A Substantial Population of Low Mass Stars in Luminous Elliptical Galaxies

The stellar initial mass function (IMF) describes the mass distribution of stars at the time of their formation and is of fundamental importance for many areas of astrophysics. The IMF is reasonably well constrained in the disk of the Milky Way but we have very little direct information on the form of the IMF in other galaxies and at earlier cosmic epochs. Here we investigate the stellar mass function in elliptical galaxies by measuring the strength of the Na I doublet and the Wing-Ford molecular FeH band in their spectra. These lines are strong in stars with masses <0.3 Msun and weak or absent in all other types of stars. We unambiguously detect both signatures, consistent with previous studies that were based on data of lower signal-to-noise ratio. The direct detection of the light of low mass stars implies that they are very abundant in elliptical galaxies, making up >80% of the total number of stars and contributing >60% of the total stellar mass. We infer that the IMF in massive star-forming galaxies in the early Universe produced many more low mass stars than the IMF in the Milky Way disk, and was probably slightly steeper than the Salpeter form in the mass range 0.1 - 1 Msun.Comment: To appear in Natur

Central Powering of the Largest Lyman-alpha Nebula is Revealed by Polarized Radiation

High-redshift Lyman-alpha blobs are extended, luminous, but rare structures that appear to be associated with the highest peaks in the matter density of the Universe. Their energy output and morphology are similar to powerful radio galaxies, but the source of the luminosity is unclear. Some blobs are associated with ultraviolet or infrared bright galaxies, suggesting an extreme starburst event or accretion onto a central black hole. Another possibility is gas that is shock excited by supernovae. However some blobs are not associated with galaxies, and may instead be heated by gas falling into a dark matter halo. The polarization of the Ly-alpha emission can in principle distinguish between these options, but a previous attempt to detect this signature returned a null detection. Here we report on the detection of polarized Ly-alpha from the blob LAB1. Although the central region shows no measurable polarization, the polarized fraction (P) increases to ~20 per cent at a radius of 45 kpc, forming an almost complete polarized ring. The detection of polarized radiation is inconsistent with the in situ production of Ly-alpha photons, and we conclude that they must have been produced in the galaxies hosted within the nebula, and re-scattered by neutral hydrogen.Comment: Published in the August 18 issue of Nature. 1750 words, 3 figures, and full Supplementary Information. Version has not undergone proofing. Reduced and processed data products are available here: http://obswww.unige.ch/people/matthew.hayes/LymanAlpha/LabPol

Galaxy-Induced Transformation of Dark Matter Halos

We use N-body/gasdynamical LambdaCDM cosmological simulations to examine the effect of the assembly of a central galaxy on the shape and mass profile of its dark halo. Two series of simulations are compared; one that follows only the evolution of the dark matter component and a second one where a baryonic component is added. These simulations include radiative cooling but neglect star formation and feedback, leading most baryons to collect at the halo center in a disk which is too small and too massive when compared with typical spiral. This unrealistic model allows us, nevertheless, to gauge the maximum effect that galaxies may have in transforming their dark halos. We find that the shape of the halo becomes more axisymmetric: halos are transformed from triaxial into essentially oblate systems, with well-aligned isopotential contours of roughly constant flattening (c/a ~ 0.85). Halos always contract as a result of galaxy assembly, but the effect is substantially less pronounced than predicted by the "adiabatic contraction" hypothesis. The reduced contraction helps to reconcile LambdaCDM halos with constraints on the dark matter content inside the solar circle and should alleviate the long-standing difficulty of matching simultaneously the scaling properties of galaxy disks and the luminosity function. The halo contraction is also less pronounced than found in earlier simulations, a disagreement that suggests that halo contraction is not solely a function of the initial and final distribution of baryons. Not only how much baryonic mass has been deposited at the center of a halo matters, but also the mode of its deposition. It might prove impossible to predict the halo response without a detailed understanding of a galaxy's assembly history. (Abriged)Comment: 11 pages and 9 figure

The Star Formation History and Dust Content in the Far Outer Disc of M31

We present a detailed analysis of two fields located 26 kpc (~5 scalelengths) from the centre of M31. One field samples the major axis populations--the Outer Disc field--while the other is offset by ~18' and samples the Warp in the stellar disc. The CMDs based on HST/ACS imaging reach old main-sequence turn-offs (~12.5 Gyr). We apply the CMD-fitting technique to the Warp field to reconstruct the star formation history (SFH). We find that after undergoing roughly constant SF until about 4.5 Gyr ago, there was a rapid decline in activity and then a ~1.5 Gyr lull, followed by a strong burst lasting 1.5 Gyr and responsible for 25% of the total stellar mass in this field. This burst appears to be accompanied by a decline in metallicity which could be a signature of the inflow of metal-poor gas. The onset of the burst (~3 Gyr ago) corresponds to the last close passage of M31 and M33 as predicted by detailed N-body modelling, and may have been triggered by this event. We reprocess the deep M33 outer disc field data of Barker et al. (2011) in order to compare consistently-derived SFHs. This reveals a similar duration burst that is exactly coeval with that seen in the M31 Warp field, lending further support to the interaction hypothesis. The complex SFHs and the smoothly-varying age-metallicity relations suggest that the stellar populations observed in the far outer discs of both galaxies have largely formed in situ rather than migrated from smaller galactocentric radii. The strong differential reddening affecting the CMD of the Outer Disc field prevents derivation of the SFH. Instead, we quantify this reddening and find that the fine-scale distribution of dust precisely follows that of the HI gas. This indicates that the outer HI disc of M31 contains a substantial amount of dust and therefore suggests significant metal enrichment in these parts, consistent with inferences from our CMD analysis.Comment: Abstract shortened. 17 pages, 12 figures (+ 6 pages & 5 figures in Appendix). MNRAS, in pres

The Baryonic Assembly of Dark Matter Halos

We use a suite of cosmological hydrodynamic simulations to quantify the accretion rates of baryons into dark matter halos and the resulting baryon mass fractions, as a function of halo mass, redshift, and baryon type (including cold and hot gas). We find that the net baryonic accretion rates through the virial radius are sensitive to galactic outflows and explore a range of outflow parameters to illustrate the effects. We show that the cold gas accretion rate is in general not a simple universal factor of the dark matter accretion rate, and that galactic winds can cause star formation rates to deviate significantly from the external gas accretion rates, both via gas ejection and re-accretion. Furthermore, galactic winds can inject enough energy and momentum in the surrounding medium to slow down accretion altogether, especially in low-mass halos and at low redshift. By resolving the accretion rates versus radius from the halo centers, we show how cold streams penetrate the hot atmospheres of massive halos at z>2, but gradually disappear at lower redshift. The total baryon mass fraction is also strongly suppressed by outflows in low-mass halos, but is nearly universal in the absence of feedback in halos above the UV background suppression scale. The transition halo mass, at which the gas mass in halos is equal for the cold and hot components, is roughly constant at ~10^11.5 Msun and does not depend sensitively on the wind prescription. We provide simple fitting formulae for the cold gas accretion rate into halos in the no-wind case. Finally, we show that cold accretion is broadly consistent with driving the bulk of the highly star-forming galaxies observed at z~2, but that the more intense star formers likely sample the high end of the accretion rate distribution, and may be additionally fueled by a combination of gas recycling, gas re-accretion, hot mode cooling, and mergers.Comment: 20 pages, 10 figures. MNRAS, in pres

Investigating the Andromeda Stream: I. Simple Analytic Bulge-Disk-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 calculation of orbits in M31's halo. We use an NFW dark halo, an exponential disk, 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 modeling 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 disk 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 $M/L$ ratio, finding a smaller region where the parameters are physically plausible. Our proposed mass model for M31 has M_{bulge} = 3.2 \times 10^{10} \Msun, M_{disk} = 7.2 \times 10^{10} \Msun, and M_{200} = 7.1\times 10^{11} \Msun, with uncorrected (for internal and foreground extinction) mass-to-light ratios of $M/L_R = 3.9$ and 3.3 for the bulge and disk, 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 disk and halo masses.Comment: 17 pages, 8 color figures, 2 tables. Accepted by Monthly Notices; Models listed in tables modified, text reorganized and shortene

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.Comment: 7 pages, 1 figure. To be published in the proceedings of the IGM/Galaxy Connection- The Distribution of Baryons at z=0 conferenc

The physical scale of the far-infrared emission in the most luminous submillimetre galaxies II: evidence for merger-driven star formation

We present high-resolution 345 GHz interferometric observations of two extreme luminous (L_{IR}>10^{13} L_sun), submillimetre-selected galaxies (SMGs) in the COSMOS field with the Submillimeter Array (SMA). Both targets were previously detected as unresolved point-sources by the SMA in its compact configuration, also at 345 GHz. These new data, which provide a factor of ~3 improvement in resolution, allow us to measure the physical scale of the far-infrared in the submillimetre directly. The visibility functions of both targets show significant evidence for structure on 0.5-1 arcsec scales, which at z=1.5 translates into a physical scale of 5-8 kpc. Our results are consistent with the angular and physical scales of two comparably luminous objects with high-resolution SMA followup, as well as radio continuum and CO sizes. These relatively compact sizes (<5-10 kpc) argue strongly for merger-driven starbursts, rather than extended gas-rich disks, as the preferred channel for forming SMGs. For the most luminous objects, the derived sizes may also have important physical consequences; under a series of simplifying assumptions, we find that these two objects in particular are forming stars close to or at the Eddington limit for a starburst.Comment: 9 pages, 3 Figures, submitted to MNRA

The evolution of galaxies from primeval irregulars to present-day ellipticals

The current understanding of galaxy formation is that it proceeds in a 'bottom up' way, with the formation of small clumps of gas and stars that merge hierarchically until giant galaxies are built up. The baryonic gas loses the thermal energy by radiative cooling and falls towards the centres of the new galaxies, while supernovae (SNe) blow gas out. Any realistic model therefore requires a proper treatment of these processes, but hitherto this has been far from satisfactory. Here we report an ultra-high-resolution simulation that follows evolution from the earliest stages of galaxy formation through the period of dynamical relaxation. The bubble structures of gas revealed in our simulation ($< 3\times10^8$ years) resemble closely the high-redshift Lyman $\alpha$ emitters (LAEs). After $10^9$ years these bodies are dominated by stellar continuum radiation and look like the Lyman break galaxies (LBGs) known as the high-redshift star-forming galaxies at which point the abundance of elements heavier than helium ("metallicity") appears to be solar. After $1.3\times10^{10}$ years, these galaxies resemble present-day ellipticals.Comment: 27 pages and 4 figures, Supplementary Information included, movie available on http://www.isc.senshu-u.ac.jp/~thj0613/natur

Constraining Galaxy Formation and Cosmology with the Conditional Luminosity Function of Galaxies

We use the conditional luminosity function (CLF), which gives the number of galaxies with luminosities in the range [L, L+dL] that reside in a halo of mass M, to link the distribution of galaxies to that of dark matter haloes. We seek the CLF that reproduces the galaxy luminosity function and the luminosity dependence of the galaxy clustering strength and test the models by comparing the resulting mass-to-light ratios (M/L) with constraints from the Tully-Fisher (TF) relation. We obtain a number of stringent constraints on both galaxy formation and cosmology. In particular, this method can break the degeneracy between Omega_0 and the power-spectrum normalization sigma_8, inherent in current weak-lensing and cluster-abundance studies. For flat LCDM cosmogonies with sigma_8 normalized by recent weak lensing observations, the best results are obtained for Omega_0~0.3; Omega_0 < 0.2 leads to too large galaxy correlation lengths, while Omega_0 > 0.4 gives too high M/L. The best-fit model for the LCDM concordance cosmology (Omega_0=0.3) predicts M/L that are slightly too high. We discuss a number of possible effects that might remedy this problem, including small modification of cosmological parameters, warm in stead of cold dark matter, systematic errors in current observational data, and the existence of dark galaxies. Finally we use the CLF to predict several statistics about the distribution of galaxy light in the local Universe.Comment: 25 pages, 10 figures. Submitted for publication in MNRA