Star formation exists in all early-type galaxies – evidence from ubiquitous structure in UV images
arxiv(2024)
摘要
Recent surveys have demonstrated the widespread presence of UV emission in
early-type (elliptical/S0) galaxies, suggesting the presence of star formation
in many of these systems. However, potential UV contributions from old and
young stars, together with model uncertainties, makes it challenging to confirm
the presence of young stars using integrated photometry alone. This is
particularly true in ETGs that are fainter in the UV and have red UV-optical
colours. An unambiguous way of disentangling the source of the UV is to look
for structure in UV images. Optical images of ETGs, which are dominated by old
stars, are smooth and devoid of structure. If the UV is also produced by these
old stars, then the UV images will share this smoothness, while, if driven by
young stars, they will exhibit significant structure. We compare the UV and
optical morphologies of 32 ETGs (93 percent of which are at z<0.03) using
quantitative parameters (concentration, asymmetry, clumpiness and the Sérsic
index), calculated via deep UV and optical images with similar resolution.
Regardless of stellar mass, UV-optical colour or the presence of interactions,
the asymmetry and clumpiness of ETGs is significantly (often several orders of
magnitudes) larger in the UV than in the optical, while the UV Sérsic indices
are typically lower than their optical counterparts. The ubiquitous presence of
structure demonstrates that the UV flux across our entire ETG sample is
dominated by young stars and indicates that star formation exists in all ETGs
in the nearby Universe.
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