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L. Hernquist

Publications and source records attributed to L. Hernquist.

At least 37 records · Page 2Linked to original sources
arXiv2009

A New Calculation of the Ionizing Background Spectrum and the Effects of HeII Reionization

The ionizing background determines the ionization balance and the thermodynamics of the cosmic gas. It is therefore a fundamental ingredient to theoretical and empirical studies of both the IGM and galaxy formation. We present here a new calculation of its spectrum that satisfies the empirical constraints we recently obtained by combining state-of-the-art luminosity functions and intergalactic opacity measurements. In our preferred model, star-forming galaxies and quasars each contribute substantially to the HI ionizing field at z<3, with galaxies rapidly overtaking quasars at higher redshifts as quasars become rarer. In addition to our fiducial model, we explore the physical dependences of the calculated background and clarify how recombination emission contributes to the ionization rates. We find that recombinations do not simply boost the ionization rates by the number of reemitted ionizing photons as many of these rapidly redshift below the ionization edges and have a distribution of energies. A simple analytic model that captures the main effects seen in our numerical radiative transfer calculations is given. Finally, we discuss the effects of HeII reionization by quasars on both the spectrum of the ionizing background and on the thermal history of the IGM. In regions that have yet to be reionized, the spectrum is expected to be almost completely suppressed immediately above 54.4 eV while a background of higher-energy (>~0.5 keV) photons permeates the entire universe owing to the frequency-dependence of the photoionization cross section. We provide an analytic model of the heat input during HeII reionization and its effects on the temperature-density relation.

astro-ph.CO↗
arXiv2009

The merger-driven evolution of warm infrared luminous galaxies

We present a merger-driven evolutionary model for the production of luminous (LIRGs) and ultraluminous infrared galaxies (ULIRGs) with warm IR colours. Our results show that simulations of gas--rich major mergers including star formation, black hole growth, and feedback can produce warm (U)LIRGs. We also find that while the warm evolutionary phase is associated with increased AGN activity, star formation alone may be sufficient to produce warm IR colours. However, the transition can be suppressed entirely - even when there is a significant AGN contribution - when we assume a single-phase ISM, which maximizes the attenuation. Finally, our evolutionary models are consistent with the 25-to-60 micron flux density ratio vs. L_HX/L_IR relation for local LIRGs and ULIRGs, and predict the observed scatter in IR color at fixed L_HX/L_IR. Therefore, our models suggest a cautionary note in the interpretation of warm IR colours: while associated with periods of active black hole growth, they are probably produced by a complex mix of star formation and AGN activity intermediate between the cold star formation dominated phase and the birth of a bright, unobscured quasar.

astro-ph.CO↗
arXiv2008

Evolution of the Intergalactic Opacity: Implications for the Ionizing Background, Cosmic Star Formation, and Quasar Activity

We investigate the implications of the intergalactic opacity for the evolution of the cosmic UV luminosity density and its sources. Our main constraint is our measurement of the Lya forest opacity at redshifts 2 ~3. Combined with direct measurements of the galaxy UV luminosity function, this requires only a small fraction f_esc~0.5% of galactic hydrogen ionizing photons to escape their source for galaxies to solely account for the entire ionizing background. Under the assumption that the galactic UV emissivity traces the star formation rate, current state-of-the-art observational estimates of the star formation rate density appear to underestimate the total photoionization rate at z~4 by a factor ~4, are in tension with recent determinations of the UV luminosity function, and fail to reionize the Universe by z~6 if extrapolated to arbitrarily high redshift. A theoretical star formation history peaking earlier fits the Lya forest photoionization rate well, reionizes the Universe in time, and is in better agreement with the rate of z~4 gamma-ray bursts observed by Swift. Quasars suffice to doubly ionize helium by z~3 and likely contribute a non-negligible and perhaps dominant fraction of the hydrogen ionizing background at their z~2 peak. [Abridged]

astro-ph↗
arXiv2008

A Direct Precision Measurement of the Intergalactic Lyman-alpha Opacity at 2<z<4.2

We directly measure the evolution of the intergalactic Lya effective optical depth, tau_eff, over the redshift range 2 ~3 based on directly fitting the quasar continua in the Lya forest, which have generally neglected this effect, are therefore likely biased low. We provide estimates of the level of absorption arising from metals in the Lya forest based on both direct and statistical metal removal results in the literature, finding that this contribution is ~6-9% at z=3 and decreases monotonically with redshift. The high precision of our measurement, attaining 3% in redshift bins of width Delta z=0.2 around z=3, indicates significant departures from the best-fit power-law redshift evolution (tau_eff=0.0018(1+z)^3.92, when metals are left in), particularly near z=3.2. The observed downward departure is statistically consistent with a similar feature detected in a precision statistical measurement using Sloan Digital Sky Survey spectra by Bernardi and coworkers using an independent approach.

astro-ph↗
arXiv2008

A Flat Photoionization Rate at 2<z<4.2: Evidence for a Stellar-Dominated UV Background and Against a Decline of Cosmic Star Formation Beyond z~3

We investigate the implications of our measurement of the Lyman-alpha forest opacity at redshifts 2 ~3. Our measurement argues against a star formation rate density declining beyond z~3, in contrast with existing state-of-the-art determinations of the cosmic star formation history from direct galaxy counts. Stellar emission from galaxies therefore likely reionized the Universe.

astro-ph↗
arXiv2008

Constraining the quasar population with the broad-line width distribution

We measure the width of the MgII $\lambda2799$ line in quasar spectra from the SDSS, 2QZ and 2SLAQ surveys and, by invoking an unnormalised virial mass estimator, relate the scatter in line width to the scatter in mass in the underlying black hole population. We find conclusive evidence for a trend such that there is less scatter in line width, and hence black hole mass, in more luminous objects. However, the most luminous objects in our sample show such a low degree of scatter in line width that, when combined with measures for the intrinsic scatter in the radius-luminosity relation for the broad-line region in active galaxies, an inconsistency arises in the virial technique for estimating black hole masses. This analysis implies that, at least for the most luminous quasars, either there is little-to-no intrinsic scatter in the radius-luminosity relation or the MgII broad emission line region is not totally dominated by virial velocities. Finally we exploit the measured scatter in line widths to constrain models for the velocity field of the broad-line region. We show that the lack of scatter in broad line-widths for luminous quasars is inconsistent with a pure planar/disk-like geometry for the broad-line region... (abridged)

astro-ph↗
arXiv2008

Numerical Simulations Unravel the Cosmic Web

The universe is permeated by a network of filaments, sheets, and knots collectively forming a "cosmic web.'' The discovery of the cosmic web, especially through its signature of absorption of light from distant sources by neutral hydrogen in the intergalactic medium, exemplifies the interplay between theory and experiment that drives science, and is one of the great examples in which numerical simulations have played a key and decisive role. We recount the milestones in our understanding of cosmic structure, summarize its impact on astronomy, cosmology, and physics, and look ahead by outlining the challenges faced as we prepare to probe the cosmic web at new wavelengths.

astro-ph↗
arXiv2007

A Century of Cosmology: A Direct Precision Measurement of the Intergalactic Lyman-alpha Opacity at 2<z<4.2

We directly measure the evolution of the intergalactic Lyman-alpha effective optical depth, tau_eff, over the redshift range 2 is <1% at z=2, 4% at z=3, and 12% at z=4. We provide estimates of the level of absorption arising from metals in the Ly-alpha forest based on both direct and statistical metal removal results in the literature, finding that this contribution is ~6-9% at z=3 and decreases monotonically with redshift. The high precision of our measurement indicates significant departures from the best-fit power-law redshift evolution, particularly near z=3.2.

astro-ph↗
arXiv2007

The Line-of-Sight Proximity Effect and the Mass of Quasar Host Halos

We show that the Lyman-alpha optical depth statistics in the proximity regions of quasar spectra depend on the mass of the dark matter halos hosting the quasars. This is owing to both the overdensity around the quasars and the associated infall of gas toward them. For a fiducial quasar host halo mass of (3.0+/-1.6) h^-1 x 10^12 Msun, as inferred by Croom et al. from clustering in the 2dF QSO Redshift Survey, we show that estimates of the ionizing background (Gamma^bkg) from proximity effect measurements could be biased high by a factor of ~2.5 at z=3 owing to neglecting these effects alone. The clustering of galaxies and other active galactic nuclei around the proximity effect quasars enhances the local background, but is not expected to skew measurements by more than a few percent. Assuming the measurements of Gamma^bkg based on the mean flux decrement in the Ly-alpha forest to be free of bias, we demonstrate how the proximity effect analysis can be inverted to measure the mass of the dark matter halos hosting quasars. In ideal conditions, such a measurement could be made with a precision comparable to the best clustering constraints to date from a modest sample of only about 100 spectra. We discuss observational difficulties, including continuum flux estimation, quasar systematic redshift determination, and quasar variability, which make accurate proximity effect measurements challenging in practice. These are also likely to contribute to the discrepancies between existing proximity effect and flux decrement measurements of Gamma^bkg.

astro-ph↗
arXiv2005

The Fundamental Scaling Relations of Elliptical Galaxies

(ABRIDGED) We examine the fundamental scaling relations of elliptical galaxies formed through mergers. Using hundreds of simulations to judge the impact of progenitor galaxy properties on merger remnants, we find that gas dissipation provides an important contribution to tilt in the Fundamental Plane relation. Dissipationless mergers of disks produce remnants that occupy the virial plane. As the gas content of disk galaxies is increased, the tilt of the Fundamental Plane relation increases and the slope of the Re-M_* relation steepens. For gas fractions fgas > 30%, the simulated Fundamental Plane scalings approach those observed in the K-band. In our simulations, feedback from supermassive black hole growth has only a minor influence on the stellar-mass scaling relations of spheroidal galaxies, but may play a role in maintaining the observed Fundamental Plane tilt at optical wavelengths by suppressing residual star formation in merger remnants. We estimate that \approx 40-100% of the Fundamental Plane tilt induced by structural properties owes to trends in the central total-to-stellar mass ratio M_total/M_* produced by dissipation. Lower mass systems obtain greater phase- space densities than higher mass systems, producing a galaxy mass-dependent central M_total/M_* and a corresponding tilt in the Fundamental Plane.

astro-ph↗
arXiv2005

The Physical and Photometric Properties of High-Redshift Galaxies in Cosmological Hydrodynamic Simulations

We study the physical and photometric properties of galaxies at z=4 in cosmological hydrodynamic simulations of a lambda-CDM universe. We focus on galaxies satisfying the GOODS "B-dropout" criteria. Our goals are: (1) to study the nature of high-redshift galaxies; (2) to test the simulations against published measurements of high-redshift galaxies; (3) to find relations between photometric measurements by HST/ACS (0.4 -- 1 micron) and Spitzer/IRAC (3.6 -- 8 micron) and the intrinsic physical properties of GOODS "B-dropouts" such as stellar mass, stellar age, dust reddening, and star-formation rate; and (4) to assess how representative the GOODS survey is at this epoch. Our simulations predict that high-redshift galaxies show strong correlations in star formation rate versus stellar mass, and weaker correlations versus environment and age, such that GOODS galaxies are predicted to be the most massive, most rapidly star-forming galaxies at that epoch, living preferentially in dense regions. The simulated rest-frame UV luminosity function (LF) and integrated luminosity density are in broad agreement with observations at z~4. The predicted rest-frame optical (observed 3.6 micron) LF is similar to the rest-frame UV LF, shifted roughly one magnitude brighter. We predict that GOODS detects less than 50% of the total stellar mass density formed in galaxies more massive than 10^8.7 M_sun by z=4, mainly because of brightness limits in the HST/ACS bands. The most rapidly star forming galaxies in our simulations have rates exceeding 1000 M_sun yr^-1, similar to observed sub-mm galaxies. The star formation rates of these galaxies show at most a mild excess (2--3x) over the rates that would be expected for their stellar mass. Whether these bright galaxies would be observable as LBGs depends on the uncertain effects of dust reddening.

astro-ph↗
arXiv2005

Simulation of Soft X-ray Emission Lines from the Missing Baryons

We study the soft X-ray emission (0.1 - 1 keV) from the Warm-Hot Intergalactic Medium (WHIM) in a hydrodynamic simulation of a Cold Dark Matter universe. Our main goal is to investigate how such emission can be explored with a combination of imaging and spectroscopy, and to motivate future X-ray missions. We first present high resolution images of the X-ray emission in several energy bands, in which emission from different ion species dominates. We pick three different areas to study the high resolution spectra of X-rays from the warm-hot IGM: (a) a galaxy group; (b) a filament and (c) an underluminous region. By taking into account the background X-ray emission from AGNs and foreground emission from the Galaxy, we compute composite X-ray spectra of the selected regions. We briefly investigate angular clustering of the soft-X-ray emission, finding a strong signal. Most interestingly, the combination of high spectral resolution and angular information allows us to map the emission from the WHIM in 3 dimensions. We cross-correlate the positions of galaxies in the simulation with this redshift map of emission and detect the presence of six different ion species (Ne IX, Fe XVII, O VII, O VIII, N VII, C VI) in the large-scale structure traced by the galaxies. Finally we show how such emission can be detected and studied with future X-ray satellites, with particular attention to a proposed mission, the Missing Baryon Explorer, or MBE. We present simulated observations of the WHIM gas with MBE.

astro-ph↗
arXiv2005

Where Are All The Fallback Disks? Constraints on Propeller Systems

Fallback disks are expected to form around new-born neutron stars following a supernova explosion. In almost all cases, the disk will pass through a propeller stage. If the neutron star is spinning rapidly (initial period $\sim 10$ ms) and has an ordinary magnetic moment ($\sim 10^{30}$ G cm$^3$), the rotational power transferred to the disk by the magnetic field of the neutron star will exceed the Eddington limit by many orders of magnitude, and the disk will be rapidly disrupted. Fallback disks can thus survive only around slow-born neutron stars and around black holes, assuming the latter do not torque their surrounding disks as strongly as do neutron stars. This might explain the apparent rarity of fallback disks around young compact objects.

astro-ph↗
arXiv2004

The Lyman-alpha forest according to LUQAS

We use the LUQAS sample of 27 high resolution, high signal-to-noise QSO absorption spectra (Kim et al. 2004) and the results from Croft et al. (2002) together with a suite of high-resolution hydro-dynamical simulations run with the GADGET-II code, to infer the linear dark matter power spectrum on scales of \~ 0.3-30 Mpc at z=2.125 and at z=2.72.

astro-ph↗
arXiv2004

Changes in the X-ray Emission from the Magnetar Candidate 1E 2259+586 during its 2002 Outburst

(abridged) An outburst of more than 80 individual bursts, similar to those seen from Soft Gamma Repeaters (SGRs), was detected from the Anomalous X-ray Pulsar (AXP) 1E 2259+586 in 2002 June. Coincident with this burst activity were gross changes in the pulsed flux, persistent flux, energy spectrum, pulse profile and spin down of the underlying X-ray source. We present RXTE and XMM-Newton observations of 1E 2259+586 that show the evolution of the aforementioned source parameters during and following this episode. Specifically, we observe an X-ray flux increase by more than an order of magnitude having two distinct components. The first component is linked to the burst activity and decays within ~2 days during which the energy spectrum is considerably harder than during the quiescent state of the source. The second component decays over the year following the glitch according to a power law in time with an exponent -0.22 +/- 0.01. The pulsed fraction decreased initially to ~15% RMS, and the pulse profile changed significantly during the outburst. A glitch was observed in 1E 2259+586 that preceded the observed burst activity. A fraction of the glitch (~19%) recovered, although the recovery was not purely exponential. An exponential rise of ~20% of the frequency jump with a time scale of ~14 days results in a significantly better fit to the data, however, contamination from a systematic drift in the phase of the pulse profile cannot be excluded. The long-term post-glitch spin-down rate decreased in magnitude relative to the pre-glitch value. A comparison with SGR outburst properties, a physical interpretation of the results, and implications on the number of magnetar candidates in our Galaxy are discussed.

astro-ph↗
arXiv2004

Disk Galaxy Formation in a LambdaCDM Universe

We describe hydrodynamical simulations of galaxy formation in a Lambda cold dark matter (CDM) cosmology performed using a subresolution model for star formation and feedback in a multiphase interstellar medium (ISM). In particular, we demonstrate the formation of a well-resolved disk galaxy. The surface brightness profile of the galaxy is exponential, with a B-band central surface brightness of 21.0 mag arcsec^-2 and a scale-length of R_d = 2.0 h^-1 kpc. We find no evidence for a significant bulge component. The simulated galaxy falls within the I-band Tully-Fisher relation, with an absolute magnitude of I = -21.2 and a peak stellar rotation velocity of V_rot=121.3 km s^-1. While the total specific angular momentum of the stars in the galaxy agrees with observations, the angular momentum in the inner regions appears to be low by a factor of ~2. The star formation rate of the galaxy peaks at ~7 M_sun yr^-1 between redshifts z=2-4, with the mean stellar age decreasing from \~10 Gyrs in the outer regions of the disk to ~7.5 Gyrs in the center, indicating that the disk did not simply form inside-out. The stars exhibit a metallicity gradient from 0.7 Z_sun at the edge of the disk to 1.3 Z_sun in the center. Using a suite of idealized galaxy formation simulations with different models for the ISM, we show that the effective pressure support provided by star formation and feedback in our multiphase model is instrumental in allowing the formation of large, stable disk galaxies. If ISM gas is instead modeled with an isothermal equation of state, or if star formation is suppressed entirely, growing gaseous disks quickly violate the Toomre stability criterion and undergo catastrophic fragmentation.

astro-ph↗
arXiv2003

NGC 4261 and NGC 4697: rejuvenated elliptical galaxies

Chandra images present evidence for a non-uniform spatial distribution of discrete X-ray sources in the elliptical galaxy NGC 4261. This non-uniform distribution is inconsistent with the optical morphology of NGC 4261 at greater than a 99.9% confidence level. Similar evidence is seen in one more elliptical galaxy (NGC 4697; 98% confidence level) out of five cases we investigated. NGC 4261 and NGC 4697 have old stellar populations (9-15 Gyrs) and fine structure parameters of 1 and 0 respectively, suggesting no recent merging activity. On the basis of simulations of galaxy interactions, we propose that the X-ray sources responsible for the non-uniform distribution are associated with young stellar populations, related to the rejuvenating fall-back of material in tidal tails onto a relaxed merger remnant, or shock induced star-formation along the tidal tails.

astro-ph↗