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Register a new userThe Formation of Dust Lanes: Implications for Galaxy Evolution
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We find that disk galaxies show a sharp, mass-dependent transition in the structure of their dusty ISM. Dust lanes are a generic feature of massive disks with V_rot>120km/s, but are completely absent in galaxies with V_rot<120km/s. The transition reflects an increase in the scale height of the cold ISM in low mass galaxies, driven by larger turbulent velocities supporting the gas layer, rather than sharp drops in the gas surface density. We identify the V_rot=120km/s transition with the onset of gravitational instabilities in high mass galaxies. The instabilities lead to fragmentation and gravitational collapse along spiral arms, smaller gas scale heights, lower turbulent velocities, and thus to narrow dust lanes. The drop in velocity dispersion may be due either to a switch in the driving mechanism for turbulence or to a change in the response of the ISM to supernovae after the ISM has collapsed to a dense layer. The resulting smaller gas scale height can lead to significant increases in the star formation rate when disk instabilities are present, and may explain the Kennicutt surface density threshold for star formation. Our data suggest that star formation will be systematically less efficient in low mass disks with V_c<120km/s, leading to star formation timescales longer than the gas accretion timescale. This effect can suppress the metallicity and nucleosynthetic yields of low mass disks, and thus explain the disk mass-metallicity relationship without invoking galactic SN-driven outflows. The transitions in disk stability, dust structure, and/or star formation efficiency may also be responsible for observed changes in the slope of the Tully-Fisher relation, in the sharp increase in the thickness of dwarf galaxy disks, and in the onset of bulges in galaxies with V_rot>120km/s. (Abridged)
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Star-forming regions that are visible at 3.6 microns and Halpha but not in the u,g,r,i,z bands of the Sloan Digital Sky survey (SDSS), are measured in five nearby spiral galaxies to find extinctions averaging ~3.8 mag and stellar masses averaging ~5x10^4 Msun. These regions are apparently young star complexes embedded in dark filamentary shock fronts connected with spiral arms. The associated cloud masses are ~10^7 Msun. The conditions required to make such complexes are explored, including gravitational instabilities in spiral shocked gas and compression of incident clouds. We find that instabilities are too slow for a complete collapse of the observed spiral filaments, but they could lead to star formation in the denser parts. Compression of incident clouds can produce a faster collapse but has difficulty explaining the semi-regular spacing of some regions along the arms. If gravitational instabilities are involved, then the condensations have the local Jeans mass. Also in this case, the near-simultaneous appearance of equally spaced complexes suggests that the dust lanes, and perhaps the arms too, are relatively young.
The intermediate results of the ongoing study of deep samples of ~200 galaxies residing in nearby voids, are presented. Their properties are probed via optical spectroscopy, ugri surface photometry, and HI 21-cm line measurements, with emphasis on their evolutionary status. We derive directly the hydrogen mass M(HI), the ratio M(HI)/L_B and the evolutionary parameter gas-phase O/H. Their luminosities and integrated colours are used to derive stellar mass M(*) and the second evolutionary parameter -- gas mass-fraction f_g. The colours of the outer parts, typically representative of the galaxy oldest stellar population, are used to estimate the upper limits on time since the beginning of the main SF episode. We compare properties of void galaxies with those of the similar late-type galaxies in denser environments. Most of void galaxies show smaller O/H for their luminosity, in average by ~30%, indicating slower evolution. Besides, the fraction of ~10% of the whole void sample or ~30% of the least luminous void LSB dwarfs show the oxygen deficiency by a factor of 2--5. The majority of this group appear very gas-rich, with f_g ~(95-99)%, while their outer parts appear rather blue, indicating the time of onset of the main star-formation episode of less than 1-4 Gyr. Such unevolved LSBD galaxies appear not rare among the smallest void objects, but turned out practically missed to date due to the strong observational selection effects. Our results evidense for unusual evolutionary properties of the sizable fraction of void galaxies, and thus, pose the task of better modelling of dwarf galaxy formation and evolution in voids.
Though observationally rare, damped Lya absorption systems dominate the mass density of neutral gas in the Universe. Eleven high redshift damped Lya systems covering 2.8<z<4.4 were discovered in 26 QSOs from the APM z>4 QSO Survey, extending these absorption system surveys to the highest redshifts currently possible. Combining our new data set with previous surveys we find that the cosmological mass density in neutral gas, omega_g, does not rise as steeply prior to z~2 as indicated by previous studies. There is evidence in the observed omega_g for a flattening at z~2 and a possible turnover at z~3. When combined with the decline at z>3.5 in number density per unit redshift of damped systems with column densities log N(HI)>21 atoms cm^-2, these results point to an epoch at z>3 prior to which the highest column density damped systems are still forming. We find that over the redshift range 2<z<4 the total mass in neutral gas is marginally comparable with the total visible mass in stars in present day galaxies. However, if one considers the total mass visible in stellar disks alone, ie excluding galactic bulges, the two values are comparable. We are observing a mass of neutral gas comparable to the mass of visible disk stars. Lanzetta, Wolfe & Turnshek (1995) found that omega_g(z~3.5) was twice omega_g(z~2), implying a much larger amount of star formation must have taken place between z=3.5 and z=2 than is indicated by metallicity studies. This created a `cosmic G-dwarf problem. The more gradual evolution of omega_g we find alleviates this. These results have profound implications for theories of galaxy formation.
We present cosmological zoom-in hydro-dynamical simulations for the formation of disc galaxies, implementing dust evolution and dust promoted cooling of hot gas. We couple an improved version of our previous treatment of dust evolution, which adopts the two-size approximation to estimate the grain size distribution, with the MUPPI star formation and feedback sub-resolution model. Our dust evolution model follows carbon and silicate dust separately. To distinguish differences induced by the chaotic behaviour of simulations from those genuinely due to different simulation set-up, we run each model six times, after introducing tiny perturbations in the initial conditions. With this method, we discuss the role of various dust-related physical processes and the effect of a few possible approximations adopted in the literature. Metal depletion and dust cooling affect the evolution of the system, causing substantial variations in its stellar, gas and dust content. We discuss possible effects on the Spectral Energy Distribution of the significant variations of the size distribution and chemical composition of grains, as predicted by our simulations during the evolution of the galaxy. We compare dust surface density, dust-to-gas ratio and small-to-big grain mass ratio as a function of galaxy radius and gas metallicity predicted by our fiducial run with recent observational estimates for three disc galaxies of different masses. The general agreement is good, in particular taking into account that we have not adjusted our model for this purpose.
We report a comprehensive statistical analysis of the observational data of the cosmic evolution of supernova (SN) rate density, to derive constraints on cosmic star formation history and the nature of type Ia supernova (SN Ia) progenitor. We use all available information of magnitude, SN type, and redshift information of both type Ia and core-collapse (CC) SNe in GOODS and SDF, as well as SN Ia rate densities reported in the literature. Furthermore, we also add 157 SN candidates in the past Subaru/Suprime-Cam data that are newly reported here, to increase the statistics. We find that the current data set of SN rate density evolution already gives a meaningful constraint on the evolution of the cosmic star formation rate (SFR) at z <~ 1, though strong constraints cannot be derived for the delay time distribution (DTD) of SNe Ia. We derive a constraint of the evolutionary index of SFR density alpha ~ 3--4 [(1+z)^alpha at z <~ 1] with an evidence for a significant evolution of mean extinction of CC SNe [E(B-V) ~ 0.5 at z ~ 0.5 compared with ~ 0.2 at z = 0], which does not change significantly within a reasonable range of various DTD models. This result is nicely consistent with the systematic trend of alpha estimates based on galactic SFR indicators in different wavelengths (ultraviolet, H_alpha, and infrared), indicating that there is a strong evolution in mean extinction of star forming regions in galaxies at relatively low redshift range of z <~ 0.5. These results are obtained by a method that is completely independent of galaxy surveys, and especially, there is no detection limit about the host galaxy luminosity in our analysis, giving a strong constraint on the star formation activity in high-z dwarf galaxies or intergalactic space.
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