Mark A. Fardal

Galaxies in a simulated ΛCDM Universe – I. Cold mode and hot cores

We study the formation of galaxies in a large volume (50 h −1 Mpc, 2 × 288 3 particles) cosmological simulation, evolved using the entropy and energy-conserving smoothed particle hydrodynamics (SPH) code GADGET-2. Most of the baryonic mass in galaxies of all masses is originally acquired through filamentary ‘cold mode’ accretion of gas that was never shock heated to its halo virial temperature, confirming the key feature of our earlier results obtained with a different SPH code. Atmospheres of hot, virialized gas develop in haloes above 2–3 × 10 11 M � , a transition mass that is nearly constant from z = 3 to 0. Cold accretion persists in haloes above the transition mass, especially at z ≥ 2. It dominates the growth of galaxies in low-mass haloes at all times, and it is the main driver of the cosmic star formation history. Our results suggest that the cooling of shock-heated virialized gas, which has been the focus of many analytic models of galaxy growth spanning more than three decades, might be a relatively minor element of galaxy formation. At high redshifts, satellite galaxies have gas accretion rates similar to central galaxies of the same baryonic mass, but at z < 1t he accretion rates of low-mass satellites are well below those of comparable central galaxies. Relative to our earlier simulations, the GADGET-2 simulations predict much lower rates of ‘hot mode’ accretion from the virialized gas component. Hot accretion rates compete with cold accretion rates near the transition mass, but only at z ≤ 1. Hot accretion is inefficient in haloes

The remnants of galaxy formation from a panoramic survey of the region around M31.

In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10–100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31’s satellites brighter than Mv = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

Cooling Radiation and the Lyα Luminosity of Forming Galaxies

We examine the cooling radiation from forming galaxies in hydrodynamic simulations of the LCDM model (cold dark matter with a cosmological constant), focusing on the Ly? line luminosities of high-redshift systems. Primordial composition gas condenses within dark matter potential wells, forming objects with masses and sizes comparable to the luminous regions of observed galaxies. As expected, the energy radiated in this process is comparable to the gravitational binding energy of the baryons, and the total cooling luminosity of the galaxy population peaks at z ? 2. However, in contrast to the classical picture of gas cooling from the ~106 K virial temperature of a typical dark matter halo, we find that most of the cooling radiation is emitted by gas with T < 20,000 K. As a consequence, roughly 50% of this cooling radiation emerges in the Ly? line. While a galaxys cooling luminosity is usually smaller than the ionizing continuum luminosity of its young stars, the two are comparable in the most massive systems, and the cooling radiation is produced at larger radii, where the Ly? photons are less likely to be extinguished by dust. We suggest, in particular, that cooling radiation could explain the two large (~100 kpc), luminous (LLy? ~ 1044 ergs s-1) blobs of Ly? emission found in the narrowband survey of a z = 3 protocluster by Steidel and collaborators. Our simulations predict objects of the observed luminosity at about the right space density, and radiative transfer effects can account for the observed sizes and line widths. We discuss observable tests of this hypothesis for the nature of the Ly? blobs, and we present predictions for the contribution of cooling radiation to the Ly? luminosity function of galaxies as a function of redshift.

Galaxies in a simulated ΛCDM universe – II. Observable properties and constraints on feedback

We compare the properties of galaxies that form in a cosmological simulation without strong feedback to observations of the z = 0 galaxy population. We confirm previous findings that models without strong feedback overproduce the observed galaxy baryonic mass function, especially at the low- and high-mass extremes. Through post-processing we investigate what kinds of feedback would be required to reproduce the statistics of observed galaxy masses and star formation rates. To mimic an extreme form of ‘preventive’ feedback, such as a highly efficient active galactic nucleus ‘radio mode’, we remove all baryonic mass that was originally accreted from shock-heated gas (‘hot-mode’ accretion). This removal does not bring the highmass end of the galaxy mass function into agreement with observations because much of the stellar mass in these systems formed at high redshift from baryons that originally accreted via ‘cold mode’ on to lower mass progenitors. An efficient ‘ejective’ feedback mechanism, such as supernova-driven galactic winds, must reduce the masses of these progenitors before they merge to form today’s massive galaxies. Feedback must also reduce the masses of lower mass z = 0 galaxies, which assemble at lower redshifts and have much lower star formation rates. If we monotonically remap galaxy masses to reproduce the observed mass function, but retain the simulation-predicted star formation rates, we obtain fairly good agreement with the observed sequence of star-forming galaxies. However, we fail to recover the observed population of passive, low star formation rate galaxies, especially at the high-mass end. Suppressing all hotmode accretion improves the agreement for high-mass galaxies, but it worsens the agreement at intermediate masses. Reproducing these z = 0 observations requires a feedback mechanism that dramatically suppresses star formation in a fraction of galaxies, increasing with mass, while leaving star formation rates of other galaxies essentially unchanged.

THE LARGE-SCALE STRUCTURE OF THE HALO OF THE ANDROMEDA GALAXY. I. GLOBAL STELLAR DENSITY, MORPHOLOGY AND METALLICITY PROPERTIES*

We present an analysis of the large-scale structure of the halo of the Andromeda galaxy, based on the Pan-Andromeda Archeological Survey (PAndAS), currently the most complete map of resolved stellar populations in any galactic halo. Despite the presence of copious substructures, the global halo populations follow closely power-law profiles that become steeper with increasing metallicity. We divide the sample into stream-like populations and a smooth halo component (defined as the population that cannot be resolved into spatially distinct substructures with PAndAS). Fitting a three-dimensional halo model reveals that the most metal-poor populations () are distributed approximately spherically (slightly prolate with ellipticity c/a = 1.09 ± 0.03), with only a relatively small fraction residing in discernible stream-like structures (f stream = 42%). The sphericity of the ancient smooth component strongly hints that the dark matter halo is also approximately spherical. More metal-rich populations contain higher fractions of stars in streams, with f stream becoming as high as 86% for . The space density of the smooth metal-poor component has a global power-law slope of γ = –3.08 ± 0.07, and a non-parametric fit shows that the slope remains nearly constant from 30 kpc to ~300 kpc. The total stellar mass in the halo at distances beyond 2° is ~1.1 × 1010 M ☉, while that of the smooth component is ~3 × 109 M ☉. Extrapolating into the inner galaxy, the total stellar mass of the smooth halo is plausibly ~8 × 109 M ☉. We detect a substantial metallicity gradient, which declines from [Fe/H] = –0.7 at R = 30 kpc to [Fe/H] = –1.5 at R = 150 kpc for the full sample, with the smooth halo being ~0.2 dex more metal poor than the full sample at each radius. While qualitatively in line with expectations from cosmological simulations, these observations are of great importance as they provide a prototype template that such simulations must now be able to reproduce in quantitative detail.

THE M31 VELOCITY VECTOR. II. RADIAL ORBIT TOWARD THE MILKY WAY AND IMPLIED LOCAL GROUP MASS

We determine the velocity vector of M31 with respect to the Milky Way and use this to constrain the mass of the Local Group, based on Hubble Space Telescope proper-motion measurements of three fields presented in Paper I. We construct N-body models for M31 to correct the measurements for the contributions from stellar motions internal to M31. This yields an unbiased estimate for the M31 center-of-mass motion. We also estimate the center-of-mass motion independently, using the kinematics of satellite galaxies of M31 and the Local Group, following previous work but with an expanded satellite sample. All estimates are mutually consistent, and imply a weighted average M31 heliocentric transverse velocity of (vW , vN ) = (– 125.2 ± 30.8, –73.8 ± 28.4) km s–1. We correct for the reflex motion of the Sun using the most recent insights into the solar motion within the Milky Way, which imply a larger azimuthal velocity than previously believed. This implies a radial velocity of M31 with respect to the Milky Way of V rad, M31 = –109.3 ± 4.4 km s–1, and a tangential velocity of V tan, M31 = 17.0 km s–1, with a 1σ confidence region of V tan, M31 ≤ 34.3 km s–1. Hence, the velocity vector of M31 is statistically consistent with a radial (head-on collision) orbit toward the Milky Way. We revise prior estimates for the Local Group timing mass, including corrections for cosmic bias and scatter, and obtain M LG ≡ M MW, vir + M M31, vir = (4.93 ± 1.63) × 1012 M ☉. Summing known estimates for the individual masses of M31 and the Milky Way obtained from other dynamical methods yields smaller uncertainties. Bayesian combination of the different estimates demonstrates that the timing argument has too much (cosmic) scatter to help much in reducing uncertainties on the Local Group mass, but its inclusion does tend to increase other estimates by ~10%. We derive a final estimate for the Local Group mass from literature and new considerations of M LG = (3.17 ± 0.57) × 1012 M ☉. The velocity and mass results at 95% confidence imply that M33 is bound to M31, consistent with expectation from observed tidal deformations.

Investigating the Andromeda stream – III. A young shell system in M31

Published maps of red giant stars in the halo region of M31 exhibit a giant stellar stream to the south of this galaxy, as well as a giant ‘shelf’ to the northeast of M31s centre. Using these maps, we find that there is a fainter shelf of comparable size on the western side as well. By choosing appropriate structural and orbital parameters for an accreting dwarf satellite within the accurate M31 potential model of Geehan et al., we produce a very similar structure in an N-body simulation. In this scenario, the tidal stream produced at pericentre of the satellites orbit matches the observed southern stream, while the forward continuation of this tidal stream makes up two orbital loops, broadened into fan-like structures by successive pericentric passages; these loops correspond to the north-eastern and western shelves. The tidal debris from the satellite also reproduces a previously observed ‘stream’ of counterrotating planetary nebulae and a related stream seen in red giant stars. The debris pattern in our simulation resembles the shell systems detected around many elliptical galaxies, though this is the first identification of a shell system in a spiral galaxy and the first in any galaxy close enough to allow measurements of stellar velocities and relative distances. We discuss the physics of these partial shells, highlighting the role played by spatial and velocity caustics in the observations. We show that kinematic surveys of the tidal debris will provide a sensitive measurement of M31s halo potential, while quantifying the surface density of debris in the shelves will let us reconstruct the original mass and time of disruption of the progenitor satellite.

Investigating the Andromeda stream — I. Simple analytic bulge—disc—halo model for M31

This paper is the first in a series which studies interactions between M31 and its satellites, including the origin of the giant southern stream. We construct accurate yet simple analytic models for the potential of the M31 galaxy to provide an easy basis for the calculation of orbits in M31s halo. We use a Navarro, Frenk and White (NFW) dark halo, an exponential disc, a Hernquist bulge, and a central black hole point mass to describe the galaxy potential. We constrain the parameters of these functions by comparing to existing surface-brightness, velocity-dispersion, and rotation-curve measurements of M31. Our description provides a good fit to the observations, and agrees well with more sophisticated modelling of M31. While in many respects the parameter set is well constrained, there is substantial uncertainty in the outer halo potential and a near-degeneracy between the disc and halo components, producing a large, nearly two-dimensional allowed region in parameter space. We limit the allowed region using theoretical expectations for the halo concentration, baryonic content, and stellar mass-to-light ratio (M/L R ), finding a smaller region where the parameters are physically plausible. Our proposed mass model for M31 has M bulge = 3.2 x 10 10 M ⊙ , M disc = 7.2 x 10 10 M ⊙ , and M 200 = 7.1 x 10 11 M ⊙ , with uncorrected (for internal and foreground extinction) mass-to-light ratios of M/L R = 3.9 and 3.3 for the bulge and disc, respectively. We present some illustrative test-particle orbits for the progenitor of the stellar stream in our galaxy potential, highlighting the effects of the remaining uncertainty in the disc and halo masses.

The Metagalactic Ionizing Radiation Field at Low Redshift

We compute the ionizing radiation field at low redshift, arising from Seyferts, QSOs, and starburst galaxies. This calculation combines recent Seyfert luminosity functions, extrapolated ultraviolet fluxes from our IUE-AGN database, and a new intergalactic opacity model based on Hubble Space Telescope and Keck Lyα absorber surveys. At z = 0 for AGNs only, our best estimate for the specific intensity at 1 ryd is I0 = 1.3 × 10-23 ergs cm-2 s-1 Hz-1 sr-1 , independent of H0, Ω0, and Λ. The one-sided ionizing photon flux is Φion ≈ 3400 photons cm-2 s-1, and the H I photoionization rate is Γ = 3.2 × 10-14 s-1, for αs = 1.8. We also derive ΓH I for z = 0–4. These error ranges reflect uncertainties in the spectral indexes for the ionizing EUV (αs = 1.8 ± 0.3) and the optical/UV (αUV = 0.86 ± 0.05), the IGM opacity model, the range of Seyfert luminosities (0.001L*–100L*), and the completeness of the luminosity functions. Our estimate is a factor of 3 lower than the most stringent upper limits on the ionizing background (Φion < 104 photons cm-2 s-1) obtained from Hα observations in external clouds, and it lies within the range implied by other indirect measures. Starburst galaxies with a sufficiently large Lyman continuum escape fraction, fesc ≥ 0.05, may provide a comparable background to AGNs, I0(z = 0) = 1.1 × 10-23 ergs cm-2 s-1 Hz-1 sr-1 . An additional component of the ionizing background of this magnitude would violate neither upper limits from Hα observations nor the acceptable range from other measurements.

Investigating the Andromeda stream – II. Orbital fits and properties of the progenitor

We construct test-particle orbits and simple N-body models that match the properties of the giant stellar stream observed to the south of M31, using the model of M31’s potential derived in the companion paper by Geehan et al. (2005). We introduce a simple approximation to account for the difference in position between the stream and the orbit of the progenitor; this significantly affects the best-fitting orbits. The prog enitor orbits we derive have orbital apocenter ∼ 60 kpc and pericenter ∼ 3 kpc, though these quantities vary somewhat with the current orbital phase of the progenitor which is as yet unknown. Our best combined fit to the stream and galaxy properties implies a mass within 125 kpc of M31 of (7.4±1.2)×10 11 M⊙. Based on its length, width, luminosity, and velocity disper sion, we conclude that the stream originates from a progenitor satellite with mass Ms ∼ 10 9 M⊙, and at most modest amounts of dark matter; the estimate of Ms is again correlated with the phase of the progenitor. M31 displays a large number of faint features in its inner halo wh ich may be progenitors or continuations of the stream. While the orbital fits are not constrai ned enough for us to conclusively identify the progenitor, we can identify several plausible candidates, of which a feature in the planetary nebula distribution found by Merrett et al. is the most plausible, and rule out several others. We make predictions for the kinematic properties of the successful candidates. These may aid in observational identification of the progenitor ob ject, which would greatly constrain the allowed models of the stream.