Luminosity Function of Faint Globular Clusters in M87

We present the luminosity function to very faint magnitudes for the globular clusters in M87, based on a 30 orbit \textit{Hubble Space Telescope (HST)} WFPC2 imaging program. The very deep images and corresponding improved false source rejection allow us to probe the mass function further beyond the turnover than has been done before. We compare our luminosity function to those that have been observed in the past, and confirm the similarity of the turnover luminosity between M87 and the Milky Way. We also find with high statistical significance that the M87 luminosity function is broader than that of the Milky Way. We discuss how determining the mass function of the cluster system to low masses can constrain theoretical models of the dynamical evolution of globular cluster systems. Our mass function is consistent with the dependence of mass loss on the initial cluster mass given by classical evaporation, and somewhat inconsistent with newer proposals that have a shallower mass dependence. In addition, the rate of mass loss is consistent with standard evaporation models, and not with the much higher rates proposed by some recent studies of very young cluster systems. We also find that the mass-size relation has very little slope, indicating that there is almost no increase in the size of a cluster with increasing mass.Comment: 22 pages, 5 figures, Accepted for publication in Ap

Galaxy clusters at high redshift and evolution of brightest cluster galaxies

Identification of high redshift clusters is important for studies of cosmology and cluster evolution. Using photometric redshifts of galaxies, we identify 631 clusters from the Canada-France-Hawaii Telescope (CFHT) Wide field, 202 clusters from the CHFT Deep field, 187 clusters from the Cosmic Evolution Survey (COSMOS) and 737 clusters from the Spitzer Wide-area InfraRed Extragalactic survey (SWIRE) field. The redshifts of these clusters are in the range of 0.1<z<1.6. Merging these cluster samples gives 1644 clusters in the four survey fields, of which 1088 are newly identified and more than half are from the large SWIRE field. Among 228 clusters of z>1, 191 clusters are newly identified, and most of them from the SWIRE field. With this large sample of high redshift clusters, we study the color evolution of the brightest cluster galaxies (BCGs). The colors r'-z' and r^+-m_{3.6\mu m} of the BCGs are consistent with a stellar population synthesis model in which the BCGs are formed at redshift z_f>2 and evolved passively. The colors g'-z' and B-m_{3.6\mu m} of the BCGs at redshifts z>0.8 are systematically bluer than the passive evolution model for galaxy formed at z_f~2, indicating star formation in high redshift BCGs.Comment: 10 pages, 14 figures, 2 tables; added reference, corrected typos; Table 2 is available at http://zmtt.bao.ac.cn/wzl/CV_wen.ht

The Race Between Stars and Quasars in Reionizing Cosmic Hydrogen

The cosmological background of ionizing radiation has been dominated by quasars once the Universe aged by ~2 billion years. At earlier times (redshifts z>3), the observed abundance of bright quasars declined sharply, implying that cosmic hydrogen was reionized by stars instead. Here, we explain the physical origin of the transition between the dominance of stars and quasars as a generic feature of structure formation in the concordance LCDM cosmology. At early times, the fraction of baryons in galaxies grows faster than the maximum (Eddington-limited) growth rate possible for quasars. As a result, quasars were not able to catch up with the rapid early growth of stellar mass in their host galaxies.Comment: 5 pages, 1 figure, Accepted for publication in JCA

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.Comment: 27 pages, 9 figures, Corrected typos and reference

Observational constraints on the co-evolution of supermassive black holes and galaxies

The star formation rate (SFR) and black hole accretion rate (BHAR) functions are measured to be proportional to each other at z < ~3. This close correspondence between SF and BHA would naturally yield a BH mass-galaxy mass correlation, whereas a BH mass-bulge mass correlation is observed. To explore this apparent contradiction we study the SF in spheroid-dominated galaxies between z=1 and the present day. We use 903 galaxies from the COMBO-17 survey with M* >2x10^10M_sun, ultraviolet and infrared-derived SFRs from Spitzer and GALEX, and morphologies from GEMS HST/ACS imaging. Using stacking techniques, we find that <25% of all SF occurs in spheroid-dominated galaxies (Sersic index n>2.5), while the BHAR that we would expect if the global scalings held is three times higher. This rules out the simplest picture of co-evolution, in which SF and BHA trace each other at all times. These results could be explained if SF and BHA occur in the same events, but offset in time, for example at different stages of a merger event. However, one would then expect to see the corresponding star formation activity in early-stage mergers, in conflict with observations. We conclude that the major episodes of SF and BHA occur in different events, with the bulk of SF happening in isolated disks and most BHA occurring in major mergers. The apparent global co-evolution results from the regulation of the BH growth by the potential well of the galactic spheroid, which includes a major contribution from disrupted disk stars.Comment: 14 pages, 5 figures, accepted for publication in Ap

An Improved Search for the Neutron Electric Dipole Moment

A permanent electric dipole moment of fundamental spin-1/2 particles violates both parity (P) and time re- versal (T) symmetry, and hence, also charge-parity (CP) symmetry since there is no sign of CPT-violation. The search for a neutron electric dipole moment (nEDM) probes CP violation within and beyond the Stan- dard Model. The experiment, set up at the Paul Scherrer Institute (PSI), an improved, upgraded version of the apparatus which provided the current best experimental limit, dn < 2.9E-26 ecm (90% C.L.), by the RAL/Sussex/ILL collaboration: Baker et al., Phys. Rev. Lett. 97, 131801 (2006). In the next two years we aim to improve the sensitivity of the apparatus to sigma(dn) = 2.6E-27 ecm corresponding to an upper limit of dn < 5E-27 ecm (95% C.L.), in case for a null result. In parallel the collaboration works on the design of a new apparatus to further increase the sensitivity to sigma(dn) = 2.6E-28 ecm.Comment: APS Division for particles and fields, Conference Proceedings, Two figure

The Panchromatic Hubble Andromeda Treasury

The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going HST Multicycle Treasury program to image ~1/3 of M31's star forming disk in 6 filters, from the UV to the NIR. The full survey will resolve the galaxy into more than 100 million stars with projected radii from 0-20 kpc over a contiguous 0.5 square degree area in 828 orbits, producing imaging in the F275W and F336W filters with WFC3/UVIS, F475W and F814W with ACS/WFC, and F110W and F160W with WFC3/IR. The resulting wavelength coverage gives excellent constraints on stellar temperature, bolometric luminosity, and extinction for most spectral types. The photometry reaches SNR=4 at F275W=25.1, F336W=24.9, F475W=27.9, F814W=27.1, F110W=25.5, and F160W=24.6 for single pointings in the uncrowded outer disk; however, the optical and NIR data are crowding limited, and the deepest reliable magnitudes are up to 5 magnitudes brighter in the inner bulge. All pointings are dithered and produce Nyquist-sampled images in F475W, F814W, and F160W. We describe the observing strategy, photometry, astrometry, and data products, along with extensive tests of photometric stability, crowding errors, spatially-dependent photometric biases, and telescope pointing control. We report on initial fits to the structure of M31's disk, derived from the density of RGB stars, in a way that is independent of the assumed M/L and is robust to variations in dust extinction. These fits also show that the 10 kpc ring is not just a region of enhanced recent star formation, but is instead a dynamical structure containing a significant overdensity of stars with ages >1 Gyr. (Abridged)Comment: 48 pages including 22 pages of figures. Accepted to the Astrophysical Journal Supplements. Some figures slightly degraded to reduce submission siz

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 ($R \propto L^\alpha$) and Faber-Jackson ($L \propto \sigma^\beta$) relations than the bulk of early type galaxies. Furthermore, the power-law indices ($\alpha$ and $\beta$) 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.Comment: 15 pages, 15 figures, and 2 tables; in press in MNRAS. Accepted 2007 November 23, Received 2007 November 23; in original form of 2007 August

Massive Galaxies in COSMOS: Evolution of Black hole versus bulge mass but not versus total stellar mass over the last 9 Gyrs?

We constrain the ratio of black hole (BH) mass to total stellar mass of type-1 AGN in the COSMOS survey at 1<z<2. For 10 AGN at mean redshift z~1.4 with both HST/ACS and HST/NICMOS imaging data we are able to compute total stellar mass M_(*,total), based on restframe UV-to-optical host galaxy colors which constrain mass-to-light ratios. All objects have virial BH mass-estimates available from the COSMOS Magellan/IMACS and zCOSMOS surveys. We find zero difference between the M_BH--M_(*,total)-relation at z~1.4 and the M_BH--M_(*,bulge)-relation in the local Universe. Our interpretation is: (a) If our objects were purely bulge-dominated, the M_BH--M_(*,bulge)-relation has not evolved since z~1.4. However, (b) since we have evidence for substantial disk components, the bulges of massive galaxies (logM_(*,total)=11.1+-0.25 or logM_BH~8.3+-0.2) must have grown over the last 9 Gyrs predominantly by redistribution of disk- into bulge-mass. Since all necessary stellar mass exists in the galaxy at z=1.4, no star-formation or addition of external stellar material is required, only a redistribution e.g. induced by minor and major merging or through disk instabilities. Merging, in addition to redistributing mass in the galaxy, will add both BH and stellar/bulge mass, but does not change the overall final M_BH/M_(*,bulge) ratio. Since the overall cosmic stellar and BH mass buildup trace each other tightly over time, our scenario of bulge-formation in massive galaxies is independent of any strong BH-feedback and means that the mechanism coupling BH and bulge mass until the present is very indirect.Comment: Published in ApJL; 7 pages, 2 figures; updated to accepted version (methods changed, results unchanged

Renormalized spin coefficients in the accumulated orbital phase for unequal mass black hole binaries

We analyze galactic black hole mergers and their emitted gravitational waves. Such mergers have typically unequal masses with mass ratio of the order 1/10. The emitted gravitational waves carry the inprint of spins and mass quadrupoles of the binary components. Among these contributions, we consider here the quasi-precessional evolution of the spins. A method of taking into account these third post-Newtonian (3PN) effects by renormalizing (redefining) the 1.5 PN and 2PN accurate spin contributions to the accumulated orbital phase is developed.Comment: 10 pages, to appear in Class. Quantum Grav. GWDAW13 Proceedings Special Issue, v2: no typos conjectur