P. Di Matteo

Star formation efficiency in galaxy interactions and mergers: a statistical study

We investigate the enhancement of star formation efficiency in galaxy interactions and mergers by numerical simulations of several hundred galaxy collisions. All morphological types along the Hubble sequence are considered in the initial conditions of the two colliding galaxies, with varying bulge-to-disk ratios and gas mass fractions. Different types of orbits are simulated, direct and retrograde, according to the initial relative energy and impact parameter, and the resulting star formation history is compared to that occuring in the two galaxies when they are isolated. Our principal results are (1) retrograde encounters have greater star formation efficiency (SFE) than direct encounters, (2) the amount of gas available in the galaxy is not the main parameter governing the SFE in the burst phase, (3) there is a negative correlation between the amplitude of the star forming burst and the tidal forces exerted per unit of time, which is due to the large amount of gas dragged outside the galaxy by tidal tails in strong interactions, (4) globally, the Kennicutt-Schmidt law is seen to apply statistically for isolated galaxies, interacting pairs and mergers, (5) enhanced star formation occurs essentially in nuclear starbursts, triggered by inward gas flows driven by non-axisymmetries in the galaxy disks. Direct encounters develop more pronounced asymmetries than retrograde ones. Based on these statistical results we derive general laws for the enhancement of star formation in galaxy interactions and mergers, as a function of the main parameters of the encounter.

On the frequency, intensity and duration of starburst episodes triggered by galaxy interactions and mergers

We investigate the intensity enhancement and the duration of starburst episodes triggered by major galaxy interactions and mergers. We analyze two large statistical datasets of numerical simulations. These have been obtained using two independent and different numerical techniques to model baryonic and dark matter evolution that are extensively compared for the first time. One is a Tree-SPH code, the other one is a grid-based N-body sticky-particles code. We show that, at low redshift, galaxy interactions and mergers in general trigger only moderate star formation enhancements. Strong starbursts where the star formation rate is increased by a factor greater than 5 are rare and found only in about 15% of major galaxy interactions and mergers. Merger-driven starbursts are also rather short-lived, with a typical duration of activity of a few 10 8 yr. These conclusions are found to be robust, independent of the numerical techniques and star formation models. At higher redshifts where galaxies contain more gas, gas inflow-induced starbursts are neither stronger nor longer than their local counterparts. In turn, the formation of massive gas clumps, results of local Jeans instability that can occur spontaneously in gas-rich disks or be indirectly favored by galaxy interactions, could play a more important role in determining the duration and intensity of star formation episodes.

Observations and modeling of a clumpy galaxy at z = 1.6 : Spectroscopic clues to the origin and evolution of chain galaxies

We investigate the properties of a clump-cluster galaxy at redshift 1.57. In optical observations, the morphology of this galaxy is dominated by eight star-forming clumps, and its photometric properties are typical of most clump-cluster and chain galaxies. Its complex asymmetrical morphology has led to the suggestion that this system is a group merger of several initially separate protogalaxies. We performed Ha integral field spectroscopy of this system using SINFONI on VLT UT4. These observations reveal a large-scale velocity gradient throughout the system, but with large local kinematic disturbances. Using a numerical model of gas-rich disk fragmentation, we find that clump interactions and migration can explain the observed disturbed rotation. On the other hand, the global rotation would not be expected for a multiply merging system. We also find that this system follows the relations of stellar mass versus metallicity, star formation rate, and size that are expected for a disk at this redshift. Furthermore, the galaxy exhibits a disk-like radial metallicity gradient. A formation scenario of internal disk fragmentation is therefore the most likely one. A red and metallic central concentration appears to be a bulge in this proto-spiral clumpy galaxy. A chain galaxy at redshift 2.07 in the same field also shows disk-like rotation. Such systems are likely progenitors of present-day bright spiral galaxies, which shape their exponential disks through clump migration and disruption, a process that in turn fuels their bulges. Our results show that disturbed morphologies and kinematics are not necessarily signs of galaxy mergers and interactions, but may instead be produced by the internal evolution of primordial disks.

Signatures of radial migration in barred galaxies: Azimuthal variations in the metallicity distribution of old stars

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.

Mapping a stellar disk into a boxy bulge: The outside-in part of the Milky Way bulge formation

By means of idealized, dissipationless N-body simulations which follow the formation and subsequent buckling of a stellar bar, we study the characteristics of boxy/peanut-shaped bulges and compare them with the properties of the stellar populations in the Milky Way bulge. The main results of our modeling, valid for the general family of boxy/peanut shaped bulges, are the following: (i) because of the spatial redistribution in the disk initiated at the ep och of bar formation, stars from the innermost regions to the outer Lindblad resonance of the stellar bar are mapped into a boxy bulge; (ii) the contribution of stars to the local bulge density depends on their birth radius: stars born in the innermost disk tend to dominate the innermost regions of the boxy bulge, while stars originating closer to the OLR are preferentially found in the outer regions of the boxy/peanut structure; (iii) stellar birth radii are imprinted in the bulge kinematics, the larger the birth radii of stars ending up in t he bulge, the greater their rotational support and the highe r their line-ofsight velocity dispersions (but note that this last trend de pends on the bar viewing angle); (iv) the higher the classical bulge-over-disk ratio, the larger its fractional contribution of stars at la rge vertical distance from the galaxy mid-plane. Comparing these results with the properties of the stellar populations of the Milky Way’s bulge recently revealed by the ARGOS survey, we conclude that: (I) the two most metal-rich populations of the MW bulge, labeled A and B in the ARGOS survey, originate in the disk, with the population of A having formed on average closer to the Galaxy center than the population of component B; (II) a massive (B/D∼0.25) classical spheroid can be excluded for the Milky Way, thus confirming pr evious findings that the Milky Way bulge is composed of popula tions that mostly have a disk origin. On the basis of their chemical and kinematic characteristics, the results of our modeling suggests that the populations A, B and C, as defined by the ARGOS survey, can b e associated, respectively, with the inner thin disk, to the young thick and to the old thick disk, following the nomenclature recently suggested for stars in the solar neighborhood by Haywood et al. (2013).

The dilution peak, metallicity evolution, and dating of galaxy interactions and mergers

Strong inflows of gas from the outer disk to the inner kiloparsecs are induced during the interaction of disk galaxies. This inflow of relatively low-metallicity gas dilutes the metallicity of the circumnuclear gas. This process is critical for the galaxy evolution. We have investigated several aspects of the process as the timing and duration of the dilution and its correlation with the induced star formation. We analysed major (1:1) gas-rich interactions and mergers, spanning a range of initial orbital characteristics. Star formation and metal enrichment from SNe are included in our model. Our results show that the strongest trend is between the star formation rate and the dilution of the metals in the nuclear region; i.e., the more intense the central burst of star formation, the more the gas is diluted. This trend comes from strong inflows of relatively metal-poor gas from the outer regions of both disks, which fuels the intense star formation and lowers the overall metallicity for a time. The strong inflows happen on timescales of about 10 8 years or less (i.e., on an internal dynamical time of the disk in the simulations), and the most intense star formation and lowest gas phase metallicities are seen generally after the first pericentre passage. As the star formation proceeds and the merger advances, the dilution reduces and enrichment becomes dominant – ultimately increasing the metallicity of the circumnuclear gas to a level higher than the initial metallicities of the merging galaxies. The “fly-bys” – pairs that interact but do not merge – also cause some dilution. We even see some dilution early in the merger or in the “fly-bys” and thus do not observe a strong trend between the nuclear metallicities and separation in our simulations until the merger is well advanced. We also analyse the O and Fe enrichment of the ISM, and show that the evolution of the α/Fe ratios, as well as the dilution of the central gas metallicity, can be used as a clock for dating the interaction.

Reconstructing the star formation history of the Milky Way disc(s) from chemical abundances

We develop a chemical evolution model in order to study the star formation history of the Milky Way. Our model assumes that the Milky Way is formed from a closed box-like system in the inner regions, while the outer parts of the disc experience some accretion. Unlike the usual procedure, we do not fix the star formation prescription (e.g. Kennicutt law) in order to reproduce the chemical abundance trends. Instead, we fit the abundance trends with age in order to recover the star formation history of the Galaxy. Our method enables one to recover with unprecedented accuracy the star formation history of the Milky Way in the first Gyrs, in both the inner (R 9-10kpc) discs as sampled in the solar vicinity. We show that, in the inner disc, half of the stellar mass formed during the thick disc phase, in the first 4-5 Gyr. This phase was followed by a significant dip in the star formation activity (at 8-9 Gyr) and a period of roughly constant lower level star formation for the remaining 8 Gyr. The thick disc phase has produced as many metals in 4 Gyr as the thin disc in the remaining 8 Gyr. Our results suggest that a closed box model is able to fit all the available constraints in the inner disc. A closed box system is qualitatively equivalent to a regime where the accretion rate, at high redshift, maintains a high gas fraction in the inner disc. In such conditions, the SFR is mainly governed by the high turbulence of the ISM. By z~1 it is possible that most of the accretion takes place in the outer disc, while the star formation activity in the inner disc is mostly sustained by the gas not consumed during the thick disc phase, and the continuous ejecta from earlier generations of stars. The outer disc follows a star formation history very similar to that of the inner disc, although initiated at z~2, about 2 Gyr before the onset of the thin disc formation in the inner disc.

Characteristics of thick disks formed through minor mergers: stellar excesses and scale lengths

By means of a series of N-body/SPH simulations we investigate the morphological properties of thick stellar disks formed through minor mergers with, e.g. a range of gas-to-stellar mass rati os. We show that the vertical surface density profile of the po st-merger thick disk follows a sech function and has an excess in the regions furthest away from the disk mid-plane (z& 2 kpc). This stellar excess also follows a sech function with a larger scale height than the main thick disk component (except at large radii). It is usually dominated by stars from the primary galaxy, but this depends on the orbital configuration. Stars in the excess have a rotat ional velocity lower than that of stars in the thick disk, and they may thus be confused with stars in the inner galactic halo, which can have a similar lag. Confirming previous results, the thick disk scale heigh t increases with radius and the rate of its increase is smalle r for more gas rich primary galaxies. On the contrary, the scale height of t he stellar excess is independent of both radius and gas fract ion. We also find that the post-merger thick disk has a radial scale length which is 10− 50% larger than that of the thin disk. Two consecutive mergers have basically the same effect on heating the stellar disk as a single merger of the same total mass, i.e., the disk heating effect of a few consecutive mergers does not saturate but is cumulative. To investigate how thick disks produced through secular processes may differ from those produced by minor mergers, we also simulated gravitationally unstable gas-rich disks (“clumpy disks”). T hese clumpy disks do not produce either a stellar excess or a ratio of thick to thin disk scale lengths greater than one. Comparing our simulation results with observations of the Milky Way and nearby galaxies shows that our results for minor mergers are consistent with observations of the ratio of thick to thin disk scale len gths and with the “Toomre diagram” (the sum in quadrature of the vertical and radial versus the rotational kinetic energies) of the Mi lky Way. The simulations of clumpy disks do not show such agreement. We conclude that minor mergers are a viable mechanism for the creation of galactic thick disks and investigating stars at several kpc above the mid-plane of the Milky Way and other galaxies may provide a quantitative method for studying the (minor) merger history of galaxies.

On the self-regulation of intense star-formation in galaxies at z = 1−3

(abridged) We have analyzed the properties of the rest-frame optical emission lines of a sample of 53 intensely star forming galaxies at z=1.3 to 2.7 observed with SINFONI on the ESO-VLT. We find large velocity dispersions in the lines, sigma=30-250 km/s. Our data agree well with simulations where we applied beam-smearing and assumed a scaling relation of the form: velocity dispersion is proportional to the square root of the star-formation intensity (star-formation rate per unit area). We conclude that the dispersions are primarily driven by star formation. To explain the high surface brightness and optical line ratios, high thermal pressures in the warm ionized medium, WIM, are required (log P/k (K/cm^3)>~6-7). Such thermal pressures in the WIM are similar to those observed in nearby starburst galaxies, but occur over much larger physical scales. Moreover, the relatively low ionization parameters necessary to fit the high surface brightnesses and optical line ratios suggest that the gas is not only directly associated with regions of star formation, but is wide spread throughout the general ISM. Thus the optical emission line gas is a tracer of the large scale dynamics of the bulk of the ISM. We present a simple model for the energy input from young stars in an accreting galaxy, to argue that the intense star-formation is supporting high turbulent pressure, which roughly balances the gravitational pressure and thus enables distant gas accreting disks to maintain a Toomre disk instability parameter Q~1. For a star formation efficiency of 3%, only 5-15% of the mechanical energy from young stars that is deposited in the ISM is needed to support the level of turbulence required for maintaining this balance. Since this balance is maintained by energy injected into the ISM by the young stars themselves, this suggests that star formation in high redshift galaxies is self-regulating.

The formation of a thick disk through the heating of a thin disk: Agreement with orbital eccentricities of stars in the solar neighborhood

We study the distribution of orbital eccentricities of stars in thick disks generated by the heating of a pre-existing thin stellar disk through a minor merger (mass ratio 1:10), using N-body/SPH numerical simulations of interactions that span a range of gas fractions in the primary disk and initial orbital configurations. The resulting eccentricity distributions have an approximately triangular shape, with a peak at 0.2-0.35, and a relatively smooth decline towards higher values. Stars originally in the satellite galaxy tend to have higher eccentricities (on average from e = 0.45 to e = 0.75), which is in general agreement with the models of Sales and collaborators, although in detail we find fewer stars with extreme values and no evidence of their secondary peak around e = 0.8. The absence of this high-eccentricity feature results in a distribution that qualitatively matches the observations. Moreover, the increase in the orbital eccentricities of stars in the solar neighborhood with vertical distance from the Galactic mid-plane found by Dierickx and collaborators can be qualitatively reproduced by our models, but only if the satellite is accreted onto a direct orbit. We thus speculate that if minor mergers were the dominant means of forming the Milky Way thick disk, the primary mechanism should be merging with satellite(s) on direct orbits.