Abstract
Context. Local spiral galaxies contain roughly two-thirds of the present-day stellar mass density. However, the formation process of disks is still poorly understood. Aims. We present and analyze observations of J033241.88-274853.9 at z=0.6679 to understand how its stellar disk was formed. Methods. We combine multi-wavelength EIS, HST/ACS, Spitzer/IRAC, and GALEX imaging with FLAMES/GIRAFFE 3D spectroscopy to study its color-morphology and spatially-resolved kinematics. A spectral energy distribution (SED) is constructed and physical properties extracted using stellar population mo dels. Results. J033241.88-274853.9 is a blue, young (320 +590 −260 Myr, 90% confidence interval) stellar disk embedded in a very gas-rich (fgas=73-82% with log(Mstellar/M⊙) = 9.45± 0.28 0.14 ) and turbulent phase that is found to be rotating on large spatial scales. We identified two unusual properties of J033241.88-274853.9. (1) The spatial distributions of the ionized gaseous and young stars sh ow a strong decoupling; while almost no stars can be detected in the southern part down to the very deep detection limit of ACS/UDF images (accounting for the light spread by seeing effects), significant emission from the [OII] ionized gas is det ected. (2) We detect an excess of velocity dispersion in the southern part of J033241.88-274853.9 in comparison to expectations from a rotating disk model. Conclusions. We considered two disk formation scenarios, depending on the gaseous phase geometry. In the first one, we examined whether J033241.88-274853.9 could be a young rotating disk that has been recently collapsed from a pre-existing, very gas-rich rotating disk. This scenario requires two (unknown) additional assumptions to explain the decoupling between the distribution of stars and gas and the excess of velocity dispersion in the same region. In a second scenario, we examine whether J033241.88-274853.9 could be a merger remnant of two gas-rich disks. In this case, the asymmetry observed between the gas and star distributions, as well as the excess of velocity dispersion, find a common explanation. Sh ocks produced during the merger in this region can be ionized easily and heat the gas while preventing star formation. This makes this scenario more satisfactory than the collapse of a pre-exis ting, gas-rich rotating disk.
Highlights
While the majority of the local stellar mass density is locked into spiral galaxies, our understanding of the formation of galactic disks still remains incomplete (Mayer et al 2008)
This “spiral rebuilding” scenario might seem inconsistent with numerical simulations, which often predict that the product of a major merger between two spirals is an elliptical galaxy
If one further assumes that the [OII] equivalent width is spatially constant, comparing the GIRAFFE [OII] distribution to the smoothed ACS V-band image underpredicts the EW0([OII]) in the leftmost pixel by a factor of 10: we find that the flux ratio between the region encompassed by the brightest GIRAFFE pixel and the region encompassed by leftmost GIRAFFE pixel is ten times larger than the same ratio directly measured in the GIRAFFE [OII] emission map
Summary
While the majority of the local stellar mass density is locked into spiral galaxies, our understanding of the formation of galactic disks still remains incomplete (Mayer et al 2008). In the framework of an ESO large program called IMAGES (“Intermediate-MAss Galaxy Evolution Sequence”, Ravikumar et al 2007; Yang et al 2008, hereafter Paper I), we have been gathering multi-wavelength data on a representative sample of emission line, intermediate-mass galaxies at z ∼ 0.6, i.e., 6 Gyr ago These galaxies are the progenitors of present-day spirals, which contain approximately two-thirds of the present-day stellar mass density (Hammer et al 2007). In Puech et al (2008) (Paper III), we analyzed their dynamical properties through the evolution of the near-infrared Tully-Fisher relation This series of papers has revealed a surprisingly high fraction of kinematically nonrelaxed galaxies (Paper I), while well-relaxed spiral rotating disks appear to be evolving since z ∼ 0.6, i.e., over the past 6 Gyr, by a factor as much as two, both in number (Paper II) and stellar mass (Paper III).
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