arXiv subjects

F. S. Liu

Publications and source records attributed to F. S. Liu.

33 records · Page 2Linked to original sources

arXiv2013

Thermal and non-thermal emission in the Cygnus X region

Radio continuum observations detect non-thermal synchrotron and thermal bremsstrahlung radiation. Separation of the two different emission components is crucial to study the properties of diffuse interstellar medium. The Cygnus X region is one of the most complex areas in the radio sky which contains a number of massive stars and HII regions on the diffuse thermal and non-thermal background. More supernova remnants are expected to be discovered. We aim to develop a method which can properly separate the non-thermal and thermal radio continuum emission and apply it to the Cygnus X region. The result can be used to study the properties of different emission components and search for new supernova remnants in the complex. Multi-frequency radio continuum data from large-scale surveys are used to develop a new component separation method. Spectral analysis is done pixel by pixel for the non-thermal synchrotron emission with a realistic spectral index distribution and a fixed spectral index of beta = -2.1 for the thermal bremsstrahlung emission. With the new method, we separate the non-thermal and thermal components of the Cygnus X region at an angular resolution of 9.5arcmin. The thermal emission component is found to comprise 75% of the total continuum emission at 6cm. Thermal diffuse emission, rather than the discrete HII regions, is found to be the major contributor to the entire thermal budget. A smooth non-thermal emission background of 100 mK Tb is found. We successfully make the large-extent known supernova remnants and the HII regions embedded in the complex standing out, but no new large SNRs brighter than Sigma_1GHz = 3.7 x 10^-21 W m^-2 Hz^-1 sr^-1 are found.

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arXiv2013

Serendipitous discovery of a massive cD galaxy at z=1.096: Implications for the early formation and late evolution of cD galaxies

We have made a serendipitous discovery of a massive cD galaxy at z=1.096 in a candidate rich cluster in the HUDF area of GOODS-South. This brightest cluster galaxy is the most distant cD galaxy confirmed to date. Ultra-deep HST/WFC3 images reveal an extended envelope starting from ~10 kpc and reaching ~70 kpc in radius along the semi-major axis. The spectral energy distributions indicate that both its inner component and outer envelope are composed of an old, passively-evolving stellar population. The cD galaxy lies on the same mass-size relation as the bulk of quiescent galaxies at similar redshifts. The cD galaxy has a higher stellar mass surface density but a similar velocity dispersion to those of more-massive, nearby cDs. If the cD galaxy is one of the progenitors of today's more massive cDs, its size and stellar mass have had to increase on average by factors of $3.4\pm1.1$ and $3.3\pm1.3$ over the past ~8 Gyrs, respectively. Such increases in size and stellar mass without being accompanied by significant increases in velocity dispersion are consistent with evolutionary scenarios driven by both major and minor dry mergers. If such cD envelopes originate from dry mergers, our discovery of even one example proves that some BCGs entered the dry merger phase at epochs earlier than z=1. Our data match theoretical models which predict that the continuance of dry mergers at z<1 can result in structures similar to those of massive cD galaxies seen today. Moreover, our discovery is a surprise given that the extreme depth of the HUDF is essential to reveal such an extended cD envelope at z>1 and, yet, the HUDF covers only a minuscule region of sky. Adding that cDs are rare, Our serendipitous discovery hints that such cDs may be more common than expected. [Abridged]

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arXiv2012

The properties of a large volume-limited sample of face-on low surface brightness disk galaxies

We select a large volume-limited sample of low surface brightness galaxies (LSBGs, 2,021) to investigate their statistical properties and their differences from high surface brightness galaxies (HSBGs, 3,639) in details. The distributions of stellar masses of LSBGs and HSBGs are nearly the same and they have the same median values. Thus this volume-limited sample have good completeness and further remove the effect of stellar masses on their other properties when we compare LSBGs and HSBGs. We found that LSBGs tend to have lower stellar metallicities, and lower effect dust attenuations indicating that they have lower dust, than HSBGs. The LSBGs have relatively higher stellar mass-to-light ratios, higher gas fraction, lower star forming rates (SFRs), and lower specific SFRs than HSBGs. Moreover, with the decreasing surface brightness, gas fraction increase, while the SFRs and specific SFRs decrease rapidly for the sample galaxies. This could mean that the star formation histories between LSBGs and HSBGs are different, HSBGs may have stronger star forming activities than LSBGs.

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arXiv2012

A catalog of 132,684 clusters of galaxies identified from Sloan Digital Sky Survey III

Using the photometric redshifts of galaxies from the Sloan Digital Sky Survey III (SDSS-III), we identify 132,684 clusters in the redshift range of 0.05 1.0*10^{14} M_{\odot} in the redshift range of 0.05 0.42 are less complete and have a biased smaller richness than the real one due to incompleteness of member galaxies. We compare our sample with other cluster samples, and find that more than 90% of previously known rich clusters of 0.05<z<0.42 are matched with clusters in our sample. Richer clusters tend to have more luminous brightest cluster galaxies (BCGs). Correlating with X-ray and the Planck data, we show that the cluster richness is closely related to the X-ray luminosity, temperature and Sunyaev-Zel'dovich measurements. Comparison of the BCGs with the SDSS luminous red galaxy (LRG) sample shows that 25% of LRGs are BCGs of our clusters and 36% of LRGs are cluster member galaxies. In our cluster sample, 63% of BCGs of r_{petro}<19.5 satisfy the SDSS LRG selection criteria.

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arXiv2012

Star formation activities in early-type brightest cluster galaxies

We identify a total of 120 early-type Brightest Cluster Galaxies (BCGs) at 0.1<z<0.4 in two recent large cluster catalogues selected from the Sloan Digital Sky Survey (SDSS). They are selected with strong emission lines in their optical spectra, with both Hα and [O II]λ3727 line emission, which indicates significant ongoing star formation. They constitute about ~ 0.5% of the largest, optically-selected, low-redshift BCG sample, and the fraction is a strong function of cluster richness. Their star formation history can be well described by a recent minor and short starburst superimposed on an old stellar component, with the recent episode of star formation contributing on average only less than 1 percent of the total stellar mass. We show that the more massive star-forming BCGs in richer clusters tend to have higher star formation rate (SFR) and specific SFR (SFR per unit galaxy stellar mass). We also compare their statistical properties with a control sample selected from X-ray luminous clusters, and show that the fraction of star-forming BCGs in X-ray luminous clusters is almost one order of magnitude larger than that in optically-selected clusters. BCGs with star formation in cooling flow clusters usually have very flat optical spectra and show the most active star formation, which may be connected with cooling flows.

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arXiv2010

Mass Function of Rich Galaxy Clusters and Its Constraint on sigma_8

The mass function of galaxy clusters is a powerful tool to constrain cosmological parameters, e.g., the mass fluctuation on the scale of 8 h^{-1} Mpc, sigma_8, and the abundance of total matter, Omega_m. We first determine the scaling relations between cluster mass and cluster richness, summed r-band luminosity and the global galaxy number within a cluster radius. These relations are then used to two complete volume-limited rich cluster samples which we obtained from the Sloan Digital Sky Survey (SDSS). We estimate the masses of these clusters and determine the cluster mass function. Fitting the data with a theoretical expression, we get the cosmological parameter constraints in the form of sigma_8(Omega_m/0.3)^{alpha}=beta and find out the parameters of alpha=0.40-0.50 and beta=0.8-0.9, so that sigma_8=0.8-0.9 if Omega_m=0.3. Our sigma_8 value is slightly higher than recent estimates from the mass function of X-ray clusters and the Wilkinson Microwave Anisotropy Probe (WMAP) data, but consistent with the weak lensing statistics.

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arXiv2010

A large sample of low surface brightness disc galaxies from the SDSS- II. Metallicities in surface brightness bins

We study the spectroscopic properties of a large sample of Low Surface Brightness galaxies (LSBGs) (with B-band central surface brightness mu0(B)>22 mag arcsec^(-2)) selected from the Sloan Digital Sky Survey Data Release 4 (SDSS-DR4) main galaxy sample. A large sample of disk-dominated High Surface Brightness galaxies (HSBGs, with mu0(B)<22 mag arcsec^(-2)) are also selected for comparison simultaneously. To study them in more details, these sample galaxies are further divided into four subgroups according to mu0(B) (in units of mag arcsec^(-2)): vLSBGs (24.5-22.75),iLSBGs (22.75-22.0), iHSBGs (22.0-21.25), and vHSBGs (<21.25). The diagnostic diagram from spectral emission-line ratios shows that the AGN fractions of all the four subgroups are small (<9%). The 21,032 star-forming galaxies with good quality spectroscopic observations are further selected for studying their dust extinction, strong-line ratios, metallicities and stellar mass-metallicities relations. The vLSBGs have lower extinction values and have less metal-rich and massive galaxies than the other subgroups. The oxygen abundances of our LSBGs are not as low as those of the HII regions in LSBGs studied in literature, which could be because our samples are more luminous, and because of the different metallicity calibrations used. We find a correlation between 12+log(O/H) and mu0(B) for vLSBGs, iLSBGs and iHSBGs but show that this could be a result of correlation between mu0(B) and stellar mass and the well-known mass-metallicity relation. This large sample shows that LSBGs span a wide range in metallicity and stellar mass, and they lie nearly on the stellar mass vs. metallicity and N/O vs. O/H relations of normal galaxies. This suggests that LSBGs and HSBGs have not had dramatically different star formation and chemical enrichment histories.

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arXiv2009

Galaxy clusters identified from the SDSS DR6 and their properties

Clusters of galaxies in most previous catalogs have redshifts z<0.3. Using the photometric redshifts of galaxies from the Sloan Digital Sky Survey Data Release 6 (SDSS DR6), we identify 39,668 clusters in the redshift range 0.05< z <0.6 with more than eight luminous (M_r<-21) member galaxies. Cluster redshifts are estimated accurately with an uncertainty less than 0.022. The contamination rate of member galaxies is found to be roughly 20%, and the completeness of member galaxy detection reaches to ~90%. Monte Carlo simulations show that the cluster detection rate is more than 90% for massive (M_{200}>2\times10^{14} M_{\odot}) clusters of z<0.42. The false detection rate is ~5%. We obtain the richness, the summed luminosity, and the gross galaxy number within the determined radius for identified clusters. They are tightly related to the X-ray luminosity and temperature of clusters. Cluster mass is related to the richness and summed luminosity with M_{200}\propto R^{1.90\pm0.04} and M_{200}\propto L_r^{1.64\pm0.03}, respectively. In addition, 685 new candidates of X-ray clusters are found by cross-identification of our clusters with the source list of the ROSAT X-ray survey.

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arXiv2009

The properties of a large sample of low surface brightness galaxies from SDSS

A large sample of low surface brightness (LSB) disk galaxies is selected from SDSS with B-band central surface brightness mu_0(B) from 22 to 24.5 mag arcsec^(-2). Some of their properties are studied, such as magnitudes, surface brightness, scalelengths, colors, metallicities, stellar populations, stellar masses and multiwavelength SEDs from UV to IR etc. These properties of LSB galaxies have been compared with those of the galaxies with higher surface brightnesses. Then we check the variations of these properties following surface brightness.

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arXiv2009

Mergers of luminous early-type galaxies

We have made a careful selection of a large complete volume-limited sample (1209) of projected close pairs (7<r_p<50 kpc) of luminous early-type galaxies (M_r<-21.5) in the local universe (z<0.12) from the SDSS data. 249 (21%) of them show interaction features, which suggests that about 0.8% of the galaxies are merging. We derived a comoving volume merger rate of ~(1.0+/-0.4)\times 10^{-5} Mpc^{-3} Gyr^{-1} for luminous early-type galaxies. This is a direct observational determination of the merger rate of luminous galaxies in the local universe. We also obtained the chirp mass distribution of supermassive black hole (SMBH) binary following log[Phi(log M /M_{odot})]=(21.7+/-4.2)-(3.0+/-0.5)log M/M_{odot}. With less assumptions than previous works, we estimated the strain amplitude of the gravitational wave (GW) background from coalescence of SMBH binaries at frequency 10^{-9}-10^{-7} Hz.

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arXiv2009

Major Dry-Mergers In Early-Type Brightest Cluster Galaxies

We search for ongoing major dry-mergers in a well selected sample of local Brightest Cluster Galaxies (BCGs) from the C4 cluster catalogue. 18 out of 515 early-type BCGs with redshift between 0.03 and 0.12 are found to be in major dry-mergers, which are selected as pairs (or triples) with $r$-band magnitude difference $\dm<1.5$ and projected separation $\rp<30$ kpc, and showing signatures of interaction in the form of significant asymmetry in residual images. We find that the fraction of BCGs in major dry-mergers increases with the richness of the clusters, consistent with the fact that richer clusters usually have more massive (or luminous) BCGs. We estimate that present-day early-type BCGs may have experienced on average $\sim 0.6 (\tmerge/0.3\Gyr)^{-1}$ major dry-mergers and through this process increases their luminosity (mass) by $15% (\tmerge/0.3\Gyr)^{-1} (\fmass/0.5)$ on average since $z=0.7$, where $\tmerge$ is the merging timescale and $\fmass$ is the mean mass fraction of companion galaxies added to the central ones. We also find that major dry-mergers do not seem to elevate radio activities in BCGs. Our study shows that major dry-mergers involving BCGs in clusters of galaxies are not rare in the local Universe, and they are an important channel for the formation and evolution of BCGs.

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arXiv2009

Mergers of luminous early-type galaxies in the local universe and gravitational wave background

Supermassive black hole (SMBH) coalescence in galaxy mergers is believed to be one of the primary sources of very low frequency gravitational waves (GWs). Significant contribution of the GWs comes from mergers of massive galaxies with redshifts z<2. Very few previous studies gave the merger rate of massive galaxies. % We selected a large sample (1209) of close pairs of galaxies with projected separations 7<r_p<50 kpc from 87,889 luminous early-type galaxies (M_r<-21.5) from the Sloan Digital Sky Survey Data Release 6. These pairs constitute a complete volume-limited sample in the local universe (z<0.12). Using our newly developed technique, 249 mergers have been identified by searching for interaction features. From them, we found that the merger fraction of luminous early-type galaxies is 0.8%, and the merger rate in the local universe is % R_g=(1.0+/-0.4)*10^{-5} Mpc^{-3} Gyr^{-1}} % with an uncertainty mainly depending on the merging timescale. % We estimated the masses of SMBHs in the centers of merging galaxies based on their luminosities. We found that the chirp mass distribution of the SMBH binaries follows a power law with an index of -3.0+/-0.5 in the range 5*10^8--5*10^{9} M_{\odot}. % Using the SMBH population in the mergers and assuming that the SMBHs can be efficiently driven into the GW regime, we investigated the stochastic GW background in the frequency range 10^{-9}--10^{-7} Hz. We obtained the spectrum of the GW background of h_c(f)=10^{-15}(f/yr^{-1})^{-2/3}, which is one magnitude higher than that obtained by Jaffe & Backer in 2003, but consistent with those calculated from galaxy-formation models.

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arXiv2008

A large sample of low surface brightness disk galaxies from the SDSS. I: The sample and the stellar populations

We present the properties of a large sample (12,282) of nearly face-on low surface brightness (LSB) disk galaxies selected from the main galaxy sample of SDSS-DR4. These properties include B-band central surface brightness mu_0(B), scale lengths h, integrated magnitudes, colors, and distances D. This sample has mu_0(B) values from 22 to 24.5 mag arcsec^{-2} with a median value of 22.42 mag arcsec^{-2}, and disk scale lengths ranging from 2 to 19 kpc. They are quite bright with M_B taking values from -18 to -23 mag with a median value of -20.08 mag. There exist clear correlations between logh and M_B, logh and logD, logD and M_B. However, no obvious correlations are found between mu_0(B) and logh, colors etc. The correlation between colors and logh is weak even though it exists. Both the optical-optical and optical-NIR color-color diagrams indicate that most of them have a mixture of young and old stellar populations. They also satisfy color-magnitude relations, which indicate that brighter galaxies tend generally to be redder. The comparison between the LSBGs and a control sample of nearly face-on disk galaxies with higher surface brightness (HSB) with mu_0(B) from 18.5 to 22 mag arcsec^{-2} show that, at a given luminosity or distance, the observed LSB galaxies tend to have larger scale lengths. These trends could be seen gradually by dividing both the LSBGs and HSBGs into two sub-groups according to surface brightness. A volume-limited sub-sample was extracted to check the incompleteness of surface brightness. The only one of the property relations having an obvious change is the relation of logh versus mu_0(B), which shows a correlation in this sub-sample.

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arXiv2007

Photometric Properties and Scaling Relations of Early Type Brightest Cluster Galaxies

We investigate the photometric properties of the early type Brightest Cluster Galaxies (BCGs) using a carefully selected sample of 85 BCGs from the C4 cluster catalogue with redshift less than 0.1. We perform accurate background subtractions, and surface photometry for these BCGs to 25 $\magsec$ in the Sloan $r$-band. By quantitatively analysing the gradient of the Petrosian profiles of BCGs, we find that a large fraction of BCGs have extended stellar envelopes in their outskirts; more luminous BCGs tend to have more extended stellar halos that are likely connected with mergers. A comparison sample of elliptical galaxies was chosen with similar apparent magnitude and redshift ranges, for which the same photometric analysis procedure is applied. We find that BCGs have steeper size-luminosity ($R \propto L^α$) and Faber-Jackson ($L \propto σ^β$) relations than the bulk of early type galaxies. Furthermore, the power-law indices ($α$ and $β$) in these relations increase as the isophotal limits become deeper. For isophotal limits from 22 to 25 $\magsec$, BCGs are usually larger than the bulk of early type galaxies, and a large fraction ($\sim 49%$) of BCGs have disky isophotal shapes. The differences in the scaling relations are consistent with a scenario where the dynamical structure and formation route of BCGs may be different from the bulk of early type galaxies, in particular dry (dissipationless) mergers may play a more important role in their formation; we highlight several possible dry merger candidates in our sample.

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arXiv2007

SDSS J121811.0+465501.2: a new Low Surface Brightness Galaxy with low metallicity

We serendipitously find a new nearby Low Surface Brightness (LSB) galaxy from SDSS database. We estimate oxygen abundance of its H II region SDSS J121811.0+465501.2 from electron temperature, as well as for another H II region, SDSS J135440.5+535309.6, located in irregular LSB galaxy UGC 8837. These two extragalactic H II regions were classified as stars in the SDSS-DR4 database, and were found occasionally by us in the automatic recognition and classification on stellar spectra.Their optical spectra show obvious emission lines, i.e., strong [O III]4959, 5007, Balmer emission lines, but very weak [N II]6548,6583 and [S II]6317,6731, which could indicate that they are metal-poor star-forming regions. The derived oxygen abundances of the two objects are 12+log(O/H) ~ 7.88+-0.30 and 7.70+-0.30, respectively. The host of the H II region SDSS J121811.0+465501.2 is identified as a new nearly edge-on LSB disc galaxy (almost without bulge) with the B-band central surface brightness mu_0(B) as 23.68 mag arcsec^{-2} and inclination angle as ~75 degree by using the GIM2D software to analyze its g- and r-band images independently. It is a nearby dwarf galaxy with redshift z~0.00157, disk scale-length ~0.40 kpc and B-band absolute magnitude M_B ~ -13.51 mag. The very low oxygen abundances of these two objects confirm the low metallicities of LSB galaxies.

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