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Register a new userDynamics of galaxies and clusters in textit{refracted gravity}
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We investigate the proof of concept and the implications of textit{refracted gravity}, a novel modified gravity aimed to solve the discrepancy between the luminous and the dynamical mass of cosmic structures without resorting to dark matter. Inspired by the behavior of electric fields in matter, refracted gravity introduces a gravitational permittivity that depends on the local mass density and modifies the standard Poisson equation. The resulting gravitational field can become more intense than the Newtonian field and can mimic the presence of dark matter. We show that the refracted gravitational field correctly describes (1) the rotation curves and the Tully-Fisher relation of disk galaxies; and (2) the observed temperature profile of the X-ray gas of galaxy clusters. According to these promising results, we conclude that refracted gravity deserves further investigation.
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Using N-body simulations we study the formation and evolution of tidally induced bars in disky galaxies in clusters. Our progenitor is a massive, late-type galaxy similar to the Milky Way, composed of an exponential disk and an NFW dark matter halo. We place the galaxy on four different orbits in a Virgo-like cluster and evolve it for 10 Gyr. As a reference case we also evolve the same model in isolation. Tidally induced bars form on all orbits soon after the first pericenter passage and survive until the end of the evolution. They appear earlier, are stronger, longer and have lower pattern speeds for tighter orbits. Only for the tightest orbit the properties of the bar are controlled by the orientation of the tidal torque from the cluster at pericenters. The mechanism behind the formation of the bars is the angular momentum transfer from the galaxy stellar component to its halo. All bars undergo extended periods of buckling instability that occur earlier and lead to more pronounced boxy/peanut shapes when the tidal forces are stronger. Using all simulation outputs of galaxies at different evolutionary stages we construct a toy model of the galaxy population in the cluster and measure the average bar strength and bar fraction as a function of clustercentric radius. Both are found to be mildly decreasing functions of radius. We conclude that tidal forces can trigger bar formation in cluster cores, but not in the outskirts, and thus cause larger concentrations of barred galaxies towards cluster center.
We quantify the evolution of the spiral, S0 and elliptical fractions in galaxy clusters as a function of cluster velocity dispersion ($sigma$) and X-ray luminosity ($L_X$) using a new database of 72 nearby clusters from the WIde-Field Nearby Galaxy-cluster Survey (WINGS) combined with literature data at $z=0.5-1.2$. Most WINGS clusters have $sigma$ between 500 and 1100 $rm km s^{-1}$, and $L_X$ between 0.2 and $5 times 10^{44} rm erg/s$. The S0 fraction in clusters is known to increase with time at the expense of the spiral population. We find that the spiral and S0 fractions have evolved more strongly in lower $sigma$, less massive clusters, while we confirm that the proportion of ellipticals has remained unchanged. Our results demonstrate that morphological evolution since $z=1$ is not confined to massive clusters, but is actually more pronounced in low mass clusters, and therefore must originate either from secular (intrinsic) evolution and/or from environmental mechanisms that act preferentially in low-mass environments, or both in low- and high-mass systems. We also find that the evolution of the spiral fraction perfectly mirrors the evolution of the fraction of star-forming galaxies. Interestingly, at low-z the spiral fraction anticorrelates with $L_X$. Conversely, no correlation is observed with $sigma$. Given that both $sigma$ and $L_X$ are tracers of the cluster mass, these results pose a challenge for current scenarios of morphological evolution in clusters.
The low-metallicity, kinematically interesting dwarf stars studied by Stephens & Boesgaard (2002, SB02) are re-examined using Gaia DR2 astrometry, and updated model atmospheres and atomic line data. New stellar parameters are determined based on the Gaia DR2 parallactic distances and Dartmouth Stellar Evolution Database isochrones. These are in excellent agreement with spectroscopically determined stellar parameters for stars with [Fe/H]$>-2$; however, large disagreements are found for stars with [Fe/H]$le-2$, with offsets as large as $Delta$T$_{rm eff}sim+500$ K and $Delta$log,$gsim+1.0$. A subset of six stars (test cases) are analysed ab initio using high resolution spectra with Keck HIRES and Gemini GRACES. This sub-sample is found to include two $alpha$-challenged dwarf stars, suggestive of origins in a low mass, accreted dwarf galaxy. The orbital parameters for the entire SB02 sample are re-determined using textit{Gaia} DR2 data. We find 11 stars that are dynamically coincident with the textit{Gaia}-Sausage accretion event and another 17 with the textit{Gaia}-Sequoia event in action space. Both associations include low-mass, metal-poor stars with isochrone ages older than 10 Gyr. Two dynamical subsets are identified within textit{Gaia}-Sequoia. When these subsets are examined separately, a common knee in [$alpha$/Fe] is found for the textit{Gaia}-Sausage and low orbital energy textit{Gaia}-Sequoia stars. A lower metallicity knee is tentatively identified in the textit{Gaia}-Sequoia high orbital energy stars. If the metal-poor dwarf stars in these samples are true members of the textit{Gaia}-Sausage and textit{Gaia}-Sequoia events, then they present a unique opportunity to probe the earlier, more pristine, star formation histories of these systems.
We examine the relationship between star formation and AGN activity by constructing matched samples of local ($0<z<0.6$) radio-loud and radio-quiet AGN in the $textit{Herschel}$-ATLAS fields. Radio-loud AGN are classified as high-excitation and low-excitation radio galaxies (HERGs, LERGs) using their emission lines and $textit{WISE}$ 22-$mu$m luminosity. AGN accretion and jet powers in these active galaxies are traced by [OIII] emission-line and radio luminosity, respectively. Star formation rates (SFRs) and specific star formation rates (SSFRs) were derived using $textit{Herschel}$ 250-$mu$m luminosity and stellar mass measurements from the SDSS$-$MPA-JHU catalogue. In the past, star formation studies of AGN have mostly focused on high-redshift sources to observe the thermal dust emission that peaks in the far-infrared, which limited the samples to powerful objects. However, with $textit{Herschel}$ we can expand this to low redshifts. Our stacking analyses show that SFRs and SSFRs of both radio-loud and radio-quiet AGN increase with increasing AGN power but that radio-loud AGN tend to have lower SFR. Additionally, radio-quiet AGN are found to have approximately an order of magnitude higher SSFRs than radio-loud AGN for a given level of AGN power. The difference between the star formation properties of radio-loud and -quiet AGN is also seen in samples matched in stellar mass.
Polar ring galaxies are ideal objects with which to study the three-dimensional shapes of galactic gravitational potentials since two rotation curves can be measured in two perpendicular planes. Observational studies have uncovered systematically larger rotation velocities in the extended polar rings than in the associated host galaxies. In the dark matter context, this can only be explained through dark halos that are systematically flattened along the polar rings. Here, we point out that these objects can also be used as very effective tests of gravity theories, such as those based on Milgromian dynamics (MOND). We run a set of polar ring models using both Milgromian and Newtonian dynamics to predict the expected shapes of the rotation curves in both planes, varying the total mass of the system, the mass of the ring with respect to the host, as well as the size of the hole at the center of the ring. We find that Milgromian dynamics not only naturally leads to rotation velocities being typically higher in the extended polar rings than in the hosts, as would be the case in Newtonian dynamics without dark matter, but that it also gets the shape and amplitude of velocities correct. Milgromian dynamics thus adequately explains this particular property of polar ring galaxies.
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