Rest-frame UV versus optical morphologies of galaxies using Sersic profile fitting: the importance of morphological K-correction

We show a comparison of the rest-frame UV morphologies of a sample of 162 intermediate redshift (median redshift 1.02) galaxies with their rest-frame optical morphologies. We select our sample from the deepest near-UV image obtained with the Hubble Space Telescope (HST) using the WFPC2 (F300W) as part of the parallel observations of the Hubble Ultra Deep Field campaign overlapping with the HST/ACS GOODS dataset. We perform single component Sersic fits in both WFPC2/F300W (rest-frame UV) and ACS/F850LP (rest-frame optical) bands and deduce that the Sersic index $n$ is estimated to be smaller in the rest-frame UV compared to the rest-frame optical, leading to an overestimation of the number of merger candidates by ~40-100% compared to the rest-frame optical depending upon the cutoff in $n$ employed for identifying merger candidates. This effect seems to be dominated by galaxies with low values of n(F300W) <= 0.5 that have a value of n(F850LP) ~ 1.0. We argue that these objects are probably clumpy starforming galaxies or minor mergers, both of which are essentially contaminants, if one is interested in identifying major mergers. In addition we also find evidence that the axis ratio b/a is lower, i.e. ellipticity (1-b/a) is higher in rest-frame UV compared to the rest-frame optical. Moreover, we find that in the rest-frame UV, the number of high ellipticity (e >= 0.8) objects are higher by a factor of ~2.8 compared to the rest-frame optical. This indicates that the reported dominance of elongated morphologies among high-z LBGs might just be a bias related to the use of rest-frame UV datasets in high-z studies.

💡 Research Summary

The paper presents a systematic comparison of rest‑frame ultraviolet (UV) and optical morphologies for a sample of 162 intermediate‑redshift galaxies (median z ≈ 1.02). The authors selected these objects from the deepest Hubble Space Telescope (HST) near‑UV image obtained with WFPC2 (F300W) during the parallel observations of the Hubble Ultra Deep Field (HUDF) campaign, which overlaps with the ACS GOODS dataset. For each galaxy they performed single‑component Sérsic profile fitting in both the WFPC2/F300W band (probing the rest‑frame UV) and the ACS/F850LP band (probing the rest‑frame optical).

The key quantitative result is that the Sérsic index n measured in the UV is systematically lower than that measured in the optical. In particular, a substantial fraction of galaxies have n(F300W) ≤ 0.5 while their n(F850LP) values cluster around ≈ 1.0. Because many studies use a low Sérsic index (e.g., n < 2.5 or n < 2.0) as a proxy for disturbed or merging systems, the authors demonstrate that relying on UV‑based n leads to an over‑estimation of merger candidates by roughly 40 % to 100 % compared with an optical‑based selection, depending on the exact n cutoff. The authors argue that the UV‑low n objects are most likely clumpy star‑forming galaxies or minor mergers, which are contaminants when the scientific goal is to identify major mergers.

In addition to the Sérsic index, the axis ratio b/a (and consequently the ellipticity e = 1 − b/a) also shows a wavelength dependence. The UV images yield systematically lower b/a values, i.e., higher ellipticities. The number of high‑ellipticity objects (e ≥ 0.8) is larger by a factor of ≈ 2.8 in the UV relative to the optical. This finding suggests that the frequently reported dominance of elongated morphologies among high‑z Lyman‑break galaxies (LBGs) may be largely a selection bias introduced by using rest‑frame UV data.

The authors discuss the physical origin of these biases. UV light traces the distribution of young, massive stars and is therefore dominated by bright star‑forming clumps. These clumps can dominate the surface‑brightness profile, causing the fitted Sérsic index to be artificially low and the apparent shape to be more elongated. In contrast, the optical band samples older stellar populations that are more smoothly distributed, yielding higher n values and rounder isophotes.

Recognizing the importance of a “morphological K‑correction,” the paper proposes several mitigation strategies. First, multi‑wavelength Sérsic fitting (UV, optical, and near‑infrared) should be performed to quantify the systematic offsets in n and b/a as a function of rest‑frame wavelength, allowing empirical correction factors to be derived. Second, simulated galaxy insertions into real images can be used to calibrate the measurement bias for each band. Third, fitting more complex models (e.g., adding a second Sérsic component or a clump‑residual term) can separate the underlying smooth component from the clumpy star‑forming regions, reducing the contamination of merger samples.

The implications are significant for studies of galaxy evolution. Without accounting for the wavelength‑dependent bias, merger rates derived from UV‑selected samples will be inflated, and the inferred prevalence of highly elongated systems at high redshift will be over‑estimated. By applying the recommended morphological K‑corrections, future surveys—especially those targeting the rest‑frame UV of distant galaxies—can obtain more reliable estimates of the true structural parameters, merger fractions, and the physical processes shaping galaxies across cosmic time.