Rotation Curves of Galaxies and Their Dependence on Morphology and Stellar Mass

We study how stellar rotation curves (RCs) of galaxies are correlated on average with morphology and stellar mass (M star) using the final release of Sloan Digital Sky Survey IV MaNGA data. We use the visually assigned T-types for the morphology indicator, and adopt a functional form for the RC that can model non-flat RCs at large radii. We discover that within the radial coverage of the MaNGA data, the popularly known flat rotation curve at large radii applies only to the particular classes of galaxies, i.e., massive late types (T-type ≥ 1, M star ≳ 1010.8 M ⊙) and S0 types (T-type = −1 or 0, M star ≳ 1010.0 M ⊙). The RC of late-type galaxies at large radii rises more steeply as M star decreases, and its slope increases to about +9 km s−1 kpc−1 at M star ≈ 109.7 M ⊙. By contrast, elliptical galaxies (T-type ≤ −2) have descending RCs at large radii. Their slope becomes more negative as M star decreases, and reaches as negative as −15 km s−1 kpc−1 at M star ≈ 1010.2 M ⊙. We also find that the inner slope of the RC is highest for elliptical galaxies with M star ≈ 1010.5 M ⊙, and decreases as T-type increases or M star changes away from 1010.5 M ⊙. The velocity at the turnover radius R t is higher for higher M star, and R t is larger for higher M star and later T-types. We show that the inner slope of the RC is coupled with the central surface stellar mass density, which implies that the gravitational potential of central regions of galaxies is dominated by baryonic matter. With the aid of simple models for matter distribution, we discuss what determines the shapes of RCs.

Evolution of the Ultraviolet Upturn at 0.3 < z < 1: Exploring Helium-rich Stellar Populations

We measure the near-UV (rest-frame ∼2400 Å) to optical color for early-type galaxies in 12 clusters at 0.3 < z < 1.0. We show that this is a suitable proxy for the more common far-ultraviolet bandpass used to measure the ultraviolet upturn and find that the upturn is detected to z = 0.6 in these data, in agreement with previous work. We find evidence that the strength of the upturn starts to wane beyond this redshift and largely disappears at z = 1. Our data are most consistent with models where early-type galaxies contain minority stellar populations with non-cosmological helium abundances, up to around 46%, formed at z ≥ 3, resembling globular clusters with multiple stellar populations in our Galaxy. This suggests that elliptical galaxies and globular clusters share similar chemical evolution and star formation histories. The vast majority of the stellar mass in these galaxies also must have been in place at z > 3.

On the Mass Loading of AGN-driven Outflows in Elliptical Galaxies and Clusters

Outflows driven by active galactic nuclei (AGNs) are an important channel for accreting supermassive black holes (SMBHs) to interact with their host galaxies and clusters. Properties of the outflows are however poorly constrained due to the lack of kinetically resolved data of the hot plasma that permeates the circumgalactic and intracluster space. In this work, we use a single parameter, outflow-to-accretion mass-loading factor m = M ̇ jet / M ̇ BH , to characterize the outflows that mediate the interaction between SMBHs and their hosts. By modeling both M87 and Perseus, and comparing the simulated thermal profiles with the X-ray observations of these two systems, we demonstrate that m can be constrained between 200 and 500. This parameter corresponds to a bulk flow speed between 4000 and 7000 km s−1 at around 1 kpc, and a thermalized outflow temperature between 108.7 and 109 K. Our results indicate that the dominant outflow speeds in giant elliptical galaxies and clusters are much lower than in the close vicinity of the SMBH, signaling an efficient coupling with and deceleration by the surrounding medium on length scales below 1 kpc. Consequently, AGNs may be efficient at launching outflows ∼10 times more massive than previously uncovered by measurements of cold, obscuring material. We also examine the mass and velocity distribution of the cold gas, which ultimately forms a rotationally supported disk in simulated clusters. The rarity of such disks in observations indicates that further investigations are needed to understand the evolution of the cold gas after it forms.

Magnetic fields in elliptical galaxies: using the Laing–Garrington effect in radio galaxies and polarized emission from background radio sources

ABSTRACT Magnetic fields in elliptical galaxies are poorly constrained because of a lack of significant synchrotron emission from them. In this paper, we explore the properties of magnetic fields in ellipticals using two methods. First, we exploit the Laing–Garrington effect (asymmetry in the observed polarization fraction between radio galaxy jets) for 57 galaxies with redshifts up to 0.5. We use the differences in polarization fraction and rotation measure between the jet and counterjet to estimate the small- and large-scale magnetic fields in and around ellipticals (including their circumgalactic medium). We find that the small-scale field (at scales smaller than the driving scale of turbulence, approximately 300 pc) lies in the range 0.1–2.75 $\mu{\rm G}$. The large-scale field (at scales of 100 kpc) is an order of magnitude smaller than the small-scale field. In the second method, we cross-match the Faraday rotation measures (RMs) of a few hundred (out of 3098) extragalactic radio sources with galaxy catalogues to explore the effect of the number and morphology of intervening galaxies on the observed RM distribution. We use both Gaussian and non-Gaussian functions to describe the RM distribution and to derive its statistical properties. Finally, using the difference in the observed polarization fraction between the intervening spirals and ellipticals, we estimate the small-scale magnetic fields at the centre of ellipticals to be ∼6 $\mu{\rm G}$. Both methods with different observations and analysis techniques give magnetic field strengths consistent with previous studies (≤10 $\mu{\rm G}$). The results can be used to constrain dynamo theories and galaxy evolution simulations.

Galactic traversability: a new concept for extragalactic SETI

Interstellar travel in the Milky Way is commonly thought to be a long and dangerous enterprise, but are all galaxies so hazardous? I introduce the concept of galactic traversability to address this question. Stellar populations are one factor in traversability, with higher stellar densities and velocity dispersions aiding rapid spread across a galaxy. The interstellar medium (ISM) is another factor, as gas, dust grains and cosmic rays all pose hazards to starfarers. I review the current understanding of these components in different types of galaxies, and conclude that red quiescent galaxies without star formation have favourable traversability. Compact elliptical galaxies and globular clusters could be ‘super-traversable’, because stars are packed tightly together and there are minimal ISM hazards. Overall, if the ISM is the major hindrance to interstellar travel, galactic traversability increases with cosmic time as gas fractions and star formation decline. Traversability is a consideration in extragalactic surveys for the Search for Extraterrestrial Intelligence (SETI).

STAR FORMATION HISTORIES OF BRIGHTEST CLUSTER GALAXIES WITH INTERMEDIATE CENTRAL AGES

This work is devoted to the study of the star formation histories (SFHs) of the brightest cluster galaxies (BCGs) with intermediate central ages (from 5 to 10Gyr), to confirm if BCGs with these ages represent different accretion histories or simply a stochastic effect. The sample is composed of 6 BCGs with intermediate central ages and 3 BCGs with old central ages (> 12Gyr) as comparison galaxies. The galaxies were observed with the integrated field spectrograph VIMOS installed in the Very Large Telescope (VLT). The SFHs were obtained with the full spectrum fitting technique using the star population code STARLIGHT. The BCGs of intermediate central age analyzed formed almost 100% of their stars at z > 2 and their SFHs are similar to the SFHs of BCGs of old central ages and elliptical galaxies of similar mass (MDyn > 1011 Mʘ); therefore, these BCGs do not represent different SFHs.