Investigating the need for a robust ultraviolet filter set aboard the Habitable Worlds Observatory

TL;DR

This study addresses how ultraviolet filter choices influence galaxy SED fitting, focusing on constraining the UV slope parameter $\beta$ with $f_\lambda \propto \lambda^{\beta}$ and hence the star formation history. Using cross-matched data from Zou et al. (GALEX FUV/NUV) and Nagaraj et al. (Swift UVOT W2, M2, W1) for 51 galaxies in the CDF-S, they estimate $\beta$ with Monte Carlo resampling and compare to an IUE baseline. Results show GALEX yields large uncertainties and weaker age constraints, whereas UVOT provides tighter $\beta$ estimates and better sensitivity to the 2175Å attenuation bump. These findings motivate Habitable Worlds Observatory to adopt at least a FUV filter with three medium-band NUV filters to maximize UV science; the authors also plan mock-SED fitting to validate these design implications.

Abstract

High resolution, ultraviolet imaging is often unavailable across the sky, even in heavily studied fields such as the Chandra Deep Field - South. The Habitable Worlds Observatory is one of two upcoming missions with the possibility of significant UV capabilities, and the only one early enough in development to consider suggestions to its design. In this paper, we conduct an initial study of how current common UV filter sets affect the results of spectral energy distribution fitting for the estimation of galaxy parameter. This initial look is intended to motivate the need for future, more robust, SED fitting of mock galaxies. We compare the broad near UV and far UV filters used by the GALEX mission to the three more narrow Swift UVOT filters. We find that the GALEX filters result in larger errors when calculating the UV beta parameter compared to UVOT, and provide little constraint on the star formation age of a galaxy. We further note the ability of the UVOT filters to investigate the 2175Å attenuation bump; GALEX has a reduced capacity to trace this same feature. Ultimately, we recommend that in order to optimize the effectiveness of HWO's ultraviolet capacity for transformative astrophysics, a minimum of a FUV filter with three medium band NUV filters should be adopted. This will combine the power of GALEX's wavelength range with the finer sampling of UVOT around an important dust feature.

Figures (5)

• Figure 1: The GALEX (solid line) filter response curves and the Swift (dashed lines) UVOT filter response curves. The vertical dot-dash line is the rest frame location of the 2175Å absorption bump. It should be noted that this feature falls near the peak of the UVOT M2 filter.
• Figure 2: The $\beta_{\textit{GALEX}}$ (blue) and $\beta_{UVOT}$ (red) values as a function of redshift. In most cases, the UVOT and GALEX values agree within the error.
• Figure 3: Models generated with B agpipes and the corresponding values of $\beta$ calculated from these models. At the top is the model with the most recent star formation "turn-on" with each model down being an older star formation age as shown in the legend. The dashed vertical lines show the range that $\beta$ is fit over from the expected IUE photometry returned by B agpipes, with the left and right line centered on the peak wavelength of the IUE bands in which $\beta$ is defined.
• Figure 4: Bottom: The UVOT filter response curves. Top: Seven galaxies with their magnitudes over each UVOT filter. The color of the line is a relative NUV color (W2-W1), with the galaxies being plotted with the reddest color at the top and bluest at the bottom. Additionally, stellar masses (log scale) are shown on the left and the FUV attenuation (as predicted by CIGALE on the right). For each galaxy, the magnitudes of each filter are offset by a constant to separate the sources for easier viewing and to achieve the progression of NUV color. For this reason the y-axis in the top panel is not shown. The shape of the SED over this narrow range shows an evolution over NUV color, stellar mass, and attenuation; all are consistent with expectations for the presence or absence of 2175Å absorption. Numbers 1-7 are identifiers only and match the numbers in Figure \ref{['f:stamps']}.
• Figure 5: DESI Legacy Survey DR10 cutouts of the 7 galaxies plotted in Figure \ref{['f:colors']}. Numbers in the upper left corner, correspond to the numbers used previously. Redshifts are overlaid on the cutouts. Cutouts are 0.75' squares.