arXiv subjects

Neal Katz

Publications and source records attributed to Neal Katz.

At least 109 records · Page 6Linked to original sources

arXiv2003

The Optical and Near-Infrared Properties of Galaxies: I. Luminosity and Stellar Mass Functions

We use a large sample of galaxies from the Two Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) to calculate galaxy luminosity and stellar mass functions in the local Universe. We estimate k-corrections, evolution corrections, and stellar mass-to-light ratios (M/Ls) by fitting the galaxy colors with simple models. Our optical and near-infrared luminosity functions agree with most recent literature optical and near-infrared determinations within the uncertainties. We argue that 2MASS is biased against low surface brightness galaxies, and use SDSS plus our knowledge of stellar populations to estimate that the true K-band luminosity function has a steeper faint end slope than the direct estimate. Assuming a universally-applicable stellar initial mass function (IMF), we derive the stellar mass function of galaxies. The faint end slope slope for the stellar mass function is steeper than -1.1, reflecting the low stellar M/Ls characteristic of low-mass galaxies. We estimate an upper limit to the stellar mass density in the local Universe Omega* h = 0.0020+/-0.0006 by assuming an IMF as rich in low-mass stars as allowed by observations of galaxy dynamics. Finally, we find that the characteristic luminosity or mass of early-type galaxies is larger than for later types, and the faint end slope is steeper for later types than for earlier types. Accounting for typing uncertainties, we estimate that at least half, and perhaps as much as 3/4, of the stellar mass in the Universe is in early-type galaxies. We present also SDSS/2MASS color-M/L correlations, an updated discussion of near-infrared stellar M/L estimates, and the volume-corrected distribution of g and K-band stellar M/Ls as a function of stellar mass. [Abridged]

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arXiv2003

Bar-Induced Evolution of Dark Matter Cusps

The evolution of a stellar bar transforms not only the galactic disk, but also the host dark matter halo. We present high resolution, fully self-consistent N-body simulations that clearly demonstrate that dark matter halo central density cusps flatten as the bar torques the halo. This effect is independent of the bar formation mode and occurs even for rather short bars. The halo and bar evolution is mediated by resonant interactions between orbits in the halo and the bar pattern speed, as predicted by linear Hamiltonian perturbation theory. The bar lengthens and slows as it loses angular momentum, a process that occurs even in rather warm disks. We demonstrate that the bar and halo response can be critically underestimated for experiments that are unable to resolve the relevant resonant dynamics; this occurs when the phase space in the resonant region is under sampled or plagued by noise.

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arXiv2003

Lyman Break Galaxies and the Lyman-alpha Forest

We use hydrodynamic simulations to predict correlations between Lya forest absorption and galaxies at redshift z~3. The probability distribution function (PDF) of Lya flux decrements shifts systematically towards higher values in the vicinity of galaxies, reflecting the overdense environments in which these galaxies reside. The predicted signal remains strong in spectra smoothed over 50-200 km/s, allowing tests with moderate resolution quasar spectra. The strong bias of high redshift galaxies towards high density regions imprints a clear signature on the flux PDF, but the predictions are not sensitive to galaxy baryon mass or star formation rate, and they are similar for galaxies and for dark matter halos. The dependence of the flux PDF on galaxy proximity is sensitive to redshift determination errors, with rms errors of 150-300 km/s substantially weakening the predicted trends. On larger scales, the mean galaxy overdensity in a cube of 5 or 10 Mpc/h (comoving) is strongly correlated with the mean Lya flux decrement on a line of sight through the cube center. The slope of the correlation is ~3 times steeper for galaxies than for dark matter as a result of galaxy bias. The predicted large scale correlation is in qualitative agreement with recently reported observational results. However, observations also show a drop in absorption in the immediate vicinity of galaxies, which our models do not predict even if we allow the galaxies or AGNs within them to be ionizing sources. This decreased absorption could be a signature of galaxy feedback on the surrounding IGM, perhaps via galactic winds. Peculiar velocities often allow gas at comoving distances ~1.5 Mpc/h to produce saturated absorption at the galaxy redshift, so any feedback mechanism must suppress neutral hydrogen out to these radii to match the data. (Abridged)

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arXiv2003

X-ray Absorption by the Low-redshift Intergalactic Medium: A Numerical Study of the Lambda CDM model

Using a hydrodynamic simulation of a LCDM universe, we investigate the "X-ray forest" absorption imprinted on the spectra of background quasars by the intervening intergalactic medium (IGM). In agreement with previous studies, we find that OVII and OVIII produce the strongest absorption features. The strong oxygen absorbers that might be detectable with Chandra or XMM-Newton arise in gas with T ~ 10^6 K and overdensities delta >~ 100 that are characteristic of galaxy groups. Future X-ray missions could detect weaker oxygen absorption produced by gas with a wider range of temperatures and the lower densities of unvirialized structures; they could also detect X-ray forest absorption by C, N, Ne, Fe, and possibly Si. If the IGM metallicity is 0.1 solar, then the predicted number of systems strong enough for a ~5σdetection with Chandra or XMM-Newton is extremely low, though scatter in metallicity would increase the number of strong absorbers even if the mean metallicity remained the same. Our simulation reproduces the high observed incidence of OVI absorbers (in the UV), and the most promising strategy for finding the X-ray forest is to search at the redshifts of known OVI systems, thus reducing the signal-to-noise threshold required for a significant detection. However, while many OVI absorbers have associated OVII or OVIII absorption, the OVI systems trace only the low temperature phases of the X-ray forest, and a full accounting of the strong OVII and OVIII systems will require a mission with the anticipated capabilities of Constellation-X. The large effective area of the XEUS satellite would make it an extremely powerful instrument for studying the IGM, measuring X-ray forest absorption by a variety of elements and revealing the shock-heated filaments that may be an important reservoir of cosmic baryons.

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arXiv2003

The Halo Occupation Distribution and the Physics of Galaxy Formation

The halo occupation distribution (HOD) describes the bias between galaxies and dark matter by specifying (a) the probability P(N|M) that a halo of virial mass M contains N galaxies of a particular class and (b) the relative distributions of galaxies and dark matter within halos. We calculate predicted HODs for a Lambda-CDM cosmological model using an SPH hydrodynamic simulation and a semi-analytic (SA) galaxy formation model. Although the two methods predict different galaxy mass functions, their HOD predictions agree remarkably well. For mass-selected samples, the mean occupation exhibits a sharp cutoff at low halo masses, a slowly rising plateau for ~1-2, and a more steeply rising high occupancy regime. At low , the mean pair and triple counts are well below Poisson expectations, with important consequences for small scale behavior of 2- and 3-point correlation functions. The HOD depends strongly on galaxy age, with high mass halos populated mainly by old galaxies and low mass halos by young galaxies. The SPH simulation supports several simplifying assumptions about HOD bias: the most massive galaxy in a halo usually lies close to the center and moves near the halo's mean velocity; satellite galaxies have the same radial profile and velocity dispersion as the dark matter; and the mean occupation at fixed halo mass is independent of the halo's larger scale environment. By applying the SPH and SA HODs to a large volume N-body simulation, we show that both methods predict slight, observable departures from a power-law galaxy correlation function. The predicted HODs are closely tied to the underlying galaxy formation physics, they offer useful guidance to theoretical models of galaxy clustering, and they will be tested empirically by ongoing analyses of galaxy redshift surveys. (Shortened)

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arXiv2003

The Clustering Dipole of the Local Universe from the Two Micron All Sky Survey

The unprecedented sky coverage and photometric uniformity of the Two Micron All Sky Survey (2MASS) provides a rich resource for investigating the galaxies populating the local Universe. A full characterization of the large-scale clustering distribution is important for theoretical studies of structure formation. 2MASS offers an all-sky view of the local galaxy population at 2.15 micron, unbiased by young stellar light and minimally affected by dust. We use 2MASS to map the local distribution of galaxies, identifying the largest structures in the nearby universe. The inhomogeneity of these structures causes an acceleration on the Local Group of galaxies, which can be seen in the dipole of the Cosmic Microwave Background (CMB). We find that the direction of the 2MASS clustering dipole is 11 degrees from the CMB dipole, confirming that the local galaxy distribution accelerates the Local Group. From the magnitude of the dipole we find a value of the linear bias parameter b=1.37 +/- 0.3 in the K_s-band. The 2MASS clustering dipole is 19 degrees from the latest measurement of the dipole using galaxies detected by the Infrared Astronomical Satellite (IRAS) suggesting that bias may be non-linear in some wavebands.

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arXiv2003

On the Relationship between Cooling Flows and Bubbles

A common feature of the X-ray bubbles observed in Chandra images of some "cooling flow" clusters is that they appear to be surrounded by bright, cool shells. Temperature maps of a few nearby luminous clusters reveal that the shells consist of the coolest gas in the clusters -- much cooler than the surrounding medium. Using simple models, we study the effects of this cool emission on the inferred cooling flow properties of clusters. We find that the introduction of bubbles into model clusters that do NOT have cooling flows results in temperature and surface brightness profiles that resemble those seen in nearby "cooling flow" clusters. They also approximately reproduce the recent XMM-Newton and Chandra observations of a high minimum temperature of 1-3 keV. Hence, bubbles, if present, must be taken into account when inferring the physical properties of the ICM. In the case of some clusters, bubbles may account entirely for these observed features, calling into question their designation as clusters with cooling flows. However, since not all nearby "cooling flow" clusters show bubble-like features, we suggest that there may be a diverse range of physical phenomena that give rise to the same observed features.

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arXiv2003

A First Estimate of the Baryonic Mass Function of Galaxies

We estimate the baryonic (stellar+cold gas) mass function of galaxies in the local Universe by assigning a complete sample of Two Micron All Sky Survey and Sloan Digital Sky Survey galaxies a gas fraction based on a statistical sample of the entire population, under the assumption of a universally-applicable stellar initial mass function. The baryonic mass function is similar to the stellar mass function at the high mass end, and has a reasonably steep faint-end slope owing to the typically high cold gas fractions and low stellar mass-to-light ratios characteristic of low-mass galaxies. The Schechter Function fit parameters are phi* = 0.0108(6) per cubic Mpc per dex of mass, M* = 5.3(3)x10^10 solar masses, and alpha = -1.21(5), with formal error estimates given in parentheses (for a Hubble constant of 100 km/s per Mpc). We show that the neutral and molecular hydrogen mass functions derived using this indirect route are in agreement with direct estimates, validating our indirect method. Integrating under the baryonic mass function and incorporating all sources of uncertainty, we find that the baryonic (stellar+cold gas) mass density implied by this estimate is Omega_cold baryon = 0.0024+0.0007-0.0014, or 8% +4% -5% of the Big Bang nucleosynthesis expectation.

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arXiv2003

The Lyman-alpha Forest as a Cosmological Tool

We review recent developments in the theory of the Lyman-alpha forest and their implications for the role of the forest as a test of cosmological models. Simulations predict a relatively tight correlation between the local Lya optical depth and the local gas or dark matter density. Statistical properties of the transmitted flux can constrain the amplitude and shape of the matter power spectrum at high redshift, test the assumption of Gaussian initial conditions, and probe the evolution of dark energy by measuring the Hubble parameter H(z). Simulations predict increased Lya absorption in the vicinity of galaxies, but observations show a Lya deficit within Delta_r ~ 0.5 Mpc/h (comoving). We investigate idealized models of "winds" and find that they must eliminate neutral hydrogen out to comoving radii ~1.5 Mpc/h to marginally explain the data. Winds of this magnitude suppress the flux power spectrum by \~0.1 dex but have little effect on the distribution function or threshold crossing frequency. In light of the stringent demands on winds, we consider the alternative possibility that extended Lya emission from target galaxies replaces absorbed flux, but we conclude that this explanation is unlikely. Taking full advantage of the data coming from large telescopes and from the Sloan Digital Sky Survey will require more complete understanding of the galaxy proximity effect, careful attention to continuum determination, and more accurate numerical predictions, with the goal of reaching 5-10% precision on key cosmological quantities.

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arXiv2002

Lyman Alpha Absorber Correlations and the Bias of the Lyman Alpha Forest

Lyman alpha absorber correlations contain information about the underlying density distribution associated with a particular class of absorbers. As such, they provide an opportunity to independently measure the "bias" of the Lyman alpha forest, i.e. the relationship between HI column density and underlying dark matter density. In these proceedings we use hydrodynamic simulations to investigate whether the evolution of this bias is measurable from observable correlations. Unfortunately, the increasingly complex physics in the IGM at z<1 makes a direct measurement of the bias difficult. Nevertheless, current simulations do make predictions for HI absorber correlations that are in broad agreement with observations at both high and low redshift, thus reinforcing the bias evolution predictions given by these models.

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arXiv2002

The Galaxy Proximity Effect in the Lyman-alpha Forest

Hydrodynamic cosmological simulations predict that the average opacity of the Ly-alpha forest should increase in the neighborhood of galaxies because galaxies form in dense environments. Recent observations (Adelberger et al. 2002) confirm this expectation at large scales, but they show a decrease of absorption at comoving separations Delta_r <~ 1 Mpc/h. We show that this discrepancy is statistically significant, especially for the innermost data point at Delta_r <= 0.5 Mpc/h, even though this data point rests on three galaxy-quasar pairs. Galaxy redshift errors of the expected magnitude are insufficient to resolve the conflict. Peculiar velocities allow gas at comoving distances >~ 1 Mpc/h to produce saturated absorption at the galaxy redshift, putting stringent requirements on any ``feedback'' solution. Local photoionization is insufficient, even if we allow for recurrent AGN activity that keeps the neutral hydrogen fraction below its equilibrium value. A simple ``wind'' model that eliminates all neutral hydrogen in spheres around the observed galaxies can marginally explain the data, but only if the winds extend to comoving radii ~1.5 Mpc/h.

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arXiv2002

Galaxy Clustering and Galaxy Bias in a Lambda-CDM Universe

We investigate galaxy clustering and galaxy-mass correlations in the LCDM cosmological model using a large volume SPH simulation. For the most part, the predicted biases between galaxies and dark matter lead to good agreement with current observations, including: (1) a nearly constant comoving correlation length from z=3 to z=0 for mass-selected galaxy samples of constant comoving space density; (2) an rms bias factor b~1 at z=0; (3) a scale-dependent bias on small scales that transforms the curved dark matter correlation function into a nearly power-law galaxy correlation function; (4) galaxy pairwise dispersion and hierarchical skewness ratio S_3 in good agreement with observed values, and lower than values for the dark matter by ~20%; (5) a ratio of galaxy-galaxy to galaxy-mass correlation functions consistent with recent measurements from the RCS survey; and (6) a mean excess mass dM(260/h kpc) approximately proportional to galaxy baryon mass M_b, in agreement with lensing estimates from the SDSS. These clustering properties vary with galaxy baryon mass and, more strongly, with stellar population age, in good qualitative agreement with the observed dependence of galaxy clustering and galaxy-mass correlations on galaxy type. The predicted ratio dM(260)/M_b is lower than the SDSS estimates for galaxies with M_b>~2e11 Msun, but numerical resolution effects account for most of this discrepancy, leaving little room for feedback or other astrophysical processes to reduce the stellar masses of luminous galaxies, at least given our adopted cosmological parameters. Our results show that the LCDM model and the galaxy formation physics incorporated in the SPH simulation give a good account of observed galaxy clustering, but anticipated improvements in clustering and weak lensing measurements will soon test this picture in much more detail.

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arXiv2002

How Do Galaxies Get Their Gas?

Not the way one might have thought. In hydrodynamic simulations of galaxy formation, some gas follows the traditionally envisioned route, shock heating to the halo virial temperature before cooling to the much lower temperature of the neutral ISM. But most gas enters galaxies without ever heating close to the virial temperature, gaining thermal energy from weak shocks and adiabatic compression, and radiating it just as quickly. This ``cold mode'' accretion is channeled along filaments, while the conventional, ``hot mode'' accretion is quasi-spherical. Cold mode accretion dominates high redshift growth by a substantial factor, while at z<1 the overall accretion rate declines and hot mode accretion has greater relative importance. The decline of the cosmic star formation rate at low z is driven largely by geometry, as the typical cross section of filaments begins to exceed that of the galaxies at their intersections.

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arXiv2002

Structure of Bright 2MASS Galaxies: 2D Fits to the Ks-band Surface Brightness Profiles

The unprecedented sky coverage and photometric uniformity of the Two Micron All Sky Survey (2MASS) provides a rich resource for obtaining a detailed understanding of the galaxies populating our local (z<0.1) Universe. A full characterization of the physical structure of nearby galaxies is essential for theoretical and observational studies of galaxy evolution and structure formation. We have begun a quantified description of the internal structure and morphology of 10,000 bright (10<Ks<11) 2MASS galaxies through multi-component model fits to the 2D surface brightness profiles using GIM2D. From our initial Monte Carlo tests on 77 galaxies drawn at random from the RC3, we find that the model derived structural parameter errors due to sky uncertainies are typically less than 10%.

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arXiv2002

Towards a Precise Measurement of Matter Clustering: Lyman-alpha Forest Data at Redshifts 2-4

We measure the filling factor, correlation function, and power spectrum of transmitted flux in a large sample of Lya forest spectra, comprised of 30 Keck HIRES spectra and 23 Keck LRIS spectra. We infer the linear matter power spectrum P(k) from the flux power spectrum P_F(k), using an improved version of the method of Croft et al. (1998) that accounts for the influence of z-space distortions, non- linearity, and thermal broadening on P_F(k). The evolution of the shape and amplitude of P(k) over the range z= 2-4 is consistent with gravitational instability, implying that non-gravitational fluctuations do not make a large contribution. Our fiducial measurement of P(k) comes from data with = 2.72. It has amplitude Delta^2(k_p)=0.74^0.20_-0.16 at wavenumber k_p=0.03 (km/s)^-1 and is well described by a power-law of index -2.43 +/- 0.06 or by a CDM-like power spectrum with shape parameter Gamma'=1.3^+0.7_-0.5*10^-3 (km/s) at z=2.72. For Omega_m=0.4, Omega_Lam=0.6, the best-fit Gamma =0.16 (h^-1mpc)^-1, in good agreement with the 2dF Galaxy Redshift Survey, and the best-fit sigma_8=0.82 (Gamma/0.15)^-0.44. Matching the observed cluster mass function and our Delta^2(k_p) in spatially flat models requires Omega_m=0.38^+0.10_-0.08 + 2.2 (Gamma-0.15). Matching Delta^2(k_p) in COBE-normalized, flat CDM models with no tensor fluctuations requires Omega_m = (0.29 +/-0.04) n^-2.89 h_65^-1.9. The Lya forest complements other probes of P(k) by constraining a regime of redshift and lengthscale not accessible by other means, and the consistency of these inferred parameters with independent estimates provides further support for inflation, cold dark matter, and vacuum energy (abridged).

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arXiv2002

X-Ray Scaling Relations of Galaxy Groups in a Hydrodynamic Cosmological Simulation

We examine the scalings of X-ray luminosity, temperature, and dark matter or galaxy velocity dispersion for galaxy groups in an LCDM cosmological simulation, which incorporates gravity, gas dynamics, radiative cooling, and star formation, but no substantial non-gravitational heating. In agreement with observations, the simulated L_X-sigma and L_X-T_X relations are steeper than those predicted by adiabatic simulations or self-similar models, with L_X\propto sigma^{4.4} and L_X\propto T_X^{2.6} for massive groups and significantly steeper relations below a break at sigma~180 km/s (T_X~0.7 keV), but T_X-sigma is fairly close to self-similar, with T_X\propto sigma^{1.75}. The entropy of hot gas in low mass groups is higher than predicted by self-similar scaling, and agrees with observations that suggest an "entropy floor". The steeper scalings of the luminosity relations are driven by radiative cooling, which reduces the hot gas fraction from 50% of the total baryons at sigma \approx 500 km/s to 20% at sigma \approx 100 km/s. A secondary effect is that hot gas in smaller systems is less clumpy, further driving down L_X. A smaller volume simulation with eight times higher mass resolution predicts nearly identical X-ray luminosities at a given group mass, demonstrating the insensitivity of the predicted scaling relations to numerical resolution. There remain some quantitative discrepancies: the predicted mass scale of the L_X-T_X and L_X-sigma breaks is somewhat too low, and the luminosity-weighted temperatures are too high at a given sigma. We conclude that radiative cooling has an important quantitative impact on group X-ray properties and can account for many of the observed trends that have been interpreted as evidence for non-gravitational heating. (abridged)

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arXiv2001

Bar-driven dark halo evolution: a resolution of the cusp--core controversy

Simulations predict that the dark matter halos of galaxies should have central cusps, while those inferred from observed galaxies do not have cusps. We demonstrate, using both linear perturbation theory and n-body simulations, that a disk bar, which should be ubiquitous in forming galaxies, can produce cores in cuspy CDM dark matter profiles within five bar orbital times. Simulations of forming galaxies suggest that one of Milky Way size could have a 10 kpc primordial bar; this bar will remove the cusp out to approximately 5 kpc in approximately 1.5 gigayears, while the disk only loses approximately 8% of its original angular momentum. An inner Lindblad-like resonance couples the rotating bar to orbits at all radii through the cusp, transferring the bar pattern angular momentum to the dark matter cusp, rapidly flattening it. This resonance disappears for profiles with cores and is responsible for a qualitative difference in bar driven halo evolution with and without a cusp. This bar induced evolution will have a profound effect on the structure and evolution of almost all galaxies. Hence, both to understand galaxy formation and evolution and to make predictions from theory it is necessary to resolve these dynamical processes. Unfortunately, correctly resolving these important dynamical processes in ab initio calculations of galaxy formation is a daunting task, requiring at least 4,000,000 halo particles using our SCF code, and probably requiring many times more particles when using noisier tree, direct summation, or grid based techniques, the usual methods employed in such calculations.

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arXiv2001

Group Scaling Relations From a Cosmological Hydrodynamic Simulation: No Pre-heating Required?

We investigate the X-ray vs. optical scaling relations of poor groups to small clusters (sigma~100-700 km/s) identified in a cosmological hydrodynamic simulation of a Lambda-CDM universe, with cooling and star formation but no pre-heating. We find that the scaling relations between X-ray luminosity, X-ray temperature, and velocity dispersion show significant departures from the relations predicted by simple hydrostatic equilibrium models or simulations without cooling, having steeper L_X-sigma and L_X-T_X slopes and a "break" at \~200 km/s (~0.3 keV). These departures arise because the hot (X-ray emitting) gas fraction varies substantially with halo mass in this regime. Our predictions roughly agree with observations. Thus radiative cooling is a critical physical process in modeling galaxy groups, and may present an alternative to ad hoc models such as pre-heating or entropy floors for explaining X-ray group scaling relations.

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