The formation of galaxies and their various components can be stringent tests of dark matter models and of gravity theories. In the standard cold dark matter (CDM) model, spheroids are formed through ...mergers in a strongly hierarchical scenario and also in the early universe through dynamical friction in clumpy galaxies. More secularly, pseudo-bulges are formed by the inner vertical resonance with bars. The high efficiency of bulge formation is in tension with observations in the local universe of a large amount of bulgeless spiral galaxies. In the present work, the formation of bulges in very gas-rich galaxies, such as those in the early universe, is studied in Milgrom’s MOdified Newtonian Dynamics (MOND) through multigrid simulations of the nonlinear gravity, including gas dissipation, star formation, and feedback. Clumpy disks are rapidly formed, as in the equivalent Newtonian systems. However, the dynamical friction is not as efficient in the absence of dark matter halos, and the clumps have no time to coalesce into the center to form bulges before they are eroded by stellar feedback and shear forces. Previous work has established that mergers are less frequent in MOND, and classical bulges are expected less massive. We now show that gas-rich clumpy galaxies in the early universe do not form bulges. Disks with a low bulge fraction, which is compatible with the observations, are therefore a natural result in MOND. Since pseudo-bulges are formed by bars with a similar rate to those in the Newtonian equivalent systems, it can be expected that the contribution of pseudo-bulges is significantly higher in MOND.
Bar quenching in gas-rich galaxies Khoperskov, S.; Haywood, M.; Di Matteo, P. ...
Astronomy and astrophysics (Berlin),
01/2018, Letnik:
609
Journal Article
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Odprti dostop
Galaxy surveys have suggested that rapid and sustained decrease in the star-formation rate (SFR), “quenching”, in massive disk galaxies is frequently related to the presence of a bar. Optical and ...near-IR observations reveal that nearly 60% of disk galaxies in the local universe are barred, thus it is important to understand the relationship between bars and star formation in disk galaxies. Recent observational results imply that the Milky Way quenched about 9–10 Gyr ago, at the transition between the cessation of the growth of the kinematically hot, old, metal-poor thick disk and the kinematically colder, younger, and more metal-rich thin disk. Although perhaps coincidental, the quenching episode could also be related to the formation of the bar. Indeed the transfer of energy from the large-scale shear induced by the bar to increasing turbulent energy could stabilize the gaseous disk against wide-spread star formation and quench the galaxy. To explore the relation between bar formation and star formation in gas rich galaxies quantitatively, we simulated gas-rich disk isolated galaxies. Our simulations include prescriptions for star formation, stellar feedback, and for regulating the multi-phase interstellar medium. We find that the action of stellar bar efficiently quenches star formation, reducing the star-formation rate by a factor of ten in less than 1 Gyr. Analytical and self-consistent galaxy simulations with bars suggest that the action of the stellar bar increases the gas random motions within the co-rotation radius of the bar. Indeed, we detect an increase in the gas velocity dispersion up to 20−35 km s-1 at the end of the bar formation phase. The star-formation efficiency decreases rapidly, and in all of our models, the bar quenches the star formation in the galaxy. The star-formation efficiency is much lower in simulated barred compared to unbarred galaxies and more rapid bar formation implies more rapid quenching.
Today, the brightest cluster galaxies (BCGs) are passive and very massive galaxies at the center of their clusters, and they still accrete mass through swallowing companions and gas from cooling ...flows. However their formation history is not well known. We report CO(4→3) and continuum map observations of the SpARCS1049+56 BCG at
z
= 1.709, one of the most distant known BCGs. Our observations yield
M
H
2
< 1.1 × 10
10
M
⊙
for the BCG; while in CO(4→3), we detect two gas-rich companions at the northeast and southeast of the BCG, within 20 kpc, with
L
CO(4→3)
′
= (5.8±0.6) × 10
9
K km s
−1
pc
2
and (7.4 ± 0.7)×10
9
K km s
−1
pc
2
, respectively. The northern companion is associated with a pair of merging cluster galaxies, while the southern one shows a southern tail in CO(4→3), which was also detected in continuum, and we suggest it to be the most distant jellyfish galaxy for which ram pressure stripping is effectively able to strip off its dense molecular gas. This study probes the presence of rare gas-rich systems in the very central region of a distant cluster core, which will potentially merge into the BCG itself. Currently, we may thus be seeing the reversal of the star formation versus density relation at play in the distant universe. This is the first time the assembly of high-
z
progenitors of our local BCGs can be studied in such great detail.
We present a self-consistent hydrodynamical simulation of a Milky Way-like galaxy at a resolution of 0.05 pc. The model includes star formation and a new implementation of stellar feedback through ...photoionization, radiative pressure and supernovae. The simulation resolves the structure of the interstellar medium at sub-parsec resolution for a few cloud lifetimes and at 0.05 pc for about a cloud-crossing time. The turbulence cascade and gravitation from kpc scales are de facto included in smaller structures like molecular clouds. We show that the formation of a bar influences the dynamics of the central ∼100 pc by creating resonances. At larger radii, the spiral arms host the formation of regularly spaced clouds: beads on a string and spurs. These instabilities pump turbulent energy into the gas, generally in the supersonic regime. Because of asymmetric drift, the supernovae explode outside their gaseous nursery, which diminishes the effect of feedback on the structure of clouds. The evolution of clouds is thus mostly due to fragmentation and gas consumption, regulated mainly by supersonic turbulence. The transition from turbulence-supported to self-gravitating gas is detected in the gas density probability distribution function at ∼2000 cm−3. The power-spectrum density suggests that gravitation governs the hierarchical organization of structures from the galactic scale down to a few pc.
ABSTRACT
We present an updated and improved Mbh–σ diagram containing 64 galaxies for which Mbh measurements (not just upper limits) are available. Because of new and increased black hole masses at ...the high‐mass end, and a better representation of barred galaxies at the low‐mass end, the ‘classical’ (all morphological type) Mbh–σ relation for predicting black hole masses is log (Mbh/M⊙) = (8.13 ± 0.05) + (5.13 ± 0.34)log σ/200 km s−1, with an rms scatter of 0.43 dex. Modifying the regression analysis to correct for a hitherto overlooked sample bias in which black holes with masses <106 M⊙ are not (yet) detectable, the relation steepens further to give log (Mbh/M⊙) = (8.15 ± 0.06) + (5.95 ± 0.44)log σ/200 km s−1. We have also updated the ‘barless’ and ‘elliptical‐only’Mbh–σ relations introduced by Graham and Hu in 2008 due to the offset nature of barred galaxies. These relations have a total scatter as low as 0.34 dex and currently define the upper envelope of points in the Mbh–σ diagram. They also have a slope consistent with a value 5, in agreement with the prediction by Silk & Rees based on feedback from massive black holes in bulges built by monolithic collapse.
Using updated virial products and velocity dispersions from 28 active galactic nuclei, we determine that the optimal scaling factor f– which brings their virial products in line with the 64 directly measured black hole masses – is 2.8+0.7−0.5. This is roughly half the value reported by Onken et al. and Woo et al., and consequently halves the mass estimates of most high‐redshift quasars. Given that barred galaxies are, on average, located ∼0.5 dex below the ‘barless’ and ‘elliptical‐only’Mbh–σ relations, we have explored the results after separating the samples into barred and non‐barred galaxies, and we have also developed a preliminary corrective term to the velocity dispersion based on bar dynamics. In addition, given the recently recognized coexistence of massive black holes and nuclear star clusters, we present the first ever (Mbh+Mnc)–σ diagram and begin to explore how galaxies shift from their former location in the Mbh–σ diagram.
Multi-phase filamentary structures around brightest cluster galaxies (BCG) are likely a key step of AGN-feedback. We observed molecular gas in three cool cluster cores, namely Centaurus, Abell S1101, ...and RXJ1539.5, and gathered ALMA (Atacama Large Millimeter/submillimeter Array) and MUSE (Multi Unit Spectroscopic Explorer) data for 12 other clusters. Those observations show clumpy, massive, and long (3−25 kpc) molecular filaments, preferentially located around the radio bubbles inflated by the AGN. Two objects show nuclear molecular disks. The optical nebula is certainly tracing the warm envelopes of cold molecular filaments. Surprisingly, the radial profile of the Hα/CO flux ratio is roughly constant for most of the objects, suggesting that (i) between 1.2 and 6 times more cold gas could be present and (ii) local processes must be responsible for the excitation. Projected velocities are between 100 and 400 km s−1, with disturbed kinematics and sometimes coherent gradients. This is likely due to the mixing in projection of several thin (and as yet) unresolved filaments. The velocity fields may be stirred by turbulence induced by bubbles, jets, or merger-induced sloshing. Velocity and dispersions are low, below the escape velocity. Cold clouds should eventually fall back and fuel the AGN. We compare the radial extent of the filaments, rfil, with the region where the X-ray gas can become thermally unstable. The filaments are always inside the low-entropy and short-cooling-time region, where tcool/tff < 20 (9 of 13 sources). The range of tcool/tff of 8−23 at rfil, is likely due to (i) a more complex gravitational potential affecting the free-fall time tff (sloshing, mergers, etc.) and (ii) the presence of inhomogeneities or uplifted gas in the ICM, affecting the cooling time tcool. For some of the sources, rfil lies where the ratio of the cooling time to the eddy-turnover time, tcool/teddy, is approximately unity.
By means of N-body simulations, we show that radial migration in galaxy disks, which is induced by bar and spiral arms, leads to significant azimuthal variations in the metallicity distribution of ...old stars at a given distance from the galaxy center. Metals do not show an axisymmetric distribution during phases of strong migration. Azimuthal variations are visible during the whole strong bar phase, and they tend to disappear as the effect of radial migration diminishes, together with a reduction in the bar strength. These results suggest that the presence of inhomogeneities in the metallicity distribution of old stars in a galaxy disk can be a probe of ongoing strong migration. Such signatures may be detected in the Milky Way by Gaia (and complementary spectroscopic data), as well as in external galaxies, by IFU surveys like CALIFA and ATLAS3D. Mixing – defined as the tendency toward a homogeneous, azimuthally symmetric, stellar distribution in the disk – and migration turns out to be two distinct processes, the effects of mixing starting to be visible when strong migration is over.
We present deep observations of the 12CO (1−0) and (3−2) lines in the ultra-luminous infrared and radio galaxy 4C 12.50, carried out with the 30 m telescope of the Institut de Radioastronomie ...Millimétrique. Our observations reveal the cold molecular gas component of a warm molecular gas outflow that was previously known from Spitzer Space Telescope data. The 12CO(3−2) profile indicates the presence of absorption at −950 km s-1 from systemic velocity with a central optical depth of 0.22. Its profile is similar to that of the H i absorption that was seen in radio data of this source. A potential detection of the 0 → 1 absorption enabled us to place an upper limit of 0.03 on its central optical depth, and to constrain the excitation temperature of the outflowing CO gas to ≥65 K assuming that the gas is thermalized. If the molecular clouds fully obscure the background millimeter continuum that is emitted by the radio core, the H2 column density is ≥1.8 × 1022 cm-2. The outflow then carries an estimated cold H2 mass of at least 4.2 × 103 M⊙ along the nuclear line of sight. This mass will be even higher when integrated over several lines of sight, but if it were to exceed 3 × 109 M⊙, the outflow would most likely be seen in emission. Since the ambient cold gas reservoir of 4C 12.50 is 1.0 × 1010 M⊙, the outflowing-to-ambient mass ratio of the warm gas (37%) could be elevated with respect to that of the cold gas.
The strong time-dependence of the dynamics of galactic bars yields a complex and rapidly evolving distribution of dense gas and star forming regions. Although bars mainly host regions void of any ...star formation activity, their extremities can gather the physical conditions for the formation of molecular complexes and mini-starbursts. Using a sub-parsec resolution hydrodynamical simulation of a Milky Way-like galaxy, we probe these conditions to explore how and where bar (hydro-)dynamics favours the formation or destruction of molecular clouds and stars. The interplay between the kpc-scale dynamics (gas flows, shear) and the parsec-scale (turbulence) is key to this problem. We find a strong dichotomy between the leading and trailing sides of the bar, in term of cloud fragmentation and in the age distribution of the young stars. After orbiting along the bar edge, these young structures slow down at the extremities of the bar, where orbital crowding increases the probability of cloud–cloud collision. We find that such events increase the Mach number of the cloud, leading to an enhanced star formation efficiency and finally the formation of massive stellar associations, in a fashion similar to galaxy–galaxy interactions. We highlight the role of bar dynamics in decoupling young stars from the clouds in which they form, and discuss the implications on the injection of feedback into the interstellar medium (ISM), in particular in the context of galaxy formation.
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