The formation and evolution of massive galaxies

The discovery of massive galaxies at high redshifts,especially the passive ones,poses a big challenge for the current standard galaxy formation models.Here we use the semi-analytic galaxy formation model developed by Henriques et al.to explore the formation and evolution of massive galaxies(MGs,stellar-mass M*>1011 M⊙).Different from previous works,we focus on the ones just formed(e.g.just reach?1011 M⊙).We find that most of the MGs are formed around z=0.6,with the earliest formation at z>4.Interestingly,although most of the MGs in the local Universe are passive,we find that only 13%of the MGs are quenched at the formation time.Most of the quenched MGs at formation already host a very massive supermassive black hole(SMBH)which could power the very effective AGN feedback.For the star-forming MGs,the ones with more massive SMBH prefer to quench in shorter timescales;in particular,those with MSMBH>107.5 M⊙have a quenching timescale of~0.5 Gyr and the characteristic MSMBH depends on the chosen stellar mass threshold in the definition of MGs as a result of their co-evolution.We also find that the"in-situ"star formation dominates the stellar mass growth of MGs until they are formed.Over the whole redshift range,we find the quiescent MGs prefer to stay in more massive dark matter halos,and have more massive SMBH and less cold gas masses.Our results provide a new angle on the whole life of the growth of MGs in the Universe.

Co-evolution of nuclear rings,bars and the central intensity ratio of their host galaxies

Using a sample of 13 early-type spiral galaxies hosting nuclear rings,we report remarkable correlations between the properties of the nuclear rings and the central intensity ratio(CIR) of their host galaxies.The CIR,a function of intensity of light within the central 1.5 and 3 arcsec region,is found to be a vital parameter in galaxy evolution,as it shares strong correlations with many structural and dynamical properties of early-type galaxies,including mass of the central supermassive black hole(SMBH).We use archival HST images for aperture photometry at the centre of the galaxy image to compute the CIR.We observe that the relative sizes of nuclear rings and ring cluster surface densities strongly correlate with the CIR.These correlations suggest reduced star formation in the centres of galaxies hosting small and dense nuclear rings.This scenario appears to be a consequence of strong bars as advocated by the significant connection observed between the CIR and bar strengths.In addition,we observe that the CIR is closely related with the integrated properties of the stellar population in the nuclear rings,associating the rings hosting older and less massive star clusters with low values of CIR.Thus,the CIR can serve as a crucial parameter in unfolding the coupled evolution of bars and rings as it is intimately connected with both their properties.

AGN Lifetimes in UV-selected Galaxies: A Clue to Supermassive Black Hole-galaxy Coevolution

The coevolution between supermassive black holes(SMBHs) and their host galaxies has been proposed for more than a decade,albeit with little direct evidence about black hole accretion activities regulating galaxy star formation at z> 1.In this paper,we study the lifetimes of X-ray active galactic nuclei(AGNs) in UV-selected red sequence(RS),blue cloud(BC) and green valley(GV) galaxies,finding that AGN accretion activities are most prominent in GV galaxies at z ~1.5-2,compared with RS and BC galaxies.We also compare AGN accretion timescales with typical color transition timescales of UV-selected galaxies.We find that the lifetime of GV galaxies at z~1.5-2 is very close to the typical timescale when the AGNs residing in them stay in the high-accretion-rate mode at these redshifts;for BC galaxies,the consistency between the color transition timescale and the black hole strong accretion lifetime is more likely to happen at lower redshifts(z <1).Our results support the scenario where AGN accretion activities govern UV color transitions of host galaxies,making galaxies and their central SMBHs coevolve with each other.

Formation and Evolution of Galaxies and Its Black Holes and Quasars

Galaxy formation and evolution is one of the most active research areas in astrophysics,so many people have studied this area.But since they didn’t understand thoroughly the evolution law from satellite to planet then to star,their theories are very weak.In their theories,they proposed that large gas clouds collapsing to form a galaxy or more recently that matter started out in smaller clumps merged to form galaxy,which is incredible.Hence,the author of this paper,through studying the formation and orbit-variation of satellites,planets and stars,has put forward a new theory of galaxy formation and evolution,therefore revealing the hierarchical structure of galaxies and the formation and evolution of black holes and quasars.

Low-ionization galaxies and evolution in a pilot survey up to z=1

We present galactic spectroscopic data from a pencil beam of 10.75×7.5 centered on the X-ray cluster RXJ0054.0–2823 at z=0.29.We study the spectral evolution of galaxies from z=1 down to the cluster redshift in a magnitude-limited sample at R≤23,for which the statistical properties of the sample are well understood.We divide emission-line galaxies into star-forming galaxies,Low Ionization Nuclear Emission line Regions（LINERs） ,and Seyferts by using emission-line ratios of[OII],Hβ,and[OIII],and derive stellar fractions from population synthesis models. We focus our analysis on absorption and low-ionization galaxies.For absorption-line galaxies,we recover the well-known result that these galaxies have had no detectable evolution since z～0.6-0.7,but we also find that in the range z=0.65-1,at least 50% of the stars in bright absorption systems are younger than 2.5 Gyr.Faint absorption-line galaxies in the cluster at z=0.29 also had significant star formation during the previous 2-3 Gyr,but their brighter counterparts seem to be only composed of old stars.At z～0.8,our dynamically young cluster had a truncated red-sequence.This result seems to be consistent with a scenario where the final assembly of E/S0 took place at z1.In the volume-limited range 0.35≤z≤0.65,we find that 23% of the early-type galaxies have LINER-like spectra with Hβin absorption and have a significant component of A stars.The vast majority of LINERs in our sample have significant populations of young and intermediate-aged stars and are thus not related to AGNs,but to the population of‘retired galaxies’recently identified by Cid Fernandes et al.in the Sloan Digital Sky Survey（SDSS） .Early-type LINERs with various fractions of A stars and E＋A galaxies appear to play an important role in the formation of the red sequence.

Observational evidence for the evolution of nuclear metallicity and star formation rate with the merger stage

We investigate the evolution of nuclear gas-phase oxygen abundance and star formation rate（SFR） of local far-infrared selected star-forming galaxies along the merger sequence, as traced by their optical morphologies. The sample was drawn from a cross-correlation analysis of the IRAS Point Source Catalog Redshift Survey and 1 Jy ultraluminous infrared galaxy sample with the Sloan Digital Sky Survey Data Release 7 database. The investigation is done by comparing our sample to a control sample matched in the normalized redshift distribution in two diagnostics, which are the nuclear gas-phase metallicity vs.stellar mass and the nuclear SFR vs. stellar mass diagrams. Galaxies with different morphological types show different mass-metallicity relations（MZRs）. Compared to the MZR defined by the control sample,isolated spirals have comparable metallicities with the control sample at a given stellar mass. Spirals in pairs and interacting galaxies with projected separations of rp 〉 20 kpc show a mild metallicity dilution of0.02–0.03 dex. Interacting galaxies with rp 〈 20 kpc, pre-mergers and advanced mergers are underabundant by～0.06,～0.05 and～0.04 dex, respectively. This shows an evolutionary trend that the metallicity is increasingly depressed as the merging proceeds and it is diluted most dramatically when two galaxies are closely interacting. Afterwards, the interstellar medium（ISM） is enriched when the galaxies coalesce.This is the first time that such ISM enrichment at the final coalescence stage has been observed, which demonstrates the importance of supernova explosions in affecting the nuclear metallicity. Moreover, the central SFR enhancement relative to the control sample evolves simultaneously with the nuclear gas-phase oxygen abundance. Our results support the predictions from numerical simulations.

The Evolution of Advanced Merger (U)LIRGs on the Color-Stellar Mass Diagram

Based on a sample of 79 local advanced merger （adv-merger） （U）LIRGs, we search for evidence of quenching processes by investigating the distributions of star formation history indicators （EW（Ha）, EW（HfiA） and D,（4000）） on the NUV-r color-mass and SFR-M, diagrams. The distributions of EW（Ha） and Dn（4000） on the NUV-r color-mass diagram show clear trends that at a given stellar mass, galaxies with redder NUV-r colors have smaller EW（Ha） and larger Dn （4000）. The reddest adv-merger （U）LIRGs close to the green valley mostly have Dn（4000）〉 1.4. In addition, in the SFR-M, diagram, as the SFR decreases, the EW（Ha） decreases and the Dn （4000） increases, implying that the adv-merger （U）LIRGs on the star formation main sequence have more evolved stellar populations than those above the main sequence. These results indicate that a fraction of the adv-merger （U）LIRGs have already exhibited signs of fading from the starburst phase and that the NUV-r reddest adv-merger （U）LIRGs are likely at the initial stage of post-starbursts with an age of - 1 Gyr, which is consistent with the gas exhaustion time-scales. Therefore, our results offer additional support for the fast evolutionary track from the blue cloud to the red sequence.

A Monte Carlo Study of the Evolution of the Scale Height of Normal Pulsars in the Galaxy

Based on the undisturbed, finite thickness disk gravitational potential, we carried out 3-D Monte Carlo simulations of normal pulsars. We find that their scale height evolves in a similar way for different velocity dispersions （δv）： it first increases linearly with time, reaches a peak, then gradually decreases, and finally approaches a stable asymptotic value. The initial velocity dispersion has a very large influence on the scale height. The time evolution of the scale height is studied. When the magnetic decay age is used as the time variable, the observed scale height has a similar trend as the simulated results in the linear stage, from which we derive velocity dispersions in the range 70 - 178km s^-1, which are near the statistical result of 90 - 270km s^-1 for 92 pulsars with known transverse velocities. If the characteristic age is used as the time variable, then the observed and theoretical curves roughly agree for t 〉 10^8 yr only if av 〈 25km s^-1.

The evolution of stellar metallicity gradients of the Milky Way disk from LSS-GAC main sequence turn-off stars: a two-phase disk formation history?

Accurate measurements of stellar metallicity gradients in the radial and vertical directions of the disk and their temporal variations provide important constraints on the formation and evolution of the Milky Way disk. We use 297 042 main sequence turn-off stars selected from the LAMOST Spectroscopic Survey of the Galactic Anticenter（LSS-GAC） to determine the radial and vertical gradients of stellar metallicity,△[Fe/H]/△R and △[Fe/H]/△｜Z ｜ of the Milky Way disk in the direction of the anticenter. We determine ages of those turn-off stars by isochrone fitting and measure the temporal variations of metallicity gradients. We have carried out a detailed analysis of the selection effects resulting from the selection, observation and data reduction of LSS-GAC targets and the potential biases of a magnitude limited sample on the determinations of metallicity gradients. Our results show that the gradients, both in the radial and vertical directions, exhibit significant spatial and temporal variations. The radial gradients yielded by stars with the oldest ages（ 11 Gyr） are essentially zero at all heights from the disk midplane, while those given by younger stars are always negative. The vertical gradients deduced from stars with the oldest ages（ 11 Gyr）are negative and only show very weak variations with Galactocentric distance in the disk plane, R, while those yielded by younger stars show strong variations with R.After being essentially flat at the earliest epochs of disk formation, the radial gradients steepen as age decreases, reaching a maximum（steepest） at age 7–8 Gyr, and then they flatten again. Similar temporal trends are also found for the vertical gradients. We infer that the assembly of the Milky Way disk may have experienced at least two distinct phases. The earlier phase is probably related to a slow, pressure-supported collapse of gas, when the gas settles down to the disk mainly in the vertical direction. In the later phase, there are significant radial flows of gas in the disk, and the rate of gas inflow near the solar neighborhood reaches a maximum around a lookback time of 7–8 Gyr.The transition between the two phases occurs around a lookback time between 8 and11 Gyr. The two phases may be responsible for the formation of the Milky Way＇s thick and thin disks, respectively. Also, as a consequence, we recommend that stellar age is a natural, physical criterion to distinguish stars from the thin and thick disks. From an epoch earlier than 11 Gyr to one between 8 and 11 Gyr, there is an abrupt, significant change in magnitude of both the radial and vertical metallicity gradients, suggesting that stellar radial migration is unlikely to play an important role in the formation of the thick disk.

The role of mergers and gas accretion in black hole growth and galaxy evolution

We use a semi-analytic galaxy formation model to study the co-evolution of supermassive black holes(SMBHs) with their host galaxies.Although the coalescence of SMBHs is not important,the quasarmode accretion induced by mergers plays a dominant role in the growth of SMBHs.Mergers play a more important role in the growth of SMBH host galaxies than in the SMBH growth.It is the combined contribution from quasar mode accretion and mergers to the SMBH growth and the combined contribution from starburst and mergers to their host galaxy growth that determine the observed scaling relation between the SMBH masses and their host galaxy masses.We also find that mergers are more important in the growth of SMBH host galaxies compared to normal galaxies which share the same stellar mass range as the SMBH host galaxies.

Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution

We compile a new sample of 89 open clusters with ages, distances and metallicities available. We derive a radial iron gradient of about -0.099±0.008 dex kpc^(-1) (unweighted) for the whole sample, which is somewhat greater than the most recent determination of oxygen gradient from nebulae and young stars. By dividing the clusters into age groups, we show that the iron gradient was steeper in the past and has evolved slowly in time. Current data show a substantial scatter of the cluster metallicities indicating that the Galactic disk has undergone a very rapid, inhomogeneous enrichment. Also, based on a simple, but quite successful model of chemical evolution of the Milky Way disk, we make a detailed calculation of the iron abundance gradient and its time evolution. The predicted current iron gradient is about -0.072 dex kpc^(-1). The model also predicts a steady flattening of the iron gradient with time, which agrees with the result from our open cluster sample.

Multiwavelength study of nearly face-on low surface brightness disk galaxies

We study the ages of a large sample （1802） of nearly face-on disk low surface brightness galaxies （LSBGs） using the evolutionary population synthesis （EPS） model PEGASE with an exponentially decreasing star formation rate to fit their mul- tiwavelength spectral energy distributions （SEDs） from far-ultraviolet （FUV） to nearinfrared （NIR）. The derived ages of LSBGs are 1-5 Gyr for most of the sample no matter if constant or varying dust extinction is adopted, which are similar to most of the previous studies on smaller samples. This means that these LSBGs formed the majority of their stars quite recently. However, a small part of the sample （~2%-3%） has larger ages of 5-8 Gyr, meaning their major star forming process may have occurred earlier. At the same time, a large sample （5886） of high surface brightness galaxies （HSBGs） are selected and studied using the same method for comparisons. The de- rived ages are 1-5 Gyr for most of the sample （97%） as well. These results probably mean that these LSBGs have not much different star formation histories from their HSBGs counterparts. However, we should notice that the HSBGs are generally about 0.2 Gyr younger, which could mean that the HSBGs may have undergone more recent star forming activities than the LSBGs.

Stellar Abundance and Galactic Chemical Evolution through LAMOST Spectroscopic Survey

A project of a spectroscopic survey of Galactic structure and evolution with a Large sky Area Multi-Object fiber Spectroscopic Telescope （LAMOST） is presented. The spectroscopic survey consists of two observational modes for various targets in our Galaxy. One is a major survey of the Milky Way aimed at a systematic study of the stellar abundance and Galactic chemical evolution through low resolution （R = 1000 2000） spectroscopy. Another is a follow-up observation with medium resolution （R = 10000） spectrographs aimed at detailed studies of the selected stars with different chemical composition, kinematics and dynamics.

Preferred alignments of angular momentum vectors of galaxies in six dynamically unstable Abell clusters

A spatial orientation of angular momentum vectors of galaxies in six dynamically unstable Abell clusters（S1171, S0001, A1035, A1373, A1474 and A4053） is studied. For this, twodimensional observed parameters（e.g., positions, diameters and position angles） are converted into three-dimensional（3D） rotation axes of the galaxy using the ＇position angle-inclination＇ method. The expected isotropic distribution curves for angular momentum vectors are obtained by performing random simulations. The observed and expected distributions are compared using several statistical tests.No preferred alignments of angular momentum vectors of galaxies are noticed in all six dynamically unstable clusters, supporting the hierarchy model of galaxy formation. These clusters have a larger value of velocity dispersion. However, local effects are noticed in the clusters that have substructures in the1D-3D number density maps.

Morphology and structure of BzK-selected galaxies at z ～ 2 in the CANDELS-COSMOS field

Utilizing a BzK-selecfion technique, we obtain 14 550 star-forming galaxies （sBzKs） and 1763 passive galaxies （pBzKs） at z - 2 from the K-selected （KAB 〈 22.5） catalog in the COSMOS/UltraVISTA field. The differential number counts of sBzKs and pBzKs are consistent with the results from the literature. Compared to the observed results, semi-analytic models of galaxy formation and evolution provide too few （many） galaxies at the high （low） mass end. Moreover, we find that the star formation rate and stellar mass of sBzKs follow the relation of the main sequence. Based on HST/Wide Field Camera 3 F160W imaging, we find a wide range of morphological diversities for sBzKs, from diffuse to early-type spiral structures, with relatively high M20, large size and low G, while pBzKs have elliptical-like compact morphologies with lower M20, smaller size and higher G, indicating a more concentrated and symmetric spatial extent of stellar population distribution in pBzKs than sBzKs. Furthermore, the sizes of pBzKs （sBzKs） at z - 2 are on average two to three （one to two） times smaller than those of local early-type （late-type） galaxies with similar stellar mass. Our findings imply that the two classes have different evolution models and mass assembly histories.

The Origin of Cosmic Structures Part 3 — Supermassive Black Holes and Galaxy Cluster Evolution

In Part 1 of this work, we showed that our new model of cosmology can account for the origin of all cosmic structures ranging in size from stars up to superclusters. In this model, at the time of nucleosynthesis, an imprint embedded in the vacuum regulated the creation of the protons (and electrons) that later made up the structures. Immediately after nucleosynthesis and for a considerable period afterward, the evolution was completely determined by the expansion of the universe. Gradually, however, gravitational influences became more important until finally, the expansion of the structures-to-be ceased at their zero velocity points. Stars, galaxies, and galaxy clusters all reached their zero velocity points more or less simultaneously at the usually accepted time of the beginning of galaxy formation. From that point onward, the evolution gravitation came to dominate the evolution although the expansion still exerted its influence. In this paper, we examine the subsequent cluster evolution in some detail. We establish the conditions required to prevent a free-fall collapse of the clusters and then show that galaxies with quasar-like active nuclei located within the cluster were the sources of the necessary radiation. We also show that the required galactic supermassive black holes were a consequence of the initial free-fall collapse of all galaxies.

IRAS F21013-0739:a possible evolutionary successor of an ultraluminous infrared galaxy

We present a stellar population synthesis study of a type II luminous infrared galaxy, IRAS F21013-0739. Optical images show clear characteristics of a merger remnant. The H-band absolute magnitude is MH = -25.1, which is -2 times as luminous as L＊ galaxies. Stellar populations are obtained through the stellar synthesis code STARLIGHT. We find that it experienced a recent starburst （SB） phase - 100 Myr ago. By reconstructing the ultraviolet-to-optical spectrum, and adopting Calzetti et al. and Leitherer et al.＇s extinction curves, we estimate the past infrared （IR） luminosities of the host galaxy and find it may have experienced an ultraluminous infrared galaxy phase which lasted for about 100 Myr. Its i-band absolute magnitude is Mi = -22.463, and its spectral type shows type 2 active galactic nucleus （AGN） characteristics. The mass of the supermassive black-hole is estimated to be MBH = 1.6 × 107 M⊙ （lower- limit）. The Eddington ratio Lbol/LEdd is 0.15, which is typical of Palomar-Green （PG） quasars. Both the nuclear SB and AGN contribute to the present IR luminosity budget, and the SB contributes -67%. On the diagram of IR color versus IR/opfical excess, it is located between IR quasars and PG quasars. These results indicate that IRAS F21013-0739 has probably evolved from a ULIRG, and it can possibly evolve into an AGN.

Infrared Galaxies in the Nearby Universe

We used the Sloan Digital Sky Survey （SDSS） Data Release 5 （DR5） to study the morphological properties of 1137 nearby infrared （IR） galaxies, most of which are brighter than 15.9 mag in r-band. This sample was drawn from a cross-correlation of the Infra-Red Astronomical Satellite （IRAS） point source catalog redshift survey with DR5 at z ≤ 0.08. Based on this IR galaxy sample, we constructed five volume-limited sub-samples with IR luminosity ranging from 10^9.5 L⊙ to 10^12L⊙. By deriving the IR luminosity functions （LF） for different morphological types, we found that normal spiral galaxies are the dominant population below LIR ~ 8 ~ 10^10 L⊙; while the fraction of barred spiral galaxies increases with increasing IR luminosity and becomes dominant in spiral galaxies beyond LIR ≈ 5 × 10^10L⊙. As the IR luminosity decreases, the IR galaxies become more compact and have lower stellar masses. The analysis also shows that normal spiral galaxies give the dominant contribution to the total comoving IR energy density in the nearby universe, while, in contrast, the contribution from peculiar galaxies is only 39%.

Stellar population analysis on the stacked spectra of double-peaked emission-line galaxies

Double-peaked emission-line galaxies have long been perceived as objects related to merging galaxies or other phenomena with disturbed dynamical activities, such as outflows and disk rotation. In order to find the connection between the unique activities happening in these objects and their stellar population physics, we study the stellar populations of the stacked spectra drawn from double-peaked emission-line galaxies in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST) Data Release 4(DR4) and the Sloan Digital Sky Survey(SDSS) Data Release 7(DR7) databases. We group the selected double-peaked emission-line objects into 10 different types of pairs based on the Baldwin-Phillips-Terlevich(BPT) diagnosis for each pair of blueshifted and redshifted components, and then stack the spectra of each group for analysis. The software STARLIGHT is employed to fit each stacked spectrum, and the contributions of stars at different ages and metallicities are quantified for subsequent comparative study and analysis. To highlight the commonality and uniqueness in these double-peaked emitting objects, we compare the population synthesis results of the stacked spectra of double-peaked emission-line galaxies with those of their counterpart reference samples displaying single-peaked emission features. The reference samples are also selected from the LAMOST DR4 and SDSS DR7 databases. From the comparison results,we confirm the strong correlations between stellar populations and their spectral classes, and find that the double-peaked emitting phenomenon is more likely to occur in an ‘older’ stellar environment and the subgroups hosting different BPT components will show an obvious heterogeneous star formation history.

Neutral Hydrogen Content of Dwarf Galaxies in Different Environments

Environments play an important role in galaxy formation and evolution,particularly in regulating the content of neutral gas.However,current HI surveys have limitations in their depth,which prevents them from adequately studying low HI content galaxies in high-density regions.In this study,we address this issue by employing the Five-hundred-meter Aperture Spherical radio Telescope with extensive integration times to complement the relatively shallow Arecibo Legacy Fast Arecibo L-band Feed Array HI survey.This approach allows us to explore the gas content of dwarf galaxies across various environments.We observe a positive relationship between HI mass and stellar mass in dwarf galaxies,with a well-defined upper boundary for HI mass that holds true in both observations and simulations.Furthermore,we find a decrease in the H I-to-stellar mass ratio(M_(HI)/M_*)as the density of the environment increases,irrespective of whether it is determined by the proximity to the nearest group or the projected number density.Comparing our observations to simulations,we note a steeper slope in the relationship,indicating a gradual gas-stripping process in the observational data.Additionally,we find that the scaling relation between the M_(HI)/M_*and optical properties can be improved by incorporating galaxy environments.