The Universality of Intrinsic Flattening in Extragalactic Stellar Disks

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Favaro, Jeremy P.
Astronomy & Astrophysics , Extragalactic Astrophysics , Disk Galaxies , Stellar Disk , Intrinsic Flattening , Observational methods , Galaxies evolution , Galaxies structure
Highly inclined (edge-on) galaxies provide the unique perspective necessary to constrain the intrinsic flattening, 𝑐/π‘Ž, of galactic disks. We use β€œdust-free” 3.6 πœ‡m maps of 141 edge-on spiral galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S4G) and its early-type galaxy extension to determine the intrinsic flattening of stellar disks. The proper assessment of the intrinsic flattening of galaxies requires careful consideration of galactic structure. To this end, we take inspiration from the findings of surface brightness profile decompositions to develop a robust method for the identification and analysis of isophotes that characterize the disk. Isophote axis ratios are averaged within two axial bins between 20% of the optical radius of the 25th magnitude isophote in the 𝐡-band, 𝑅25, and 80% of 𝑅25, which we demonstrate characterises the stellar disk’s 𝑐/π‘Ž. We then test for correlation between 𝑐/π‘Ž and Hubble type. The relationships between 𝑐/π‘Ž and other galactic physical parameters – total stellar mass, concentration index, total HI mass, mass of the central mass concentration (CMC), and circular velocity – are also investigated. We find that: (i) the intrinsic flattening of spiral galaxies is βŸ¨π‘/π‘ŽβŸ© = 0.130 Β± 0.002 (stat) Β± 0.034 (intrinsic/systematic); (ii) the intrinsic flattening of spiral galaxies is similar across morphological types; (iii) intrinsic flattening shows good positive correlation with measures of CMC size; and (iv) intrinsic flattening correlates well with the model-dependent ratio of scale height to scale length.
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