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Context. Lenticular galaxies are believed to form by a combination of environmental effects and secular evolution.

The near proportionality between HI and dark matter in outer galactic disks prompted us to run N-body simulations of galactic disks in which the observed gas content is supplemented by a dark gas component representing between zero and five times the visible gas content. While adding baryons in the disk of galaxies may solve some issues, it poses the problem of disk stability. We show that the global stability is ensured if the ISM is multiphased, composed of two partially coupled phases, a visible warm gas phase and a weakly collisionless cold dark phase corresponding to a fraction of the unseen baryons. The phases are subject to stellar and UV background heating and gas cooling, and their transformation into each other is studied as a function of the coupling strength. This new model, which still possesses a dark matter halo, fits the rotation curves as well as the classical CDM halos, but is the only one to explain the existence of an open and contrasting spiral structure, as observed in the outer HI disks

Using a large suite of galaxies with a variety of concentrations and metallicity gradients, we study the evolution of non-dissipative (“dry”) equal mass mergers. Our purpose in generating these simulations is to explore how the metallicity gradient in dry mergers depends on the structure and metallicity gradients of the galaxies involved in the merger. Specifically, we would like to answer: Could dry mergers lead to metallicity gradients as observed in elliptical galaxies in the local Universe? Do dry mergers always lead to a flattening of the initial (i.e., pre-merger) gradient? From this modeling, we conclude that: The ratio of the remnant and the initial galaxy slopes span a wide range of values, up to values greater than 1 (with values greater than one resulting only when companions have gradients twice the progenitor). For a merger between two ellipticals having identical initial metallicity slopes (i.e., equal companion and galaxy slopes), the metallicity profile of the remnant flattens, with a final gradient about 0.6 times the initial one. Ellipticals can maintain their original pre-merger metallicity gradient if the companion slope is sufficiently steep. The amount of flattening neither depends on the characteristics of the orbit of the progenitors or on their initial concentration. Given the diversity in outcomes of the mergers, we conclude that dry mergers do not violate any observational constraints on the systematic characteristics of metallicity gradients in local ellipticals. In fact, dry mergers may be important within the context of the results of our simulations and may explain the large scatter in gradients for massive ellipticals and the relative paucity of massive ellipticals with no or shallow metallicity gradients.

We present new observations of a sample of blue dwarf galaxies in the lines of CO(3–2) and HCN(4–3). The observations were obtained with the 12 m APEX telescope located at an altitude of 5100 m in Chajnantor (ALMA site). We detected CO(3–2) emission in seven of nine observed galaxies in our sample. In two galaxies, NGC 5253 and He 2–10, we mapped the central $20\arcsec\times20\arcsec$in the CO(3–2) emission line. In He 2–10, we detected an extended component of molecular gas to the north-east of the main body of the galaxy. Comparing our data with previous HI and CO observations, we speculate that the presence of this detached cloud of molecular gas is produced by the merger already known in He 2–10, and could lead to the formation of a small tidal dwarf galaxy. The HCN(4–3) line was observed in NGC 5253 and He 2–10 only, but not detected.

We consider for the first time the implications for the modified gravity MOND model of galaxies, of the presence of dark baryons, in the form of cold molecular gas in galaxy discs. We show that MOND models of rotation curves are still valid and universal, but the critical acceleration a0separating the Newtonian and MONDian regimes has a lower value. We quantify this modification as a function of the scale factor cbetween the total gas of the galaxy and the measured atomic gas. The main analysis concerns 43 resolved rotation curves and allows us to find the best pair ($a_0 = 0.96\times 10^{-10}$m s-2, $c = 3$), which is also compatible with the one obtained from a second method minimizing the scatter in the baryonic Tully-Fisher relation.

The $21\,\rm{cm}$emission of neutral hydrogen is the most promising probe of the epoch of reionization (EoR). In the next few years, the SKA pathfinders will provide statistical measurements of this signal, such as its power spectrum. Within one decade, SKA should produce a full tomography of the signal. Numerical simulations predicting these observations are necessary to optimize the design of the instruments and help in the interpretation of the data. Simulations are reaching a reasonable level in terms of scale range, but still often rely on simplifications to compute the strength of the signal. The main difficulty is the computation of the spin temperature of neutral hydrogen which depends on the gas kinetic temperature and on the level of the local Lyman-αflux (The Wouthuysen-Field effect). A $T_{\rm S} \gg T_{\rm CMB}$assumption is usual. However, this assumption does not apply early in the reionization history, or even later in the history as long as the sources of X-rays are too weak to heat the intergalactic medium significantly. This work presents the first EoR numerical simulations including, beside dynamics and ionizing continuum radiative transfer, a self-consistent treatment of the Ly-αradiative transfer. This allows us to compute the spin temperature more accurately. We use two different box sizes, $20\,h^{-1}\,\rm{Mpc}$and $100\,h^{-1}\,\rm{Mpc}$, and a star source model. Using the redshift dependence of average quantities, maps, and power spectra, we quantify the effect of using different assumptions to compute the spin temperature and the influence of the box size. The first effect comes from allowing for a signal in absorption (i.e. not making the $T_{\rm S} \gg T_{\rm CMB}$approximation). The magnitude of this effect depends on the amount of heating by hydrodynamic shocks and X-rays in the intergalactic medium (IGM). With our source model we have little heating so regions seen in absorption survive almost until the end of reionization. The second effects comes from using the real, local, Lyman-αflux. This effect is important for an average ionization fraction of less than ~$ 10\%$: it changes the overall amplitude of the $21\,\rm{cm}$signal, and adds its own fluctuations to the power spectrum.

We report CO observations towards MS0735.6+7421 a distant cooling core galaxy, and towards M87, the nearest cooling core in the center of the Virgo cluster. Both galaxies contain radio cavities that are thought to be responsible for the heating that can regulate or stop the cooling of the surrounding gas. In this feedback process, there could still be some gas cooling along filaments, along the borders of the radio cavities. Molecular gas is known to exist in clusters with cooling cores, in long and thin filaments that can be formed behind the rising bubbles inflated by the central AGN. CO emission was searched for at several locations along the radio lobes of those two galaxies, but only upper limits were found. These correspond to cold gas mass limits of a few 109$M_\odot$for each pointing in MS0735.6+7421, and a few 106$M_\odot$in M87. This non detection means that either the cooling is strongly reduced by the AGN feedback or that the gas is cooling in very localized places like thin filaments, possibly diluted in the large beam for MS0735.6+7421. For M87, the AGN heating appears to have stopped the cooling completely.

Context. The gas distribution and dynamics in the inner Galaxy present many unknowns, such as the origin of the asymmetry of the lv-diagram of the Central Molecular Zone (CMZ). On the other hand, there is recent evidence in the stellar component of the presence of a nuclear bar that may be slightly lopsided.

We present 12CO(1–0) and 12CO(2–1) observations of the SA(rs)bc Seyfert 2 galaxy NGC 3147, obtained with the IRAM interferometer at 1$\farcs$9 $\times$1$\farcs$6 and 1$\farcs$6 $\times$1$\farcs$4 resolutions, respectively. We have also observed the central region of NGC 3147 with the IRAM 30 m telescope (at resolutions of 22´´ and 12´´ for 12CO(1–0) and 12CO(2–1), respectively), in order to obtain complete sampling at low spatial frequencies. These observations have been made in the context of the NUclei of GAlaxies (NUGA) project, aimed at the study of the different mechanisms for gas fueling of active galactic nuclei (AGN). A central peak seen mainly in 12CO(2–1) and a ring-like structure at $r \simeq 10$´´~ 2 kpc dominate the 12CO maps. In 12CO(1–0) an outer spiral at $r \simeq 20\arcsec\sim4$kpc is also detected, not visible in 12CO(2–1) emission because it falls outside the field-of-view of the primary beam. The average $I_{21}/I_{10}$line ratio is ~0.7 in temperature units over the region mapped in both lines, consistent with the optically thick emission expected in the nuclei of spiral galaxies. The kinematics of the molecular structures are quite regular, although there is evidence for local non-circular or streaming motions. We show that the molecular gas distribution is similar but not exactly identical to those of star formation tracers, i.e., infrared (Spitzer) and ultraviolet (GALEX) emission. This agreement is consistent with a scenario of steady, ongoing formation of stars from the molecular clouds at a rate of ~$1\,M_\odot\,{\rm yr}^{-1}$within the innermost 4 kpc in radius. Using a near-infrared (NIR) image obtained with adaptive optics at the Canada-France-Hawaii Telescope (CFHT), we identify a weak bar in NGC 3147, which is classified as non-barred galaxy in the optical. We then compute the gravity torques exerted by this stellar bar on the gas. The torque is obtained first at each point in the map, and then azimuthally averaged with a weighting determined by the gas surface density traced by the CO emission. We find that the gas inside the inner CO ring is subject to a net negative torque and loses angular momentum. This is expected for gas at the ultra-harmonic resonance (UHR), just inside the corotation resonance of the stellar bar. In contrast, the gas outside corotation, in the spiral arms comprising the outer spiral structure, suffers positive torques and is driven outwards. We conclude that some molecular gas is presently flowing into the central region, since we find negative torques down to the resolution limit of our images.

We study 13 core-collapse supernova (CCSN) host galaxies at $z=0.1{-}0.6$. Continuum observations at 250 GHz were performed using MAMBO at the IRAM-30 m telescope. No source is detected and the error-weighted mean flux is 0.25 ±0.32 mJy. Upper limits on their dust masses are derived and the corresponding sample mean corresponds to $1.4\pm 2.2 \times 10^8~M_\odot$. These results are comparable to previous submillimetre observations of SN-Ia hosts completed by Farrah et al. and Clements et al. We conclude that CCSN hosts are not extreme sources at millimetre wavelengths, and as confirmed using the optical luminosities of a subset of our sample, they are typical of the local galaxy population.

We recently found extended CO(2-1) emission from cold molecular gas embedded in the network of Hαfilaments surrounding the galaxy NGC 1275 (Salome et al. 2006). We now present CO(2-1) interferometer maps of the eastern filaments, at high spatial and spectral resolutions. The cold molecular gas is detected by the Plateau de Bure Interferometer along the eastern filaments over an extent of 15'', or with a projected length of 5 kpc. In our 2.5''beam, the main CO filament is mostly unresolved along its minor axis. The multiple peaks along the CO filaments and the low values of the observed CO brightness temperatures imply further unresolved structures that may be giant molecular clouds. These clouds have very narrow line-width emission lines (~30 km s-1). The CO emission is optically thick. It very likely traces cold clouds bound under their own self-gravity that may be falling back in the gravitational potential well of the galaxy. Such a picture would agree with current models of “positive feedback” in which some of the hot gas around NGC 1275 (a) is trapped by buoyantly rising bubbles inflated by the energy input of the 3C 84 AGN, (b) subsequently cools efficiently at a larger radius around the edges of the hot bubbles, and (c) then falls back in self-gravitating clouds of molecular gas toward the center of the galaxy.

The stability of spiral galaxies is compared in modified Newtonian Dynamics (MOND) and Newtonian dynamics with dark matter. We extend our previous simulations that involved pure stellar discs without gas, to deal with the effects of gas dissipation and star formation. We also vary the interpolating μ-function between the MOND and Newtonian regime. Bar formation is studied and compared in both dynamics, from initial conditions identical in visible component morphology and kinematics (same density profile, rotation curve, and velocity dispersion). One first result is that the MOND galaxy evolution is not affected by the choice of the μ-function, it develops bars with the same frequency and strength. The choice of the μ-function significantly changes the equivalent Newton models, in changing the dark matter to visible mass ratio and, therefore, changing the stability. The introduction of gas shortens the timescale for bar formation in the Newton model, but is not significantly shortened in the MOND model, since it was already small. As a consequence, with gas, the MOND and DM bar frequency histograms are now more similar than without gas. The thickening of the plane occurs through vertical resonance with the bar and peanut formation, and even more quickly with gas. Since the mass gets more concentrated with gas, the radius of the peanut is smaller, and the appearance of the pseudo-bulge is more boxy. The bar strength difference is moderated by saturation, and feedback effects, like the bar weakening or destruction by gas inflow due to gravity torques. Averaged over a series of models representing the Hubble sequence, the MOND models have still more bars, and stronger bars, than the equivalent Newton models, better fitting the observations. Gas inflows driven by bars produce accumulations at rings or pseudo-rings at Lindblad resonances, and MOND models can reproduce observed morphologies quite well, as was found before in the Newtonian dynamics.

Emission-lines in the form of filamentary structures is common in bright clusters characterized by short cooling times. In the Perseus cluster, cold molecular gas, tightly linked to the Hαfilaments, has been recently revealed by CO observations. In order to understand the origin of these filamentary structures, we have investigated the evolution of the hot ICM gas perturbed by the AGN central activity in a Perseus like cluster. Using very-high resolution TreeSPH simulations combined with a multiphase model and a model of plasma bubbles, we have been able to follow the density and temperature evolution of the disturbed ICM gas around the bubbles. Our simulations show that a fraction of the $1{-}2~{\rm keV}$gas present at the center of clusters is trapped and entrained by the rising buoyant bubble to higher radius where the AGN heating is less efficient. The radiative cooling makes it cool in a few tens of Myr below $10^4~{\rm K}$, forming cold filamentary structures in the wake and in the rim of the bubbles. The predicted cold gas formed outside the cluster center is in agreement with the total CO mass and density profile of the observed molecular gas as well as with the kinematics of the Hαfilaments. This scenario explains the Hαand CO filaments observed in luminous clusters without contradicting the observed lack of $1~{\rm keV}$gas. It also emphasizes that if the AGN feedback provides some heating (negative feedback) it also perturbs the ICM, increasing its cooling (positive feedback).

We report on the results of an IRAM-30 m search for CO emission lines in three galaxies at intermediate redshifts. The idea was to investigate the molecular content of galaxies that are bright in the infrared at $z=0.4{-}1.5$, a redshift desert for molecular line studies because poorly investigated as yet. We integrated 8-10 h per source and did not succeed in detecting any of the sources. From our upper limits, we were able to constrain the molecular gas content in these systems to less than 4 to 8 $\times$$10^9~M_\odot$, assuming a CO-to-H2conversion factor ($\alpha=0.8~M_\odot$/(K km s-1pc2)). We stress the current difficulty of selecting sources with a detectable molecular content, a problem that will be faced by the ALMA First Science projects.

We use numerical simulations to investigate the possibility of forming counter-rotating old stellar components by major mergers between an elliptical and a spiral galaxy. We show that counterrotation can appear both in dissipative and dissipationless retrograde mergers, and it is mostly associated to the presence of a disk component, which preserves part of its initial spin. In turn, the external regions of the two interacting galaxies acquire part of the orbital angular momentum, from the action of tidal forces exerted on each galaxy by the companion.

We investigate how different models that have been proposed for solving the dark matter problem can fit the velocity dispersion observed around elliptical galaxies, on either a small scale (~20 kpc) with stellar tracers, such as planetary nebulae, or large scale (~200 kpc) with satellite galaxies as tracers. Predictions of Newtonian gravity, either containing pure baryonic matter, or embedded in massive cold dark matter (CDM) haloes, are compared with predictions of the modified gravity of MOND. The standard CDM model has problems on a small scale, and the Newtonian pure baryonic model has difficulties on a large scale, while a fit with MOND is possible on both scales.

Multiple, sequential mergers are unavoidable in the hierarchical build-up picture of galaxies, in particular for the minor mergers that are frequent and highly likely to have occured several times for most present-day galaxies. However, the effect of repeated minor mergers on galactic structure and evolution has not been studied systematically so far. We present a numerical study of multiple, subsequent, minor galaxy mergers, with various mass ratios ranging from 4:1 to 50:1. The N-body simulations include gas dynamics and star formation. We study the morphological and kinematical properties of the remnants, and show that several so-called “minor” mergers can lead to the formation of elliptical-like galaxies that have global morphological and kinematical properties similar to that observed in real elliptical galaxies. The properties of these systems are compared with those of elliptical galaxies produced by the standard scenario of one single major merger. We thus show that repeated minor mergers can theoretically form elliptical galaxies without major mergers, and can be more frequent than major mergers, in particular at moderate redshift. This process must then have formed some elliptical galaxies seen today, and could in particular explain the high boxiness of massive ellipticals, and some fundamental relations observed in ellipticals. In addition, because repeated minor mergers, even at high mass ratios, destroy disks into spheroids, these results indicate that spiral galaxies cannot have grown only by a succession of minor mergers.

During the epoch of reionization, Ly-αphotons emitted by the first stars can couple the neutral hydrogen spin temperature to the kinetic gas temperature, providing an opportunity to observe the gas in emission or absorption in the 21-cm line. Given the bright foregrounds, it is particularly important to determine the fluctuation signature of the signal precisely, so as to be able to extract it by its correlation power. LICORICE is a Monte-Carlo radiative transfer code, coupled to the dynamics via an adaptative Tree-SPH code. We present here the Ly-αpart of the implementation and validate it through three classical tests. Unlike previous works, we do not assume that $P_\alpha$, the number of scatterings of Ly-αphotons per atom per second, is proportional to the Ly-αbackground flux, but take the scatterings in the Ly-αline wings into account. The latter have the effect of steepening the radial profile of $P_\alpha$around each source, and re-inforce the contrast of the fluctuations. In the particular geometry of cosmic filaments of baryonic matter, $\rm Ly-\alpha$photons are scattered out of the filament, and the large-scale structure of $P_\alpha$is significantly anisotropic. This could have strong implications for the possible detection of the 21-cm signal.

The gravitationally lensed Lyαemitting galaxy, HCM 6A, detected by Hu et al. (2002) at $z = 6.56$behind the Abell 370 cluster was observed with the MAMBO-2 array of bolometers at 1.2 mm wavelength. The galaxy was not detected down to 1.08 mJy ($3 \sigma$), but the depth of the observations and the lens amplification allow us to improve by approximately one order of magnitude previously published upper limits on far infrared emission of Lyαemitting galaxies at this redshift. The following upper limits are derived from our observations, assuming typical dust parameters: dust mass <5.3$\times$107$M_{\odot}$, IR luminosity <2.1$\times$1011$L_{\odot}$, and star formation rate <35 $M_{\odot}$yr-1. The observed restframe UV-optical-IR spectral energy distribution (SED) of this galaxy is compatible with that of normal spiral galaxies or blue compact dwarf galaxies. SEDs of prototypical ULIRGs, such as Arp 220, are clearly excluded. Finally, we obtain an upper limit of ${\la} $2.1$\times$10-2$M_{\odot}$yr-1Mpc-3for the dust-obscured SFR density of Lyαselected galaxies at z~ 6.6.

Context.Star formation depends strongly on both the local environment of galaxies and the internal dynamics of the interstellar medium. To disentangle the two effects, we obtained, in the framework of the AMIGA project, Hαand Gunn rphotometric data for more than 200 spiral galaxies lying in very low-density regions of the local Universe.

We report a new search for 12CO(1–0) emission in high-velocity clouds (HVCs) performed with the IRAM 30 m millimeter-wave telescope. This search was motivated by the recent detection of cold dust emission ($T\sim 10.7$K) in the HVCs of Complex C, implying a total gas column density 5 times larger than the column density measured in $\ion{H}{i}$and suggesting undetected gas, presumably in molecular form. Despite a spatial resolution which is three times better and sensitivity twice as good compared to previous studies, no CO emission is detected in the HVCs of Complex C down to a best 5σlimit of 0.16 K km s-1at a $22''$resolution. The non-detection of both the 12CO(1–0) emission and of the diffuse H2absorption with the Far Ultraviolet Spectroscopic Explorer does not provide any evidence in favor of large amounts of molecular gas in these HVCs and hence in favor of the infrared findings. We discuss different configurations which, however, allow us to reconcile the negative CO result with the presence of molecular gas and cold dust emission. H2column densities higher than our detection limit, $N({\rm H}_2) = 3\times 10^{19}$cm-2, are expected to be confined in very small and dense clumps with 20 times smaller sizes than the 0.5 pc clumps resolved in our observations according to the results obtained in cirrus clouds, and might thus still be highly diluted. As a consequence, the inter-clump gas at the 1 pc scale, as resolved in our data, has a volume density lower than 20 cm-3and already appears as too diffuse to excite the CO molecules. The observed physical conditions in the HVCs of Complex C also play an important role against CO emission detection. The sub-solar metallicity of $0.1{-}0.3$dex affects the H2formation rate onto dust grains, and it has been shown that the CO-to-H2conversion factor in low metallicity media is 60 times higher than at the solar metallicity, leading for a given H2column density to a 60 times weaker integrated CO intensity. And the very low dust temperature estimated in these HVCs implies the possible presence of gas cold enough (<20 K) to cause CO condensation onto dust grains under interstellar medium pressure conditions and thus CO depletion in gas-phase observations.

We investigate the molecular gas content and the excitation and fragmentation properties in the central region of the spiral galaxy Messier 81 in both the 12CO(1–0) and 12CO(2–1) transitions. We have recently observed the two transitions of CO in the M 81 center with A, B, and HERA receivers of the IRAM 30-m telescope. We find no CO emission in the inner ~300 pc and a weak molecular gas clump structure at a distance of around 460 pc from the nucleus. Observations of the first two CO transitions allowed us to compute the line ratio, and the average $I_{21}/I_{10}$ratio is 0.68 for the M 81 center. This low value, atypical both of the galactic nuclei of spiral galaxies and of interacting systems, is probably associated to diffuse gas with molecular hydrogen density that is not high enough to excite the CO molecules. After analyzing the clumping properties of the molecular gas in detail, we identify very massive giant molecular associations (GMAs) in CO(2–1) emission with masses of ~105$M_\odot$and diameters of ~250 pc. The deduced N(H2)/ICOratio for the individually resolved GMAs, assumed to be virialized, is a factor of ~15 higher than the standardGalactic value, showing – as suspected – that the X ratio departs significantly from the mean for galaxies with an unusual physics of the molecular gas.

Within the cold dark matter (CDM) framework tidal dwarf galaxies (TDGs) cannot contain dark matter, so the recent results by Bournaud et al. (2007, Science, 316, 1166) that 3 rotating TDGs do show significant evidence for being dark matter dominated is inconsistent with the current concordance cosmological theory unless yet another dark matter component is postulated. We confirm that the TDG rotation curves are consistent with Newtonian dynamics only if either an additional dark matter component is postulated, or if all 3 TDGs happen to be viewed nearly edge-on, which is unlikely given the geometry of the tidal debris. We also find that the observed rotation curves are very naturally explained without any free parameters within the modified Newtonian dynamics (MOND) framework if inclinations are adopted as derived by Bournaud et al. We explore different inclination angles and two different assumptions about the external field effect. The results do not change significantly, and we conclude therefore that Newtonian dynamics has severe problems while MOND does exceedingly well in explaining the observed rotation curves of the 3 TDGs studied by Bournaud et al.

Aims.The influence of a time-varying ram pressure on spiral galaxies in clusters is explored with a new simulation method based on the N-body SPH/tree code GADGET.

Context.The AMIGA project aims to build a well defined and statistically significant reference sample of isolated galaxies in order to estimate the environmental effects on the formation and evolution of galaxies.

This work is part of the NUGA survey of CO emission in nearby active galaxies. We present observations of NGC 4569, a member of the Virgo Cluster. We analyse the molecular gas distribution and kinematics in the central region and we investigate a possible link to the strong starburst present at the nucleus. 70% of the [ 1.1±0.2] $\times$109$M_{\odot}$of molecular gas detected in the inner 20” is found to be concentrated within the inner 800 pc and is distributed along the large scale stellar bar seen in near-infrared observations. A hole in the CO distribution coincides with the nucleus where most of the Hαemission and blue light are emitted. The kinematics are modelled in three different ways, ranging from the purely geometrical to the most physical. This approach allows us to constrain progressively the physical properties of the galaxy and eventually to emerge with a reasonable fit to an analytical model of orbits in a barred potential. Fitting an axisymmetric model shows that the non-circular motions must be comparable in amplitude to the circular motions (120 km s-1). Fitting a model based on elliptical orbits allows us to identify with confidence the single inner Lindblad resonance (ILR) of the large scale bar. Finally, a model based on analytical solutions for the gas particle orbits in a weakly barred potential constrained by the ILR radius reproduces the observations well. The mass inflow rate is then estimated and discussed based on the best fit model solution. The gravitational torques implied by this model are able to efficiently funnel the gas inside the ILR down to 300 pc, although another mechanism must take over to fuel the nuclear starburst inside 100 pc.