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Register a new userGas inflows, star formation and metallicity evolution in galaxy pairs
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It has been known since many decades that galaxy interactions can induce star formation (hereafter SF) enhancements and that one of the driving mechanisms of this enhancement is related to gas inflows into the central galaxy regions, induced by asymmetries in the stellar component, like bars. In the last years many evidences have been accumulating, showing that interacting pairs have central gas-phase metallicities lower than those of field galaxies, by {sim} 0.2-0.3 dex on average. These diluted ISM metallicities have been explained as the result of inflows of metal-poor gas from the outer disk to the galaxy central regions. A number of questions arises: Whats the timing and the duration of this dilution? How and when does the SF induced by the gas inflow enrich the circumnuclear gas with re-processed material? Is there any correlation between the timing and strength of the dilution and the timing and intensity of the SF? By means of Tree-SPH simulations of galaxy major interactions, we have studied the effect that gas inflows have on the ISM dilution, and the effect that the induced SF has, subsequently, in re-enriching the nuclear gas. In this contribution, we present the main results of this study.
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The infrared (IR) emission of M_* galaxies (10^{10.4} < M_{star} < 10^{11.0} M_sun) in galaxy pairs, derived using data obtained in Herschel (PEP/HerMES) and Spitzer (S-COSMOS) surveys, is compared to that of single disk galaxies in well matched control samples to study the cosmic evolution of the star-formation enhancement induced by galaxy-galaxy interaction. Both the mean IR SED and mean IR luminosity of star-forming galaxies (SFGs) in SFG+SFG (S+S) pairs in the redshift bin of 0.6 < z < 1 are consistent with no star-formation enhancement. SFGs in S+S pairs in a lower redshift bin of 0.2 < z < 0.6 show marginal evidence for a weak star-formation enhancement. Together with the significant and strong sSFR enhancement shown by SFGs in a local sample of S+S pairs (obtained using previously published Spitzer observations), our results reveal a trend for the star-formation enhancement in S+S pairs to decrease with increasing redshift. Between z=0 and z=1, this decline of interaction-induced star-formation enhancement occurs in parallel with the dramatic increase (by a factor of ~10) of the sSFR of single SFGs, both can be explained by the higher gas fraction in higher z disks. SFGs in mixed pairs (S+E pairs) do not show any significant star-formation enhancement at any redshift. The difference between SFGs in S+S pairs and in S+E pairs suggests a modulation of the sSFR by the inter-galactic medium IGM in the dark matter halos (DMH) hosting these pairs.
How high-mass stars form remains unclear currently. Calculation suggests that the radiation pressure of a forming star can halt spherical infall, preventing its further growth when it reaches 10 M$_{odot}$. Two major theoretical models on the further growth of stellar mass were proposed. One model suggests the mergence of less massive stellar objects, and the other is still through accretion but with the help of disk. Inflow motions are the key evidence of how forming stars further gain mass to build up massive stars. Recent development in technology has boosted the search of inflow motion. A number of high-mass collapse candidates were obtained with single dish observations, mostly showed blue profile. The infalling signatures seem to be more common in regions with developed radiation pressure than in younger cores, which opposes the theoretical prediction and is also very different from that of low mass star formation. Interferometer studies so far confirm such tendency with more obvious blue profile or inverse P Cygni profile. Results seem to favor the accretion model. However, the evolution tendency of the infall motion in massive star forming cores needs to be further explored. Direct evidence for monolithic or competitive collapse processes is still lack. ALMA will enable us to probe more detail of gravity process.
(abridged) Here we present HI line and 20-cm radio continuum data of the nearby galaxy pair NGC1512/1510 as obtained with the Australia Telescope Compact Array. These are complemented by GALEX UV-, SINGG Halpha- and Spitzer mid-infrared images, allowing us to compare the distribution and kinematics of the neutral atomic gas with the locations and ages of the stellar clusters within the system. For the barred, double-ring galaxy NGC1512 we find a very large HI disk, about 4x its optical diameter, with two pronounced spiral/tidal arms. Both its gas distribution and the distribution of the star-forming regions are affected by gravitational interaction with the neighbouring blue compact dwarf galaxy NGC1510. The two most distant HI clumps, at radii of about 80 kpc, show signs of star formation and are likely tidal dwarf galaxies. Star formation in the outer disk of NGC1512 is revealed by deep optical- and two-color ultraviolet images. Using the latter we determine the properties of about 200 stellar clusters and explore their correlation with dense HI clumps in the even larger 2XHI disk. The multi-wavelength analysis of the NGC1512/1510 system, which is probably in the first stages of a minor merger having started about 400 Myr ago, links stellar and gaseous galaxy properties on scales from one to 100 kpc.
We study the impact of different galaxy statistics and empirical metallicity scaling relations on the merging rates and on the properties of compact objects binaries. First, we analyze the similarities and differences of using the star formation rate functions or the stellar mass functions as galaxy statistics for the computation of the cosmic star formation rate density. Then we investigate the effects of adopting the Fundamental Metallicity Relation or a classic Mass Metallicity Relation to assign metallicity to galaxies with given properties. We find that when the Fundamental Metallicity Relation is exploited, the bulk of the star formation occurs at relatively high metallicities even at high redshift; the opposite holds when the Mass Metallicity Relation is employed, since in this case the metallicity at which most of the star formation takes place strongly decreases with redshift. We discuss the various reasons and possible biases originating this discrepancy. Finally, we show the impact that these different astrophysical prescriptions have on the merging rates and on the properties of compact objects binaries; specifically, we present results for the redshift dependent merging rates and for the chirp mass and time delay distributions of the merging binaries.
Using reconstructed galaxy star formation histories, we calculate the instantaneous efficiency of galaxy star formation (i.e., the star formation rate divided by the baryon accretion rate) from $z=8$ to the present day. This efficiency exhibits a clear peak near a characteristic halo mass of 10^11.7 Msun, which coincides with longstanding theoretical predictions for the mass scale relevant to virial shock heating of accreted gas. Above the characteristic halo mass, the efficiency falls off as the mass to the minus four-thirds power; below the characteristic mass, the efficiency falls off at an average scaling of mass to the two-thirds power. By comparison, the shape and normalization of the efficiency change very little since z=4. We show that a time-independent star formation efficiency simply explains the shape of the cosmic star formation rate since z=4 in terms of dark matter accretion rates. The rise in the cosmic star formation from early times until z=2 is especially sensitive to galaxy formation efficiency. The mass dependence of the efficiency strongly limits where most star formation occurs, with the result that two-thirds of all star formation has occurred inside halos within a factor of three of the characteristic mass, a range that includes the mass of the Milky Way.
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