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Etude approfondie de la galaxie spirale de type floculent NGC 4414. Dynamique, milieu interstellaire et formation d'étoiles.

Abstract : My PhD thesis deals with the study of the dynamics of the spiral galaxy NGC 4414 for which I have used a six-field CO(1-0) mosaic obtained from IRAM Plateau de Bure observations and deep multiwavelength Hubble Space Telescope Observations. The goals of the thesis were to determine whether the spiral armlets in NGC 4414 are long-lived and to obtain a measure of the stellar mass-to-light ratio, the ultimate goal being to estimate the mass and extent of the dark matter surrounding NGC 4414. The starlight and gas in NGC 4414 are quite axisymmetric so I could use the analytical 3D expressions for gravitational potential and density proposed by Myamoto-Nagai (1975). The mass model is strongly constrained by our high-resolution (interferometric) CO(1-0) rotation curve and HST observations in the B, V and I bands.
By assuming that the stellar mass follows I-band light in the unobscured center and beyond the highly extincted molecular ring and fitting the well-determined CO rotation curve in the nuclear ring region, the mass distribution was determined quite precisely and the stellar M/L ratio measured as a function of radius. A posteriori, the mass model was found to be equivalent to the K' band light distribution with a constant mass-to-light ratio at nearly 0.5 M⊙/L⊙, because the extinction has little effect at 2.1 mm. I show that it is nearly worthless to use a constant M/L ratio at optical wavelengths for a spiral galaxy, as is often done with the so-called maximum disk models.
The interferometric CO observations not only yield a high-resolution rotation curve but can be used to measure non-circular, possibly streaming, motions which could be due to the spiral armlets if long-lived. No sign of streamng motions is found, such that any streaming is a factor 5-10 lower in velocity than in the grand design spiral M 51. No emission was detected in the H2S(1-0) line, indicating a lack of strong shocks. A major structural difference was found between NGC 4414 and M 51 (and other grand design spirals) despite their similar luminosity, gas mass, and star formation rates. The arm-interarm contrast is much lower in NGC 4414 than M 51, as expected, but also the contrast rises dramatically with radius in M 51 whereas it remains constant in NGC 4414. I reach the conclusion that there is no persistent spiral structure in NGC 4414. The star formation likely propagates over kiloparsec scales and these regions are stretched by the differential rotation to give the spiral pattern.
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Contributor : Olivier Vallejo <>
Submitted on : Thursday, May 22, 2003 - 5:06:48 PM
Last modification on : Saturday, April 20, 2019 - 1:23:47 AM
Long-term archiving on: : Friday, April 2, 2010 - 7:07:13 PM

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  • HAL Id : tel-00002865, version 1

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Olivier Vallejo. Etude approfondie de la galaxie spirale de type floculent NGC 4414. Dynamique, milieu interstellaire et formation d'étoiles.. Astrophysique [astro-ph]. Université Sciences et Technologies - Bordeaux I, 2003. Français. ⟨tel-00002865⟩

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