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Register a new userStar-Forming, Rotating Spheroidal Galaxies in the GAMA and SAMI Surveys
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The Galaxy And Mass Assembly (GAMA) survey has morphologically identified a class of Little Blue Spheroid (LBS) galaxies whose relationship to other classes of galaxies we now examine in detail. Considering a sample of 868 LBSs, we find that such galaxies display similar but not identical colours, specific star formation rates, stellar population ages, mass-to-light ratios, and metallicities to Sd-Irr galaxies. We also find that LBSs typically occupy environments of even lower density than those of Sd-Irr galaxies, where ~65% of LBS galaxies live in isolation. Using deep, high-resolution imaging from VST KiDS and the new Bayesian, two-dimensional galaxy profile modeling code PROFIT, we further examine the detailed structure of LBSs and find that their Sersic indices, sizes, and axial ratios are compatible with those of low-mass elliptical galaxies. We then examine SAMI Galaxy survey integral field emission line kinematics for a subset of 62 LBSs and find that the majority (42) of these galaxies display ordered rotation with the remainder displaying disturbed/non-ordered dynamics. Finally, we consider potential evolutionary scenarios for a population with this unusual combination of properties, concluding that LBSs are likely formed by a mixture of merger and accretion processes still recently active in low-redshift dwarf populations. We also infer that if LBS-like galaxies were subjected to quenching in a rich environment, they would plausibly resemble cluster dwarf ellipticals.
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We explore the radial distribution of star formation in galaxies in the SAMI Galaxy Survey as a function of their local group environment. Using a sample of galaxies in groups (with halo masses less than $ simeq 10^{14} , mathrm{M_{odot}}$) from the Galaxy And Mass Assembly Survey, we find signatures of environmental quenching in high-mass groups ($M_{G} > 10^{12.5} , mathrm{M_{odot}}$). The mean integrated specific star formation rate of star-forming galaxies in high-mass groups is lower than for galaxies in low-mass groups or that are ungrouped, with $Delta log(sSFR/mathrm{yr^{-1}}) = 0.45 pm 0.07$. This difference is seen at all galaxy stellar masses. In high-mass groups, star-forming galaxies more massive than $M_{*} sim 10^{10} , mathrm{M_{odot}}$ have centrally-concentrated star formation. These galaxies also lie below the star-formation main sequence, suggesting they may be undergoing outside-in quenching. Lower mass galaxies in high-mass groups do not show evidence of concentrated star formation. In groups less massive than $M_{G} = 10^{12.5} , mathrm{M_{odot}}$ we do not observe these trends. In this regime we find a modest correlation between centrally-concentrated star formation and an enhancement in total star formation rate, consistent with triggered star formation in these galaxies.
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Galaxy internal structure growth has long been accused of inhibiting star formation in disc galaxies. We investigate the potential physical connection between the growth of dispersion-supported stellar structures (e.g. classical bulges) and the position of galaxies on the star-forming main sequence at $zsim0$. Combining the might of the SAMI and MaNGA galaxy surveys, we measure the $lambda_{Re}$ spin parameter for 3781 galaxies over $9.5 < log M_{star} [rm{M}_{odot}] < 12$. At all stellar masses, galaxies at the locus of the main sequence possess $lambda_{Re}$ values indicative of intrinsically flattened discs. However, above $log M_{star}[rm{M}_{odot}]sim10.5$ where the main sequence starts bending, we find tantalising evidence for an increase in the number of galaxies with dispersion-supported structures, perhaps suggesting a connection between bulges and the bending of the main sequence. Moving above the main sequence, we see no evidence of any change in the typical spin parameter in galaxies once gravitationally-interacting systems are excluded from the sample. Similarly, up to 1 dex below the main sequence, $lambda_{Re}$ remains roughly constant and only at very high stellar masses ($log M_{star}[rm{M}_{odot}]>11$), do we see a rapid decrease in $lambda_{Re}$ once galaxies decline in star formation activity. If this trend is confirmed, it would be indicative of different quenching mechanisms acting on high- and low-mass galaxies. The results suggest that while a population of galaxies possessing some dispersion-supported structure is already present on the star-forming main sequence, further growth would be required after the galaxy has quenched to match the kinematic properties observed in passive galaxies at $zsim0$.
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A model of magnetic field structure is presented to help test the prevalence of flux freezing in star-forming clouds of various shapes, orientations, and degrees of central concentration, and to estimate their magnetic field strength. The model is based on weak-field flux freezing in centrally condensed Plummer spheres and spheroids of oblate and prolate shape. For a spheroid of given density contrast, aspect ratio, and inclination, the model estimates the local field strength and direction, and the global field pattern of hourglass shape. Comparisons with a polarization simulation indicate typical angle agreement within 1 - 10 degrees. Scalable analytic expressions are given to match observed polarization patterns, and to provide inputs to radiative transfer codes for more accurate predictions. The model may apply to polarization observations of dense cores, elongated filamentary clouds, and magnetized circumstellar disks.
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