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Register a new userPushing back the limits: detailed properties of dwarf galaxies in a LCDM universe
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Yves Revaz
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We present the results of a set of high resolution chemo-dynamical simulations of dwarf galaxies in a $Lambda$CDM cosmology. Out of an original 3.4 Mpc$^3$/h$^3$ cosmological box, a sample of 27 systems are zoomed-in from z=70 to z=0. Gas and stellar properties are confronted to the observations in the greatest details: in addition to the galaxy global properties, we investigate the model galaxy velocity dispersion profiles, half-light radii, star formation histories, metallicity distributions, and [Mg/Fe] abundance ratios. The formation and sustainability of the metallicity gradients and kinetically distinct stellar populations are also tackled. We show how the properties of six Local Group dwarf galaxies, NGC 6622, Andromeda II, Sculptor, Sextans, Ursa Minor and Draco are reproduced, and how they pertain to three main galaxy build-up modes. Our results indicate that the interaction with a massive central galaxy could be needed for a handful of Local Group dwarf spheroidal galaxies only, the vast majority of the systems and their variety of star formation histories arising naturally from a $Lambda$CDM framework. We find that models fitting well the local Group dwarf galaxies are embedded in dark haloes of mass between $5times 10^8$ to a few $10^9,rm{M_odot}$, without any missing satellite problem. We confirm the failure of the abundance matching approach at the mass scale of dwarf galaxies. Some of the observed faint however gas-rich galaxies with residual star formation, such as Leo T and Leo P, remain challenging. They point out the need of a better understanding of the UV-background heating.
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We present the largest publicly available catalog of interacting dwarf galaxies. It includes 177 nearby merging dwarf galaxies of stellar mass M$_{*}$ $<$ 10$^{10}$M$_{sun}$ and redshifts z $<$ 0.02. These galaxies are selected by visual inspection of publicly available archival imaging from two wide-field optical surveys (SDSS III and the Legacy Survey), and they possess low surface brightness features that are likely the result of an interaction between dwarf galaxies. We list UV and optical photometric data which we use to estimate stellar masses and star formation rates. So far, the study of interacting dwarf galaxies has largely been done on an individual basis, and lacks a sufficiently large catalog to give statistics on the properties of interacting dwarf galaxies, and their role in the evolution of low mass galaxies. We expect that this public catalog can be used as a reference sample to investigate the effects of the tidal interaction on the evolution of star-formation, morphology/structure of dwarf galaxies. Our sample is overwhelmingly dominated by star-forming galaxies, and they are generally found significantly below the red-sequence in the color-magnitude relation. The number of early-type galaxies is only 3 out of 177. We classify them, according to observed low surface brightness features, into various categories including shells, stellar streams, loops, antennae or simply interacting. We find that dwarf-dwarf interactions tend to prefer the low density environment. Only 41 out of the 177 candidate dwarf-dwarf interaction systems have giant neighbors within a sky projected distance of 700 kpc and a line of sight radial velocity range $pm$700 km/s and, compared to the LMC-SMC, they are generally located at much larger sky-projected distances from their nearest giant neighbor.
Associations of dwarf galaxies are loose systems composed exclusively of dwarf galaxies. These systems were identified in the Local Volume for the first time more than thirty years ago. We study these systems in the cosmological framework of the $Lambda$ Cold Dark Matter ($Lambda$CDM) model. We consider the Small MultiDark Planck simulation and populate its dark matter haloes by applying the semi-analytic model of galaxy formation SAG. We identify galaxy systems using a friends of friends algorithm with a linking length equal to $b=0.4 ,{rm Mpc},h^{-1}$, to reproduce the size of dwarf galaxy associations detected in the Local Volume. Our samples of dwarf systems are built up removing those systems that have one (or more) galaxies with stellar mass larger than a maximum threshold $M_{rm max}$. We analyse three different samples defined by ${rm log}_{10}(M_{rm max}[{rm M}_{odot},h^{-1}]) = 8.5, 9.0$ and $9.5$. On average, our systems have typical sizes of $sim 0.2,{rm Mpc},h^{-1}$, velocity dispersion of $sim 30 {rm km,s^{-1}} $ and estimated total mass of $sim 10^{11} {rm M}_{odot},h^{-1}$. Such large typical sizes suggest that individual members of a given dwarf association reside in different dark matter haloes and are generally not substructures of any other halo. Indeed, in more than 90 per cent of our dwarf systems their individual members inhabit different dark matter haloes, while only in the remaining 10 per cent members do reside in the same halo. Our results indicate that the $Lambda$CDM model can naturally reproduce the existence and properties of dwarf galaxies associations without much difficulty.
Our knowledge about galaxy evolution comes from transforming observed galaxy properties at different redshifts to co-moving physical scales. This transformation depends on using a cosmological model. Here the effects of unintentional mixing of two different cosmological models on the size evolution of galaxies is studied. As a gedanken experiment, a galaxy of fixed proper size and luminosity is moved across different redshifts. The apparent size of this galaxy is then interpreted with a cosmological model presumed by the observer, which is different compared to the cosmology exhibited by the Universe. In such a case, a spurious size evolution of the galaxy is observed. A galaxy behaving according to the R_h=ct and Neumanns cosmology, when interpreted with the LCDM cosmological model, shows an increase in size by a factor of 1.1 and 1.3 from z=7.5 to z approx. 0, respectively. The apparent size of a galaxy in a static Euclidean cosmology, when interpreted in the LCDM model, shows a factor of 23.8 increase in size between z=7.5 to z approx. 0. This is in close agreement with the observational data with a size increase of a factor of 6.8 between z=3.2 to z approx. 0. Furthermore, using the apparent size data, it is shown that the difference between the derived proper sizes in R_h=ct, Neumanns and LCDM cosmological models are minimal.
The phenomenological basis for Modified Newtonian Dynamics (MOND) is the radial-acceleration-relation (RAR) between the observed acceleration, $a=V^2_{rot}(r)/r$, and the acceleration accounted for by the observed baryons (stars and cold gas), $a_{bar}=V_{bar}^2(r)/r$. We show that the RAR arises naturally in the NIHAO sample of 89 high-resolution LCDM cosmological galaxy formation simulations. The overall scatter from NIHAO is just 0.079 dex, consistent with observational constraints. However, we show that the scatter depends on stellar mass. At high masses ($10^9 <M_{star} <10^{11}$ Msun) the simulated scatter is just $simeq 0.04$ dex, increasing to $simeq 0.11$ dex at low masses ($10^7 < M_{star} <10^{9}$Msun). Observations show a similar dependence for the intrinsic scatter. At high masses the intrinsic scatter is consistent with the zero scatter assumed by MOND, but at low masses the intrinsic scatter is non-zero, strongly disfavoring MOND. Applying MOND to our simulations yields remarkably good fits to most of the circular velocity profiles. In cases of mild disagreement the stellar mass-to-light ratio and/or distance can be tuned to yield acceptable fits, as is often done in observational mass models. In dwarf galaxies with $M_{star}sim10^6$Msun MOND breaks down, predicting lower accelerations than observed and in our LCDM simulations. The assumptions that MOND is based on (e.g., asymptotically flat rotation curves, zero intrinsic scatter in the RAR), are approximately, but not exactly, true in LCDM. Thus if one wishes to go beyond Newtonian dynamics there is more freedom in the RAR than assumed by MOND.
Employing hydrodynamic simulations of structure formation in a LCDM cosmology, we study the history of cosmic star formation from the dark ages at redshift z~20 to the present. In addition to gravity and ordinary hydrodynamics, our model includes radiative heating and cooling of gas, star formation, supernova feedback, and galactic winds. By making use of a comprehensive set of simulations on interlocking scales and epochs, we demonstrate numerical convergence of our results on all relevant halo mass scales, ranging from 10^8 to 10^15 Msun/h. The predicted density of cosmic star formation is broadly consistent with measurements, given observational uncertainty. From the present epoch, it gradually rises by about a factor of ten to a peak at z~5-6, which is beyond the redshift range where it has been estimated observationally. 50% of the stars are predicted to have formed by redshift z~2.1, and are thus older than 10.4 Gyr, while only 25% form at redshifts lower than z~1. The mean age of all stars at the present is about 9 Gyr. Our model predicts a total stellar density at z=0 of Omega_*=0.004, corresponding to about 10% of all baryons being locked up in long-lived stars, in agreement with recent determinations of the luminosity density of the Universe. We determine the multiplicity function of cosmic star formation as a function of redshift; i.e. the distribution of star formation with respect to halo mass. We also briefly examine possible implications of our predicted star formation history for reionisation of hydrogen in the Universe. We find that the star formation rate predicted by the simulations is sufficient to account for hydrogen reionisation by z~6, but only if a high escape fraction close to unity is assumed. (abridged)
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