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<jats:title>Abstract</jats:title> <jats:p>The strong alignment of small-scale turbulent Alfvénic motions with the direction of magnetic field that percolates the small-scale eddies and imprints the direction of the magnetic field is a property that follows from the MHD theory and the theory of turbulent reconnection. The Alfvénic eddies mix magnetic fields perpendicular to the direction of the local magnetic field, and this type of motion is used to trace magnetic fields with the velocity gradient technique (VGT). The other type of turbulent motion, fast modes, induces anisotropies orthogonal to Alfvénic eddies and interferes with the tracing of the magnetic field with the VGT. We report a new effect, i.e., in a magnetically dominated low-<jats:italic>β</jats:italic> subsonic medium, fast modes are very intermittent, and in a volume with a small filling factor the fast modes dominate other turbulent motions. We identify these localized regions as the cause of the occasional change of direction of gradients in our synthetic observations. We show that the new technique of measuring the gradients of gradient amplitudes suppresses the contribution from the fast-mode-dominated regions, improving the magnetic field tracing. In addition, we show that the distortion of the gradient measurements by fast modes is also applicable to the synchrotron intensity gradients, but the effect is reduced compared to the VGT.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We apply the helioseismic methodology of Fourier Legendre decomposition to 88 months of Dopplergrams obtained by the Helioseismic and Magnetic Imager (HMI) as the basis of inferring the depth variation of the mean meridional flow, as averaged between 20° and 60° latitude and in time, in both the northern and southern hemispheres. We develop and apply control procedures designed to assess and remove center-to-limb artifacts using measurements obtained by performing the analysis with respect to artificial poles at the east and west limbs. Forward modeling is carried out using sensitivity functions proportional to the mode kinetic energy density to evaluate the consistency of the corrected frequency shifts with models of the depth variation of the meridional circulation in the top half of the convection zone. The results, taken at face value, imply substantial differences between the meridional circulation in the northern and southern hemispheres. The inferred presence of a return (equatorward propagating) flow at a depth of approximately 40 Mm below the photosphere in the northern hemisphere is surprising and appears to be inconsistent with many other helioseismic analyses. This discrepancy may be the result of the inadequacy of our methodology to remove systematic errors in HMI data. Our results appear to be at least qualitatively similar to those by Gizon et al., which point to an anomaly in HMI data that is not present in MDI or GONG data.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report results from a Far Ultraviolet Spectrographic Explorer (FUSE) survey of interstellar molecular hydrogen (H<jats:sub>2</jats:sub>) in the Galactic disk toward 139 O-type and early B-type stars at Galactic latitudes ∣<jats:italic>b</jats:italic>∣ ≤ 10°, with updated photometric and parallax distances. H<jats:sub>2</jats:sub> absorption is measured using the far-UV Lyman and Werner bands, including strong R(0), R(1), and P(1) lines from rotational levels <jats:italic>J</jats:italic> = 0 and <jats:italic>J</jats:italic> = 1 and excited states up to <jats:italic>J</jats:italic> = 5 (sometimes <jats:italic>J</jats:italic> = 6 and 7). For each sight line, we report column densities <jats:italic>N</jats:italic> <jats:sub>H2</jats:sub>, <jats:italic>N</jats:italic> <jats:sub>HI</jats:sub>, <jats:italic>N</jats:italic>(<jats:italic>J</jats:italic>), and <jats:italic>N</jats:italic> <jats:sub>H</jats:sub> = <jats:italic>N</jats:italic> <jats:sub>HI</jats:sub> + 2<jats:italic>N</jats:italic> <jats:sub>H2</jats:sub> and the molecular fraction <jats:italic>f</jats:italic> <jats:sub>H2</jats:sub> = 2<jats:italic>N</jats:italic> <jats:sub>H2</jats:sub>/<jats:italic>N</jats:italic> <jats:sub>H</jats:sub>. Our survey extends the 1977 Copernicus H<jats:sub>2</jats:sub> survey up to <jats:italic>N</jats:italic> <jats:sub>H</jats:sub> ≈ 5 × 10<jats:sup>21</jats:sup> cm<jats:sup>−2</jats:sup>. The lowest rotational states have excitation temperatures and rms dispersions 〈<jats:italic>T</jats:italic> <jats:sub>01</jats:sub>〉 = 88 ± 20 K and 〈<jats:italic>T</jats:italic> <jats:sub>02</jats:sub>〉 = 77 ± 18 K, suggesting that <jats:italic>J</jats:italic> = 0, 1, 2 are coupled to the gas kinetic temperature. Populations of higher-<jats:italic>J</jats:italic> states exhibit mean excitation temperatures 〈<jats:italic>T</jats:italic> <jats:sub>24</jats:sub>〉 = 237 ± 91 K and 〈<jats:italic>T</jats:italic> <jats:sub>35</jats:sub>〉 = 304 ± 108 K, produced primarily by UV radiative pumping. Correlations of <jats:italic>f</jats:italic> <jats:sub>H2</jats:sub> with <jats:italic>E</jats:italic>(<jats:italic>B</jats:italic> − <jats:italic>V</jats:italic>) and <jats:italic>N</jats:italic> <jats:sub>H</jats:sub> show a transition to <jats:italic>f</jats:italic> <jats:sub>H2</jats:sub> ≥ 0.1 at <jats:italic>N</jats:italic> <jats:sub>H</jats:sub> ≳ 10<jats:sup>21</jats:sup> cm<jats:sup>−2</jats:sup> and <jats:italic>E</jats:italic>(<jats:italic>B</jats:italic> − <jats:italic>V</jats:italic>) ≳ 0.2, interpreted with an analytic model of H<jats:sub>2</jats:sub> formation–dissociation equilibrium and attenuation of the far-UV radiation field by self-shielding and dust opacity. Results of this disk survey are compared to previous FUSE studies of H<jats:sub>2</jats:sub> in translucent clouds, at high Galactic latitudes, and in the Magellanic Clouds. Using updated distances to the target stars, we find average sight-line values 〈<jats:italic>f</jats:italic> <jats:sub>H2</jats:sub>〉 = 0.20 and 〈<jats:italic>N</jats:italic> <jats:sub>H</jats:sub>/<jats:italic>E</jats:italic>(<jats:italic>B</jats:italic> − <jats:italic>V</jats:italic>)〉 = 6.07 × 10<jats:sup>21</jats:sup> cm<jats:sup>−2</jats:sup> mag<jats:sup>−1</jats:sup>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present new deep, high-resolution, 1.5 GHz observations of the prototypical nearby Perseus galaxy cluster from the Karl G. Jansky Very Large Array. We isolate for the first time the complete tail of radio emission of the bent-jet radio galaxy NGC 1272, which had been previously mistaken to be part of the radio mini-halo. The possibility that diffuse radio galaxy emission contributes to mini-halo emission may be a general phenomenon in relaxed cool-core clusters, and should be explored. The collimated jets of NGC 1272 initially bend to the west, and then transition eastward into faint, 60 kpc long extensions with eddy-like structures and filaments. We suggest interpretations for these structures that involve bulk motions of intracluster gas, the galaxy’s orbit in the cluster including projection effects, and the passage of the galaxy through a sloshing cold front. Instabilities and turbulence created at the surface of this cold front and in the turbulent wake of the infalling host galaxy most likely play a role in the formation of the observed structures. We also discover a series of faint rings, southeast of NGC 1272, which are a type of structure that has never been seen before in galaxy clusters.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The phenomenological study of evolving galaxy populations in Peng et al. has shown that star forming galaxies can be quenched by two distinct processes: mass quenching and environment quenching. To explore the mass quenching process in local galaxies, we study the massive central disk galaxies with stellar mass above the Schechter characteristic mass. In Zhang et al., we showed that during the quenching of the massive central disk galaxies as their star formation rate decreases, their molecular gas mass and star formation efficiency drop rapidly but their H <jats:sc>i</jats:sc> gas mass remains surprisingly constant. To identify the underlying physical mechanisms, in this work we analyze the change during quenching of various structure parameters, bar frequency, and active galactic nucleus (AGN) activity. We find three closely related facts. On average, as star formation rate decreases in these galaxies: (1) they become progressively more compact, indicated by their significantly increasing concentration index, bulge-to-total mass ratio, and central velocity dispersion, which are mainly driven by the growth and compaction of their bulge component; (2) the frequency of barred galaxies increases dramatically, and at a given concentration index the barred galaxies have a significantly higher quiescent fraction than unbarred galaxies, implying that the galactic bar may play an important role in mass quenching; and (3) the “AGN” frequency increases dramatically from 10% on the main sequence to almost 100% for the most quiescent galaxies, which is mainly driven by the sharp increase of LINERs. These observational results lead to a self-consistent picture of how mass quenching operates.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>It has been suggested that a huge amount of baryon resides in the circumgalactic medium (CGM) extending out to the virial radii of galaxies. In this work we attempt to measure the baryon mass in the CGM with a novel method based on the gamma-ray observations of the extended halo of the Andromeda galaxy Messier 31 (M31). Since cosmic-ray particles generated inside the galaxy will eventually escape to the CGM, they will produce gamma-ray emission via the proton–proton collision with the CGM and produce gamma-rays. Different from some traditional measurements that are sensitive only to certain metallic ions in specific temperature range, the hadronic gamma-ray flux is sensitive to baryonic gases in all phases and does not rely on the metallicity in the halo, hence the measured gamma-ray flux from the galaxy’s halo can be used to constrain the mass of the CGM. By dealing with the cosmic-ray transport in the halo and calculating the hadronic gamma-ray intensity, we find that the total baryon mass contained within the virial radius is less than (1.4–5) × 10<jats:sup>10</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> according to the gamma-ray intensity obtained with a model-dependent analysis. It implies that the CGM of Andromeda galaxy may not account for more than 30% of the missing baryons, but the result is subject to uncertainties from the gamma-ray intensity upper limit, diffusion coefficient of the cosmic-rays in the halo, as well as the stellar mass and dark matter halo mass of the galaxy. This method will become more constraining provided better understandings on these issues and more sensitive gamma-ray telescopes in the future.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>To investigate the lifetimes of red sequence (RS), blue cloud (BC), and green valley (GV) galaxies, we derive their lifetimes using clustering analyses at 0.5 ≤ <jats:italic>z</jats:italic> ≤ 2.5 in the COSMOS/UltraVISTA field. Several essentials that may influence the lifetime estimation have been explored, including the dark matter (DM) halo mass function (HMF), the width of the redshift bin, the growth of DM halos within each redshift bin, and the stellar mass. We find that the HMF difference results in scatters of ∼0.2 dex on the lifetime estimation, adopting a redshift bin width of Δ<jats:italic>z</jats:italic> = 0.5 is good enough to estimate the lifetime, and no significant effect on lifetime estimation is found due to the growth of DM halos within each redshift bin. The galaxy subsamples with higher stellar masses generally have shorter lifetimes, but the lifetimes in different subsamples at <jats:italic>z</jats:italic> &gt; 1.5 tend to be independent of stellar mass. Consistently, the clustering-based lifetime for each galaxy subsample agrees well with that inferred using the spectral energy distribution modeling. Moreover, the lifetimes of the RS and BC galaxies also coincide well with their typical gas-depletion timescales attributed to the consumption of star formation. Interestingly, the distinct lifetime behaviors of the GV galaxies at <jats:italic>z</jats:italic> ≤ 1.5 and <jats:italic>z</jats:italic> &gt; 1.5 cannot be fully accounted for by their gas-depletion timescales. Instead, this discrepancy between the lifetimes and gas-depletion timescales of the GV galaxies suggests that there are additional physical processes, such as feedback of active galactic nuclei, which accelerates the quenching of GV galaxies at high redshifts.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this paper, we study the sizes of quasar proximity zones with synthetic quasar absorption spectra obtained by postprocessing a Cosmic Reionization On Computers (CROC) simulation. CROC simulations have both relatively large box sizes and high spatial resolution, allowing us to resolve Lyman limit systems (LLSs), which are crucial for modeling the quasar absorption spectra. We find that before reionization, most quasar proximity zone sizes grow steadily for ∼10 Myr, while after reionization, they grow rapidly but only for ∼0.1 Myr. We also find a slow growth of <jats:italic>R</jats:italic> <jats:sub>obs</jats:sub> with decreasing turn-on redshift. In addition, we find that ∼1%–2% of old quasars (30 Myr old) display extremely small proximity zone sizes (&lt;1 proper Mpc), the vast majority of which are due to the occurrence of a damped Ly<jats:italic>α</jats:italic> absorber (DLA) or an LLS along the line of sight. These DLAs and LLSs are contaminated with metal, which offers a way to distinguish them from the normal proximity zones of young quasars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Voyager 1 has been moving through the very local interstellar medium (VLISM) from the time that it crossed the heliopause on 2012/DOY 238 to 2020/DOY 292. Three notable objects in the magnetic field of the VLISM have been observed: two shocks and one pressure front. This paper reports the observation of a fourth object observed near 2020/DOY 147. There were no upstream electron plasma oscillations of the type often observed ahead of shocks, abrupt increases in energetic particles, or fluctuations in the 48 s increments of the magnetic field associated with this feature, suggesting that it was probably not a shock. This feature was associated with a relatively large increase in the magnetic field strength (<jats:italic>B</jats:italic>2/<jats:italic>B</jats:italic>1 = 1.35) and in the electron density determined by the Plasma Wave Science experiment (<jats:italic>N</jats:italic>2/<jats:italic>N</jats:italic>1 = 1.36) using a new method described in this paper. This feature appears to be a pressure front associated with a compressive wave in the VLISM. The two shocks and the two pressure fronts were associated with the four largest maxima observed in <jats:italic>B</jats:italic>(<jats:italic>t</jats:italic>) between 2012/DOY 238 and 2020/DOY 292. Each feature was associated with a jump–ramp structure. The jump–ramp structures were separated by long relatively undisturbed quiet intervals.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present new WFC3/UVIS observations of UGC 4483, the closest example of a metal-poor blue compact dwarf galaxy, with a metallicity of <jats:italic>Z</jats:italic> ≃ 1/15 <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub> and located at a distance of <jats:italic>D</jats:italic> ≃ 3.4 Mpc. The extremely high quality of our new data allows us to clearly resolve the multiple stellar evolutionary phases populating the color–magnitude diagram (CMD), to reach more than 4 mag deeper than the tip of the red giant branch, and to detect for the first time core He-burning stars with masses ≲2 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, populating the red clump and possibly the horizontal branch (HB) of the galaxy. By applying the synthetic CMD method to our observations, we determine an average star formation rate over the whole Hubble time of at least <jats:inline-formula> <jats:tex-math> <?CDATA $(7.01\pm 0.44)\times {10}^{-4}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">(</mml:mo> <mml:mn>7.01</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.44</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabea16ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\odot }\,{{yr}}^{-1}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mi mathvariant="italic">yr</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabea16ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, corresponding to a total astrated stellar mass of <jats:inline-formula> <jats:tex-math> <?CDATA $(9.60\pm 0.61)\times {10}^{6}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">(</mml:mo> <mml:mn>9.60</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.61</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabea16ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, 87% of which went into stars at epochs earlier than 1 Gyr ago. With our star formation history recovery method we find the best fit with a distance modulus of DM = 27.45 ± 0.10, slightly lower than previous estimates. Finally, we find strong evidence of an old (≳10 Gyr) stellar population in UGC 4483 thanks to the detection of an HB phase and the identification of six candidate RR Lyrae variable stars.</jats:p>