arXiv subjects

Renyue Cen

Publications and source records attributed to Renyue Cen.

At least 181 records · Page 10Linked to original sources

arXiv2005

Where Are the Baryons? III: Non-Equilibrium Effects and Observables

Numerical simulations of the intergalactic medium have shown that at the present epoch a significant fraction (40-50%) of the baryonic component should be found in the (T~10^6K) Warm-Hot Intergalactic Medium (WHIM) - with several recent observational lines of evidence indicating the validity of the prediction. We here recompute the evolution of the WHIM with the following major improvements: (1) galactic superwind feedback processes from galaxy/star formation are explicitly included; (2) major metal species (O V to O IX) are computed explicitly in a non-equilibrium way; (3) mass and spatial dynamic ranges are larger by a factor of 8 and 2, respectively, than in our previous simulations. We find: (1) non-equilibrium calculations produce significantly different results from ionization equilibrium calculations. (2) The abundance of O VI absorption lines based on non-equilibrium simulations with galactic superwinds is in remarkably good agreement with latest observations, implying the validity of our model, while the predicted abundances for O VII and O VIII absorption lines appear to be lower than observed but the observational errorbars are currently very large. The expected abundances for O VI (as well as Lyman alpha), O VII and O VIII absorption systems are in the range 50-100 per unit redshift at EW=1km/s decreasing to 10-20 per unit redshift at EW=10km/s. The number of O VI absorption lines with EW>100km/s is very small, while there are about 1-3 lines per unit redshift for O VII and O VIII absorption lines at EW=100km/s. (3) Emission lines, primarily O VI and \lya in the UV and O VII and O VIII in the soft X-rays are potentially observable by future missions. The number of emission lines per unit redshift that may be detectable by planned UV and soft X-ray missions are in the order of 0.1-1.

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arXiv2005

Massive galaxies & EROs at z=1-3 in cosmological hydrodynamic simulations: near-IR properties

Recent observations have revealed a population of red massive galaxies at high redshift which are challenging to explain in hierarchical galaxy formation models. We analyze this ``massive galaxy problem'' with two different types of hydrodynamic simulations -- Eulerian TVD and SPH -- of a concordance Lambda CDM universe. We consider two separate but connected aspects of the problem posed by these extremely red objects (EROs): (1) the mass-scale, and (2) red colors. We perform spectrophotometric analyses of simulated galaxies in B, z, R, I, Js, Ks, K filters, and compare their near-IR properties with observations at redshift z=1-3. We find that the simulated galaxies brighter than the magnitude limit of K_vega=20 mag have stellar masses Mstar >= 10^{11} Msun/h and a number density of a few \times 10^{-4} h^3 Mpc^{-3} at z~2, in good agreement with the observed number density in the K20 survey. Therefore, our hydrodynamic simulations do not exhibit the ``mass-scale problem''. The answer to the ``redness problem'' is less clear because of our poor knowledge of the amount of dust extinction in EROs and the uncertain fraction of star-forming EROs. However, our simulations can account for the observed comoving number density of ~ 1e-4 Mpc^{-3} at z=1-2 if we assume a uniform extinction of E(B-V)=0.4 for the entire population of simulated galaxies. The critical question of the existence of a significant population of old, passively evolving massive EROs should continue to be tested with future hydrodynamic simulations. Upcoming observations of the thermal emission of dust in 24 micron by the Spitzer Space Telescope will help to better estimate the dust content of EROs at z=1-3, and thus to further constrain the star formation history of the Universe, and theoretical models of galaxy formation. (abridged)

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arXiv2005

Very Large H II Regions Around Proto-Clusters of Galaxies During Reionization

The physical basis for the belief that the abundant old dwarf galaxies seen in present-day galaxy clusters formed before reionization of the universe is very compelling, because (1) the observed faint end slope of the galaxy luminosity function ascends from a shallower slope at brighter magnitudes and tracks that of dark matter halo mass function, and (2) that steep power-law slope is observed to extend all the way to galaxies of inferred velocity dispersion of ~10km/s. We then show that the number of ionizing photons emitted by these dwarf galaxies should be able to produce an H II region at least as large as each proto-cluster region, occupying at least 20% of the entire IGM volume at z >= 6. A more likely scenario is that our estimate of the ionizing photon production rate based on these dwarfs under-estimates the true rate from proto-clusters by a factor of ~5 for a variety of physically plausible reasons, in which case a fully self- consistent picture emerges. Specific implications include (1) Strong clustering of sources would collectively produce very large individual H II regions of sizes up to ~100Mpc before overlapping with others, having important implications for upcoming radio and CMB observations. There should be large spatial fluctuations of neutral fraction in the intergalactic medium up to cluster scales of ~100Mpc. We give an H II region size function. (2) The ionizing sources at z~6 are expected to be highly biased with a bias factor >=15. (3) Large H II regions would enable largely unattenuated transmission of flux of a significant number of sources prior to reionization. When more sensitive observations become available, one should expect to see clusters of fainter sources in the vicinity of known galaxies at z>6.

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arXiv2005

The Rise of Dwarfs and the Fall of Giants: Galaxy Formation Feedback Signatures in the Halo Satellite Luminosity Function

The observed luminosity function (LF) of satellite galaxies shows several interesting features that require a better understanding of gas-thermodynamic processes and feedback effects related to reionization and galaxy formation. In galaxy clusters, the abundance of dwarf galaxies is in good agreement with the expectation based on the subhalo mass function, whereas in galaxy groups, the relatively small abundance of dwarfs conflicts with theoretical expectations. In all halo systems, there is a dip in the abundance of galaxies with luminosities in the range ~ 2x10^8 L_sun to 10^10 L_sun, corresponding to subhalo mass scales between ~ 5x10^10 M_sun to few times 10^11 M_sun. Photoionization from reionization has been used to explain statistics of the dwarf population, with larger systems forming prior to, and smaller systems forming subsequent to, reionization. The observed dip in the LF is an imprint of small dwarf galaxies (<2x10^8 L_sun) that formed prior to reionization. The galactic winds powered by supernovae in these dwarf galaxies propagate energy and metals to large distances such that the intergalactic medium is uniformly enriched to a level of 10^-3 zsun, as observed in the low-redshift Ly-alpha forest. The associated energy related to this metallicity raises the intergalactic medium temperature and the Jeans mass to a range 10^10-10^11 M_sun at z ~ 3.4-6.0. Because the epoch of nonlinearity for halos in this mass range is at z > 3.4-4.4, their gas content, hence star formation, is greatly suppressed on average and leads to a dip in the observed LF at $z=0$. Larger halos (M > 10^11 M_sun), becoming nonlinear at z < 3.4-4.4, have masses topping the Jeans mass, where subhalo mass function based LF is again in agreement with observations.

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arXiv2005

Galactic Wind Effects on the Lyman-alpha Absorption in the Vicinity of Galaxies

We present predictions of Ly-alpha forest-galaxy correlations at z=3 from Eulerian simulations that include the effects of galactic winds, driven primarily by supernova explosions. Galactic winds produce expanding bubbles of shock-heated gas within ~0.5 comoving Mpc/h of luminous galaxies in the simulation, which have space density similar to that of observed Lyman break galaxies (LBGs). However, most of the low-density intergalactic gas that determines the observed properties of the Ly-alpha forest is unaffected by winds. The impact of winds on the Ly-alpha optical depth near galaxies is less dramatic than their impact on gas temperature because winds heat only a small fraction of the gas present in the turnaround regions surrounding galaxies. Hence, Ly-alpha absorption from gas outside the wind bubbles is spread out over the same velocity range occupied by the wind-heated gas. In general, Ly-alpha absorption is expected to be stronger than average near galaxies because of the high gas density. Winds result in a modest reduction of this expected increase of Ly-alpha absorption. Our predictions can be compared to future observations to detect the wind effects and infer their strength, although with the caveat that the results are still dependent on the correspondence of simulated galaxies and observed LBGs. We find that wind effects in our simulations are not strong enough to reproduce the high Ly-alpha transmission within 0.5 Mpc/h comoving of galaxies that has been suggested by recent observations; powerful galactic explosions or ejecta with hyper-escape velocities would be required, but these are unlikely to be produced by ordinary star formation and supernovae alone

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arXiv2005

Transition From Population III to Population II Stars

The transition from Population III to Population II stars is determined by the presence of a sufficient amount of metals, in particular, oxygen and carbon. The vastly different yields of these relevant metals between different initial stellar mass functions would then cause such a transition to occur at different times. We show that the transition from Pop III to Pop II stars is likely to occur before the universe can be reionized, if the IMF is entirely very massive stars (M > 140 M_sun). A factor of about 10 more ionizing photons would be produced in the case with normal top-heavy IMF (e.g., M ~ 10-100 M_sun), when such a transition occurs. Thus, a high Thomson optical depth (tau_e >= 0.11-0.14) may be indication that the Population III stars possess a more conventional top-heavy IMF.

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arXiv2004

Massive galaxies at redshift 2 in cosmological hydrodynamic simulations

We study the properties of galaxies at z=2 in a Lambda cold dark matter universe, using two different types of hydrodynamic simulation methods -- Eulerian TVD and smoothed particle hydrodynamics (SPH) -- and a spectrophotometric analysis in the U_n, G, R filter set. The simulated galaxies at z=2 satisfy the color-selection criteria proposed by Adelberger et al. (2004) and Steidel et al. (2004) when we assume Calzetti extinction with E(B-V)=0.15. We find that the number density of simulated galaxies brighter than R<25.5 at z=2 is about 1e-2 h^3 Mpc^-3 for E(B-V)=0.15, which is roughly twice that of the number density found by Erb et al. (2004) for the UV bright sample. This suggests that roughly half of the massive galaxies with M*>10^{10} Msun/h at z=2 are UV bright population, and the other half is bright in the infra-red wavelengths. The most massive galaxies at z=2 have stellar masses >= 10^{11-12} Msun. They typically have been continuously forming stars with a rate exceeding 30 Msun/yr over a few Gyrs from z=10 to z=2, together with significant contribution by starbursts reaching up to 1000 Msun/yr which lie on top of the continuous component. TVD simulations indicate a more sporadic star formation history than the SPH simulations. Our results do not imply that hierarchical galaxy formation fails to account for the observed massive galaxies at z>=1. The global star formation rate density in our simulations peaks at z>=5, a much higher redshift than predicted by the semianalytic models. This star formation history suggests early build-up of the stellar mass density, and predicts that 70 (50, 30)% of the total stellar mass at z=0 had already been formed by z=1 (2, 3). Upcoming observations by Spitzer and Swift might help to better constrain the star formation history at high redshift.

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arXiv2004

Constraining Reionization with the Evolution of the Luminosity Function of Lyman Alpha mitting Galaxies

At redshifts beyond z>6, as the mean fraction of neutral hydrogen x_HI in the intergalactic medium (IGM) increases, the line flux of Lyman alpha (Lya) emitters can be significantly suppressed, which can result in a decrease in the observed number of emitters at a given Lya flux. However, cosmological HII regions surrounding the Lya emitting galaxies alleviate these effects. We use simple models of the Lya line suppression that incorporate the presence of HII regions to predict the overall effect of the Lya absorption on the Lya luminosity function. We find, in agreement with other recent studies, that unless ionizing sources are unusually strongly clustered, a fully neutral IGM may be inconsistent with the large abundance of confirmed z=6.5 Lya emitters. However, the presence of local HII regions prohibits placing a tight constraint on the mean neutral fraction. We find x_HI < 0.25; the presence of strong winds and/or the clustering of ionizing sources would further weaken this constraint. We conclude that the evolution of the Lya LF is consistent with reionization occurring near this redshift. Finally, we suggest that a measurement of observed Lya line width as a function of the Lya luminosity, in a future, larger sample of Lya emitters, may serve as a robust diagnostic of the neutral fraction in the IGM.

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arXiv2004

Shock-Heated Gas in the Large Scale Structure of the Universe

Cosmological shock waves play a major role in heating baryonic gas in the large scale structure of the universe. In this contribution we study the shock-heated gas in connection with shocks themselves. The distributions of shock speed and temperature of shock-hated gas both should reflect the depth of gravitational potential of associated nonlinear structures, so their morphology. From their distributions we find, in addition to hot gas in and around clusters/groups and warm-hot intergalactic medium (WHIM) with T=10^5-10^7 K mostly in filaments, there is a significant amount of low temperature WHIM with T < 10^5 K distributed mostly as sheet-like structures. Not only the WHIM with T=10^5-10^7 K but also the WHIM with T < 10^5 K make up a significant fraction of gas mass, implying the low temperature WHIM could be important in resolving the missing baryon problem. The shock-heated gas in filaments and sheets are manifested best through emissions and absorptions in soft X-ray and far UV. We confirm that the WHIM with T=10^5-10^7 K makes significant contributions to the soft X-ray background, absorptions of highly ionized species such as OVII and OVIII in AGN spectra, and line emissions from OVII and OVIII ions, as pointed by previous studies. However, the WHIM with T < 10^5 K is the major contributor to absorptions of lower ionized species such as OV and OVI, because these photo-ionized ions are most abundant in sheets of low density and temperature. (abridged)

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arXiv2004

Massive galaxies in cosmological simulations: UV-selected sample at redshift z=2

We study the properties of galaxies at z=2 in a Lambda CDM universe, using two different types of hydrodynamic simulation methods (Eulerian TVD and SPH) and a spectrophotometric analysis in the Un, G, R filter set. The simulated galaxies at z=2 satisfy the color-selection criteria proposed by Adelberger et al. (2004) when we assume Calzetti extinction with E(B-V)=0.15. We find that the number density of simulated galaxies brighter than R<25.5 at z=2 is about 2e-2 h^3/Mpc^3, roughly one order of magnitude larger than that of Lyman break galaxies at z=3. The most massive galaxies at z=2 have stellar masses >~1e11 Msun, and their observed-frame G-R colors lie in the range 0.0 = 1.0 and Mstar>1e10 Msun/h finished the build-up of their stellar mass by z~3. Interestingly, our study suggests that the majority of the most massive galaxies at z=2 should be detectable at rest-frame UV wavelengths, contrary to some recent claims made on the basis of near-IR studies of galaxies at the same epoch, provided the median extinction is less than E(B-V)<0.3. However, our results also suggest that the fraction of stellar mass contained in galaxies that pass the color-selection criteria could be as low as 50% of the total stellar mass in the Universe at z=2. Our simulations suggest that the missing stellar mass is contained in fainter (R>25.5) and intrinsically redder galaxies. Our results do not suggest that hierarchical galaxy formation fails to account for the massive galaxies at z>=1. (abridged)

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arXiv2004

Signatures of Galactic Superwinds: Inhomogeneous Metal Enrichment of the Lyman Alpha Forest

We investigate possible signatures of feedback from galactic superwinds on the metallicity of the Lyman alpha forest, using a set of high resolution hydrodynamic simulations of a Lambda cold dark matter model. Simulations produce metals self-consistently, based on one single parameter, the metal yield, which in turn is constrained by metallicity in the intra-cluster gas. We follow metals as a separate density species. For the metallicity of Lyman alpha clouds with column density of N_{HI}~10^{14.5}-10^{15.5}/cm^2 at z=2-4 we find reasonable agreement between simulations, both with and without GSW, and observations (Schaye et al). A unique signature and sensitive test of GSW is, however, provided by lower density regions with gas density of ρ/<ρ> = 0.01-1.0 and a corresponding column density of 10^{12}-10^{14}/cm^2. Without GSW we predict that both the mean and median metallicity of \lya clouds in this column density range at z=2-4 should have Z<10^{3}Zsun, since these small systems support little star formation. GSW contaminate these regions, however, and also there is a significant fraction (~25%) of Lyman alpha clouds in this column density range which have a high metallicity excess of 10^{-2}Zsun, resulting in a mean metallicity of ~10^{-2}Zsun. In addition, we find that there is a minimum in the median metallicity for clouds of N_{HI}~10^{13}-10^{14}/cm^2 in the case with GSW, whereas without GSW the metallicity decreases monotonically and rapidly with decreasing column density. Finally, we predict that the ratio of secondary (e.g., N) to primary metals (e.g., O,C) is expected to be smaller by a factor of 10 in clouds of N_{HI}~10^{14.5}/cm^2 compared to that in large galaxies; this factor increases to >50 for N_{HI}< 10^{13.5}/cm^2.

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arXiv2004

Probing the Reionization History Using the Spectra of High-Redshift Sources

We quantify and discuss the footprints of neutral hydrogen in the intergalactic medium (IGM) on the spectra of high-redshift (z ~ 6) sources, using mock spectra generated from hydrodynamical simulations of the IGM. We show that it should be possible to extract relevant parameters, including the mean neutral fraction in the IGM, and the radius of the local cosmological Stromgren region, from the flux distribution in the observed spectra of distant sources. We focus on quasars, but a similar analysis is applicable to galaxies and gamma ray burst (GRB) afterglows. We explicitly include uncertainties in the spectral shape of the assumed source template near the Lyman alpha line. Our results suggest that a mean neutral hydrogen fraction, x(HI) of unity can be statistically distinguished from x(HI)<0.01, by combining the spectra of tens of bright (M = -27) quasars. Alternatively, the same distinction can be achieved using the spectra of several hundred sources that are ~100 times fainter. Furthermore, if the radius of the Stromgren sphere can be independently constrained to within ~10 percent, this distinction can be achieved using a single source. The information derived from such spectra will help in settling the current debate as to what extent the universe was reionized at redshifts near z=6.

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arXiv2004

A Two-Fluid Thermally-Stable Cooling Flow Model

A new model for cooling flows in X-ray clusters, capable of naturally explaining salient features observed, is proposed. The only requirement is that a significant relativistic component, in the form of cosmic rays (CR), be present in the intra-cluster medium and significantly frozen to the thermal gas. Such an addition qualitatively alters the conventional isobaric thermal instability criterion such that a fluid parcel becomes thermally stable when its thermal pressure drops below a threshold fraction of its CR pressure. Consequently, the lowest possible temperature at any radius is about one third of the ambient temperature {\it at that radius}, exactly as observed, In addition, we suggest that dissipation of internal gravity waves, excited by radial oscillatory motions of inward drifting cooling clouds about their radial equilibrium positions, may be responsible for heating up cooling gas. With the ultimate energy source for powering the cooling X-ray luminosity and heating up cooling gas being gravitational due to inward drifting cooling clouds as well as the general inward flow, heating is spatially distributed and energetically matched with cooling. One desirable property of this heating mechanism is that heating energy is strongly centrally concentrated, providing the required heating for emission-line nebulae.

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arXiv2004

Is There a Missing Galaxy Problem at High Redshift?

We study the evolution of the global stellar mass density in a Lambda cold dark matter universe using two different types of hydrodynamical simulations (Eulerian TVD and SPH) and the analytical model of Hernquist & Springel (2003). We find that the theoretical calculations all predict both a higher stellar mass density at z~3 than indicated by current observations, and that the peak of the cosmic star formation rate history should lie at z~5. Such a star formation history implies that as much as (70%, 30%) of the total stellar mass density today must already have formed by z=(1, 3). Our results suggest that current observations at z~3 are missing as much as 50% of the total stellar mass density in the Universe, perhaps owing to an inadequate allowance for dust obscuration in star-forming galaxies, limited sample sizes, or cosmic variance. We also compare our results with some of the updated semi-analytic models of galaxy formation.

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arXiv2004

Implications of the Lyman alpha Emission Line from a Candidate z=10 Galaxy

The recently discovered z=10 galaxy (Pello et al 2004) has a strong Lyman alpha emission line that is consistent with being surprisingly symmetric, even given the relatively poor quality of its spectrum. The blue wing of a Lyman alpha line originating at high redshift should be strongly suppressed by resonant hydrogen absorption along the line of sight, an expectation borne out by the observed asymmetric shapes of the existing sample of Lyman alpha emitting sources at lower redshifts (3< z < 6.7). Absorption on the blue side of the line of the Pello et al. source could be reduced if the intergalactic medium (IGM) in the vicinity of the galaxy is highly ionized, but we show that this requires an unrealistically high ionizing emissivity. We suggest instead that the Lyman alpha emitting gas be receding relative to the surrounding gas with a velocity of >= 35km/s, a large velocity that is plausible only if the galaxy is part of a larger system (group of galaxies) with a velocity dispersion >= 35km/s. We find that with this velocity shift, the observed strength and shape of the line is still consistent with the galaxy being surrounded by its own Stromgren sphere embedded in a fully neutral IGM. More generally, we predict that at any given redshift, the bright Lyman alpha emitters with broader lines would exhibit stronger asymmetry than fainter ones. Bright galaxies with symmetric Lyman alpha lines may be signposts for groups and clusters of galaxies, within which they can acquire random velocities comparable to or larger than their linewidths.

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arXiv2004

Properties of Cold Dark Matter Halos at z>6

We compute the properties of dark matter halos with mass $10^{6.5}-10^9\msun$ at redshift $z=6-11$ in the standard cold dark matter cosmological model, utilizing a very high resolution N-body simulation. We find that dark matter halos in these mass and redshift ranges are significantly biased over matter with a bias factor in the range 2-6. The dark matter halo mass function displays a slope of $2.05\pm 0.15$ at the small mass end. We do not find a universal dark matter density profile. Instead, we find a significant dependence of the central density profile of dark matter halos on halo mass and epoch with $α_0=0.4-1.0$; the high-mass ($M\ge 10^8\msun$) low-redshift ($z\sim 6$) halos occupy the high end of the range and low-mass ($M\sim 10^{7}\msun$) high-redshift ($z\sim 11$) halos occupy the low end. Additionally, for fixed mass and epoch there is a significant dispersion in $α_0$ due to the stochastic assembly of halos. Our results fit a relationship of the form $α_0=0.75((1+z)/7.0)^{-1.25}(M/10^7\msun)^{0.11(1+z)/7.0}$ with a dispersion about this fit of $\pm 0.5$ and no systematic dependence of variance correlated with environment. The median spin parameter of dark matter halos is $0.03-0.04$ but with a large lognormal dispersion of $\sim 0.4$. Various quantities are tabulated or fitted with empirical formulae.

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arXiv2003

Dependence of Star Formation Rate on Overdensity

We use a large-scale Lambda-CDM hydrodynamical simulation to assess the dependence of the cosmic Star Formation Rate (SFR) on overdensity of luminosity. The `morphology-density relation' is qualitatively captured when the SFR is plotted as a function of redshift, divided into the quartiles of the overdensity distribution.

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arXiv2003

Formation of First Stars Triggered by Collisions and Shockwaves: Prospect for High Star Formation Efficiency and High Ionizing Photon Escape Fraction

We show that large, high-redshift (z>10) galaxies with virial temperature in excess of 10^4K may be mostly comprised of cold atomic clouds which were formerly minihalos. These clouds move at a speed of 15-30km/s and collide with one another on a time scale of 10^7yr. The supersonic collisions result in spatially distributed star formation with high efficiency. We then show that most, subsequent star formation in cold atomic clouds may be triggered by shockwaves launched from the first stars formed in collisions. Those shockwave-compressed clouds are even more widespread spatially because of large imparted velocities and some can possibly escape into the intergalactic medium. More importantly, with respect to cosmological reionization, widespread star formation would allow a much higher ionizing photon escape fraction. These favorable conditions may form the physical basis to enable the standard cosmological model to produce a reasonably high Thomson optical depth τ_e=0.10-0.14. A chain reaction of star formation in minihalos in the intergalactic space may be triggered by explosions in the intergalactic medium, if minihalos are strongly clustered. In this case, a still higher τ_e would be achievable.

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