Sub-galactic Scales Research Articles

Spatial Variations and Evolution of PAH, Gas, and Dust Properties in Nearby Major Mergers

AbstractInteractions and mergers of gas-rich galaxies are important drivers of galaxy evolution, in particular in reshaping spirals and fueling powerful starbursts and supermassive nuclear black holes. Hubble Space Telescope observations of nuclei in merging galaxies have shown an increasing nuclear luminosity density in the optical/near-infrared light with advancing merger stage (Laine et al. 2003, Rossa et al. 2007, Veilleux et al. 2009, Haan et al. 2011a), suggesting an increase of nuclear starburst activity. To address the physical conditions in major mergers and excitation mechanisms such as shocks and starbursts, it is necessary to resolve the spatial variations of the warm dust, molecular gas, and ionized gas on subgalactic scales. Here we summarize the main results based on Spitzer IRS spectral mapping observations of a sample of eight nearby, IR-bright galaxies of the Toomre Sequence (Toomre & Toomre 1972) that span the range from early through mid- to late-stage major mergers (Haan et al. 2011b).We find that the cold molecular gas component as observed in the CO emission line does not correlate with the warm gas in early merger systems, leading to large spatial variations in the ratio of cold-to-warm molecular gas. In particular, the cold molecular gas distribution in NGC 4676 and NGC 6621 show a significant offset not only from the warm gas component, but also from the stellar (NIR) and dust distribution (PAH, mid-IR continuum). On locally resolved scales within a merger, we find an intrinsic relation between H2 and PAH flux as given by a power law with a very similar exponent (0.61 ± 0.05) for all galaxies but with a constant that can vary up to a factor of ten between different galaxies. These findings suggest a common dominant excitation mechanism for H2 emission over a large range of global H2/PAH flux ratios in major mergers. The spatial extent of the warm gas component (H2) is larger than for the PAH emission (in all galaxies but more extrem in mid-stage merger), which leads to an increase of the H2/PAH ratio as function of radius. Two merger systems (NGC 7592 and NGC 6621) show evidence for buried star formation in overlap regions between the nuclei, namely strong PAH, [Ne II], [Ne III] and warm gas (H2) emission, but no apparent corresponding CO (J = 1−0) emission. Our findings also demonstrate that the variations of the physical conditions within a merger are much larger than any systematic trends along the Toomre Sequence.

THE DARK MAGNETISM OF THE UNIVERSE

Despite the success of Maxwell's electromagnetism in the description of the electromagnetic interactions on small scales, we know very little about the behavior of electromagnetic fields on cosmological distances. Thus, it has been suggested recently that the problems of dark energy and the origin of cosmic magnetic fields could be pointing to a modification of Maxwell's theory on large scales. Here, we review such a proposal in which the scalar state which is usually eliminated by means of the Lorenz condition is allowed to propagate. On super-Hubble scales, the new mode is essentially given by the temporal component of the electromagnetic potential and contributes as an effective cosmological constant to the energy–momentum tensor. The new state can be generated from quantum fluctuations during inflation and it is shown that the predicted value for the cosmological constant agrees with observations, provided inflation took place at the electroweak scale. We also consider more general theories including non-minimal couplings to the spacetime curvature in the presence of the temporal electromagnetic background. We show that both in the minimal and non-minimal cases, the modified Maxwell's equations include new effective current terms which can generate magnetic fields from sub-galactic scales up to the present Hubble horizon. The corresponding amplitudes could be enough to seed a galactic dynamo or even to account for observations just by collapse and differential rotation in the protogalactic cloud.

Cosmological magnetic fields from inflation in extended electromagnetism

In this work we consider an extended electromagnetic theory in which the scalar state which is usually eliminated by means of the Lorenz condition is allowed to propagate. This state has been shown to generate a small cosmological constant in the context of standard inflationary cosmology. Here we show that the usual Lorenz gauge-breaking term now plays the role of an effective electromagnetic current. Such a current is generated during inflation from quantum fluctuations and gives rise to a stochastic effective charge density distribution. Because of the high electric conductivity of the cosmic plasma after inflation, the electric charge density generates currents which give rise to both vorticity and magnetic fields on sub-Hubble scales. Present upper limits on vorticity coming from temperature anisotropies of the CMB are translated into lower limits on the present value of cosmic magnetic fields. We find that, for a nearly scale invariant vorticity spectrum, magnetic fields ${B}_{\ensuremath{\lambda}}>{10}^{\ensuremath{-}12}\text{ }\text{ }\mathrm{G}$ are typically generated with coherence lengths ranging from subgalactic scales up to the present Hubble radius. Those fields could act as seeds for a galactic dynamo or even account for observations just by collapse and differential rotation of the protogalactic cloud.

On the Electromagnetic Nature of Dark Energy and the Origin of Cosmic Magnetic Fields

In this work we consider quantum electromagnetic fields in an expanding universe. We start by reviewing the difficulties found when trying to impose the Lorenz condition in a time dependent geometry. Motivated by this fact, we explore the possibility of extending the electromagnetic theory by allowing the scalar state which is usually eliminated by means of the Lorenz condition to propagate, preserving at the same time the dynamics of ordinary transverse photons. We show that the new state cannot be generated by charged currents, but it breaks conformal invariance and can be excited gravitationally. In fact, primordial quantum fluctuations produced during inflation can give rise to super-Hubble temporal electromagnetic modes whose energy density behaves as a cosmological constant. The value of the effective cosmological constant is shown to agree with observations provided inflation took place at the electroweak scale. The theory is compatible with all the local gravity tests and is free from classical or quantum instabilities. Thus we see that, not only the true nature of dark energy can be established without resorting to new physics, but also the value of the cosmological constant finds a natural explanation in the context of standard inflationary cosmology. On sub-Hubble scales, the new state generates an effective charge density which, due to the high electric conductivity of the cosmic plasma after inflation, gives rise to both vorticity and magnetic fields. Present upper limits on vorticity coming from CMB anisotropies are translated into lower limits on the present value of cosmic magnetic fields. We find that magnetic fields B(lambda) > 1 (-12) G can be typically generated with coherence lengths ranging from sub-galactic scales up to the present Hubble radius. Those fields could act as seeds for a galactic dynamo or even account for observations just by collapse and differential rotation of the protogalactic cloud.

EvoL: the new Padova Tree-SPH parallel code for cosmological simulations

<i>Context. <i/>We present the new release of the Padova <i>N<i/>-body code for cosmological simulations of galaxy formation and evolution, EvoL. The basic Tree + SPH code is presented and analysed, together with an overview of the software architectures.<i>Aims. <i/>EvoL is a flexible parallel Fortran95 code, specifically designed for simulations of cosmological structure formations on cluster, galactic and sub-galactic scales.<i>Methods. <i/>EvoL is a fully Lagrangian self-adaptive code, based on the classical oct-tree by Barnes & Hut (1986, Nature, 324, 446) and on the smoothed particle hydrodynamics algorithm (SPH, Lucy 1977, AJ, 82, 1013). It includes special features like adaptive softening lengths with correcting extra-terms, and modern formulations of SPH and artificial viscosity. It is designed to be run in parallel on multiple CPUs to optimise the performance and save computational time.<i>Results. <i/>We describe the code in detail, and present the results of a number of standard hydrodynamical tests.

Dwarf Cosmology with the Stromlo Missing Satellites Survey

The standard Lambda Cold Dark Matter model is considered to be a triumph of theoretical astrophysics but observations of the Milky Way and its system of satellite galaxies irresistibly signal that theory is incomplete on galactic and subgalactic scales. The Stromlo Missing Satellites (SMS) Survey is a critical endeavor to investigate at what level predictions of CDM cosmology are consistent with the observed matter distribution in the Milky Way halo. It will be the deepest, most extended search for optically elusive satellite galaxies to date, covering 20 000 square degrees of sky. The international SMS Survey collaboration will exploit 150 TB of CCD images in six filters acquired by the new SkyMapper telescope of the Australian National University over the next five years, expecting on completion photometric limits 0.5–1.0 mag fainter than the Sloan Digital Sky Survey. The primary objective of the program is to characterise the baryonic and dark matter components of a complete sample of MW satellites in the Southern hemisphere to provide stringent observational constraints for improving our understanding of how the Milky Way formed and what physical processes governed galaxy formation and evolution in general.

Chandraobservation of the edge-on spiral NGC 5775: probing the hot galactic disc/halo connection

We study the edge-on galaxy NGC 5775, utilizing a 58.2 ks {\sl Chandra} ACIS-S observation together with complementary {\sl HST} ACS, {\sl Spitzer} IRAC and other multi-wavelength data sets. This edge-on galaxy, with its disk-wide active star formation, is particularly well-suited for studying the disk/halo interaction on sub-galactic scales. We detect 27 discrete X-ray sources within the $D_{25}$ region of the galaxy, including an ultra-luminous source with a 0.3-7 keV luminosity of $\sim7\times10^{40}\rm ergs s^{-1}$. The source-removed diffuse X-ray emission shows several prominent extraplanar features, including a $\sim10\rm kpc$ diameter ``shell-like'' feature and a ``blob'' reaching a projected distance of $\sim25\rm kpc$ from the galactic disk. The bulk of the X-ray emission in the halo has a scale height of $\sim$1.5 kpc and can be characterized by a two-temperature optically thin thermal plasma with temperatures of $\sim$ 0.2 and 0.6 keV and a total 0.3-2 keV luminosity of $\sim3.5\times10^{39}\rm ergs s^{-1}$. The high-resolution, multi-wavelength data reveal the presence of several extraplanar features around the disk, which appear to be associated with the in-disk star formation. We suggest that hot gas produced with different levels of mass loading can have different temperatures, which may explain the characteristic temperatures of hot gas in the halo. We have obtained a sub-galactic scale X-ray-intensity-star formation relation, which is consistent with the integrated version in other star forming galaxies.

The violent interstellar medium of the Magellanic Cloud System

AbstractThe interstellar gas of the Magellanic System is subject to the harassment of tidal interactions on galaxy-wide scales and stellar energy feedback on sub-galactic scales. Hisurveys of the Magellanic System have produced spectacular images of the tidally displaced interstellar gas in the Magellanic Bridge and Streams. Multi-wavelength observations of the interstellar gas in the Magellanic Clouds have revealed gas components in physical conditions ranging from cold molecular cloud to hot ionized coronal gas. While stellar energy feedback is responsible for heating and dispersing interstellar gas, it can also compress ambient cloud to form stars. I will useChandra,XMM-Newton,FUSE,HST,Spitzer,ATCA, and other ground-based observations to illustrate the interplay among massive stars, interstellar medium, and star formation.

Angular signatures of annihilating dark matter in the cosmic gamma-ray background

The extragalactic cosmic gamma-ray background (CGB) is an interesting channel to look for signatures of dark matter annihilation. In particular, besides the imprint in the energy spectrum, peculiar anisotropy patterns are expected compared to the case of a pure astrophysical origin of the CGB. We take into account the uncertainties in the dark matter clustering properties on subgalactic scales, deriving two possible anisotropy scenarios. A clear dark matter angular signature is achieved when the annihilation signal receives only a moderate contribution from subgalactic clumps and/or cuspy haloes. Experimentally, if galactic foreground systematics are efficiently kept under control, the angular differences are detectable with the forthcoming GLAST observatory, provided that the annihilation signal contributes to the CGB for a fraction $\ensuremath{\gtrsim}10\char21{}20%$. If, instead, subgalactic structures have a more prominent role, the astrophysical and dark matter anisotropies become degenerate, correspondingly diluting the dark matter signature. As complementary observables we also introduce the cross correlation between surveys of galaxies and the CGB and the cross correlation between different energy bands of the CGB, and we find that they provide a further sensitive tool to detect the dark matter angular signatures.

Constraining warm dark matter using QSO gravitational lensing

Warm Dark Matter (WDM) has been invoked to resolve apparent conflicts of Cold Dark Matter (CDM) models with observations on subgalactic scales. In this work we provide a new and independent lower limit for the WDM particle mass (e.g. sterile neutrino) through the analysis of image fluxes in gravitationally lensed QSOs. Starting from a theoretical unperturbed cusp configuration we analyze the effects of intergalactic haloes in modifying the fluxes of QSO multiple images, giving rise to the so-called anomalous flux ratio. We found that the global effect of such haloes strongly depends on their mass/abundance ratio and it is maximized for haloes in the mass range $10^6-10^8 \Msun$. This result opens up a new possibility to constrain CDM predictions on small scales and test different warm candidates, since free streaming of warm dark matter particles can considerably dampen the matter power spectrum in this mass range. As a consequence, while a ($\Lambda$)CDM model is able to produce flux anomalies at a level similar to those observed, a WDM model, with an insufficiently massive particle, fails to reproduce the observational evidences. Our analysis suggests a lower limit of a few keV ($m_{\nu} \sim 10$) for the mass of warm dark matter candidates in the form of a sterile neutrino. This result makes sterile neutrino Warm Dark Matter less attractive as an alternative to Cold Dark Matter, in good agreement with previous findings from Lyman-$\alpha$ forest and Cosmic Microwave Background analysis.

Cold dark matter as compact composite objects

Dark matter (DM) being the vital ingredient in the cosmos, still remains a mystery. The standard assumption is that the collisionless cold dark matter (CCDM) particles are represented by some weakly interacting fundamental fields which cannot be associated with any standard quarks or leptons. However, recent analyses of structure on galactic and subgalactic scales have suggested discrepancies and stimulated numerous alternative proposals including, e.g. self-interacting dark matter, self-annihilating dark matter, decaying dark matter, to name just a few. We propose the alternative to the standard assumption about the nature of DM particles (which are typically assumed to be weakly interacting fundamental pointlike particles, yet to be discovered). Our proposal is based on the idea that DM particles are strongly interacting composite macroscopically large objects which made of well-known light quarks (or even antiquarks). The required weakness of the DM particle interactions is guaranteed by a small geometrical factor {epsilon}{approx}(area/volume){approx}B{sup -1/3} >1, rather than by a weak coupling constant of a new field. We argue that the interaction between hadronic matter and composite dark objects does not spoil the desired properties of the latter as cold matter. We also argue that suchmore » a scenario does not contradict to the current observational data. Rather, it has natural explanations of many observed data, such as {omega}{sub DM}/{omega}{sub B}{approx}1 or 511 KeV line from the bulge of our galaxy. We also suggest that composite dark matter may modify the dynamics of structure formation in the central overdense regions of galaxies. We also present a number of other cosmological/astrophysical observations which indirectly support the novel concept of DM nature.« less

Dark matter self-interactions from higher dimensional gravity

It has recently been suggested that, in a brane-world scenario with large extra dimensions, short-distance gravitational interactions can enhance the dark matter scattering cross section in a velocity-dependent way. Such a modification may then help to address possible problems with noninteracting cold dark matter on galactic and subgalactic scales. We argue that, considering the singular character of the higher-dimensional Newtonian potential, the scattering cross section is UV dependent, depending ultimately on the underlying quantum gravity theory considered. We demonstrate that, for a wide class of unitary short-distance regularizations, the actual cross section is velocity independent and does not significantly affect the dark matter substructure. We comment on the problem of thermalization of ultralight cold dark matter by gravitational interactions in the early universe.

Small scale structure formation in chameleon cosmology

Chameleon fields are scalar fields whose mass depends on the ambient matter density. We investigate the effects of these fields on the growth of density perturbations on sub-galactic scales and the formation of the first dark matter halos. Density perturbations on comoving scales R<1 pc go non-linear and collapse to form structure much earlier than in standard ΛCDM cosmology. The resulting mini-halos are hence more dense and resilient to disruption. We therefore expect (provided that the density perturbations on these scales have not been erased by damping processes) that the dark matter distribution on small scales would be more clumpy in chameleon cosmology than in the ΛCDM model.

The first WIMPy halos

Dark matter direct and indirect detection signals depend crucially on the dark matterdistribution. While the formation of large scale structure is independent of the nature ofthe cold dark matter (CDM), the fate of inhomogeneities on subgalactic scales, and hencethe present day CDM distribution on these scales, depends on the microphysics of theCDM particles. We study the density contrast of weakly interacting massive particles(WIMPs) on subgalactic scales. We calculate the damping of the primordial powerspectrum due to collisional damping and free streaming of WIMPy CDM and show thatfree streaming leads to a CDM power spectrum with a sharp cut-off at about. We also calculate the transfer function for the growth of the inhomogeneities in the linearregime, taking into account the suppression in the growth of the CDM densitycontrast after matter–radiation equality due to baryons and show that our analyticresults are in good agreement with numerical calculations. Combining the transferfunction with the damping of the primordial fluctuations we produce a WMAPnormalized primordial CDM power spectrum, which can serve as an input for highresolution CDM simulations. We find that the smallest inhomogeneities typically havecomoving radius of about 1 pc and enter the non-linear regime at a red-shift of60 ± 20. We study the effect of scale dependence of the primordial power spectrum on thesenumbers and also use the spherical collapse model to make simple estimates of theproperties of the first generation of WIMP halos to form. We find that the very firstWIMPy halos may have a significant impact on indirect dark matter searches.

The connection between the formation of galaxies and that of their central supermassive black holes

Massive black holes appear to be an essential ingredient of massive galactic bulges but little is known yet to what extent massive black holes reside in dwarf galaxies and globular clusters. Massive black holes most likely grow by a mixture of merging and accretion of gas in their hierarchically merging host galaxies. While the hierarchical merging of dark matter structures extends to sub-galactic scales and very high redshift, it is uncertain if the same is true for the build-up of massive black holes. I discuss here some of the relevant problems and open questions.

COSMIC RAYS AND GAMMA-RAYS IN LARGE-SCALE STRUCTURE

During the hierarchical formation of large scale structure in the universe, the progressive collapse and merging of dark matter should inevitably drive shocks into the gas, with nonthermal particle acceleration as a natural consequence. Two topics in this regard are discussed, emphasizing what important things nonthermal phenomena may tell us about the structure formation (SF) process itself. 1. Inverse Compton gamma-rays from large scale SF shocks and non-gravitational effects, and the implications for probing the warm-hot intergalactic medium. We utilize a semi-analytic approach based on Monte Carlo merger trees that treats both merger and accretion shocks self-consistently. 2. Production of <TEX>$^6Li$</TEX> by cosmic rays from SF shocks in the early Galaxy, and the implications for probing Galaxy formation and uncertain physics on sub-Galactic scales. Our new observations of metal-poor halo stars with the Subaru High Dispersion Spectrograph are highlighted.

The power spectrum of SUSY-CDM on subgalactic scales

ABSTRACT The formation of large scale structure is independent of the nature of the cold darkmatter (CDM), however the fate of very small scale inhomogeneities depends on themicro-physics of the CDM particles. We investigate the matter power spectrum forscales that enter the Hubble radius well before matter-radiation equality, and followits evolution until the time when the first inhomogeneities become non-linear. Ourfocus lies on weakly interacting massive particles (WIMPs), and as a concrete exam-ple we analyze the case when the lightest supersymmetric particle is a bino. We showthat collisional damping and free-streaming of WIMPs lead to a matter power spec-trum with a sharp cut-off at about 10 −6 M ⊙ and a maximum close to that cut-off. Wealso calculate the transfer function for the growth of the inhomogeneities in the lin-ear regime. These three effects (collisional damping, free-streaming and gravitationalgrowth) are combined to provide a WMAP normalized primordial CDM power spec-trum, which could serve as an input for high resolution CDM simulations. The smallestinhomogeneities typically enter the non-linear regime at a redshift of about 60.Key words: cosmology: theory – dark matter – early Universe

Inhomogeneous primordial baryon distributions on subgalactic scales: high-zgalaxy formation with warm dark matter

For the warm dark matter (WDM) cosmological model, the implications of a strongly inhomogeneous, primordial baryon distribution on subgalactic scales for big bang nucleosynthesis (BBN), cosmic microwave background (CMB) anisotropies and galaxy formation (including fully non-linear evolution to z = 0) are discussed, and the inflationary theory leading to such distributions is briefly reviewed. It is found that BBN is essentially unaffected relative to standard BBN, and that the change in recombination history at z ∼ 1500-700 relative to 'standard' theory leads to differences in the anisotropy and polarization power spectra, which should be detectable by the Planck satellite provided systematic effects can be accounted for. Moreover, it is shown by fully cosmological, hydro/gravity simulations that the formation of galactic discs is only weakly affected by going from smooth to highly non-homogeneous, initial baryon distributions. In particular, the final disc angular momenta at z = 0 are as large as for the standard case and the 'disc angular momentum problem' is solved to within a factor of 2 or better without invoking (hypothetical) energetic feedback events. A very desirable difference relative to the standard WDM model, however, is that the onset of star (and active galactic nucleus) formation happens earlier. For the 'optimal' free-streaming mass scale of M f ∼ 1.5 x 10 11 h -1 M ○. , the redshift of formation of the first stars increases from z *,i = 4-5 to ≥6.5, in much better agreement with observational data on high-redshift galaxies and quasi-stellar objects. However, it will not be possible to push z *,i above 10, because at higher redshifts the gas flow is nowhere compressive. Probing the 'dark ages' will hence enable a direct test of this theory.

GENERATING SMALL NUMBERS BY TUNNELING IN MULTI-THROAT COMPACTIFICATIONS

A generic F-theory compactification containing many D3 branes develops multiple brane throats. The interaction of observers residing inside different throats involves tunneling suppression and as a result, is very weak. This suggests a new mechanism for generating small numbers in Nature. One application is to the hierarchy problem: large supersymmetry breaking near the unification scale inside a shallow throat causes TeV-scale SUSY-breaking inside the standard-model throat. Another application, inspired by nuclear-decay, is in designing naturally long-lived particles: a cold dark matter particle residing near the standard model brane decays to an approximate CFT-state of a longer throat within a Hubble time. This suggests that most of the mass of the universe today could consist of CFT-matter and may soften structure formation at sub-galactic scales. The tunneling calculation demonstrates that the coupling between two throats is dominated by higher dimensional modes and consequently is much larger than a naive application of holography might suggest.

Theory of non-thermal phenomena in thin plasma sources: galaxy clusters, galaxies and supernova remnants

Abstract We review current models of energetic particle acceleration and nonthermal emission from objects of very different scales from clusters of galaxies through massive galaxies to supernova remnants. The nonthermal phenomena in clusters and massive galaxies are considered in the context of the hierarchical model of cosmic structure formation by accretion and merging of the dark matter (DM) substructures. The hierarchical model predicts interaction of sub-galactic scale DM halos with massive galaxies. The accretion and merging processes are producing gas shocks. Being the main gas-heating agent, large-scale shocks in the course of cluster/galaxy structure aggregation, could accelerate energetic particles by the same collisionless plasma relaxation processes. Nonthermal emission of the energetic particles in clusters and massive galaxies could be a test to constrain the DM minihalos properties. We also discuss X-ray emission from supernova remnants in a dense galactic environment with applications to deep observations of the Galactic Center region.