs. Our quantitative estimates indicate that major mergers have significant impact on the stellar mass assembly of the most massive galaxies, but for less massive galaxies the stellar mass assembly is dominated by the star formation. Comparison with the mass dependent (U)LIRG rates suggests that the frequency of major-merger events is comparable to or higher than that of (U)LIRGs.

ALMA OBSERVATIONS OF WARM DENSE GAS IN NGC 1614-BREAKING OF THE STAR FORMATION LAW IN THE CENTRAL KILOPARSEC

Recently, research performed by two groups has revealed that the magnetar spin-down energy injection model with full energy trapping can explain the early-time light curves of SN 2010gx, SN 2013dg, LSQ12dlf, SSS120810, and CSS121015 but fails to fit the late-time light curves of these superluminous supernovae (SLSNe). These results imply that the original magnetar-powered model is challenged in explaining these SLSNe. Our paper aims to simultaneously explain both the early-and late-time data/upper limits by considering the leakage of hard emissions. We incorporate quantitatively the leakage effect into the original magnetar-powered model and derive a new semianalytical equation. Comparing the light curves reproduced by our revised magnetar-powered model with the observed data and/or upper limits of these five SLSNe, we found that the late-time light curves reproduced by our semianalytical equation are in good agreement with the late-time observed data and/or upper limits of SN 2010gx, CSS121015, SN 2013dg, and LSQ12dlf and the late-time excess of SSS120810, indicating that the magnetar-powered model might be responsible for these SLSNe and that the gamma-ray and X-ray leakages are unavoidable when the hard photons were down-Comptonized to softer photons. To determine the details of the leakage effect and unveil the nature of SLSNe, more high-quality bolometric light curves and spectra of SLSNe are required.

LUMINOSITY-METALLICITY RELATIONS FOR BLUE COMPACT DWARF GALAXIES IN THE OPTICAL AND NEAR-INFRARED

In this paper, we present systematic studies on the B-, R- and K(s)-band luminosity-metallicity (L-Z) relations for a set of metal-poor, blue compact dwarf galaxies (BCDs). Metallicity is derived by using both the empirical N2 and the direct T(e) methods. Our work reconciles contradictory results obtained by different authors and shows that the L-Z relationship does also hold for BCDs. The empirical N2-based slope of the L-Z relation, for each photometric band, is consistent with the T(e)-based one. We confirm that the slope of the L-Z relation is shallower in the near-infrared than that in the optical. Our investigations on the correlations between the LB-Z relation residuals and different galactic parameters show that the star formation activities could be a cause of the large scatter in the optical L-Z relationships, whereas the internal absorption might be another possible contributing factor.

Spin period evolution of a recycled pulsar in an accreting binary

We investigate the spin period evolutions of recycled pulsars in binary accreting systems. Taking both the accretion induced field decay and spin-up into consideration, we calculate their spin period evolutions influenced by the initial magnetic field strengths, initial spin periods, and accretion rates. The results indicate that the minimum spin period (or maximum spin frequency) of a millisecond pulsar (MSP) is independent of the initial conditions and accretion rate when the neutron star (NS) accretes the mass of similar to 0.2 M-circle dot. The accretion torque with the fastness parameter and gravitational wave (GW) radiation torque may be responsible for the formation of the minimum spin period (maximum spin frequency). The fastest spin frequency (716 Hz) of MSP can be inferred to associate with a critical fastness parameter of about omega(c) = 0.55. Furthermore, comparisons with the observational data are presented in the field period (B - P) diagram.

Comparing six evolutionary population synthesis models by performing spectral synthesis for galaxies

Aims. We compare six popularly used evolutionary population synthesis (EPS) models by fitting the full optical spectra of six representative types of galaxies (star-forming and composite galaxies, Seyfert 2s, LINERs, E+A, and early-type galaxies) taken from the Sloan Digital Sky Survey (SDSS). We explore the dependence of stellar population synthesis results on the main ingredients of the EPS models and study whether there is an age sequence among these types of galaxies. Methods. We use the simple stellar populations (SSPs) of each EPS model and the software STARLIGHT to perform our fits. Firstly, we explore the dependence of stellar population synthesis on EPS models by fixing the age, metallicity, and initial mass function (IMF) to construct a standard SSP group. We then use the standard SSP group of each EPS model (BC03, CB07, Ma05, GALEV, GRASIL, and Vazdekis/Miles) to fit the spectra of star-forming and E+A galaxies. Secondly, we fix the IMF and alter the age and metallicity to construct eight additional SSP groups. We then use these SSP groups to fit the spectra of star-forming and E+A galaxies to verify the effects of age and metallicity on stellar populations. Finally, we study the effect of stellar evolution tracks and stellar spectral libraries on our results, and present a possible age sequence among these types of galaxies. Results. Using different EPS models, the numerical values of contributing light fractions obviously change, even though the dominant populations are unaltered. The stellar population synthesis does depend on the selection of age and metallicity, but does not depend significantly on the stellar evolution track. The importance of young populations decreases from star-forming, composite, Seyfert 2, LINER, to early-type galaxies, and the properties of E+A galaxies are between composite galaxies and Seyfert 2s in most cases. Conclusions. Different EPS models infer different stellar population parameters, so that it is not reasonable to directly compare stellar populations estimated from different EPS models. To obtain reliable results, we should use the same EPS model to derive the parameter values that we compare.

A Herschel Survey of the [N II] 205 μm Line in Local Luminous Infrared Galaxies: The [N II] 205 μm Emission as a Star Formation Rate Indicator

We present, for the first time, a statistical study of [N II] 205 mu m line emission for a large sample of local luminous infrared galaxies using Herschel Spectral and Photometric Imaging Receiver Fourier Transform Spectrometer (SPIRE FTS) data. For our sample of galaxies, we investigate the correlation between the [N II] luminosity (L-[N II]) and the total infrared luminosity (L-IR), as well as the dependence of L-[N II]/L-IR ratio on L-IR, far-infrared colors (IRAS f(60)/f(100)), and the [O III] 88 mu m to [N II] luminosity ratio. We find that L-[N II] correlates almost linearly with L-IR for non-active galactic nucleus galaxies (all having L-IR < 10(12) L-circle dot) in our sample, which implies that L-[N II] can serve as a star formation rate tracer which is particularly useful for high-redshift galaxies that will be observed with forthcoming submillimeter spectroscopic facilities such as the Atacama Large Millimeter/submillimeter Array. Our analysis shows that the deviation from the mean L-[N II]-L-IR relation correlates with tracers of the ionization parameter, which suggests that the scatter in this relation is mainly due to the variations in the hardness, and/or ionization parameter, of the ambient galactic UV field among the sources in our sample.

Warm Molecular Gas in Luminous Infrared Galaxies

We present our initial results on the CO rotational spectral line energy distribution (SLED) of the J to J-1 transitions from J = 4 up to 13 from Herschel SPIRE spectroscopic observations of 65 luminous infrared galaxies (LIRGs) in the Great Observatories All-Sky LIRG Survey. The observed SLEDs change on average from one peaking at J <= 4 to a broad distribution peaking around J similar to 6 to 7 as the IRAS 60-to-100 mu m color, C(60/100), increases. However, the ratios of a CO line luminosity to the total infrared luminosity, L-IR, show the smallest variation for J around 6 or 7. This suggests that, for most LIRGs, ongoing star formation (SF) is also responsible for a warm gas component that emits CO lines primarily in the mid-J regime (5 less than or similar to J less than or similar to 10). As a result, the logarithmic ratios of the CO line luminosity summed over CO (5-4), (6-5), (7-6), (8-7) and (10-9) transitions to L-IR, log R-midCO, remain largely independent of C(60/100), and show a mean value of -4.13 ( log R-midCO(SF)) and a sample standard deviation of only 0.10 for the SF-dominated galaxies. Including additional galaxies from the literature, we show, albeit with a small number of cases, the possibility that galaxies, which bear powerful interstellar shocks unrelated to the current SF, and galaxies, in which an energetic active galactic nucleus contributes significantly to the bolometric luminosity, have their R-midCO higher and lower than R-midCO(SF), respectively.

THE STELLAR POPULATION AND STAR FORMATION PROPERTIES OF BLUE COMPACT DWARF GALAXIES

We study stellar populations, star formation histories (SFIIs), and star formation properties for a sample of blue compact dwarf galaxies (BCDs) selected by cross-correlating the Gil de Paz et al. sample with the Sloan Digital Sky Survey Data Release 6. The sample includes 31 BCDs, which span a large range of galactic parameters. Using a stellar population synthesis method, we derive stellar populations and reconstruct SFHs for these BCDs. Our studies confirm that BCDs are not young systems experiencing their first star formation, but old systems undergoing a starburst activity. The stellar mass-weighted ages can be up to 10 Gyr, while the luminosity-weighted ages might be up to approximately three orders of magnitude younger (similar to 10 Myr) for most galaxies. Based on multiwavelength data, we also study the integrated star formation properties. The star formation rate (SFR) for our sample galaxies spans nearly three orders of magnitude, from a few 10(-3) to similar to 1M(circle dot) yr(-1), with a median value of similar to 0.1M(circle dot) yr(-1). We find that about 90% of BCDs in our sample have their birthrate parameter (the ratio of the current SFR to the averaged past SFR) b > 2-3. We further discuss correlations of the current SFR with the integrated galactic stellar mass and explore the connection between SFR and metallicity.

A STUDY ON THE CHEMICAL PROPERTIES OF BLUE COMPACT DWARF GALAXIES

In this paper, we report our studies on the gaseous and chemical properties of a relatively large sample ( 53 members) of blue compact dwarf galaxies (BCDs). The results of correlations among the oxygen abundance, stellar mass, gas mass, baryonic mass, and gas fraction are present both for E-and I-type BCDs, which are classified according to Loose & Thuan and show elliptical and irregular outer haloes, respectively. These correlations of I-type BCDs show slopes similar to those of E-type ones. However, in general, E-type BCDs are more gas-poor and metal-rich than I-type ones at a given baryonic mass. Based on these results, we suggest that E-type BCDs, at least a part of them, and I-type ones might be likely at different evolutionary phases and/or having different progenitors. Our investigation of the correlation between oxygen abundance and gas fraction shows that BCDs appear to have not evolved as isolated systems, but to have experienced some gas flows and/or mergers.

Stellar population analysis on local infrared-selected galaxies

Aims. We study the stellar population of local infrared galaxies, which contain star-forming galaxies, composite galaxies, LINERs, and Seyfert 2s. We also want to find whether infrared luminosi ty and spectral class have any effect on their stellar populations. Methods. The sample galaxies are selected from the main galaxy sample of SDSS-DR4 and then cross-correlated with the IRASPSCz catalog. We fit our spectra (stellar absorption lines an d continua) using the spectral synthesis code STARLIGHT based on the templates of simple stellar population and the spectra of st ar clusters. Results. Among the 4 spectral classes, LINERs present the oldest stellar populations, and the other 3 subsamples all present substantial young and intermediate age populations and very few old populations. The importance of young populations decreases from starforming, composite, Seyfert 2 to LINER. As for the different infrared luminosity bins, ULIGs & LIGs (log(LIR/L⊙) ≥11) present younger populations than do starbursts and normal galaxies; however, the dominant contributors to mass are old populations in all sample galaxies. The fittings also give consistent results b y using the spectra of star clusters with different ages and metallicities as templates. The dominated populations in star-forming and composite galaxies are those with metallicity Z = 0.2Z⊙, while LINERs and Seyfert 2s are more metal-rich. The normal galaxies are more metal-rich than the ULIGs & LIGs and starbursts for the star-forming galaxies within different infrared luminosity bins. Additionally, we compare some synthesis results with other parameters obtained from the MPA/JHU catalog. First we find that the stellar and nebular extinctions are correlated, and the ion ized gas suffers twice as much extinction as stars. Second we confirm that Dn(4000) is a much better age indicator than HδA. Following the evolution of galaxies, Dn(4000) monotonously varies. Finally we investigate some relationships between mean stellar age, mean stellar metallicity, and nebular metallicity for the su bsample of starforming galaxies. In star-forming galaxies, the nebular metallicity Zneb is correlated with the light-weighted mean stellar ageh log t∗i L in an intermediate strength, and Zneb is weakly correlated with the mass-weighted mean stellar metallicityh Z∗i M.