arXiv subjects

Jesper Sommer-Larsen

Publications and source records attributed to Jesper Sommer-Larsen.

At least 37 records · Page 2Linked to original sources

arXiv2007

Modeling Lyman continuum emission from young galaxies

Based on cosmological simulations, we model Lyman continuum emission from a sample of 11 high-redshift star forming galaxies spanning a mass range of a factor 20. Each of the 11 galaxies has been simulated both with a Salpeter and a Kroupa initial mass function (IMF). We find that the Lyman continuum (LyC) luminosity of an average star forming galaxy in our sample declines from z=3.6 to 2.4 due to the steady gas infall and higher gas clumping at lower redshifts, increasingly hampering the escape of ionizing radiation. The galaxy-to-galaxy variation of apparent LyC emission at a fixed redshift is caused in approximately equal parts by the intrinsic variations in the LyC emission and by orientation effects. The combined scatter of an order of magnitude can explain the variance in the far-UV spectra of high-redshift galaxies detected by Shapley et al. (2006). Our results imply that the cosmic galactic ionizing UV luminosity would be monotonically decreasing from z=3.6 to 2.4, curiously anti-correlated with the star formation rate in the smaller galaxies, which on average rises during this redshift interval.

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arXiv2007

Coincident, 100 kpc-scale damped Lyman alpha absorption towards a binary QSO: how large are galaxies at z ~ 3?

We report coincident damped Lyman alpha (DLA) and sub-DLA absorption at z = 2.66 and z = 2.94 towards the z ~ 3 13.8 arcsecond separation binary quasar SDSS 1116+4118 AB. At the redshifts of the absorbers, this angular separation corresponds to a proper transverse separation of ~ 110 kpc. A third absorber, a sub-DLA at z = 2.47, is detected towards SDSS 1116+4118 B, but no corresponding high column density absorber is present towards SDSS 1116+4118 A. We use high resolution galaxy simulations and a clustering analysis to interpret the coincident absorption and its implications for galaxy structure at z ~ 3. We conclude that the common absorption in the two lines of sight is unlikely to arise from a single galaxy, or a galaxy plus satellite system, and is more feasibly explained by a group of two or more galaxies with separations ~ 100 kpc. The impact of these findings on single line of sight observations is also discussed; we show that abundances of DLAs may be affected by up to a few tenths of a dex by line of sight DLA blending. From a Keck ESI spectrum of the two quasars, we measure metal column densities for all five absorbers and determine abundances for the three absorbers with log N(HI) > 20. For the two highest N(HI) absorbers, we determine high levels of metal enrichment, corresponding to 1/3 and 1/5 solar. These metallicities are amongst the highest measured for DLAs at any redshift and are consistent with values measured in Lyman break galaxies at 2 < z < 3. For the DLA at z = 2.94 we also infer an approximately solar ratio of alpha-to-Fe peak elements from [S/Zn] = +0.05, and measure an upper limit for the molecular fraction in this particular line of sight of log f(H_2)< -5.5.

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arXiv2007

Baryonic Pinching of Galactic Dark Matter Halos

High resolution cosmological N-body simulations of four galaxy-scale dark matter halos are compared to corresponding N-body/hydrodynamical simulations containing dark matter, stars and gas. The simulations without baryons share features with others described in the literature in that the dark matter density slope continuously decreases towards the center, with a density ρ~r^{-1.3+/-0.2}, at about 1% of the virial radius for our Milky Way sized galaxies. The central cusps in the simulations which also contain baryons steepen significantly, to ρ~r^{-1.9+/-0.2}, with an indication of the inner logarithmic slope converging. Models of adiabatic contraction of dark matter halos due to the central build-up of stellar/gaseous galaxies are examined. The simplest and most commonly used model, by Blumenthal et al., is shown to overestimate the central dark matter density considerably. A modified model proposed by Gnedin et al. is shown to be a considerable improvement, but not perfect. Moreover it is found that the contraction parameters not only depend on the orbital structure of the dark-matter-only halos but also on the stellar feedback prescription which is most relevant for the baryonic distribution. Implications for dark matter annihilation at the galactic center are discussed and it is found that although our simulations show a considerable reduced halo contraction as compared to the Blumenthal et al. model, the fluxes from dark matter annihilation is still expected to be enhanced by at least a factor of a hundred as compared to dark-matter-only halos. Finally, it is shown that while dark-matter-only halos are typically prolate, the dark matter halos containing baryons are mildly oblate with minor-to-major axis ratios of c/a=0.73+/-0.11, with their flattening aligned with the central baryonic disks.

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arXiv2006

The Tully-Fisher relation and its evolution with redshift in cosmological simulations of disc galaxy formation

We present predictions on the evolution of the Tully-Fisher (TF) relation with redshift, based on cosmological N-body/hydrodynamical simulations of disc galaxy formation and evolution. The simulations invoke star formation and stellar feedback, chemical evolution with non-instantaneous recycling, metallicity dependent radiative cooling and effects of a meta-galactic UV field, including simplified radiative transfer. At z=0, the simulated and empirical TF relations are offset by about 0.4 magnitudes (1 sigma) in the B and I bands. The origin of these offsets is somewhat unclear, but it may not necessarily be a problem of the simulations only. As to evolution, we find a brightening of the TF relation between z=0 and z=1 of about 0.85 mag in rest-frame B band, with a non-evolving slope. The brightening we predict is intermediate between the (still quite discrepant) observational estimates. This evolution is primarily a luminosity effect, while the stellar mass TF relation shows negligible evolution. The individual galaxies do gain stellar mass between z=1 and z=0, by a 50-100%; but they also correspondingly increase their characteristic circular speed. As a consequence, individually they mainly evolve ALONG the stellar mass TF relation, while the relation as such does not show any significant evolution.

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arXiv2006

Investigating hot gas in the halos of two massive spirals: Observations and cosmological simulations

Models of disk galaxy formation commonly predict the existence of an extended reservoir of hot gas surrounding massive spirals at low redshift. As a test of these models, we have obtained X-ray and optical data of the two massive edge-on spirals NGC 5746 and NGC 5170, in order to investigate the amount and origin of hot gas in their disks and halos. Chandra observations of NGC 5746 reveal evidence for diffuse X-ray emission with a total luminosity of ~7 x 10^39 erg/s surrounding this galaxy out to at least ~20 kpc from the disk, whereas an identical study of the less massive NGC 5170 fails to detect any extraplanar X-ray emission. Unlike the case for other disk galaxies with detected X-ray halos, the halo emission around NGC 5746 is not accompanied by extraplanar H-alpha or radio emission, and there is no evidence for significant nuclear or starburst activity in the disk. In contrast to these other cases, the emission around NGC 5746 therefore appears to arise from the cooling of externally accreted material rather than from disk outflows. To verify this idea, we present results of cosmological simulations of galaxy formation and evolution, showing our observations to be in good agreement with expectations for cosmological accretion, while also confirming that the X-ray halos of other spirals do not fit well into an accretion scenario. We find that the estimated cooling rate of hot halo gas around NGC 5746 would provide sufficient material for star formation in the disk to proceed at its present rate. This lends support to the idea that a supply of hot ambient gas is potentially available as fuel for star formation in massive, nearby spirals, and suggests that accretion of hot gas could be important for maintaining the stellar disks of such galaxies.

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arXiv2006

Broadband UBVRI Photometry of Horizontal-Branch and Metal-Poor Candidates from the HK and Hamburg/ESO Surveys. I

We report broadband UBV and/or BVRI CCD photometry for a total of 1857 stars in the thick-disk and halo populations of the Galaxy. The majority of our targets were selected as candidate field horizontal-branch or other A-type stars (FHB/A, N = 576), or candidate low-metallicity stars (N = 1221), from the HK and Hamburg/ESO objective-prism surveys. Similar data for a small number of additional stars from other samples are also reported. These data are being used for several purposes. In the case of the FHB/A candidates they are used to accurately separate the lower-gravity FHB stars from various higher-gravity A-type stars, a subsample that includes the so-called Blue Metal Poor stars, halo and thick-disk blue stragglers, main-sequence A-type dwarfs, and Am and Ap stars. These data are also being used to derive photometric distance estimates to high-velocity hydrogen clouds in the Galaxy and for improved measurements of the mass of the Galaxy. Photometric data for the metal-poor candidates are being used to refine estimates of stellar metallicity for objects with available medium-resolution spectroscopy, to obtain distance estimates for kinematic analyses, and to establish initial estimates of effective temperature for analysis of high-resolution spectroscopy of the stars for which this information now exists.

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arXiv2006

Escape of ionizing radiation from star forming regions in young galaxies

Using results from high-resolution galaxy formation simulations in a standard Lambda-CDM cosmology and a fully conservative multi-resolution radiative transfer code around point sources, we compute the energy-dependent escape fraction of ionizing photons from a large number of star forming regions in two galaxies at five different redshifts from z=3.8 to 2.39. All escape fractions show a monotonic decline with time, from (at the Lyman-limit) ~6-10% at z=3.6 to ~1-2% at z=2.39, due to higher gas clumping at lower redshifts. It appears that increased feedback can lead to higher f_esc at z>3.4 via evacuation of gas from the vicinity of star forming regions and to lower f_esc at z<2.39 through accumulation of swept-up shells in denser environments. Our results agree well with the observational findings of \citet{inoue..06} on redshift evolution of f_esc in the redshift interval z=2-3.6.

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arXiv2006

The density profiles of hot galactic halo gas

Extended gas haloes around galaxies are a ubiquitous prediction of galaxy formation scenarios. However, the density profiles of this hot halo gas is virtually unknown, although various profiles have been suggested on theoretical grounds. In order to quantitatively address the gas profile, we compare galaxies from direct cosmological simulations with analytical solutions of the underlying gas equations. We find remarkable agreement between simulations and theoretical predictions. We present an expression for this gas profile with a non-trivial dependence on the total mass profile. This expression is useful when setting up equilibrium galaxy models for numerical experiments.

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arXiv2006

A Lyman-alpha blob in the GOODS South field: evidence for cold accretion onto a dark matter halo

We report on the discovery of a z = 3.16 Lyman-alpha emitting blob in the GOODS South field. The blob has a total Ly-alpha luminosity of ~ 10^(43) erg s^(-1) and a diameter larger than 60 kpc. The available multi-wavelength data in the GOODS field consists of 13 bands from X-rays (Chandra) to infrared (Spitzer). Unlike other discovered Ly-alpha blobs, this blob shows no obvious continuum counter-part in any of the broad-bands. In particular, no optical counter-parts are found in the deep HST/ACS imaging available. For previously published blobs, AGN (Active Galactic Nuclei) or 'superwind' models have been found to provide the best match with the data. We here argue that the most probable origin of the extended Ly-alpha emission from the blob in the GOODS South field is cold accretion onto a dark matter halo.

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arXiv2006

Properties of Intra-group Stars and Galaxies in Galaxy Groups: "Normal" versus "Fossil" Groups

Cosmological LCDM simulations of 12 M_vir~10^14 Msun galaxy groups have been performed, invoking star formation, chemical evolution with non-instantaneous recycling, metallicity dependent radiative cooling, strong star-burst driven galactic super-winds and effects of a meta-galactic UV field. At z=0, intra-group light (IGL) fractions are found to be 12-45%. Low values refer to groups with only a small difference between the R-band magnitudes of the first and second ranked group galaxy, large are typical of "fossil" groups (FGs). The IG stars in the 4 FGs are 0.3-0.5 Gyr older than in the 8 nonFGs. For the IGL, B-R=~1.4, in good agreement with observations. For FGs/nonFGs the iron abundance of the IG stars is slightly sub-solar in the central parts (r~100 kpc) decreasing to about 40% solar at about 0.5 r_vir The IG stars are alpha-element enhanced with [O/Fe] increasing with r, and an overall [O/Fe]~0.45, indicating predominant SNII enrichment. The velocity distributions of the IG stars and group galaxies are, at r>~30 kpc, significantly more radially anisotropic for FGs than for nonFGs. So a characteristic of FG formation, apart from formation time (D'Onghia et al.), may be the "initial" velocity distribution of the group galaxies. For FGs one can dynamically infer the (dark matter dominated) mass distribution of the groups all the way to r_vir, from the kinematics of the IG stars or group galaxies. For the nonFGs this method overestimates the group mass at r>~200 kpc, by up to a factor of two at r_vir. This is interpreted as FGs being, in general, more relaxed than nonFGs. Finally, FGs of the above M_vir should host ~500 planetary nebulae at projected distances between 100 and 1000 kpc from the first ranked galaxy. All results appear consistent with the FG formation scenario of D'Onghia et al.

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arXiv2006

Formation of Disk Galaxies: Warm Dark Matter and the Angular Momentum problem

We have performed TreeSPH simulations of disk galaxy formation in various warm dark matter (WDM) cosmologies. Our results indicate that for a range of WDM free-streaming masses, the disk galaxy formation angular momentum problem can be completely resolved by going to the WDM structure formation scenario, without having to invoke stellar feedback processes at all. We also confirm our previous suspicion, that part of the angular momentum problem is due to numerical effects, most likely related to the shock capturing, artificial viscosity used in SPH. Furthermore we find that we can match the observed I-band Tully-Fisher (TF) relation, provided that the I-band mass-to-light ratio of disk galaxies is about 0.8. We argue that this is quite a reasonable value in comparison with various dynamical and spectrophotometric estimates, including one given in this paper. We speculate that our success in matching the TF relation may be due to WDM halos being less centrally concentrated than CDM halos and suggest to check this exciting possibility with high resolution simulations, in particular in low Omega_M, WDM cosmologies. Finally, we discuss possible physical candidates for WDM particles extensively. We find that the most promising are neutrinos with weaker or stronger interactions than normal, majorons (light pseudogoldstone bosons) or mirror or shadow world neutrinos.

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arXiv2006

Where are the ``Missing'' Galactic Baryons?

Based on 19 high-resolution N-body/gas-dynamical galaxy formation simulations in the LCDM cosmology it is shown, that for a galaxy like the Milky Way, in addition to the baryonic mass of the galaxy itself, about 70% extra baryonic mass should reside around the galaxy (inside of the virial radius), chiefly in the form of hot gas. Averaging over the entire field galaxy population, this ``external'' component amounts to 64-85% of the baryonic mass of the population itself. These results are supported by the recent detection of very extended, soft X-ray emission from the halo of the quiescent, massive disk galaxy NGC 5746. Some of the hot gas may, by thermal instability, have condensed into mainly pressure supported, warm clouds, similar to the Galactic High Velocity Clouds (HVCs). Based on an ultra-high resolution cosmological test simulation of a Milky Way like galaxy (with a gas particle mass and gravity softening length of only 7600 h^-1 Msun and 83 h^-1 pc,respectively), it is argued, that the hot gas phase dominates over the warm gas phase, in the halo. Finally, an origin of HVCs as ``leftovers'' from filamentary, ``cold'' accretion events, mainly occurring early in the history of galaxies, is proposed.

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arXiv2006

Discovery of a very extended X-ray halo around a quiescent spiral galaxy - the "missing link" of galaxy formation

Hot gaseous haloes surrounding galaxies and extending well beyond the distribution of stars are a ubiquitous prediction of galaxy formation scenarios. The haloes are believed to consist of gravitationally trapped gas with a temperature of millions of Kelvin. The existence of such hot haloes around massive elliptical galaxies has been established through their X-ray emission. While gas out-flowing from starburst spiral galaxies has been detected, searches for hot haloes around normal, quiescent spiral galaxies have so far failed, casting doubts on the fundamental physics in galaxy formation models. Here we present the first detection of a hot, large-scale gaseous halo surrounding a normal, quiescent spiral galaxy, NGC 5746, alleviating a long-standing problem for galaxy formation models. In contrast to starburst galaxies, where the X-ray halo can be powered by the supernova energy, there is no such power source in NGC 5746. The only compelling explanation is that we are here witnessing a galaxy forming from gradually in-flowing hot and dilute halo gas.

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arXiv2005

Soft X-rays from disk galaxy halos, Ly-alpha emission from forming galaxies, and the z~1 Tully-Fisher relation

Extended, soft X-ray emission from the halo of a very large disk galaxy has been detected out to r~20-40 kpc. The luminosity and surface brightness distribution is in very good agreement with predictions by recent, cosmological galaxy formation models. Predictions of Ly-alpha emission, associated with ``cold'' accretion of filamentary gas onto galaxies, are presented. In particular these predictions are compared to the properties of a recently detected Ly-alpha ``blob'', which shows no obvious continuum counter-part in any of 12 broad-bands, from X-rays (Chandra) to infrared (Spitzer). Finally, the predicted evolution of the Tully-Fisher relation, going from z=0 to 1, is discussed in relation to recent observations.

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arXiv2005

Simulating galaxy Clusters -II: global star formation histories and galaxy populations

Cosmological (LambdaCDM) TreeSPH simulations of the formation and evolution of galaxy groups and clusters have been performed. The simulations invoke star formation, chemical evolution with non-instantaneous recycling, metal dependent radiative cooling, strong star burst and (optionally) AGN driven galactic super winds, effects of a meta-galactic UV field and thermal conduction. The properties of the galaxy populations in two clusters, one Virgo-like (T~3 keV) and one (sub) Coma-like (T~6 keV), are discussed. The global star formation rates of the cluster galaxies are found to decrease very significantly with time from redshift z=2 to 0, in agreement with observations. The total K-band luminosity of the cluster galaxies correlates tightly with total cluster mass, and for models without additional AGN feedback, the zero point of the relation matches the observed one fairly well. The match to observed galaxy luminosity functions is reasonable, except for a deficiency of bright galaxies (M_B < -20), which becomes increasingly significant with super-wind strength. Results of a high resolution test indicate that this deficiency is not due to ``over--merging''. The redshift evolution of the luminosity functions from z=1 to 0 is mainly driven by luminosity evolution, but also by merging of bright galaxies with the cD. The colour--magnitude relation of the cluster galaxies matches the observed "red sequence" very well and, on average, galaxy metallicity increases with luminosity. As the brighter galaxies are essentially coeval, the colour--magnitude relation results from metallicity rather than age effects, as observed.

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arXiv2004

Simulating Galaxies and Galaxy Clusters

The X-ray luminosities of the hot halo gas around simulated, Milky Way like disk galaxies have been determined, as a function of redshift. The X-ray luminosity increases significantly with redshift, in some cases as much as a factor 30 going from z=0 to 2. Consequently, the optimal detection redshift can be >~0.5. Results of fully cosmological simulations of galaxy groups and clusters, incorporating star formation, chemical evolution with non-instantaneous recycling, metal-dependent radiative cooling, galactic super-winds and thermal conduction are presented. X-ray luminosities at z=0 are somewhat high and central entropies somewhat low compared to observations. This is likely a combined effect of chemical evolution and metal-dependent, radiative cooling. Central ICM abundance profiles are somewhat steep, and the observed level of ICM enrichment can only be reproduced with IMFs more top-heavy than the Salpeter IMF. In agreement with observations it is found that the iron in the ICM is in place already at z~1. The [Si/Fe] of the ICM decreases with time, and increases slightly with radius at a given time. The cluster galaxies match the observed "red sequence" very well, and the metallicity of cluster galaxies increases with galaxy mass, as observed.

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arXiv2004

High-redshift X-ray properties of the haloes of simulated disc galaxies

X-ray luminosities and surface brightness profiles of the hot gas haloes of simulated disc galaxies at redshifts z=0-2 are presented. The galaxies are extracted from fully cosmological simulations and correspond in mass to the Milky Way. We find that the bolometric X-ray luminosities of the haloes decrease by a factor 4-10 from z~1 to z~0, reflecting the decrease in the rate at which hot halo gas cools out on to the disc. At all redshifts, most of the emission is found to originate within 10-15 kpc of the disc. When combined with models in which the evolution of disc X-ray luminosity is dominated by X-ray binaries, the predicted halo luminosities at z~1 show good agreement with constraints from spiral galaxies in Chandra Deep Field data. There is an indication that haloes with a metal abundance of 0.3 solar overpredict observed X-ray luminosities at z~1, suggesting that halo metallicities are lower than this value. Prospects for direct detection of the haloes of Milky Way-sized galaxies with current and future X-ray instrumentation are discussed. It is found that XEUS should be able to single out the halo emission of highly inclined Milky Way-sized disc galaxies out to approximately z=0.3. For such galaxies in this redshift interval, we estimate a lower limit to the surface density of detectable haloes on the sky of ~10 deg^(-2). More generally, owing to their luminosity evolution, the optimum redshifts at which to observe such haloes could be 0.5<z<1, depending on their assembly history.

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arXiv2003

Galaxy Formation: CDM, Feedback and the Hubble Sequence

TreeSPH simulations of galaxy formation in a LCDM cosmology, with star formation, energetic stellar feedback and a meta-galactic UV field have been performed, resulting in realistic disk, S0 and E galaxies at z=0. The disk galaxies are deficient in angular momentum by only about a factor of two compared to observations for runs where fairly strong star-bursts in early, proto-galactic clouds lead to "blow-away" of the remaining gas. The surface density profiles of the stellar disks are approximately exponential and those of the bulges range from exponential to r^(1/4). B/D ratios and integrated B-V colours are consistent with observations. The observed I-band TF relation can be matched with M/L_I ~ 0.8, in fair agreement with recent determinations. The (E/S0)s have approximately r^(1/4) profiles, non disk-like kinematics and are flattened due to non-isotropic stellar velocity distributions. We predict hot halo gas to cool out and accrete onto the Galactic disk at 0.5-1 Msun/yr at z=0, consistent with upper limits from FUSE observations of O VI. We analyzed two Milky Way-sized galaxies and find accretion rates, and X-ray halo luminosities, 6-7 times larger at z=1 than at z=0. The gas infall declines nearly exponentially with time, supporting the approximation often used in chemical evolution models. The infall time-scales are comparable to what is used to solve the "G-dwarf problem". One disk forms "inside-out", the other "outside-in". For both, the mean stellar ages in the outskirts agree with findings for the disk of M31. The amount of hot gas in disk galaxy haloes is consistent with observational upper limits, as are dispersion measures to pulsars in the globular cluster M53 and the LMC, which were "inserted" in the disk galaxy haloes.

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