Keywords

The structural parameters of a galaxy can be used to gain insight into its formation and evolution history. In this paper, we strive to compare the Milky Way's structural parameters to other, primarily edge-on, spiral galaxies in order to determine how our Galaxy measures up to the Local Universe. For our comparison, we use the galaxy structural parameters gathered from a variety of literature sources in the optical and near-infrared wave bands. We compare the scale length, scale height, and disk flatness for both the thin and thick disks, the thick-to-thin disk mass ratio, the bulge-to-total luminosity ratio, and the mean pitch angle of the Milky Way's spiral arms to those in other galaxies. We conclude that many of the Milky Way's structural parameters are largely ordinary and typical of spiral galaxies in the Local Universe, though the Galaxy's thick disk appears to be appreciably thinner and less extended than expected from zoom-in cosmological simulations of Milky Way-mass galaxies with a significant contribution of galaxy mergers involving satellite galaxies.

We analyzed the relationship between several basic parameters describing supermassive black holes such as jet power, black hole spin, accretion disk magnetic field, black hole mass, etc. We found that there is a general correlation between these parameters, such as jet power is significantly positively correlated with black hole spin, while black hole mass is significantly negatively correlated with black hole spin. To apprehend these relationships, we consider the Blandford–Znajek model to be superior to the Blandford–Payne model. It is also found that the intrinsic gamma luminosity of the FSRQs has a positive correlation with the accretion disk magnetic field, while the intrinsic gamma luminosity of the BL Lacs has a negative correlation with the accretion disk magnetic field. A feedback effect may exist between accretion disk accretion rate and magnetic field, which may be the key to the evolution between BL Lacs and FSRQs. There is no significant difference in the jet power and jet generation efficiency of FSRQs and BL Lacs, which suggests that the jets are generated by the same mechanism. The contribution rate of accretion rate to jet generation efficiency is high, while the contribution rate of accretion rate to jet power is very low.

We analyze how the column density of hydrogen atoms in the H i regions, observed in dark matter halos of a number of galaxies, can be determined. Specifically we study how the determination of the column density of hydrogen atoms from the observed astrophysical data would be affected by the possible presence of the Second Flavor of Hydrogen Atoms (SFHA), whose existence had been previously demonstrated in four different types of atomic experiments and had helped in explaining two puzzling astrophysical observations: the anomalous absorption in the 21 cm line from the early Universe and the smoother, less clumpy distribution of dark matter in the Universe than predicted by Einstein's gravity. By a model example we demonstrate that the neglect of the SFHA leads to the overestimation of the column density of hydrogen atoms in dark matter halos by about 30%. We perform these relatively simple estimates just to get the message across and to motivate further corresponding theoretical and experimental studies.

The Next Generation Virgo Cluster Survey (NGVS) was designed to provide a deep census of baryonic structures in the Virgo cluster. The survey covers the 104 deg² area from the core of Virgo out to one virial radius, in the u*griz bandpasses, to a point-source depth of g ∼ 25.9 mag (10σ) and a single pixel surface brightness limit of μ_g ∼ 29 mag arcsec⁻² (2σ above the sky). Here we present the final catalog of 404 Virgo galaxies located within a 3.71 deg² (0.3 Mpc²) region centered on M87, Virgo's dominant galaxy. Of these, 154 were previously uncataloged and span the range 17.8 mag < g < 23.7 mag (−13.4 mag < M_g < −7.4 mag at the 16.5 Mpc distance of Virgo). Extensive simulations show that the NGVS catalog is complete down to g = 18.6 mag (M_g = −12.5 mag, corresponding to a stellar mass ${ \mathcal M }\sim 1.6\times {10}^{7}{{ \mathcal M }}_{\odot }$ for an old stellar population), and 50% complete at g = 22.0 mag (M_g = −9.1 mag, ${ \mathcal M }\sim 6.2\times {10}^{5}{{ \mathcal M }}_{\odot }$). The NGVS 50% completeness limit is 3 mag deeper than that of the Virgo Cluster Catalog (VCC), which has served as Virgo's reference standard for over a quarter century, and 2 mag deeper than the VCC detection limit. We discuss the procedure adopted for the identification of objects and the criteria used to assess cluster membership. For each of the 404 galaxies in the NGVS Virgo Cluster core catalog, we present photometric and structural parameters based on a nonparametric curve-of-growth and isophotal analysis, as well as parametric (Sérsic, double-Sérsic, and/or core-Sérsic) fits to the one-dimensional surface brightness profiles and two-dimensional light distributions.

Optically compact star-forming galaxies (SFGs) have been proposed as immediate progenitors of quiescent galaxies, although their origin and nature are debated. Were they formed in slow secular processes or in rapid merger-driven starbursts? Answering this question would provide fundamental insight into how quenching occurs. We explore the location of the general population of galaxies with respect to fundamental star-forming and structural relations, identify compact SFGs based on their stellar core densities, and study three diagnostics of the burstiness of star formation: (1) star formation efficiency, (2) interstellar medium (ISM), and (3) radio emission. The overall distribution of galaxies in the fundamental relations points toward a smooth transition toward quiescence while galaxies grow their stellar cores, although some galaxies suddenly increase their specific star formation rate when they become compact. From their star formation efficiencies compact and extended SFGs appear similar. In relation to the ISM diagnostic, study of the CO excitation, the density of the neutral gas, and the strength of the ultraviolet radiation field shows that compact SFGs resemble galaxies located in the upper envelope of the main sequence of SFGs, although this is based on a small sample size. Regarding the radio emission diagnostic, we find that galaxies become increasingly compact as the starburst ages, implying that at least some compact SFGs are old starbursts. We suggest that compact SFGs could be starbursts winding down and eventually crossing the main sequence toward quiescence.

The Arecibo L-band Feed Array Zone of Avoidance (ALFAZOA) Shallow Survey is a blind H i survey of the extragalactic sky behind the northern Milky Way conducted with the ALFA receiver on the 305 m Arecibo Radio Telescope. ALFAZOA Shallow covered 900 square degrees at full sensitivity from 30° ≤ l ≤ 75° and $| b| \,\leqslant \,10^\circ$ and an additional 460 square degrees at limited sensitivity at latitudes up to 20°. It has an rms sensitivity of 5–7 mJy and a velocity resolution of 9–20.6 km s⁻¹, and detected 403 galaxies out to a recessional velocity of 12,000 km s⁻¹, with an angular resolution of 34 and a positional accuracy between 02 and 17. The survey is complete above an integrated line flux of F_{H i} = 2.0 Jy km s⁻¹ for half the survey, and above F_{H i} = 2.8 Jy km s⁻¹ for the other half. Of the ALFAZOA H i detections, 43% have at least one possible optical/near-infrared counterpart in the literature, and an additional 16% have counterparts that only included previous H i measurements. There are fewer counterparts in regions of high extinction and for galaxies with lower H i mass. Comparing the results of the survey to the predictions of Erdoǧdu et al., and using their nomenclature, ALFAZOA confirms the position and extent in the ZOA of the C7, Cζ, Pegasus, Corona Borealis, and Delphinus structures, but not of the Cygnus void. Two new structures are identified, both connected to the C7 overdensity; one extends to  35°, and the other crosses the ZOA.

The Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey is currently acquiring integral-field spectroscopy for the largest sample of galaxies to date. By 2020, the MaNGA Survey—which is one of three core programs in the fourth-generation Sloan Digital Sky Survey (SDSS-IV)—will have observed a statistically representative sample of 10⁴ galaxies in the local universe (z ≲ 0.15). In addition to a robust data-reduction pipeline (DRP), MaNGA has developed a data-analysis pipeline (DAP) that provides higher-level data products. To accompany the first public release of its code base and data products, we provide an overview of the MaNGA DAP, including its software design, workflow, measurement procedures and algorithms, performance, and output data model. In conjunction with our companion paper (Belfiore et al.), we also assess the DAP output provided for 4718 observations of 4648 unique galaxies in the recent SDSS Data Release 15 (DR15). These analysis products focus on measurements that are close to the data and require minimal model-based assumptions. Namely, we provide stellar kinematics (velocity and velocity dispersion), emission-line properties (kinematics, fluxes, and equivalent widths), and spectral indices (e.g., D4000 and the Lick indices). We find that the DAP provides robust measurements and errors for the vast majority (>99%) of analyzed spectra. We summarize assessments of the precision and accuracy of our measurements as a function of signal-to-noise. We also provide specific guidance to users regarding the limitations of the data. The MaNGA DAP software is publicly available and we encourage community involvement in its development.

Galaxies host a wide array of internal stellar components, which need to be decomposed accurately in order to understand their formation and evolution. While significant progress has been made with recent integral-field spectroscopic surveys of nearby galaxies, much can be learned from analyzing the large sets of realistic galaxies now available through state-of-the-art hydrodynamical cosmological simulations. We present an unsupervised machine-learning algorithm, named auto-GMM, based on Gaussian mixture models, to isolate intrinsic structures in simulated galaxies based on their kinematic phase space. For each galaxy, the number of Gaussian components allowed by the data is determined through a modified Bayesian information criterion. We test our method by applying it to prototype galaxies selected from the cosmological simulation IllustrisTNG. Our method can effectively decompose most galactic structures. The intrinsic structures of simulated galaxies can be inferred statistically by non-human supervised identification of galaxy structures. We successfully identify four kinds of intrinsic structures: cold disks, warm disks, bulges, and halos. Our method fails for barred galaxies because of the complex kinematics of particles moving on bar orbits.

Tremaine and Weinberg (TW) proposed a conceptually simple procedure relying on long-slit spectroscopy to measure the pattern speeds of bars (Ω_p) in disk galaxies. Using a simulated galaxy, we investigate the potential biases and uncertainties of TW measurements using increasingly popular integral-field spectrographs (IFSs), for which multiple pseudo-slits (and thus independent measurements) can be constructed with a single observation. Most importantly, to establish the spatial coverage required and ensure the validity of the measurements, the inferred Ω_p must asymptotically converge as the (half-)length of each pseudo-slit used is increased. The requirement for our simulation is to reach ≈1.3 times the half-light radius, but this may vary from galaxy to galaxy. Only those slits located within the bar region yield accurate measurements. We confirm that the position angle of the disk is the dominant source of systematic error in TW Ω_p measurements, leading to under/overestimates of tens of percent for inaccuracies of even a few degrees. Recasting the data so that the data grid aligns with the disk major axis leads to slightly reduced uncertainties. Accurate measurements are obtained only for well-defined ranges of the bar angle (relative to the galaxy major axis) ϕ_bar and the inclination angle i, here 10° ≲ ϕ_bar ≲ 75° and 105° ≲ ϕ_bar ≲ 170° and 15° ≲ i ≲ 70°. The adopted (pseudo-)slit widths, spatial resolution, and (unless extremely aggressive) spatial binning of IFS data have no significant impact on the measurements. Our results thus provide useful guidelines for reliable and accurate direct Ω_p measurements with IFS observations.

We assemble an unbiased sample of 29 galaxies with [O ii] λ3727 and/or [O iii] λ5007 detections at z < 0.15 from the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) Pilot Survey (HPS). The HPS finds galaxies without preselection based on their detected emission lines via integral field spectroscopy. Sixteen of these objects were followed up with the second-generation, low-resolution spectrograph on the upgraded HET. Oxygen abundances were then derived via strong emission lines using a Bayesian approach. We find that most of the galaxies fall along the mass–metallicity relation derived from photometrically selected star-forming galaxies in the Sloan Digital Sky Survey (SDSS). However, two of these galaxies have low metallicity (similar to the very rare green pea galaxies in mass–metallicity space). The star formation rates (SFRs) of this sample fall in an intermediate space between the SDSS star-forming main sequence and the extreme green pea galaxies. We conclude that spectroscopic selection fills a part of the mass–metallicity–SFR phase space that is missed in photometric surveys with preselection like SDSS; i.e., we find galaxies that are actively forming stars but are faint in continuum. We use the results of this pilot investigation to make predictions for the upcoming unbiased, large spectroscopic sample of local line emitters from HETDEX. With the larger HETDEX survey, we will determine if galaxies selected spectroscopically without continuum brightness preselection have metallicities that fall on a continuum that bridges typical star-forming and rarer, more extreme systems like green peas.