arXiv subjects

J. Sommer-Larsen

Publications and source records attributed to J. Sommer-Larsen.

At least 19 recordsLinked to original sources

arXiv2016

The high-redshift evolution of the Red Sequence scatter from joint simulations and HAWK-I Cluster Survey

We study the evolution of the Red Sequence (RS) scatter in galaxy clusters and groups simultaneously using predictions from our simulations (cosmological hydrodynamic + semi-analytical) as well as observational data from the HAWK-I Cluster Survey (HCS), a sample of galaxy clusters at redshifts $0.8 < z < 1.5$. We analyze the intrinsic scatter of the RS to investigate whether the stellar age can be its main effective driver, at the same time assessing the role of metallicity variations in shaping the RS building at around epoch $z\sim$1 and beyond. To this purpose we rely on various methods to derive the average age and age spread from the RS colour scatter of the HCS sample, with the aid of population synthesis models. The RS scatter predicted by the models at $z< 0.7$ is found to not depend on the star formation history adopted, whilst at $z>the correlation found between age and rest-frame colour scatters is quite robust, although all age scatter estimations ultimately depend on the definition of RS as well as on the completeness limits adopted. We find that the age spread of RS galaxies predicted by both hydrodynamical simulations and SAM increases with cosmic epoch, while the ratio between the age spread and the average age remains approximately constant. Both trends are in agreement with observational results from both the HCS and other literature samples.

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arXiv2016

The galaxy counterpart of the high-metallicity and 16 kpc impact parameter DLA towards Q0918+1636 - a challenge to galaxy formation models?

The quasar Q0918+1636 (z=3.07) has an intervening high-metallicity Damped Lyman-alpha Absorber (DLA) along the line of sight, at a redshift of z=2.58. The DLA is located at a large impact parameter of 16.2 kpc, and has an almost solar metallicity. It is shown, that a novel type of cosmological galaxy formation models, invoking a new SNII feedback prescription, the Haardt & Madau (2012) UVB field and explicit treatment of UVB self-shielding, can reproduce the observed characteristics of the DLA. UV radiation from young stellar populations in the galaxy, in particular in the photon energy range 10.36-13.61 eV (relating to Sulfur II abundance), are also considered in the analysis. It is found that a) for L~L* galaxies (at z=2.58), about 10% of the sight-lines through the galaxies at impact parameter 16.2 kpc will display a Sulfur II column density N(SII)$>$ 10$^{15.82}$ cm$^{-2}$ (the observed value for the DLA), and b) considering only cases where a near-solar metallicity will be detected at 16.2 kpc impact parameter, the probability distribution of galaxy SFR peaks near the value observed for the DLA galaxy counterpart of ~27 Msun/yr. It is argued, that the bulk of the alpha-elements, like Sulfur, traced by the high metal column density, b=16.2 kpc absorption lines, were produced by evolving young stars in the inner galaxy, and later transported outward by galactic winds.

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arXiv2015

A study on the multicolour evolution of Red Sequence galaxy populations: insights from hydrodynamical simulations and semi-analytical models

By means of our own cosmological-hydrodynamical simulation and semi-analytical model we studied galaxy population properties in clusters and groups, spanning over 10 different bands from UV to NIR, and their evolution since redshift z=2. We compare our results in terms of galaxy red/blue fractions and luminous-to-faint ratio (LFR) on the Red Sequence (RS) with recent observational data reaching beyond z=1.5. Different selection criteria were tested in order to retrieve galaxies belonging to the RS: either by their quiescence degree measured from their specific SFR ("Dead Sequence"), or by their position in a colour-colour plane which is also a function of sSFR. In both cases, the colour cut and the limiting magnitude threshold were let evolving with redshift, in order to follow the natural shift of the characteristic luminosity in the LF. We find that the Butcher-Oemler effect is wavelength-dependent, with the fraction of blue galaxies increasing steeper in optical colours than in NIR. Besides, only when applying a lower limit in terms of fixed absolute magnitude, a steep BO effect can be reproduced, while the blue fraction results less evolving when selecting samples by stellar mass or an evolving magnitude limit. We then find that also the RS-LFR behaviour, highly debated in the literature, is strongly dependent on the galaxy selection function: in particular its very mild evolution recovered when measured in terms of stellar mass, is in agreement with values reported for some of the highest redshift confirmed (proto)clusters. As to differences through environments, we find that normal groups and (to a lesser extent) cluster outskirts present the highest values of both star forming fraction and LFR at low z, while fossil groups and cluster cores the lowest: this separation among groups begins after z~0.5, while earlier all group star forming properties are undistinguishable.

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arXiv2014

Evolution of the Mass-Metallicity relations in passive and star-forming galaxies from SPH-cosmological simulations

We present results from SPH-cosmological simulations, including self-consistent modelling of SN feedback and chemical evolution, of galaxies belonging to two clusters and twelve groups. We reproduce the mass-metallicity (ZM) relation of galaxies classified in two samples according to their star-forming activity, as parametrized by their sSFR, across a redshift range up to z=2. Its slope shows irrelevant evolution in the passive sample, being steeper in groups than in clusters. However, the sub-sample of high-mass passive galaxies only is characterized by a steep increase of the slope with redshift, from which it can be inferred that the bulk of the slope evolution of the ZM relation is driven by the more massive passive objects. (...ABRIDGED...) The ZM relation for the star-forming sample reveals an increasing scatter with redshift, indicating that it is still being built at early epochs. The star-forming galaxies make up a tight sequence in the SFR-M_* plane at high redshift, whose scatter increases with time alongside with the consolidation of the passive sequence. We also confirm the anti-correlation between sSFR and stellar mass, pointing at a key role of the former in determining the galaxy downsizing, as the most significant means of diagnostics of the star formation efficiency. Likewise, an anti-correlation between sSFR and metallicity can be established for the star-forming galaxies, while on the contrary more active galaxies in terms of simple SFR are also metal-richer. We discuss these results in terms of the mechanisms driving the evolution within the high- and low-mass regimes at different epochs: mergers, feedback-driven outflows and the intrinsic variation of the star formation efficiency.

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arXiv2010

High-Redshift Star-Forming Galaxies: Angular Momentum and Baryon Fraction, Turbulent Pressure Effects and the Origin of Turbulence

The structure of a sample of high-redshift (z=2), rotating galaxies with high star formation rates and turbulent gas velocities of sigma=40-80 km/s is investigated. Fitting the observed disk rotational velocities and radii with a Mo, Mao, White (1998) (MMW) model requires unusually large disk spin parameters lambda_d>0.1 and disk-to-dark halo mass fraction m_d=0.2, close to the cosmic baryon fraction. The galaxies segregate into dispersion-dominated systems with 1 200 km/s, vmax/sigma>3 and rd=4-8 kpc. For the dispersion-dominated sample, radial pressure gradients partly compensate the gravitational force, reducing the rotational velocities. Including this pressure effect in the MMW model, dispersion-dominated galaxies can be fitted well with spin parameters lf lambda_d=0.03-0.05 for high disk mass fractions of m_d=0.2 and with lambda_d=0.01-0.03 for m_d=0.05. These values are in good agreement with cosmological expectations. For the rotation-dominated sample however pressure effects are small and better agreement with theoretically expected disk spin parameters can only be achieved if the dark halo mass contribution in the visible disk regime (2-3*rd) is smaller than predicted by the MMW model. We argue that these galaxies can still be embedded in standard cold dark matter halos if the halos did not contract adiabatically in response to disk formation. It is shown that the observed high turbulent gas motions of the galaxies are consistent with a Toomre instability parameter Q=1 which is equal to the critical value, expected for gravitational disk instability to be the major driver of turbulence. The dominant energy source of turbulence is then the potential energy of the gas in the disk.

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arXiv2010

Stellar population gradients from cosmological simulations: dependence on mass and environment in local galaxies

The age and metallicity gradients for a sample of group and cluster galaxies from N-body+hydrodynamical simulation are analyzed in terms of galaxy stellar mass. Dwarf galaxies show null age gradient with a tail of high and positive values for systems in groups and cluster outskirts. Massive systems have generally zero age gradients which turn to positive for the most massive ones. Metallicity gradients are distributed around zero in dwarf galaxies and become more negative with mass; massive galaxies have steeper negative metallicity gradients, but the trend flatten with mass. In particular, fossil groups are characterized by a tighter distribution of both age and metallicity gradients. We find a good agreement with both local observations and independent simulations. The results are also discussed in terms of the central age and metallicity, as well as the total colour, specific star formation and velocity dispersion.

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arXiv2009

Lyman alpha Resonant Scattering in Young Galaxies - Predictions from Cosmological Simulations

We present results obtained with a 3D, Ly alpha radiative transfer code, applied to a fully cosmological galaxy formation simulation. The developed Monte Carlo code is capable of treating an arbitrary distribution of source Ly alpha emission, neutral hydrogen density, temperature, and peculiar velocity of the interstellar medium. We investigate the influence of resonant scattering on the appearance and properties of young galaxies by applying the code to a simulated "Lyman Break Galaxy" at redshift z = 3.6, and of star formation rate 22 M_sun/yr and total Ly alpha luminosity 2.0 X 10^43 erg/s. It is found that resonant scattering of Ly alpha radiation can explain that young galaxies frequently are observed to be more extended on the sky in Ly alpha than in the optical. Moreover, it is shown that, for the system investigated, due to the anisotropic escape of the photons, the appearent maximum surface brightness can differ by a factor of ~15, and the total derived luminosity by a factor of ~4, depending on the orientation of the system relative to the observer.

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arXiv2009

The SINS survey: modeling the dynamics of z~2 galaxies and the high-z Tully-Fisher relation

We present the modeling of SINFONI integral field dynamics of 18 star forming galaxies at z ~ 2 from Halpha line emission. The galaxies are selected from the larger sample of the SINS survey, based on the prominence of ordered rotational motions with respect to more complex merger induced dynamics. The quality of the data allows us to carefully select systems with kinematics dominated by rotation, and to model the gas dynamics across the whole galaxy using suitable exponential disk models. We obtain a good correlation between the dynamical mass and the stellar mass, finding that large gas fractions Mgas~M*) are required to explain the difference between the two quantities. We use the derived stellar mass and maximum rotational velocity Vmax from the modeling to construct for the first time the stellar mass Tully-Fisher relation at z ~ 2.2. The relation obtained shows a slope similar to what is observed at lower redshift, but we detect an evolution of the zero point. We find that at z ~ 2.2 there is an offset in log(M*) for a given rotational velocity of 0.41+-0.11 with respect to the local Universe. This result is consistent with the predictions of the latest N-body/hydrodynamical simulations of disk formation and evolution, which invoke gas accretion onto the forming disk in filaments and cooling flows. This scenario is in agreement with other dynamical evidence from SINS, where gas accretion from the halo is required to reproduce the observed properties of a large fraction of the z ~ 2 galaxies.

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arXiv2008

Strong Lensing in Abell 1703: Constraints on the Slope of the Inner Dark Matter Distribution

In this article, we apply strong lensing techniques in Abell 1703, a massive X-ray luminous galaxy cluster at z=0.28. Our analysis is based on imaging data both from space and ground in 8 bands, complemented with a spectroscopic survey. Abell 1703 looks rather circular from the general shape of its multiply imaged systems and present a dominant giant elliptical cD galaxy in its centre. This cluster exhibits a remarkable bright 'central ring' formed by 4 bright images at z_{spec}=0.888 located very close to the cD galaxy, providing observational constraints that are potentially very interesting to probe the central mass distribution. The stellar contribution from the cD galaxy (~1.25 10^{12} M_{sun} within 7") is accounted for in our parametric mass modelling, and the underlying smooth dark matter component distribution is described using a generalized NFW profile parametrized with a central logarithmic slope α. We find that within the range where observational constraints are present (from ~5" to ~50"), the slope of the dark matter distribution in Abell1703 is equal to 1.09^{+0.05}_{-0.11} (3σconfidence level). The concentration parameter is equal to c_{200} ~ 3.5, and the scale radius is constrained to be larger than the region where observational constraints are available. Within this radius, the 2D mass is equal to M(50")=2.4 10^{14} M_{\sun}. We cannot draw any conclusions on cosmological models at this point since we lack results from realistic numerical simulations containing baryons to make a proper comparison. We advocate the need for a sample of observed and simulated unimodal relaxed galaxy clusters in order to make reliable comparisons, and potentially provide a test of cosmological models.

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arXiv2008

The evolution of the galaxy Red Sequence in simulated clusters and groups

N-body + hydrodynamical simulations of the formation and evolution of galaxy groups and clusters in a LambdaCDM cosmology are used in order to follow the building-up of the colour-magnitude relation in two clusters and in 12 groups. We have found that galaxies, starting from the more massive, move to the Red Sequence (RS) as they get aged over times and eventually set upon a ``dead sequence'' (DS) once they have stopped their bulk star formation activity. Fainter galaxies keep having significant star formation out to very recent epochs and lie broader around the RS. Environment plays a role as galaxies in groups and cluster outskirts hold star formation activity longer than the central cluster regions. However galaxies experiencing infall from the outskirts to the central parts keep star formation on until they settle on to the DS of the core galaxies. Merging contributes to mass assembly until z~1, after which major events only involve the brightest cluster galaxies. The emerging scenario is that the evolution of the colour-magnitude properties of galaxies within the hierarchical framework is mainly driven by star formation activity during dark matter halos assembly. Galaxies progressively quenching their star formation settle to a very sharp ``red and dead'' sequence, which turns out to be universal, its slope and scatter being almost independent of the redshift (since at least z~1.5) and environment. Differently from the DS, the operatively defined RS evolves more evidently with z, the epoch when it changes its slope being closely corresponding to that at which the passive galaxies population takes over the star forming one: this goes from z~1 in clusters down to 0.4 in normal groups.

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arXiv2008

CO line emission from Lyman Break Galaxies. Cosmological simulations and predictions for ALMA

[Abridged] The detection of the rotational lines of CO in proto-galaxies in the early Universe provides one of the most promising ways of probing the fundamental physical properties of a galaxy, such as its size, dynamical mass, gas density, and temperature. While such observations are currently limited to the most luminous galaxies, the advent of ALMA will change the situation dramatically, resulting in the detection of numerous normal galaxies at high redshifts. Maps and spectra of rotational CO line emission were calculated from a cosmological N-body/hydrodynamical TreeSPH simulation of a z ~ 3 "Lyman break galaxy'' of UV luminosity about one order of magnitude below L*. To simulate a typical observation of our system with ALMA, we imposed characteristic noise, angular, and spectral resolution constraints. The CO line properties predicted by our simulation are in good agreement with the two Lyman break systems detected in CO to date. We find that while supernovae explosions from the ongoing star formation carve out large cavities in the molecular ISM, they do not generate large enough gas outflows to make a substantial imprint on the CO line profile. This implies that for most proto-galaxies - except possibly the most extreme cases - stellar feedback effects do not affect CO as a dynamical mass tracer. Detecting CO in sub-L* galaxies at z ~3 will push ALMA to the limits of its cababilities, and whether a source is detected or not may depend critically on its inclination angle. Both these effects (sensitivity and inclination) will severely impair the ability of ALMA to infer the gas kinematics and dynamical masses using line observations.

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arXiv2006

Evolution of the Red Sequence in simulated galaxy groups

N--body + hydrodynamical simulations of formation and evolution of galaxy groups in a LambdaCDM cosmology have been performed. The properties of the galaxy populations in 12 groups are here discussed, with focus on the colour-magnitude relation in both normal and fossil groups.

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arXiv2006

Simulating galaxy clusters -- I. Thermal and chemical properties of the intra-cluster medium

We have performed a series of N-body/hydrodynamical (TreeSPH) simulations of clusters and groups of galaxies, selected from cosmological N-body simulations within a $Λ$CDM framework: these objects have been re-simulated at higher resolution to $z$=0, in order to follow also the dynamical, thermal and chemical input on to the ICM from stellar populations within galaxies. The simulations include metal dependent radiative cooling, star formation according to different IMFs, energy feedback as strong starburst-driven galactic super-winds, chemical evolution with non-instantaneous recycling of gas and heavy elements, effects of a meta-galactic UV field and thermal conduction in the ICM. In this Paper I of a series of three, we derive results, mainly at $z=0$, on the temperature and entropy profiles of the ICM, its X-ray luminosity, the cluster cold components (cold fraction as well as mass--to--light ratio) and the metal distribution between ICM and stars. In general, models with efficient super-winds, along with a top-heavy stellar IMF, are able to reproduce fairly well the observed $L_X-T$ relation, the entropy profiles and the cold fraction. Observed radial ICM temperature profiles can be matched, except for the gradual decline in temperature inside of $r\sim$~0.1$R_{\rm{vir}}$. Metal enrichment of the ICM gives rise to somewhat steep inner iron gradients; yet, the global level of enrichment compares well to observational estimates after correcting for the stars formed at late times at the base of the cooling flows; also the metal partition between stars and ICM gets into good agreement with observations.

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arXiv2006

Simulating Galaxy Clusters - III: Properties of the Intracluster Stars

Cosmological (LCDM) TreeSPH simulations of galaxy clusters have been performed, invoking star-formation, chemical evolution with non-instantaneous recycling, metal dependent radiative cooling, strong star-burst and/or AGN driven super-winds, effects of a meta-galactic UV field and thermal conduction. Results for two clusters, one Virgo-like (T~3 keV) and one (sub) Coma-like (T~6 keV), are presented. At z=0, the stellar contents of both clusters consist of a central dominant (cD) galaxy surrounded by galaxies and intracluster (IC) stars. The IC stars are found to contribute 20-40% of the total cluster B-band light and to form at a mean redshift z_f~3, on average about 0.5 Gyr prior to the stars in galaxies. UBVRIJHK surface brightness profiles of the IC star populations are presented; the profile of the larger cluster matches the observed V-band profile of the cD in Abell 1413 (T~8 keV). The IC stellar populations have B-R=1.4-1.5, comparable to larger E galaxies. The mean [Fe/H]~0.0 in the central part of the cluster (r~100 kpc) decreasing to about -0.3 at r_vir. The IC stars are alpha-element enhanced, with a weak trend of [O/Fe] increasing with r, and an overall [O/Fe]~0.4 dex. The IC stars are kinematically colder than the galaxies: The velocity dispersions of the IC stars are at r~100-500 kpc only about half of those of the galaxies increasing to about 70% at r=1-2 Mpc. The projected velocity dispersion in the Virgo-like cluster at R>~50 kpc is 300-600 km/s depending on orientation and R. Rotation is dynamically insignificant for the IC stars. The velocity distributions of IC stars and galaxies are in one cluster highly radially anisotropic, in the other close to isotropic.

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arXiv2005

The Formation of Fossil Galaxy Groups in the hierarchical Universe

We use a set of twelve high-resolution N-body/hydrodynamical simulations in the $Λ$CDM cosmology to investigate the origin and formation rate of fossil groups (FGs), which are X-ray bright galaxy groups dominated by a large elliptical galaxy, with the second brightest galaxy being at least two magnitudes fainter. The simulations invoke star formation, chemical evolution with non-instantaneous recycling, metal dependent radiative cooling, strong star burst driven galactic super winds, effects of a meta-galactic UV field and full stellar population synthesis. We find an interesting correlation between the magnitude gap between the first and second brightest galaxy and the formation time of the group. It is found that FGs have assembled half of their final dark matter mass already at $z\ga1$, and subsequently typically grow by minor merging only, wheras non-FGs on average form later. The early assembly of FGs leaves sufficient time for galaxies of $L \sim L_*$ to merge into the central one by dynamical friction, resulting in the large magnitude gap at $z=0$. A fraction of 33$\pm$16% of the groups simulated are found to be fossil, whereas the observational estimate is $\sim$10-20%. The FGs are found to be X-ray over-luminous relative to non-FGs of the same optical luminosity, in qualitative agreement with observations. Finally, from a dynamical friction analysis is found that only because infall of $L \sim L_*$ galaxies happens along filaments with small impact parameters do FGs exist at all.

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arXiv2005

Simulating galaxy clusters : the ICM and the galaxy populations

Cosmological LambdaCDM TreeSPH simulations of the formation and evolution of galaxy groups and clusters have been performed. The simulations include: 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. We report results on the temperature and entropy profiles of the ICM, the X-ray luminosity, cold fraction, M/L and IMLR ratios in gas and stars, metal abundances and gradients. Besides, the properties of the galaxy populations in the two richest clusters are discussed: global star formation rates of the cluster galaxies, the total K-band luminosity, the galaxy luminosity functions at z=0 and their redshift evolution, the colour-magnitude relation (``red sequence'') as resulting from metallicity effects, and the role of the IMF in reproducing colours and abundances of the stellar populations. Finally the contribute (20-40 %) to cluster light from the intra-cluster stars and the cD galaxy has been investigated: surface brightness profiles, mean colours and chemical abundances, kinematics (velocity dispersion and distributions) of the IC stellar populations.

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arXiv2004

Mass-to-Light ratio, Initial Mass Function and chemical evolution in disc galaxies

Cosmological simulations of disc galaxy formation, when compared to the observed Tully-Fisher relation, suggest a low Mass-to-Light (M/L) ratio for the stellar component in spirals. We show that a number of "bottom-light" Initial Mass Functions (IMFs) suggested independently in literature, do imply M/L ratios as low as required, at least for late type spirals (Sbc/Sc). However the typical M/L ratio, and correspondingly the zero-point of the Tully-Fisher relation, is expected to vary considerably with Hubble type. Bottom-light IMFs tend to have a metal production in excess of what is tipically estimated for spiral galaxies. Suitable tuning of the IMF slope and mass limits, post-supernova fallback of metals onto black holes or metal outflows must then be invoked, to reproduce the observed chemical properties of disc galaxies.

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arXiv2003

Can a "standard" Initial Mass Function explain the metal enrichment in clusters of galaxies?

It is frequently debated in literature whether a "standard" Initial Mass Function (IMF) - meaning an IMF of the kind usually adopted to explain the chemical evolution in the local Solar Neighbourhood - can account for the observed metal enrichment and Iron Mass-to-Light Ratio in clusters of galaxies. We address this problem by means of straightforward estimates that should hold independently of the details of chemical evolution models. It is crucial to compute self-consistently the amount of mass and metals locked-up in stars, by accounting for the stellar mass-to-light ratio predicted by a given IMF. It becomes then clear that a "standard" Solar Neighbourhood IMF cannot provide enough metals to account for the observed chemical properties in clusters: clusters of galaxies and the local environment must be characterized by different IMFs. Alternatively, if we require the IMF to be universal, in order to explain clusters such an IMF must be much more efficient in metal production than usually estimated for the Solar Vicinity. In this case, substantial loss of metals is required from the Solar Neighbourhood and from disc galaxies in general. This "non-standard" scenario for the local chemical evolution would challenge our present understanding of the Milky Way and of disc galaxy formation.

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