The Galactic Bulge: A Review

The Milky Way is the only galaxy for which we can resolve individual stars at all evolutionary phases, from the Galactic center to the outskirt. The last decade, thanks to the advent of near IR detectors and 8 meter class telescopes, has seen a great progress in the understanding of the Milky Way central region: the bulge. Here we review the most recent results regarding the bulge structure, age, kinematics and chemical composition. These results have profound implications for the formation and evolution of the Milky Way and of galaxies in general. This paper provides a summary on our current understanding of the Milky Way bulge, intended mainly for workers on other fields.Comment: 10 pages, 8 Postscript figures, uses iaus.cls To appear in Proceedings of IAU Symp. 245 on "Formation and Evolution of Galaxy Bulges", (held at Oxford, July 16-20 2007), Eds. Martin Bureau, Lia Athanassoula, and Beatriz Barbu

A Different Approach to Galaxy Evolution

The consequences are explored of an observationally established relation of the star formation rate (SFR) of star-forming galaxies with their stellar mass (M) and cosmic time (t), such that SFR is proportional to M x t^{-2.5}. It is shown, that small systematic differences in SFR dramatically amplify in the course of time: galaxies with above average SFR run into quasi-exponential mass and SFR growth, while galaxies with below average SFR avoid such exponential growth and evolve with moderate mass increase. It is argued that galaxies following the first path would enormously overgrow if keeping to form stars all the way to the present, hence should quench star formation and turn passive. By the same token, those instead avoiding the quasi-exponential growth may keep to form stars up to the present. Thus, it is conjectured that this divergent behaviour can help understanding the origin of the dichotomy between passive, spheroidal galaxies, and star-forming, disk galaxies.Comment: 5 Pages, 3 figures, to appear on MNRA

Testing the universal stellar IMF on the metallicity distribution in the bulges of the Milky Way and M31

We test whether the universal initial mass function (UIMF) or the integrated galaxial IMF (IGIMF) can be employed to explain the metallicity distribution (MD) of giants in the Galactic bulge. We make use of a single-zone chemical evolution model developed for the Milky Way bulge in the context of an inside-out model for the formation of the Galaxy. We checked whether it is possible to constrain the yields above 80 M_{\sun} by forcing the UIMF and required that the resulting MD matches the observed ones. We also extended the analysis to the bulge of M31 to investigate a possible variation of the IMF among galactic bulges. Several parameters that have an impact on stellar evolution (star-formation efficiency, gas infall timescale) are varied. We show that it is not possible to satisfactorily reproduce the observed metallicity distribution in the two galactic bulges unless assuming a flatter IMF ($x \leq 1.1$) than the universal one. We conlude that it is necessary to assume a variation in the IMF among the various environments.Comment: 9 pages, 4 figures, accepted for publication in A&

Star Formation during Galaxy Formation

Young galaxies are clumpy, gas-rich, and highly turbulent. Star formation appears to occur by gravitational instabilities in galactic disks. The high dispersion makes the clumps massive and the disks thick. The star formation rate should be comparable to the gas accretion rate of the whole galaxy, because star formation is usually rapid and the gas would be depleted quickly otherwise. The empirical laws for star formation found locally hold at redshifts around 2, although the molecular gas consumption time appears to be smaller, and mergers appear to form stars with a slightly higher efficiency than the majority of disk galaxies.Comment: 14 pages, 1 figure, Ecole Evry Schatzman 2010: Star Formation in the Local Universe. Lecture 5 of

Natures of a clump-origin bulge: a pseudobulge-like but old metal-rich bulge

Bulges in spiral galaxies have been supposed to be classified into two types: classical bulges or pseudobulges. Classical bulges are thought to form by galactic merger with bursty star formation, whereas pseudobulges are suggested to form by secular evolution due to spiral arms and a barred structure funneling gas into the galactic centre. Noguchi (1998,1999) suggested another bulge formation scenario, `clump-origin bulge'. He demonstrated using a numerical simulation that a galactic disc suffers dynamical instability to form clumpy structures in the early stage of disc formation since the premature disc is expected to be highly gas-rich, then the clumps are sucked into the galactic centre by dynamical friction and merge into a single bulge at the centre. This bulge formation scenario, which is expected to happen only at the high-redshift, is different from the galactic merger and the secular evolution. Therefore, clump-origin bulges may have their own unique properties. We perform a high-resolution N-body/smoothed particle hydrodynamics (SPH) simulation for the formation of the clump-origin bulge in an isolated galaxy model and study dynamical and chemical properties of the clump-origin bulge. We find that the clump-origin bulge resembles pseudobulges in dynamical properties, a nearly exponential surface density profile, a barred boxy shape and a significant rotation. We also find that this bulge consists of old and metal-rich stars, displaying resemblance to classical bulges. These natures, old metal-rich population but pseudobulge-like structures, mean that the clump-origin bulge can not be simply classified into classical bulges nor pseudobulges. From these results, we discuss similarities of the clump-origin bulge to the Milky Way bulge.Comment: 13 pages, 10 figures, Accepted for publication in MNRA

Integral field spectroscopy with SINFONI of VVDS galaxies. I. Galaxy dynamics and mass assembly at 1.2 < z < 1.6

Context. Identifying the main processes of galaxy assembly at high redshifts is still a major issue to understand galaxy formation and evolution at early epochs in the history of the Universe. Aims. This work aims to provide a first insight into the dynamics and mass assembly of galaxies at redshifts 1.2<z<1.6, the early epoch just before the sharp decrease of the cosmic star formation rate. Methods. We use the near-infrared integral field spectrograph SINFONI on the ESO-VLT under 0.65 seeing to obtain spatially resolved spectroscopy on nine emission line galaxies with 1.2<z<1.6 from the VIMOS VLT Deep Survey. We derive the velocity fields and velocity dispersions on kpc scales using the Halpha emission line. Results. Out of the nine star-forming galaxies, we find that galaxies distribute in three groups: two galaxies can be well reproduced by a rotating disk, three systems can be classified as major mergers and four galaxies show disturbed dynamics and high velocity dispersion. We argue that there is evidence for hierarchical mass assembly from major merger, with most massive galaxies with M>10^11Msun subject to at least one major merger over a 3 Gyr period as well as for continuous accretion feeding strong star formation. Conclusions. These results point towards a galaxy formation and assembly scenario which involves several processes, possibly acting in parallel, with major mergers and continuous gas accretion playing a major role. Well controlled samples representative of the bulk of the galaxy population at this key cosmic time are necessary to make further progress.Comment: 23 pages, 22 figures, accepted for publication in A&

Survival of Star-Forming Giant Clumps in High-Redshift Galaxies

We investigate the effects of radiation pressure from stars on the survival of the star-forming giant clumps in high-redshift massive disc galaxies, during the most active phase of galaxy formation. The clumps, typically of mass ~10^8-10^9 Msun and radius ~0.5-1, are formed in the turbulent gas-rich discs by violent gravitational instability and then migrate into a central bulge in ~10 dynamical times. We show that the survival or disruption of these clumps under the influence of stellar feedback depends critically on the rate at which they form stars. If they convert a few percent of their gas mass to stars per free-fall time, as observed for all local star-forming systems and implied by the Kennicutt-Schmidt law, they cannot be disrupted. Only if clumps convert most of their mass to stars in a few free-fall times can feedback produce significant gas expulsion. We consider whether such rapid star formation is likely in high-redshift giant clumps.Comment: 11 pages, 1 figure, accepted to MNRA

Star-Forming Galaxies at z~2 and the Formation of the Metal-Rich Globular Cluster Population

We examine whether the super star-forming clumps (R~1-3 kpc; M~10^8-10^9.5 Msun) now known to be a key component of star-forming galaxies at z~2 could be the formation sites of the locally observed old globular cluster population. We find that the stellar populations of these super star-forming clumps are excellent matches to those of local metal-rich globular clusters. Moreover, this globular cluster population is known to be associated with the bulges / thick disks of galaxies, and we show that its spatial distribution and kinematics are consistent with the current understanding of the assembly of bulges and thick disks from super star-forming clumps at high redshift. Finally, with the assumption that star formation in these clumps proceeds as a scaled-up version of local star formation in molecular clouds, this formation scenario reproduces the observed numbers and mass spectra of metal-rich globular clusters. The resulting link between the turbulent and clumpy disks observed in high-redshift galaxies and a local globular cluster population provides a plausible co-evolutionary scenario for several of the major components of a galaxy: the bulge, the thick disk, and one of the globular cluster populations.Comment: Accepted for publication in MNRAS Letters. 5 pages, 2 figure

A forming disk at z~0.6: Collapse of a gaseous disk or major merger remnant?

[Abridged] We present and analyze observations of J033241.88-274853.9 at z=0.6679, using multi-wavelength photometry and imaging with FLAMES/GIRAFFE 3D spectroscopy. J033241.88-274853.9 is found to be a blue, young (~320Myr) stellar disk embedded in a very gas-rich (fgas=73-82% with log(Mstellar/Mo)=9.45) and turbulent phase that is found to be rotating on large spatial scales. We identified two unusual properties of J033241.88-274853.9. (1) The spatial distributions of the ionized gaseous and young stars show a strong decoupling; while almost no stars can be detected in the southern part down to the very deep detection limit of ACS/UDF images, significant emission from the [OII] ionized gas is detected. (2) We detect an excess of velocity dispersion in the southern part of J033241.88-274853.9 in comparison to expectations from a rotating disk model. We considered two disk formation scenarios, depending on the gaseous phase geometry. In the first one, we examined whether J033241.88-274853.9 could be a young rotating disk that has been recently collapsed from a pre-existing, very gas-rich rotating disk. This scenario requires two (unknown) additional assumptions to explain the decoupling between the distribution of stars and gas and the excess of velocity dispersion in the same region. In a second scenario, we examine whether J033241.88-274853.9 could be a merger remnant of two gas-rich disks. In this case, the asymmetry observed between the gas and star distributions, as well as the excess of velocity dispersion, find a common explanation. Shocks produced during the merger in this region can be ionized easily and heat the gas while preventing star formation. This makes this scenario more satisfactory than the collapse of a pre-existing, gas-rich rotating disk.Comment: Accepted for publication in A&A. 8 pages & 5 figure

The effective stability parameter for two-component galactic discs: Is 1/Q ~ 1/Q_stars + 1/Q_gas ?

The Wang-Silk approximation, 1/Q ~ 1/Q_stars + 1/Q_gas, is frequently used for estimating the effective Q parameter in two-component discs of stars and gas. Here we analyse this approximation in detail, and show how its accuracy depends on the radial velocity dispersions and Toomre parameters of the two components. We then propose a much more accurate but still simple approximation for the effective Q parameter, which further takes into account the stabilizing effect of disc thickness. Our effective Q parameter is a natural generalization of Toomre's Q, and as such can be used in a wide variety of contexts, e.g. for predicting star formation thresholds in galaxies or for measuring the stability level of galactic discs at low and high redshifts.Comment: MNRAS, in pres