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تسجيل مستخدم جديدAzimuthal propagation of star formation in nearby spiral galaxies: NGC 628, NGC 3726 and NGC 6946
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Star formation induced by a spiral shock wave, which in turn is generated by a spiral density wave, produces an azimuthal age gradient across the spiral arm, which has opposite signs on either side of the corotational resonance. An analysis of the spatial separation between young star clusters and nearby HII regions made it possible to determine the position of the corotation radius in the studied galaxies. Fourier analysis of the gas velocity field in the same galaxies independently confirmed the corotation radius estimates obtained by the morphological method presented here.
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We present a far-UV (FUV) study of the star-forming complexes (SFCs) in three nearby galaxies using the Ultraviolet Imaging Telescope (UVIT). The galaxies are close to face-on and show significant outer disk star formation. Two of them are isolated (
NGC 628, NGC 6946), and one is interacting with distant companions (NGC 5457). We compared the properties of the SFCs inside and outside the optical radius (R$_{25}$). We estimated the sizes, star formation rates (SFRs), metallicities, and the Toomre Q parameter of the SFCs. We find that the outer disk SFCs are at least ten times smaller in area than those in the inner disk. The SFR per unit area ($Sigma_{SFR}$) in both regions have similar mean values, but the outer SFCs have a much smaller range of $Sigma_{SFR}$. They are also metal-poor compared to the inner disk SFCs. The FUV emission is well correlated with the neutral hydrogen gas (HI) distribution and is detected within and near several HI~holes. Our estimation of the Q parameter in the outer disks of the two isolated galaxies suggests that their outer disks are stable (Q$>$1). However, their FUV images indicate that there is ongoing star formation in these regions. This suggests that there may be some non-luminous mass or dark matter in their outer disks, which increases the disk surface density and supports the formation of local gravitational instabilities. In the interacting galaxy, NGC 5457, the baryonic surface density is sufficient (Q$<$1) to trigger local disk instabilities in the outer disk.
The distributions of size and luminosity for star-forming regions in the nearby spiral galaxy NGC 628 are studied over a wide range of scales using progressively blurr
We characterize the dust in NGC628 and NGC6946, two nearby spiral galaxies in the KINGFISH sample. With data from 3.6um to 500um, dust models are strongly constrained. Using the Draine & Li (2007) dust model, (amorphous silicate and carbonaceous grai
ns), for each pixel in each galaxy we estimate (1) dust mass surface density, (2) dust mass fraction contributed by polycyclic aromatic hydrocarbons (PAH)s, (3) distribution of starlight intensities heating the dust, (4) total infrared (IR) luminosity emitted by the dust, and (5) IR luminosity originating in regions with high starlight intensity. We obtain maps for the dust properties, which trace the spiral structure of the galaxies. The dust models successfully reproduce the observed global and resolved spectral energy distributions (SEDs). The overall dust/H mass ratio is estimated to be 0.0082+/-0.0017 for NGC628, and 0.0063+/-0.0009 for NGC6946, consistent with what is expected for galaxies of near-solar metallicity. Our derived dust masses are larger (by up to a factor 3) than estimates based on single-temperature modified blackbody fits. We show that the SED fits are significantly improved if the starlight intensity distribution includes a (single intensity) delta function component. We find no evidence for significant masses of cold dust T<12K. Discrepancies between PACS and MIPS photometry in both low and high surface brightness areas result in large uncertainties when the modeling is done at PACS resolutions, in which case SPIRE, MIPS70 and MIPS160 data cannot be used. We recommend against attempting to model dust at the angular resolution of PACS.
We present the results of CO(J=3-2) on-the-fly mappings of two nearby non-barred spiral galaxies NGC 628 and NGC 7793 with the Atacama Submillimeter Telescope Experiment at an effective angular resolution of 25. We successfully obtained global distri
butions of CO(J=3-2) emission over the entire disks at a sub-kpc resolution for both galaxies. We examined the spatially-resolved (sub-kpc) relationship between CO(J=3-2) luminosities (LCO(3-2)) and infrared (IR) luminosities (LIR) for NGC 628, NGC 7793, and M 83, and compared it with global luminosities of JCMT Nearby Galaxy Legacy Survey sample. We found a striking linear LCO(3-2)-LIR correlation over the 4 orders of magnitude, and the correlation is consistent even with that for ultraluminous infrared galaxies and submillimeter selected galaxies. In addition, we examined the spatially-resolved relationship between CO(J=3-2) intensities (ICO(3-2)) and extinction-corrected star formation rates (SFRs) for NGC 628, NGC 7793, and M 83, and compared it with that for GMCs in M 33 and 14 nearby galaxy centers. We found a linear ICO(3-2)-SFR correlation with 1 dex scatter. We conclude that the CO(J=3-2) star formation law (i.e., linear LCO(3-2)-LIR and ICO(3-2)-SFR correlations) is universally applicable to various types and spatial scales of galaxies, from spatially-resolved nearby galaxy disks to distant IR-luminous galaxies, within 1 dex scatter.
Relations between star formation rates along the spiral arms and the velocities of gas inflow into the arms in grand-design galaxy NGC 628 were studied. We found that the radial distribution of average star formation rate in individual star formation
regions in regular spiral arms correlates with the velocity of gas inflow into the spiral arms. Both distributions have maxima at a galactocentric distance of 4.5-5 kpc. There are no correlations between the radial distributions of average star formation rate in star formation regions in spiral arms and outside spiral arms in the main disc. We also did not find a correlation between the radial distribution of average star formation rate in star formation regions in spiral arms and HI column density.
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