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Using the atomic carbon [C  i ] (⁠|$^{3} \rm P_{1} \rightarrow {\rm ^3 P}_{0}$|⁠) and [C  i ] (⁠|$^{3} \rm P_{2} \rightarrow {\rm ^3 P}_{1}$|⁠) emission {hereafter [C  i ] (1 − 0) and [C  i ] (2 − 1), respectively} maps observed with the Herschel   Space   Observatory , and CO (1 − 0), H  i , infrared and submm maps from literatures, we estimate the [C  i ]-to-H2 and CO-to-H2 conversion factors of α[C  i ](1 − 0), α[C  i ](2 − 1), and αCO at a linear resolution |$\sim 1\,$| kpc scale for six nearby galaxies of M 51, M 83, NGC 3627, NGC 4736, NGC 5055, and NGC 6946. This is perhaps the first effort, to our knowledge, in calibrating both [C  i ]-to-H2 conversion factors across the spiral disks at spatially resolved |$\sim 1\,$| kpc scale though such studies have been discussed globally in galaxies near and far. In order to derive the conversion factors and achieve these calibrations, we adopt three different dust-to-gas ratio (DGR) assumptions that scale approximately with metallicity taken from precursory results. We find that for all DGR assumptions, the α[C  i ](1 − 0), α[C  i ](2 − 1), and αCO are mostly flat with galactocentric radii, whereas both α[C  i ](2 − 1) and αCO show decrease in the inner regions of galaxies. And the central αCO and α[C  i ](2 − 1) values are on average ∼2.2 and 1.8 times lower than its galaxy averages. The obtained carbon abundances from different DGR assumptions show flat profiles with galactocentric radii, and the average carbon abundance of the galaxies is comparable to the usually adopted value of 3 × 10−5. We find that both metallicity and infrared luminosity correlate moderately with the αCO, whereas only weakly with either the α[C  i ](1 − 0) or carbon abundance, and not at all with the α[C  i ](2 − 1). [ABSTRACT FROM AUTHOR]

To study the high-transition dense-gas tracers and their relationships to the star formation of the inner ∼2 kpc circumnuclear region of NGC 253, we present HCN J = 4−3 and HCO+ J = 4−3 maps obtained with the James Clerk Maxwell Telescope. Using the spatially resolved data, we compute the concentration indices r 90/ r 50 for the different tracers. HCN and HCO+ 4–3 emission features tend to be centrally concentrated, which is in contrast to the shallower distribution of CO 1–0 and the stellar component. The dense-gas fraction (f dense, traced by the velocity-integrated-intensity ratios of HCN/CO and HCO+/CO) and the ratio R 31 (CO 3–2/1–0) decline towards larger galactocentric distances, but increase with higher star formation rate surface density. The radial variation and the large scatter of f dense and R 31 imply distinct physical conditions in different regions of the galactic disc. The relationships of f dense versus Σstellar, and SFEdense versus Σstellar are explored. SFEdense increases with higher Σstellar in this galaxy, which is inconsistent with previous work that used HCN 1–0 data. This implies that existing stellar components might have different effects on the high-J HCN and HCO+ than their low-J emission. We also find that SFEdense seems to be decreasing with higher f dense which is consistent with previous works, and it suggests that the ability of the dense gas to form stars diminishes when the average density of the gas increases. This is expected in a scenario where only the regions with high-density contrast collapse and form stars. [ABSTRACT FROM AUTHOR]

We selected an active galactic nuclei (AGNs) sample (0 < |$z$| ≤ 0.35) from Sloan Digital Sky Survey Data Release 7, and measured the broad- (⁠|$\rm H\alpha ^{b}/H\beta ^{b}$|⁠) and narrow-line Balmer decrements (⁠|$\rm H\alpha ^{n}/H\beta ^{n}$|⁠) of 554 selected AGNs. We found that the distributions of Balmer decrements can be fitted by a Gaussian function and give the best estimates of |$\rm H\alpha ^{b}/H\beta ^{b}= 3.16$| with a standard deviation 0.07 dex, and |$\rm H\alpha ^{n}/H\beta ^{n}= 4.37$| with a standard deviation 0.10 dex. We inspected the distributions of |$\rm H\alpha ^{b}/H\beta ^{b}$| and |$\rm H\alpha ^{n}/H\beta ^{n}$| in the Baldwin−Phillips−Terlevich (BPT) diagram and found that only narrow-line Balmer decrements depend on the physical conditions of the narrow-line region (NLR). We tested the relationship between |$\rm H\alpha ^{b}/H\beta ^{b}$| and |$\rm H\alpha ^{n}/H\beta ^{n}$|⁠, and found that |$\rm H\alpha ^{b}/H\beta ^{b}$| does not correlate with |$\rm H\alpha ^{n}/H\beta ^{n}$|⁠. We investigated the relationship between Balmer decrements and Seyfert sub-type, and found that only broad-line Balmer decrements correlate with Seyfert sub-type, We also examined the dependency of Balmer decrements on AGN properties, and found that Balmer decrements have no correlation with optical luminosity, but show some dependence on accretion rate. These results indicate that the NLR is subject to more reddening by dust than the broad-line region. [ABSTRACT FROM AUTHOR]