arXiv subjects

Sadegh Khochfar

Publications and source records attributed to Sadegh Khochfar.

At least 127 records · Page 7Linked to original sources

arXiv2011

The ATLAS3D project - III. A census of the stellar angular momentum within the effective radius of early-type galaxies: unveiling the distribution of Fast and Slow Rotators

We provide a census of the apparent stellar angular momentum within 1 Re of a volume-limited sample of 260 early-type galaxies (ETGs) in the nearby Universe, using integral-field spectroscopy obtained in the course of the ATLAS3D project. We exploit the LambdaR parameter to characterise the existence of two families of ETGs: Slow Rotators which exhibit complex stellar velocity fields and often include stellar kinematically Distinct Cores (KDCs), and Fast Rotators which have regular velocity fields. Our complete sample of 260 ETGs leads to a new criterion to disentangle Fast and Slow Rotators which now includes a dependency on the apparent ellipticity (Epsilon). It separates the two classes significantly better than the previous prescription, and than a criterion based on V/Sigma: Slow Rotators and Fast Rotators have LambdaR lower and larger than kFSxSQRT(Epsilon), respectively, where kFS=0.31 for measurements made within 1 Re. We show that the vast majority of early-type galaxies are Fast Rotators: these have regular stellar rotation, with aligned photometric and kinematic axes (Paper II, Krajnovic et al. 2011}, include discs and often bars and represent 86% (224/260) of all early-type galaxies in the volume-limited ATLAS3D sample. Fast Rotators span the full range of apparent ellipticities from 0 to 0.85, and we suggest that they cover intrinsic ellipticities from about 0.35 to 0.85, the most flattened having morphologies consistent with spiral galaxies. Only a small fraction of ETGs are Slow Rotators representing 14% (36/260) of the ATLAS3D sample of ETGs. Of all Slow Rotators, 11% (4/36) exhibit two counter-rotating stellar disc-like components and are rather low mass objects (Mdyn<10^10.5 M_Sun). All other Slow Rotators (32/36) appear relatively round on the sky (Epsilon_e<0.4), tend to be massive (Mdyn>10^10.5 M_Sun), and often (17/32) exhibit KDCs.

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arXiv2011

The interplay between chemical and mechanical feedback from the first generation of stars

We study cosmological simulations of early structure formation, including non-equilibrium molecular chemistry, metal pollution from stellar evolution, transition from population III (popIII) to population II (popII) star formation, regulated by a given critical metallicity, and feedback effects. We investigate the properties of early metal spreading from the different stellar populations and its interplay with primordial molecular gas. We find that, independently of the details about popIII modeling, after the onset of star formation, regions enriched below the critical level are mostly found in isolated environments, while popII star formation regions are much more clumped. Typical star forming haloes show average SN driven outflow rates of up to 10^{-4} Msun/yr in enriched gas, initially leaving the original star formation regions almost devoid of metals. The polluted material, which is gravitationally incorporated in over-dense environments on timescales of 10^7 yr, is mostly coming from external, nearby star forming sites ("gravitational enrichment"). In parallel, the pristine-gas inflow rates are between 10^{-3} - 10^{-1} Msun/yr. However, thermal feedback from SN generates turbulence and destroys molecules within the pristine gas, and only the polluted material, incorporated via gravitational enrichment, can continue to cool by atomic metal fine-structure transitions on time scales short enough to end the initial popIII regime within less than 10^8 yr.

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arXiv2011

Gravity at Work: How the Build-Up of Environments Shape Galaxy Properties

We present results on the heating of the inter-cluster medium (ICM) by gravitational potential energy from in-falling satellites. We calculate the available excess energy of baryons once they are stripped from their satellite and added to the ICM of the hosting environment. this excess energy is a strong function of environment and we find that it can exceed the contribution from AGNs or supernovae (SN) by up to two orders of magnitude in the densest environments/haloes. Cooling by radiative losses is in general fully compensated by gravitational heating in massive groups and clusters with hot gas temperature > 1 keV. The reason for the strong environment dependence is the continued infall of substructure onto dense environments during their formation in contrast to field-like environments. We show that gravitational heating is able to reduce the number of too luminous galaxies in models and to produce model luminosity functions in agreement with observations.

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arXiv2010

The Impact of Gas Stripping and Stellar Mass Loss on Satellite Galaxy Evolution

Current semi-analytic models (SAMs) of galaxy formation over-predict the fraction of passive small late-type satellite galaxies in dense environments by a factor of two to three. We hypothesize that this is due to inaccurate prescriptions on cold gas evolution. In the hope of solving this problem we apply detailed prescriptions on the evolution of diffuse hot gases in satellites and on stellar mass loss, both of which are critical to model cold gas evolution. We replace the conventional shock-heating motivated instant stripping with a realistic gradual prescription based on ram pressure and tidal stripping. We also carefully consider stellar mass loss in our model. When both mechanisms are included, the fraction of passive late types matches the data much more closely. The satellite over-quenching problem is still present in small galaxies in massive haloes, however. In terms of the detectable residual star formation rates, gradual diffuse gas stripping appears to be much more important than stellar mass loss in our model. The implications of these results and other possibilities, such as redshift-dependent merging geometry and tidal disruption, are also discussed.

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arXiv2010

Suppression of accretion onto low-mass Population III stars

Motivated by recent theoretical work suggesting that a substantial fraction of Population (Pop) III stars may have had masses low enough for them to survive to the present day, we consider the role that the accretion of metal-enriched gas may have had in altering their surface composition, thereby disguising them as Pop II stars. We demonstrate that if weak, Solar-like winds are launched from low-mass Pop III stars formed in the progenitors of the dark matter halo of the Galaxy, then such stars are likely to avoid significant enrichment via accretion of material from the interstellar medium. We find that at early times accretion is easily prevented if the stars are ejected from the central regions of the haloes in which they form, either by dynamical interactions with more massive Pop III stars, or by violent relaxation during halo mergers. While accretion may still take place during passage through sufficiently dense molecular clouds at later times, we find that the probability of such a passage is generally low (< 0.1), assuming that stars have velocities of order the maximum circular velocity of their host haloes and accounting for the orbital decay of merging haloes. In turn, due to the higher gas density required for accretion onto stars with higher velocities, we find an even lower probability of accretion (~ 0.01) for the subset of Pop III stars formed at z > 10, which are more quickly incorporated into massive haloes than stars formed at lower redshift. While there is no a priori reason to assume that low-mass Pop III stars do not have Solar-like winds, without them surface enrichment via accretion is likely to be inevitable. We briefly discuss the implications that our results hold for stellar archaeology.

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arXiv2010

The Atlas3D project -- I. A volume-limited sample of 260 nearby early-type galaxies: science goals and selection criteria

The Atlas3D project is a multi-wavelength survey combined with a theoretical modeling effort. The observations span from the radio to the millimeter and optical, and provide multi-colour imaging, two-dimensional kinematics of the atomic (HI), molecular (CO) and ionized gas (Hbeta, [OIII] and [NI]), together with the kinematics and population of the stars (Hbeta, Fe5015 and Mgb), for a carefully selected, volume-limited (1.16*10^5 Mpc^3) sample of 260 early-type (elliptical E and lenticular S0) galaxies (ETGs). The models include semi-analytic, N-body binary mergers and cosmological simulations of galaxy formation. Here we present the science goals for the project and introduce the galaxy sample and the selection criteria. The sample consists of nearby (D<42 Mpc) morphologically-selected ETGs extracted from a parent sample of 871 galaxies (8% E, 22% S0 and 70% spirals) brighter than M_K<-21.5 mag (stellar mass M_Star>6*10^9 M_Sun). We analyze possible selection biases and we conclude that the parent sample is essentially complete and statistically representative of the nearby galaxy population. We present the size-luminosity relation for the spirals and ETGs and show that the ETGs in the Atlas3D sample define a tight red sequence in a colour-magnitude diagram, with few objects in the transition from the blue cloud. We describe the strategy of the SAURON integral-field observations and the extraction of the stellar kinematics with the pPXF method. We give an overview of the characteristics of the other main datasets already available for our sample and of the ongoing modelling projects.

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arXiv2010

The specific star formation rate of high redshift galaxies: the case for two modes of star formation

We study the specific star formation rate (SSFR) and its evolution at $z\gtsim 4$, in models of galaxy formation, where the star formation is driven by cold accretion flows. We show that constant star formation and feedback efficiencies cannot reproduce the observed trend of SSFR with stellar mass and its observed lack of evolution at $z>4$. Model galaxies with $\log(M_*) \ltsim 9.5$ M$_{\odot}$ show systematically lower specific star formation rates by orders of magnitudes, while massive galaxies with M$_* \gtsim 5 \times 10^{10}$ M$_{\odot}$ have up to an order of magnitude larger SSFRs, compared to recent observations by Stark et al.. To recover these observations we apply an empirical star formation efficiency in galaxies that scales with the host halo velocity dispersion as $\propto 1/σ^3$ during galaxy mergers. We find that this modification needs to be of stochastic nature to reproduce the observations, i.e. only applied during mergers and not during accretion driven star formation phases. Our choice of star formation efficiency during mergers allows us to capture both, the boost in star formation at low masses and the quenching at high masses, and at the same time produce a constant SSFR-stellar mass relation at $z\gtsim 4$ under the assumption that most of the observed galaxies are in a merger triggered star formation phase. Our results suggest that observed high-z low mass galaxies with high SSFRs are likely to be frequently interacting systems, which experienced bursts in their star formation rate and efficiency (mode 1), in contrast to low redshift $z \ltsim 3$ galaxies which are cold accretion-regulated star forming systems with lower star formation efficiencies (mode 2).

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arXiv2010

Mergers in Lambda-CDM: Uncertainties in Theoretical Predictions and Interpretations of the Merger Rate

Different methodologies lead to order-of-magnitude variations in predicted galaxy merger rates. We examine and quantify the dominant uncertainties. Different halo merger rates and subhalo 'destruction' rates agree to within a factor ~2 given proper care in definitions. If however (sub)halo masses are not appropriately defined or are under-resolved, the major merger rate can be dramatically suppressed. The dominant differences in galaxy merger rates owe to baryonic physics. Hydrodynamic simulations without feedback and older models that do not agree with the observed galaxy mass function propagate factor ~5 bias in the resulting merger rates. However, if the model matches the galaxy mass function, properties of central galaxies are sufficiently converged to give small differences in merger rates. But variations in baryonic physics of satellites also have dramatic effects. The known problem of satellite 'over-quenching' in most semi-analytic models (SAMs), whereby SAM satellites are too efficiently stripped of gas, could lead to order-of-magnitude under-estimates of merger rates for low-mass, gas-rich galaxies. Fixing the satellite properties to observations tends to predict higher merger rates, but with factor ~2 empirical uncertainties. Choice of mass ratio definition matters: at low masses, most true major mergers (in baryonic/dynamical galaxy mass) will appear to be minor mergers in their stellar or luminosity mass ratio. Observations and models using these criteria may underestimate major merger rates by factors ~5. Orbital parameters and gas fractions also introduce factor ~3 differences in amount of bulge formed by mergers, even for fixed mass ratio encounters.

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arXiv2010

Asymmetries in Extragalactic Double Radio Sources: Clues from 3D Simulations of Jet - Disc Interaction

Observational and theoretical studies of extragalactic radio sources have suggested that an inhomogeneous environment may be responsible for observed arm length asymmetries of jets and the properties of extended emission line regions in high redshift radio galaxies. We perform 3D hydrodynamic simulations of the interaction of a powerful extragalactic bipolar jet with a disc-shaped clumpy interstellar medium of log-normal density distribution and analyze the asymmetry. Furthermore, we compute the relation between jet asymmetry and the ISM properties by means of Monte Carlo simulations based on a 1D propagation model for the jet through the dense medium. We find that the properties of the ISM can be related to a probability distribution of jet arm length asymmetries: Disc density and height are found to have the largest effect on the asymmetry for realistic parameter ranges, while the Fourier energy spectrum of the ISM and turbulent Mach number only have a smaller effect. The hydrodynamic simulations show that asymmetries generally may be even larger than expected from the 1D model due to the complex interaction of the jet and its bow shock with gaseous clumps, which goes much beyond simple energy disposal. From our results, observed asymmetries of medium-sized local radio galaxies may be explained by gas masses of 10^9 to 10^10 solar masses in massive elliptical galaxies. Furthermore, the simulations provide a theoretical basis for the observed correlation that emission line nebulae are generally found to be brighter on the side of the shorter lobe in high redshift radio galaxies (McCarthy et al. 1991). This interaction of jets with the cold gas phase suggests that star formation in evolving high redshift galaxies may be affected considerably by jet activity.

astro-ph.HE↗

arXiv2010

Accretion onto black holes formed by direct collapse

One possible scenario for the formation of massive black holes (BHs) in the early Universe is from the direct collapse of primordial gas in atomic-cooling dark matter haloes in which the gas is unable to cool efficiently via molecular transitions. We study the formation of such BHs, as well as the accretion of gas onto these objects and the high energy radiation emitted in the accretion process, by carrying out cosmological radiation hydrodynamics simulations. In the absence of radiative feedback, we find an upper limit to the accretion rate onto the central object which forms from the initial collapse of hot (~ 10^4 K) gas of the order of 0.1 MSun per year. This is high enough for the formation of a supermassive star, the immediate precursor of a BH, with a mass of the order of 10^5 MSun. Assuming that a fraction of this mass goes into a BH, we track the subsequent accretion of gas onto the BH self-consistently with the high energy radiation emitted from the accretion disk. Using a ray-tracing algorithm to follow the propagation of ionizing radiation, we model in detail the evolution of the photoionized region which forms around the accreting BH. We find that BHs with masses of the order of 10^4 MSun initially accrete at close to the Eddington limit, but that the accretion rate drops to of order 1 percent of the Eddington limit after ~ 10^6 yr, due to the expansion of the gas near the BH in response to strong photoheating and radiation pressure. One signature of the accretion of gas onto BHs formed by direct collapse, as opposed to massive Pop III star formation, is an extremely high ratio of the luminosity emitted in He II 1640 to that emitted in H_alpha (or Ly_alpha); this could be detected by the James Webb Space Telescope. Finally, we briefly discuss implications for the coevolution of BHs and their host galaxies.

astro-ph.CO↗

arXiv2010

Mergers and Bulge Formation in Lambda-CDM: Which Mergers Matter?

We use a suite of semi-empirical models to predict galaxy merger rates and contributions to bulge growth as functions of merger mass, redshift, and mass ratio. The models use empirical halo occupation constraints to identify mergers, together with high-resolution simulations to quantify how mergers with different properties contribute to the bulge population. We find good agreement with a variety of observational constraints, and provide fitting functions for merger rates and contributions to bulge growth. We identify several robust conclusions. (1) Major mergers dominate formation and assembly of L* bulges and the spheroid mass density, minor mergers contribute ~30%. (2) This is mass-dependent: bulge formation is dominated by more minor mergers in lower-mass systems. At higher masses, bulges form in major mergers near L*, but subsequently assemble in minor mergers. (3) The minor/major contribution is also morphology-dependent: higher B/T systems form in more major mergers, lower B/T systems form in situ from minor mergers. (4) Low-mass galaxies, being gas-rich, require more major mergers to reach the same B/T as high-mass systems. (5) Absolute merger rates increase with galaxy mass. (6) Predicted rates agree well with observations, but suggest that some morphology-selected samples include contamination from minor mergers. (7) Predicted rates agree with integrated growth in bulge mass with cosmic time, but with factor ~2 uncertainty - half the bulge mass density could come from non-mergers. We consider ~1000 model variations and quantify resulting uncertainties. Conclusions regarding the major/minor contribution to bulge growth are very robust, absolute merger rates have systematic factor ~2 uncertainties.

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arXiv2010

Further constraining galaxy evolution models through the Size Function of SDSS Early-type galaxies

We discuss how the effective radius Phi(Re) function (ERF) recently worked out by Bernardi et al. (2009) represents a new testbed to improve the current understanding of Semi-analytic Models of Galaxy formation. In particular, we here show that a detailed hierarchical model of structure formation can broadly reproduce the correct peak in the size distribution of local early-type galaxies, although it significantly overpredicts the number of very compact and very large galaxies. This in turn is reflected in the predicted size-mass relation, much flatter than the observed one, due to too large (~3 kpc) low-mass galaxies (<10^11 \msun), and to a non-negligible fraction of compact (< 0.5-1 kpc) and massive galaxies (> 10^11 \msun). We also find that the latter discrepancy is smaller than previously claimed, and limited to only ultracompact (Re < 0.5 kpc) galaxies when considering elliptical-dominated samples. We explore several causes behind these effects. We conclude that the former problem might be linked to the initial conditions, given that large and low-mass galaxies are present at all epochs in the model. The survival of compact and massive galaxies might instead be linked to their very old ages and peculiar merger histories. Overall, knowledge of the galactic stellar mass {\em and} size distributions allows a better understanding of where and how to improve models.

astro-ph.CO↗

arXiv2009

Bulge n and B/T in High Mass Galaxies: Constraints on the Origin of Bulges in Hierarchical Models

We use the bulge Sersic index n and bulge-to-total ratio (B/T) to explore the fundamental question of how bulges form. We perform 2D bulge-disk-bar decomposition on H-band images of 143 bright, high stellar mass (>1.0e10 solar masses) low-to-moderately inclined (i<70 degrees) spirals. Our results are: (1) Our H-band bar fraction (~58%) is consistent with that from ellipse fits. (2) 70% of the stellar mass is in disks, 10% in bars, and 20% in bulges. (3) A large fraction (~69%) of bright spirals have B/T<0.2, and ~76% have low n<2 bulges. These bulges exist in barred and unbarred galaxies across a wide range of Hubble types. (4) About 65% (68%) of bright spirals with n<2 (B/T<0.2) bulges host bars, suggesting a possible link between bars and bulges. (5) We compare the results with predictions from a set of LCDM models. In the models, a high mass spiral can have a bulge with a present-day low B/T<0.2 only if it did not undergo a major merger since z<2. The predicted fraction (~1.6%) of high mass spirals, which have undergone a major merger since z<4 and host a bulge with a present-day low B/T<0.2, is a factor of over thirty smaller than the observed fraction (~66%) of high mass spirals with B/T<0.2. Thus, contrary to common perception, bulges built via major mergers since z<4 seriously fail to account for the bulges present in ~66% of high mass spirals. Most of these present-day low B/T<0.2 bulges are likely to have been built by a combination of minor mergers and/or secular processes since z<4.

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arXiv2009

Galaxy Zoo: A sample of blue early-type galaxies at low redshift

We report the discovery of a population of nearby, blue early-type galaxies with high star formation rates (0.5 < SFR < 50 Msun/yr). They are identified by their visual morphology as provided by Galaxy Zoo for SDSS DR6 and their u-r colour. We select a volume-limited sample in the redshift range 0.02 < z < 0.05, corresponding to luminosities of approximately L* and above, and with u-r colours significantly bluer than the red sequence. We confirm the early-type morphology of the objects in this sample and investigate their environmental dependence and star formation properties. Blue early-type galaxies tend to live in lower-density environments than `normal' red sequence early-types and make up 5.7 +/-0.4% of the low-redshift early-type galaxy population. We find that such blue early-type galaxies are virtually absent at high velocity dispersions above 200 km/s. Our analysis uses emission line diganostic diagrams and we find that ~25% of them are actively starforming, while another ~25% host both star formation and an AGN. Another ~12% are AGN. The remaining 38% show no strong emission lines. When present and uncontaminated by an AGN contribution, the star formation is generally intense. We consider star formation rates derived from Halpha, u-band and infrared luminosities, and radial colour profiles, and conclude that the star formation is spatially extended. Of those objects that are not currently undergoing star formation must have ceased doing so recently in order to account for their blue optical colours. The gas phase metallicity of the actively starforming blue early-types galaxies is supersolar in all cases. We discuss the place of these objects in the context of galaxy formation. A catalogue of all 204 blue early-type galaxies in our sample, including star formation rates and emission line classification, is provided.

astro-ph.CO↗

arXiv2009

On the formation of massive galaxies: A simultaneous study of number density, size and intrinsic colour evolution in GOODS

The evolution of number density, size and intrinsic colour is determined for a volume-limited sample of visually classified early-type galaxies selected from the HST/ACS images of the GOODS North and South fields (version 2). The sample comprises 457 galaxies over 320 arcmin2 with stellar masses above 3E10 Msun in the redshift range 0.4 3E11 Msun) do not show any appreciable change in comoving number density or size in our data. Furthermore, when including the results from 2dFGRS, we find that the number density of massive early-type galaxies is consistent with no evolution between z=1.2 and 0, i.e. over an epoch spanning more than half of the current age of the Universe. Massive galaxies show very homogeneous **intrinsic** colour distributions, featuring red cores with small scatter. The distribution of half-light radii -- when compared to z=0 and z>1 samples -- is compatible with the predictions of semi-analytic models relating size evolution to the amount of dissipation during major mergers. However, in a more speculative fashion, the observations can also be interpreted as weak or even no evolution in comoving number density **and size** between 0.4<z<1.2, thus pushing major mergers of the most massive galaxies towards lower redshifts.

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arXiv2008

The role of minor mergers in the recent star formation history of early-type galaxies

We demonstrate that the large scatter in the ultra-violet (UV) colours of intermediate-mass early-type galaxies in the local Universe and the inferred low-level recent star formation in these objects can be reproduced by minor mergers in the standard LCDM cosmology. Numerical simulations of mergers with mass ratios less than or equal to 1:4, with reasonable assumptions for the ages, metallicities and dust properties of the merger progenitors, produce good agreement to the observed UV colours of the early-type population, if the infalling satellites are assumed to have (cold) gas fractions greater than 20%. Early-types that satisfy (NUV-r) < 3.8 are likely to have experienced mergers with mass ratios between 1:4 and 1:6 within the last ~1.5 Gyrs, while those that satisfy 3.8<(NUV-r)<5.5 are consistent with either recent mergers with mass ratios < 1:6 or mergers with higher mass ratios that occurred more than ~1.5 Gyrs in the past. We demonstrate that the early-type colour-magnitude relations and colour distributions in both the UV and optical spectral ranges are consistent with the expected frequency of minor merging activity in the standard LCDM cosmology at low redshift. We present a strong plausibility argument for minor mergers to be the principal mechanism behind the large UV scatter and associated low-level recent star formation observed in early-type galaxies in the nearby Universe.

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arXiv2006

Suppression of star formation in early-type galaxies by feedback from supermassive black holes

Detailed high-resolution observations of the innermost regions of nearby galaxies have revealed the presence of supermassive black holes1. These black holes may interact with their host galaxies by means of 'feedback' in the form of energy and material jets; this feedback affects the evolution of the host and gives rise to observed relations between the black hole and the host. Here we report observations of the ultraviolet emissions of massive early-type galaxies. We derive an empirical relation for a critical black-hole mass (as a function of velocity dispersion) above which the outflows from these black holes suppress star formation in their hosts by heating and expelling all available cold gas. Supermassive black holes are negligible in mass compared to their hosts but nevertheless seem to play a critical role in the star formation history of galaxies.

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arXiv2006

A simple model for the size-evolution of elliptical galaxies

We use semi-analytical modelling of galaxy formation to predict the redshift-size-evolution of elliptical galaxies. Using a simple model in which relative sizes of elliptical galaxies of a given mass correlate with the fraction of stars formed in a star burst during a major merger event, we are able to reproduce the observed size redshif t evolution. The size evolution is a result of the amount of cold gas available during the major merger. Mergers at high redshifts are gas-rich and produce ellipticals with smaller sizes. In particular we find a power-law relation between the sizes at different redshifts, with the power-law index giving a measure of the relative amount of dissipation during the mergers that lead to the formation of an elliptical. The size evolution is found to be stronger for more massive galaxies as they involve more gas at high redshifts when they form compared to less massive ellipticals. Local ellipticals more massive than $5 \times 10^{11}$ M$_{\odot}$ will be approximately 4 times larger than their counterparts at $z=2$. Our results indicate that the scatter in the size of similar massive present day elliptical galaxies is a result of their formation epoch, with smaller ellipticals being formed earlier.

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