1,186 research outputs found
How can star formation be sustained?
Get PDFThere is overwhelming evidence that the Milky Way has formed its stars at a relatively constant rate throughout the Hubble time. This implies that its stock of cold gas was not in place since the beginning but it has been acquired slowly through gas accretion. The gas accretion must have been at low metallicity in order to reconcile the metallicities observed in the disc with the predictions of chemical evolution models. But how does this gas accretion take place? I review the current evidence of gas accretion into the Milky Way and similar galaxies through the infall of cold gas clouds and satellites. The conclusion from these studies is that the infalling gas at high column densities observed in HI emission is a least one order of magnitude below the value required to sustain star formation, thus accretion must come from a different channel. The likely reservoir for gas accretion is the cosmological corona of virial-temperature gas in which every galaxy must be embedded. At the interface between the disc and the corona the cold high-metallicity disc gas and the hot coronal medium must mix efficiently and this mixing causes the cooling and accretion of the lower corona. I show how this mechanism reproduces the kinematics of the neutral extraplanar gas in the Milky Way and other nearby galaxies and the ionised high-velocity clouds observed in HST spectra. I conclude with the speculation that the loss in efficiency of the disc-corona interaction is the ultimate cause for the evolution of disc galaxies towards the red sequence
Evolution of dwarf galaxies: A dynamical perspective
Full text linkFor a rotating galaxy, the inner circular-velocity gradient dRV(0) provides a direct estimate of the central dynamical mass density, including gas, stars, and dark matter. We consider 60 low-mass galaxies with high-quality H I and/or stellar rotation curves (including starbursting dwarfs, irregulars, and spheroidals), and estimate dRV(0) as VRd/Rd, where Rd is the galaxy scale length. For gas-rich dwarfs, we find that VRd/Rd correlates with the central surface brightness μ0, the mean atomic gas surface density Σgas, and the star formation rate surface density ΣSFR. Starbursting galaxies, such as blue compact dwarfs (BCDs), generally have higher values of VRd/Rd than dwarf irregulars, suggesting that the starburst is closely related to the inner shape of the potential well. There are, however, some "compact" irregulars with values of VRd/Rd similar to BCDs. Unless a redistribution of mass takes place, BCDs must evolve into compact irregulars. Rotating spheroidals in the Virgo cluster follow the same correlation between VRd/Rd and μ0 as gas-rich dwarfs. They have values of VRd/Rd comparable to those of BCDs and compact irregulars, pointing to evolutionary links between these types of dwarfs. Finally, we find that, as for spiral galaxies and massive starbursts, the star-formation activity in dwarfs can be parametrized as ΣSFR = ɛ Σgas/τorb, where τorb is the orbital time and ɛ ≃ 0.02. Appendices are available in electronic form at http://www.aanda.or
Accretion, radial flows and abundance gradients in spiral galaxies
Full text linkThe metal-poor gas continuously accreting on to the discs of spiral galaxies is unlikely to arrive from the intergalactic medium (IGM) with exactly the same rotation velocity as the galaxy itself and even a small angular momentum mismatch inevitably drives radial gas flows within the disc, with significant consequences to galaxy evolution. Here, we provide some general analytic tools to compute accretion profiles, radial gas flows and abundance gradients in spiral galaxies as a function of the angular momentum of the accreting material. We generalize existing solutions for the decomposition of the gas flows, required to reproduce the structural properties of galaxy discs, into direct accretion from the IGM and a radial mass flux within the disc. We then solve the equation of metallicity evolution in the presence of radial gas flows with a novel method, based on characteristic lines, which greatly reduces the numerical demand on the computation and sheds light on the crucial role of boundary conditions on the abundance profiles predicted by theoretical models. We also discuss how structural and chemical constraints can be combined to disentangle the contributions of inside-out growth and radial flows in the development of abundance gradients in spiral galaxies. Illustrative examples are provided throughout with parameters plausible for the Milky Way. We find that the material accreting on the Milky Way should rotate at 70–80 per cent of the rotational velocity of the disc, in agreement with previous estimates
The angular momentum of cosmological coronae and the inside-out growth of spiral galaxies
Full text linkMassive and diffuse haloes of hot gas (coronae) are important intermediaries between cosmology and galaxy evolution, storing mass and angular momentum acquired from the cosmic web until eventual accretion on to star-forming discs. We introduce a method to reconstruct the rotation of a galactic corona, based on its angular momentum distribution (AMD). This allows us to investigate in what conditions the angular momentum acquired from tidal torques can be transferred to star-forming discs and explain observed galaxy-scale processes, such as inside-out growth and the build-up of abundance gradients. We find that a simple model of an isothermal corona with a temperature slightly smaller than virial and a cosmologically motivated AMD is in good agreement with galaxy evolution requirements, supporting hot-mode accretion as a viable driver for the evolution of spiral galaxies in a cosmological context. We predict moderately sub-centrifugal rotation close to the disc and slow rotation close to the virial radius. Motivated by the observation that the Milky Way has a relatively hot corona (T ≃ 2 × 106 K), we also explore models with a temperature larger than virial. To be able to drive inside-out growth, these models must be significantly affected by feedback, either mechanical (ejection of low angular momentum material) or thermal (heating of the central regions). However, the agreement with galaxy evolution constraints becomes, in these cases, only marginal, suggesting that our first and simpler model may apply to a larger fraction of galaxy evolution history
Dynamics of starbursting dwarf galaxies: III. A H I study of 18 nearby objects
Full text linkWe investigate the dynamics of starbursting dwarf galaxies, using both new and archival H I observations. We consider 18 nearby galaxies that have been resolved into single stars by HST observations, providing their star formation history and total stellar mass. We find that 9 objects have a regularly rotating H I disk, 7 have a kinematically disturbed H I disk, and 2 show unsettled H I distributions. Two galaxies (NGC 5253 and UGC 6456) show a velocity gradient along the minor axis of the H I disk, which we interpret as strong radial motions. For galaxies with a regularly rotating disk we derive rotation curves, while for galaxies with a kinematically disturbed disk, we estimate the rotation velocities in their outer parts. We derive baryonic fractions within about 3 optical scale lengths and find that, on average, baryons constitute at least 30% of the total mass. Despite the star formation having injected ~1056 ergs in the ISM in the past ~500 Myr, these starbursting dwarfs have both baryonic and gas fractions similar to those of typical dwarf irregulars, suggesting that they did not eject a large amount of gas out of their potential wells. Appendices are available in electronic form at http://www.aanda.orgH I datacubes (FITS files) are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/566/A7
The Angular Momentum of the Accreting Gas
Full text linkEvery galaxy is embedded in a multiphase and extended circumgalactic medium that comprises cold high-column density gas, warm ionised filaments and a hot rarefied atmosphere (corona). This circumgalactic medium is vital for maintaining blue star-forming galaxies as it provides new fresh gas for star formation to proceed. At the interface between a galaxy disc and the surrounding corona the mixing between the two media is very efficient and produces exchanges of both matter and angular momentum. After describing the various phases of the circumgalactic medium and its kinematics, I will discuss how the interplay between the host galaxy and its environment can drive a steady flow of material (gas accretion) towards the disc and the spinning up of the inner corona
How axi-symmetric is the inner HI disc of the Milky Way?
Full text linkWe modelled the distribution and the kinematics of HI in the inner Milky Way (R < Ro) at latitude b = 0∘ assuming axi-symmetry. We fitted the line profiles of the LAB 21-cm survey using an iterative approach based on the tangent-point method. The resulting model reproduces the H I data remarkably well, with significant differences arising only for R < 2 kpc. This suggests that, despite the presence of a barred potential, the neutral gas in the inner Milky Way is distributed in a fairly axi-symmetric disc
Introduction to Galaxy Formation and Evolution. From Primordial Gas to Present-Day Galaxies
No full textPresent-day elliptical, spiral and irregular galaxies are large systems made of stars, gas and dark matter. Their properties result from a variety of physical processes that have occurred during the nearly fourteen billion years since the Big Bang. This comprehensive textbook, which bridges the gap between introductory and specialized texts, explains the key physical processes of galaxy formation, from the cosmological recombination of primordial gas to the evolution of the different galaxies that we observe in the Universe today. In a logical sequence, the book introduces cosmology, illustrates the properties of galaxies in the present-day Universe, then explains the physical processes behind galaxy formation in the cosmological context, taking into account the most recent developments in this field. The text ends on how to find distant galaxies with multi-wavelength observations, and how to extract the physical and evolutionary properties based on imaging and spectroscopic data
New evidence for halo gas accretion onto disk galaxies
No full textStudies of the halo gas in the Milky Way and in nearby spiral galaxies show the presence of gas complexes that cannot be reconciled with an internal (galactic fountain) origin and are direct evidence of gas accretion. Estimating gas accretion rates from these features consistently gives values, which are one order of magnitude lower than what is needed to feed the star formation. I show that this problem can be overcome if most of the accretion is in fact "hidden" as it mixes with the galactic fountain material coming from the disk. This model not only provides an explanation for the missing gas accretion but also reproduces the peculiar kinematics of the halo gas in particular the vertical rotation gradient. In this view this gradient becomes indirect evidence for gas accretion
Gas accretion onto galaxies: models vs observations
No full textGas accretion is a vital support for galaxy evolution and the feeding of star formation. In recent years, the study of gaseous haloes surrounding disk galaxies has shown the presence of gas complexes, analogous to the galactic High-Velocity Clouds, that can be direct evidence of gas
accretion. However, the accretion rates estimated from these features consistently give values, which are one order of magnitude lower than what is needed to feed the star formation. This problem can be overcome if most of the accretion is “hidden” and visible only indirectly through the effects it has on the kinematics of the halo gas. In this second scheme, the gas expelled from the disk through galactic fountains sweeps up ambient gas causing it to accrete. This model provides an explanation for the missing gas accretion and also reproduces the peculiar kinematics of the halo gas, in particular the vertical rotation gradient
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