[en] Feedback from active galactic nuclei (AGNs) is expected to impact the amount of cold gas in galaxies by driving strong galactic winds, by preventing external gas inflows, or by changing the thermodynamical state of the gas. We use estimates of molecular gas mass based on dust absorption (Hα/Hβ) to study gas content of large samples of type 2 AGN host galaxies in comparison with inactive galaxies. Using sparse principal component and clustering analysis, we analyze a suite of stellar and structural parameters of ∼27,100 face-on, central galaxies at redshift z = 0.02–0.15 and with stellar mass M _⋆ ≈ 10¹⁰–2 ×  10¹¹ M _⊙. We identify four galaxy groups of similar mass and morphology (mass surface density, velocity dispersion, concentration, and Sérsic index) that can be evolutionarily linked through a life cycle wherein gas content mediates their star formation rate (SFR) and level of AGN activity. Galaxies first consume their gas mostly through bursty star formation, then enter into a transition phase of intermediate gas richness in which star formation and AGNs coexist, before settling into retirement as gas-poor, quiescent systems with residual levels of AGN activity (LINERs). Strongly accreting black holes (Seyferts) live in gas-rich, star-forming hosts, but neither their gas reservoir nor their ability to form stars seems to be impacted instantaneously (timescales ≲0.5 Gyr) by AGN feedback. Our results are inconsistent with AGN feedback models that predict that central, bulge-dominated, Seyfert-like AGNs in massive galaxies have significantly lower molecular gas fractions than inactive galaxies of similar mass, morphology, and SFR.

[en] We combine ionized gas ([N II] λ6583) and stellar central velocity dispersions for a sample of 345 galaxies, with and without active galactic nuclei (AGNs), to study the dynamical state of the nuclear gas and its physical origin. The gas dispersions strongly correlate with the stellar dispersions over the velocity range of σ ∼ 30-350 km s^-1, such that σ _g/σ_* ∼ 0.6-1.4, with an average value of 0.80. These results are independent of Hubble type (for galaxies from E to Sbc), the presence or absence of a bar, or local galaxy environment. For galaxies of type Sc and later and that have σ_* ∼< 40 km s^-1, the gas seems to have a minimum threshold of σ _g ∼ 30 km s^-1, such that σ _g/σ_* always exceeds 1. Within the sample of AGNs, σ _g/σ_* increases with nuclear luminosity or Eddington ratio, a possible manifestation of AGN feedback associated with accretion disk winds or outflows. This extra source of nongravitational line broadening should be removed when trying to use σ _g to estimate σ_*. We show that the mass budget of the narrow-line region (NLR) can be accounted for by mass loss from evolved stars. The kinematics of the gas, dominated by random motions, largely reflect the velocity field of the hot gas in the bulge. Lastly, we offer a simple explanation for the correlation between line width and line luminosity observed in the NLR of AGNs.

[en] We use new central stellar velocity dispersions and nuclear X-ray and Hα luminosities for the Palomar survey of nearby galaxies to investigate the distribution of nuclear bolometric luminosities and Eddington ratios for their central black holes (BHs). This information helps to constrain the nature of their accretion flows and the physical drivers that control the spectral diversity of nearby active galactic nuclei. The characteristic values of the bolometric luminosities and Eddington ratios, which span over 7-8 orders of magnitude, from L _bol ∼< 10³⁷ to 3 x 10⁴⁴ erg s^-1 and L _bol/L _Edd ∼10^-9 to 10^-1, vary systematically with nuclear spectral classification, increasing along the sequence absorption-line nuclei → transition objects → low-ionization nuclear emission-line regions → Seyferts. The Eddington ratio also increases from early-type to late-type galaxies. We show that the very modest accretion rates inferred from the nuclear luminosities can be readily supplied through local mass loss from evolved stars and Bondi accretion of hot gas, without appealing to additional fueling mechanisms such as angular momentum transport on larger scales. Indeed, we argue that the fuel reservoir generated by local processes should produce far more active nuclei than is actually observed. This generic luminosity-deficit problem suggests that the radiative efficiency in these systems is much less than the canonical value of 0.1 for traditional optically thick, geometrically thin accretion disks. The observed values of L _bol/L _Edd, all substantially below unity, further support the hypothesis that massive BHs in most nearby galaxies reside in a low or quiescent state, sustained by accretion through a radiatively inefficient mode.

[en] Black hole accretion is widely thought to influence star formation in galaxies, but the empirical evidence for a physical correlation between star formation rate (SFR) and the properties of active galactic nuclei (AGNs) remain highly controversial. We take advantage of a recently developed SFR estimator based on the [O ii] λ3727 and [O iii] λ5007 emission lines to investigate the SFRs of the host galaxies of more than 5800 type-1 and 7600 type-2 AGNs with z < 0.35. After matching in luminosity and redshift, we find that type-1 and type-2 AGNs have a similar distribution of internal reddening, which is significant and corresponds to ∼10⁹ M _⊙ of cold molecular gas. In spite of their comparable gas content, type-2 AGNs, independent of stellar mass, Eddington ratio, redshift or molecular gas mass, exhibit intrinsically stronger star formation activity than type-1 AGNs, in apparent disagreement with the conventional AGN unified model. We observe a tight, linear relation between AGN luminosity (accretion rate) and SFR, one that becomes more significant toward smaller physical scales, suggesting that the link between the AGN and star formation occurs in the central kpc-scale region. This, along with a correlation between SFR and Eddington ratio in the regime of super-Eddington accretion, can be interpreted as evidence that star formation is impacted by positive feedback from the AGN.

[en] The mid-infrared spectrum contains rich diagnostics to probe the physical properties of galaxies, among which the pervasive emission features from polycyclic aromatic hydrocarbons (PAHs) offer promising means of estimating the star formation rate (SFR) relatively immune from dust extinction. This paper investigates the effectiveness of PAH emission as a SFR indicator on subkiloparsec scales by studying the Spitzer/IRS mapping-mode observations of the nearby grand-design spiral galaxy M51. We present a new approach of analyzing the spatial elements of the spectral data cube that simultaneously maximizes spatial resolution and spatial coverage, while yielding reliable measurements of the total, integrated 5–20 μm PAH emission. We devise a strategy of extracting robust PAH emission using spectra with only partial spectral coverage, complementing missing spectral regions with properly combined mid-infrared photometry. We find that in M51 the PAH emission correlates tightly with the extinction-corrected far-ultraviolet, near-ultraviolet, and Hα emission, from scales of ∼0.4 kpc close to the nucleus to 6 kpc out in the disk of the galaxy, indicating that PAH serves as an excellent tracer of SFR over a wide range of galactic environments. But regional differences exist. Close to the active nucleus of M51 the 6.2 μm feature is weaker, and the overall level of PAH emission is suppressed. The spiral arms and the central star-forming region of the galaxy emit stronger 7.7 and 8.6 μm PAH features than the inter-arm regions.

[en] The integrated 21 cm H i emission profile of a galaxy encodes valuable information on the kinematics, spatial distribution, and dynamical state of its cold interstellar medium. The line width, in particular, reflects the rotation velocity of the galaxy, which, in combination with a size scale, can be used to constrain the dynamical mass of the system. We introduce a new method based on the concept of the curve of growth to derive a set of robust parameters to characterize the line width, asymmetry, and concentration of the integrated H i spectra. We use mock spectra to evaluate the performance of our method, to estimate realistic systematic uncertainties for the proposed parameters, and to correct the line widths for the effects of instrumental resolution and turbulence broadening. Using a large sample of nearby galaxies with available spatially resolved kinematics, we demonstrate that the newly defined line widths can predict the rotational velocities of galaxies to within an accuracy of ≲30 km s⁻¹. We use the calibrated line widths, in conjunction with the empirical relation between the size and mass of H i disks, to formulate a prescription for estimating the dynamical mass within the H i-emitting region of gas-rich galaxies. Our formalism yields dynamical masses accurate to ∼0.3 dex based solely on quantities that can be derived efficiently and robustly from current and future extragalactic H i surveys. We further extend the dynamical mass calibration to the scale of the dark matter halo.

[en] We analyze two-armed global spiral density wave modes generated by gravitational instability in razor-thin, nonviscous, self-gravitating protoplanetary disks to understand the dependence of spiral arm morphology (pitch angle α and amplitude) on various disk conditions. The morphologies of the resulting spiral density wave modes closely resemble observations. Their pitch angles and pattern speeds are insensitive to the boundary conditions adopted. Gaussian disks exhibit more tightly wound spirals (smaller pitch angle) than power-law disks under the same conditions. We find that at a fixed disk-to-star mass ratio (M _d/M _*), the pitch angle increases with average Toomre’s stability parameter ($\stackrel{¯<!--\; ??\; -->}{Q}$) or average disk aspect ratio ($\stackrel{¯<!--\; ??\; -->}{h}$). For a given $\stackrel{¯<!--\; ??\; -->}{Q}$, density wave modes with higher M _d/M _* have larger pitch angles, while the behavior reverses for a given $\stackrel{¯<!--\; ??\; -->}{h}$. The interdependence between pitch angle and disk properties can be roughly approximated by $\mathrm{\alpha <!--\; ??\; -->}\propto <!--\; ???\; -->c_s^2/M_d$, where c _s is the sound speed. Our gravitational instability-excited spiral density waves can be distinguished from planet-launched spirals: (1) massive cool disks have spiral pitch angle falling with radius, while low-mass hot disks have spiral pitch angle rising with radius; (2) the profile of spiral amplitude presents several dips and bumps. We propose that gravitational instability-excited density waves can serve as an alternative scenario to explain the observed spiral arms in self-gravitating protoplanetary disks.

[en] The detection of polarized continuum and line emission from the nucleus of NGC 4258 by Wilkes et al. in 1995 provides an intriguing application of the unified model of Seyfert nuclei to a galaxy in which there is known to be an edge-on, rotating disk of molecular gas surrounding the nucleus. Unlike most Seyfert nuclei, however, NGC 4258 has strongly polarized narrow emission lines. To further investigate the origin of the polarized emission, we have obtained spectropolarimetric observations of the NGC 4258 nucleus at the Keck II telescope. The narrow-line polarizations range from 1.0% for [S ii] λ6716 to 13.9% for the [O ii] λλ7319, 7331 blend, and the position angle of polarization is oriented nearly parallel to the projected plane of the masing disk. A correlation between critical density and degree of polarization is detected for the forbidden lines, indicating that the polarized emission arises from relatively dense (n_e (greater-or-similar sign)10⁴ cm-3), radially stratified gas. An archival Hubble Space Telescope narrowband [O iii] image shows that the narrow-line region has a compact, nearly unresolved core, implying a FWHM size of (less-or-similar sign)2.5 pc. We discuss the possibility that the polarized emission might arise from the accretion disk itself and become polarized by scattering within the disk atmosphere. A more likely scenario is an obscuring torus or strongly warped disk surrounding the inner portion of a narrow-line region that is strongly stratified in density. The compact size of the narrow-line region implies that the obscuring structure must be smaller than about 2.5 pc in diameter. (c) (c) 1999. The American Astronomical Society

[en] We investigate the impact of spiral structure on global star formation using a sample of 2226 nearby bright disk galaxies. Examining the relationship between spiral arms, star formation rate (SFR), and stellar mass, we find that arm strength correlates well with the variation of SFR as a function of stellar mass. Arms are stronger above the star-forming galaxy main sequence (MS) and weaker below it: arm strength increases with higher $\mathrm{l}\mathrm{o}\mathrm{g}(\mathrm{S}\mathrm{F}\mathrm{R}/{\mathrm{S}\mathrm{F}\mathrm{R}}_{\mathrm{M}\mathrm{S}})$, where SFR_MS is the SFR along the MS. Likewise, stronger arms are associated with higher specific SFR. We confirm this trend using the optical colors of a larger sample of 4378 disk galaxies, whose position on the blue cloud also depends systematically on spiral arm strength. This link is independent of other galaxy structural parameters. For the subset of galaxies with cold gas measurements, arm strength positively correlates with H i and H₂ mass fraction, even after removing the mutual dependence on $\mathrm{l}\mathrm{o}\mathrm{g}(\mathrm{S}\mathrm{F}\mathrm{R}/{\mathrm{S}\mathrm{F}\mathrm{R}}_{\mathrm{M}\mathrm{S}})$, consistent with the notion that spiral arms are maintained by dynamical cooling provided by gas damping. For a given gas fraction, stronger arms lead to higher $\mathrm{l}\mathrm{o}\mathrm{g}(\mathrm{S}\mathrm{F}\mathrm{R}/{\mathrm{S}\mathrm{F}\mathrm{R}}_{\mathrm{M}\mathrm{S}})$, resulting in a trend of increasing arm strength with shorter gas depletion time. We suggest a physical picture in which the dissipation process provided by gas damping maintains spiral structure, which, in turn, boosts the star formation efficiency of the gas reservoir.

[en] The development of two-dimensional (2D) bulge-to-disk decomposition techniques has shown their advantages over traditional one-dimensional (1D) techniques, especially for galaxies with non-axisymmetric features. However, the full potential of 2D techniques has yet to be fully exploited. Secondary morphological features in nearby disk galaxies, such as bars, lenses, rings, disk breaks, and spiral arms, are seldom accounted for in 2D image decompositions, even though some image-fitting codes, such as GALFIT, are capable of handling them. We present detailed, 2D multi-model and multi-component decomposition of high-quality R -band images of a representative sample of nearby disk galaxies selected from the Carnegie-Irvine Galaxy Survey, using the latest version of GALFIT. The sample consists of five barred and five unbarred galaxies, spanning Hubble types from S0 to Sc. Traditional 1D decomposition is also presented for comparison. In detailed case studies of the 10 galaxies, we successfully model the secondary morphological features. Through a comparison of best-fit parameters obtained from different input surface brightness models, we identify morphological features that significantly impact bulge measurements. We show that nuclear and inner lenses/rings and disk breaks must be properly taken into account to obtain accurate bulge parameters, whereas outer lenses/rings and spiral arms have a negligible effect. We provide an optimal strategy to measure bulge parameters of typical disk galaxies, as well as prescriptions to estimate realistic uncertainties of them, which will benefit subsequent decomposition of a larger galaxy sample.