The Redshift Evolution of Bulges and Disks of Spiral Galaxies in COSMOS

We present a preliminary analysis of the bulge and disk properties for a sample of over 4000 L* spiral galaxies at z < 0.84 from the COSMOS 2 square degree survey. We find that for early Hubble type spiral galaxies (Sa–Sb), the bulge-to-disk ratio is roughly constant over the last 7 Gyr of lookback time. This suggests that bulges of early type spirals were in place early on, consistent with other downsizing signatures. There is a monotonic increase in the bulge-to-disk ratios of late type spirals but that likely reflects the well-known decline in the star formation rate from z ~ 1 to the present. For this sample of L* spirals, we also find that the median exponential scale length of disks remains unchanged at 3.1 kpc from z = 0.0 to z = 0.84

Evolution of the bar fraction in COSMOS: quantifying the assembly of the Hubble sequence

We have analyzed the redshift-dependent fraction of galactic bars over 0.2 < z < 0.84 in 2157 luminous face-on spiral galaxies from the COSMOS 2 deg^2 field. Our sample is an order of magnitude larger than that used in any previous investigation, and is based on substantially deeper imaging data than that available from earlier wide-area studies of high-redshift galaxy morphology. We find that the fraction of barred spirals declines rapidly with redshift. Whereas in the local universe about 65% of luminous spiral galaxies contain bars (SB+SAB), at z ~ 0.84 this fraction drops to about 20%. Over this redshift range the fraction of strong bars (SBs) drops from about 30% to under 10%. It is clear that when the universe was half its present age, the census of galaxies on the Hubble sequence was fundamentally different from that of the present day. A major clue to understanding this phenomenon has also emerged from our analysis, which shows that the bar fraction in spiral galaxies is a strong function of stellar mass, integrated color and bulge prominence. The bar fraction in very massive, luminous spirals is about constant out to z ~ 0.84, whereas for the low-mass, blue spirals it declines significantly with redshift beyond z = 0.3. There is also a slight preference for bars in bulge-dominated systems at high redshifts that may be an important clue toward the coevolution of bars, bulges, and black holes. Our results thus have important ramifications for the processes responsible for galactic downsizing, suggesting that massive galaxies matured early in a dynamical sense, and not just as a result of the regulation of their star formation rate

Convective Origin of Stellar Magnetic Fields

Magnetic fields are pervasive and complex features of many stars, yet neither their dynamics nor their origins are well understood. We propose that a possible origin of some stellar magnetic fields is the thermal gradient in a star’s convection zone preferentially driving the free electrons at a greater average speed than the other ion species present. This relative electron-ion flow would have a corresponding electric field, which could then induce a magnetic field. This mechanism for creating stellar magnetic fields is tested in the context of simulations modeling the solar convective envelope where the relative electron-ion flow is treated as a ‘difference flow’ of ‘difference particles’. Model S solar parameters are used as constraints for simulations executed using the 3D ideal-MHD ZEUS-MP code which has been modified to include magnetic fields induced via the Biot-Savart Law. Effects due to varying the surface thermal boundary conditions, artificial viscosity values, charge density, and mesh dimensions are explored

Juno Constraints on the Formation of Jupiter's Magnetospheric Cushion Region

Observations by the Pioneer, Voyager, Ulysses, and Galileo spacecraft in Jupiter's dayside magnetosphere revealed a cushion region, where the magnetic field became increasingly dipolar and the 10‐hr periodicity associated with rotation of the magnetodisc was no longer visible. Focused observations at the dawn terminator by the Juno spacecraft provide critical constraints on the formation physics of the dayside cushion. We observe a persistent 10‐hr periodicity at dawn with only minor distortions of the field near the magnetopause boundary, indicating the absence of a systematic dawn cushion region. These data suggest that the dayside cushion is not formed via mass loss associated with magnetic reconnection along a localized X line but rather may be due to the gradual compression of the dawnside magnetic field as it rotates toward local noon