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Register a new userLarge-Scale Outer Rings of Early-type Disk Galaxies
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Irina P. Kostiuk
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We have searched for presence of current star formation in outer stellar rings of early-type disk (S0-Sb) galaxies by inspecting a representative sample of nearby galaxies with rings from the recent Spitzer catalog ARRAKIS (Comeron et al. 2014). We have found that regular rings (of R-type) reveal young stellar population with the age of less than 200~Myr in about half of all the cases, while in the pseudorings (open rings, R), which inhabit only spiral galaxies, current star formation proceeds almost always.
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Dark gaps are commonly seen in early-to-intermediate type barred galaxies having inner and outer rings or related features. In this paper, the morphologies of 54 barred and oval ringed galaxies have been examined with the goal of determining what the dark gaps are telling us about the structure and evolution of barred galaxies. The analysis is based mainly on galaxies selected from the Galaxy Zoo 2 database and the Catalogue of Southern Ringed Galaxies. The dark gaps between inner and outer rings are of interest because of their likely association with the L4 and L5 Lagrangian points that would be present in the gravitational potential of a bar or oval. Since the points are theoretically expected to lie very close to the corotation resonance (CR) of the bar pattern, the gaps provide the possibility of locating corotation in some galaxies simply by measuring the radius rgp of the gap region and setting rCR=rgp. With the additional assumption of generally flat rotation curves, the locations of other resonances can be predicted and compared with observed morphological features. It is shown that this gap method provides remarkably consistent interpretations of the morphology of early-to-intermediate type barred galaxies. The paper also brings attention to cases where the dark gaps lie inside an inner ring, rather than between inner and outer rings. These may have a different origin compared to the inner/outer ring gaps.
We use a 380 h-1 pc resolution hydrodynamic AMR simulation of a cosmic filament to investigate the orientations of a sample of ~100 well-resolved galactic disks spanning two orders of magnitude in both stellar and halo mass. We find: (i) At z=0, there is an almost perfect alignment at a median angle of 18 deg, in the inner dark matter halo regions where the disks reside, between the spin vector of the gaseous and stellar galactic disks and that of their inner host haloes. The alignment between galaxy spin and spin of the entire host halo is however significantly weaker, ranging from a median of ~46 deg at z=1 to ~50 deg at z=0. (ii) The most massive galaxy disks have spins preferentially aligned so as to point along their host filaments. (iii) The spin of disks in lower-mass haloes shows, at redshifts above z~0.5 and in regions of low environmental density, a clear signature of alignment with the intermediate principal axis of the large-scale tidal field. This behavior is consistent with predictions of linear tidal torque theory. This alignment decreases with increasing environmental density, and vanishes in the highest density regions. Non-linear effects in the high density environments are plausibly responsible for establishing this density-alignment correlation. We expect that our numerical results provide important insights for both understanding intrinsic alignment in weak lensing from the astrophysical perspective and formation and evolution processes of galactic disks in a cosmological context.
Next generation CMB experiments with arcmin resolution will, for free, lay the foundations for a real breakthrough on the study of the early evolution of galaxies and galaxy clusters, thanks to the detection of large samples of strongly gravitationally lensed galaxies and of proto-clusters of dusty galaxies up to high redshifts. This has an enormous legacy value. High resolution follow-up of strongly lensed galaxies will allow the direct investigation of their structure and kinematics up to z~6, providing direct information on physical processes driving their evolution. Follow-up of proto-clusters will allow an observational validation of the formation history of the most massive dark matter halos up to z~4, well beyond the redshift range accessible via X-ray or SZ measurements. These experiments will also allow a giant leap forward in the determination of polarization properties of extragalactic sources, and will provide a complete census of cold dust available for star formation in the local universe.
The origin of S0 galaxies is discussed in the framework of early mergers in a Cold Dark Matter cosmology, and in a scenario where S0s are assumed to be former spirals stripped of gas. From an analysis of 127 early-type disk galaxies (S0-Sa), we find a clear correlation between the scale parameters of the bulge (r_eff) and the disk (h_R), a correlation which is difficult to explain if these galaxies were formed in mergers of disk galaxies. However, the stripping hypothesis, including quiescent star formation, is not sufficient to explain the origin of S0s either, because it is not compatible with our finding that S0s have a significantly smaller fraction of bars (46$pm$6 %) than their assumed progenitors, S0/a galaxies (93$pm$5 %) or spirals (64-69 %). Our conclusion is that even if a large majority of S0s were descendants of spiral galaxies, bars and ovals must play an important role in their evolution. The smaller fraction particularly of strong bars in S0 galaxies is compensated by a larger fraction of ovals/lenses (97$pm$2 % compared to 82-83 % in spirals), many of which might be weakened bars. We also found massive disk-like bulges in nine of the S0 galaxies, bulges which might have formed at an early gas-rich stage of galaxy evolution.
The HI in galaxies often extends past their conventionally defined optical extent. I report results from our team which has been probing low intensity star formation in outer disks using imaging in H-alpha and ultraviolet. Using a sample of hundreds of HI selected galaxies, we confirm that outer disk HII regions and extended UV disks are common. Hence outer disks are not dormant but are dimly forming stars. Although the ultraviolet light in galaxies is more centrally concentrated than the HI, the UV/HI ratio (the Star Formation Efficiency) is nearly constant, with a slight dependency on surface brightness. This result is well accounted for in a model where disks maintain a constant stability parameter Q. This model also accounts for how the ISM and star formation are distributed in the bright parts of galaxies, and how HI appears to trace the distribution of dark matter in galaxy outskirts.
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