Orbital structure of merger remnants: Trends with gas fraction in 1:1 mergers

Since the violent relaxation in hierarchical merging is incomplete, elliptical galaxies retain a wealth of information about their formation pathways in their present-day orbital structure. A variety of evidence indicates that gas-rich major mergers play an important role in the formation of elliptical galaxies. We simulate 1:1 disk mergers at seven different initial gas fractions ranging from 0 to 40%, using the TreeSPH code Gadget-2. We classify the stellar orbits in each remnant and construct radial profiles of the orbital content, intrinsic shape, and orientation. The dissipationless remnants are typically prolate-triaxial, dominated by box orbits within r_c ~ 1.5Reff, and by tube orbits in their outer parts. As the gas fraction increases, the box orbits within r_c are increasingly replaced by a population of short axis tubes (z-tubes) with near zero net rotation, and the remnants become progressively more oblate and round. The long axis tube (x-tube) orbits are highly streaming and relatively insensitive to the gas fraction, implying that their angular momentum is retained from the dynamically cold initial conditions. Outside r_c, the orbital structure is essentially unchanged by the gas. The 15-20% gas remnants often display disk-like kinematically distinct cores (KDCs). These remnants show an interesting resemblance, in both their velocity maps and intrinsic orbital structure, to the KDC galaxy NGC4365 (van den Bosch et al. 2008). At 30-40% gas, the remnants are rapidly rotating, with sharp embedded disks on ~ 1Reff scales. We predict a characteristic, physically intuitive orbital structure for 1:1 disk merger remnants, with a distinct transition between 1 and 3Reff that will be readily observable with combined data from the 2D kinematics surveys SAURON and SMEAGOL.Comment: 29 pages, 20 figures, ApJ submitted (abstract abridged, figures degraded

The morphology of HII regions during reionization

It is possible that the properties of HII regions during reionization depend sensitively on many poorly constrained quantities (the nature of the ionizing sources, the clumpiness of the gas in the IGM, the degree to which photo-ionizing feedback suppresses the abundance of low mass galaxies, etc.), making it extremely difficult to interpret upcoming observations of this epoch. We demonstrate that the actual situation is more encouraging, using a suite of radiative transfer simulations, post-processed on outputs from a 1024^3, 94 Mpc N-body simulation. Analytic prescriptions are used to incorporate small-scale structures that affect reionization, yet remain unresolved in the N-body simulation. We show that the morphology of the HII regions is most dependent on the global ionization fraction x_i. This is not to say that the bubble morphology is completely independent of all parameters besides x_i. The next most important dependence is that of the nature of the ionizing sources. The rarer the sources, the larger and more spherical the HII regions become. The typical bubble size can vary by as much as a factor of 4 at fixed x_i between different possible source prescriptions. The final relevant factor is the abundance of minihalos or of Lyman-limit systems. These systems suppress the largest bubbles from growing, and the magnitude of this suppression depends on the thermal history of the gas as well as the rate at which minihalos are photo-evaporated. We find that neither source suppression owing to photo-heating nor gas clumping significantly affect the large-scale structure of the HII regions. We discuss how observations of the 21cm line with MWA and LOFAR can constrain properties of reionization, and we study the effect patchy reionization has on the statistics of Lyman-alpha emitting galaxies. [abridged]Comment: 23 pages, 18 figure

The imprint of dissipation on the shapes of merger remnant LOSVDs

The properties of elliptical galaxies are broadly consistent with simulated remnants of gas-rich mergers between spirals, motivating more detailed studies of the imprint of this formation mechanism on the remnant distribution function. Gas has a strong impact on the non-Gaussian shapes of the line-of-sight velocity distributions (LOSVDs) of the merger remnant, owing to the embedded disk that forms out of the gas that retains its angular momentum during the merger, and the strong central mass concentration from the gas that falls to the center. The deviations from Gaussianity are parametrized by the Gauss-Hermite moments h_3 and h_4, which are related to the skewness and kurtosis of the LOSVDs. We quantify the dependence of the (h_3,h_4)-v/sigma relations on the initial gas fraction of the progenitor disks in 1:1 mergers, using Gadget-2 simulations including star formation, radiative cooling, and feedback from supernovae and AGN. For gas fractions f_gas < ~15% the overall correlation between h_3 and v/sigma is weak, consisting of a flat negatively correlated component arising from edge-on viewing angles plus a steep positively correlated part from face-on projections. The spread in v/sigma values decreases toward high positive h_4, and there is a trend toward lower h_4 as the gas fraction increases from 0 to 15%. For f_gas > ~20% the (h_3,4)- v/sigma distributions look quite different - there is a tight negative h_3- v/sigma correlation, and a wide spread in v/sigma values at all h_4, in better agreement with observations. Re-mergers of the high-f_gas remnants (dry mergers) produce slowly rotating systems with nearly Gaussian LOSVDs. We explain all of these trends in terms of the underlying orbit structure of the remnants, as molded by their dissipative formation histories.Comment: ApJ accepted - added some references and background on previous studies. 9 pages, 4 figure

Kinematic Structure of Merger Remnants

We use numerical simulations to study the kinematic structure of remnants formed from mergers of equal-mass disk galaxies. In particular, we show that remnants of dissipational mergers, which include the radiative cooling of gas, star formation, feedback from supernovae, and the growth of supermassive black holes, are smaller, rounder, have, on average, a larger central velocity dispersion, and show significant rotation compared to remnants of dissipationless mergers. The increased rotation speed of dissipational remnants owes its origin to star formation that occurs in the central regions during the galaxy merger. We have further quantified the anisotropy, three-dimensional shape, minor axis rotation, and isophotal shape of each merger remnant, finding that dissipational remnants are more isotropic, closer to oblate, have the majority of their rotation along their major axis, and are more disky than dissipationless remnants. Individual remnants display a wide variety of kinematic properties. A large fraction of the dissipational remnants are oblate isotropic rotators. Many dissipational, and all of the dissipationless, are slowly rotating and anisotropic. The remnants of gas-rich major mergers can well-reproduce the observed distribution of projected ellipticities, rotation parameter (V/\sigma)*, kinematic misalignments, Psi, and isophotal shapes. The dissipationless remnants are a poor match to this data. Our results support the merger hypothesis for the origin of low-luminosity elliptical galaxies provided that the progenitor disks are sufficiently gas-rich, however our remnants are a poor match to the bright ellipticals that are slowly rotating and uniformly boxy.Comment: 22 pages, 17 figures, accepted to Ap

Probing the Neutral Fraction of the IGM with GRBs during the Epoch of Reionization

We show that near-infrared observations of the red side of the Ly-alpha line from a single gamma ray burst (GRB) afterglow cannot be used to constrain the global neutral fraction of the intergalactic medium (IGM), x_H, at the GRB's redshift to better than ~0.3. Some GRB sight-lines will encounter more neutral hydrogen than others at fixed x_H owing to the patchiness of reionisation. GRBs during the epoch of reionization will often bear no discernible signature of a neutral IGM in their afterglow spectra. We discuss the constraints on x_H from the z = 6.3 burst, GRB050904, and quantify the probability of detecting a neutral IGM using future spectroscopic observations of high-redshift, near-infrared GRB afterglows. Assuming an observation with signal-to-noise similar to the Subaru FOCAS spectrum of GRB050904 and that the column density distribution of damped Ly-alpha absorbers is the same as measured at lower redshifts, a GRB from an epoch when x_H = 0.5 can be used to detect a partly neutral IGM at 98% confidence level 10% of the time (and, for an observation with three times the sensitivity, 30% of the time).Comment: 10 pages, 8 figures, submitted to MNRA

HeII Reionization and its Effect on the IGM

Observations of the intergalactic medium (IGM) suggest that quasars reionize HeII in the IGM at z ~ 3. We have run a set of 190 and 430 comoving Mpc simulations of HeII being reionized by quasars to develop an understanding of the nature of HeII reionization and its potential impact on observables. We find that HeII reionization heats regions in the IGM by as much as 25,000 K above the temperature that is expected otherwise, with the volume-averaged temperature increasing by ~ 12,000 K and with large temperature fluctuations on ~ 50 Mpc scales. Much of the heating occurs far from QSOs by hard photons. We find a temperature-density equation of state of gamma -1 ~ 0.3 during HeII reionization, but with a wide dispersion in this relation having sigma ~ 10^4 K. HeII reionization by the observed population of quasars cannot produce an inverted relation (gamma - 1 < 0). Our simulations are consistent with the observed evolution in the mean transmission of the HeII Ly-alpha forest. We argue that the heat input due to HeII reionization is unable to cause the observed depression at z = 3.2 in the HI Ly-alpha forest opacity as has been suggested. We investigate how uncertainties in the properties of QSOs and of HeII Lyman-limit systems influence our predictions.Comment: 19 pages, 15 figures, plus 9 pages of Appendix. accepted by Ap

Studying Reionization with Ly-alpha Emitters

We show that observations of high-redshift Ly-alpha emitters (LAEs) have the potential to provide definitive evidence for reionization in the near future. Using 200 Mpc radiative transfer simulations, we calculate the effect that patchy reionization has on the line profile, on the luminosity function, and, most interestingly, on the clustering of emitters for several realistic models of reionization. Reionization increases the measured clustering of emitters, and we show that this enhancement would be essentially impossible to attribute to anything other than reionization. Our results motivate looking for the signature of reionization in existing LAE data. We find that for stellar reionization scenarios the angular correlation function of the 58 LAEs in the Subaru Deep Field z = 6.6 photometric sample is more consistent with a fully ionized universe (mean volume ionized fraction x_i = 1) than a universe with x_i 2-sigma confidence level. Measurements in the next year on Subaru will increase their z = 6.6 LAE sample by a factor of five and tighten these limits. If the clustering signature of reionization is detected in a LAE survey, a comparison with a Lyman-break or a H-alpha survey in the same field would confirm the reionization hypothesis. We discuss the optimal LAE survey specifications for detecting reionization, with reference to upcoming programs.Comment: 24 pages, 17 figures, accepted by MNRA

A Measurement of Small Scale Structure in the 2.2 < z < 4.2 Lyman-alpha Forest

The amplitude of fluctuations in the Ly-a forest on small spatial scales is sensitive to the temperature of the IGM and its spatial fluctuations. The temperature of the IGM and its spatial variations contain important information about hydrogen and helium reionization. We present a new measurement of the small-scale structure in the Ly-a forest from 40 high resolution, high signal-to-noise, VLT spectra at z=2.2-4.2. We convolve each Ly-a forest spectrum with a suitably chosen wavelet filter, which allows us to extract the amount of small-scale structure in the forest as a function of position across each spectrum. We compare these measurements with high resolution hydrodynamic simulations of the Ly-a forest which track more than 2 billion particles. This comparison suggests that the IGM temperature close to the cosmic mean density (T_0) peaks near z=3.4, at which point it is greater than 20,000 K at 2-sigma confidence. The temperature at lower redshift is consistent with the fall-off expected from adiabatic cooling ($T_0 \propto (1+z)^2$), after the peak temperature is reached near z=3.4. At z=4.2 our results favor a temperature of T_0 = 15-20,000 K. However, owing mostly to uncertainties in the mean transmitted flux at this redshift, a cooler IGM model with T_0 = 10,000 K is only disfavored at the 2-sigma level here, although such cool IGM models are strongly discrepant with the z ~ 3-3.4 measurement. We do not detect large spatial fluctuations in the IGM temperature at any redshift covered by our data set. The simplest interpretation of our measurements is that HeII reionization completes sometime near z ~ 3.4, although statistical uncertainties are still large [Abridged].Comment: Submitted to ApJ. Best printed in colo

Dissipation and Extra Light in Galactic Nuclei: II. 'Cusp' Ellipticals

We study the origin and properties of 'extra' or 'excess' central light in the surface brightness profiles of cusp or power-law ellipticals. Dissipational mergers give rise to two-component profiles: an outer profile established by violent relaxation acting on stars present in the progenitors prior to the final merger, and an inner stellar population comprising the extra light, formed in a compact starburst. Combining a large set of hydrodynamical simulations with data that span a broad range of profiles and masses, we show that this picture is borne out -- cusp ellipticals are indeed 'extra light' ellipticals -- and examine how the properties of this component scale with global galaxy properties. We show how to robustly separate the 'extra' light, and demonstrate that observed cusps are reliable tracers of the degree of dissipation in the spheroid-forming merger. We show that the typical degree of dissipation is a strong function of stellar mass, tracing observed disk gas fractions at each mass. We demonstrate a correlation between extra light content and effective radius at fixed mass: systems with more dissipation are more compact. The outer shape of the light profile does not depend on mass, with a mean outer Sersic index ~2.5. We explore how this relates to shapes, kinematics, and stellar population gradients. Simulations with the gas content needed to match observed profiles also reproduce observed age, metallicity, and color gradients, and we show how these can be used as tracers of the degree of dissipation in spheroid formation.Comment: 40 pages, 32 figures, accepted to ApJ (revised to match accepted version