Galaxy UV Legacy Project: Survey Description and First Insights Into NGC 4449 Recent History of Star Formation

Abstract

The Galaxy UV Legacy Project (GULP) is a Cycle 28 Treasury program with the Hubble Space Telescope (HST) designed to characterize resolved massive stars, OB associations, and young star clusters (YSCs) in 26 nearby star-forming galaxies. Utilizing the ACS/SBC F150LP and WFC3/UVIS F218W filters, combined with extensive archival observations, GULP provides an unprecedented panchromatic 8-band view from the Far-UV to the I-band. The target galaxies were carefully selected to span a broad range of metallicities, masses, morphological types, and star formation rates, thereby enabling detailed studies of star formation processes across different galactic environments. This paper introduces the GULP survey, detailing its observational strategy, data processing, and initial scientific results for the irregular barred starburst dwarf galaxy NGC 4449, used as a test case. We derived the physical parameters and ages for thousands of stars using the Binary Populations And Spectral Synthesis (BPASS) models, and found that the younger stars and clusters are predominantly concentrated along the galaxy's central bar, and that over the past <50 Myr star formation progressively migrated from northeast to southwest. We used the F150LP, F218W, and F275W filters to investigate how the UV-bump at lambda 2175 A correlates with the intensity of the UV radiation. The UV-bump is detected in many areas of the galaxy, but is absent in the regions of most intense and recent star formation. This strongly supports the scenario where UV radiation from young, massive stars effectively destroys the small dust grains responsible for the UV-bump.

Figures (18)

• Figure 1: Upper panel: Relative transmission of the broadband filters used in the GULP survey as a function of wavelength. Lower panel: Atmosphere models for stars with different temperatures and surface gravities, including a WR star Hamann2004, an O5V star (dark blue), a B5V star (cyan), Vega (green), and a K5iii red supergiant (red). Apart from the WR star, the remaining models are from Kurucz1993. Fluxes are normalized at $\lambda = 5000\, {\textrm{\AA}}$.
• Figure 2: The Magellanic-type galaxy NGC 4449. Panel a) shows the GALEX color-composite image with the FUV filter in blue and the NUV filter in red. The white rectangle highlights the region covered by GULP with the SBC filter F150LP. Panel b) shows the color composite image of the portion of galaxy studied with the SBC. The filter F150LP is depicted in blue, the ACS/F658N filter in red, and ACS/F438W in green. Panel c) presents a $\sim 447\times 277\,\mathrm{pc}$ zoom-in of F150LP image of the central double YSC. The corresponding region in panel b) is marked with a white rectangle. In all three panels, North is up, and East is toward the left.
• Figure 3: Left Panel: the sSFR and mass, both in logarithmic scale, for 276 star-forming galaxies found within $8\, \mathrm{Mpc}$Kennicutt2008. GULP galaxies are color-coded from dark purple to light yellow, for increasing metallicity. The light blue crosses represent the four galaxies (the 2 MCs, NGC 3109, and Sextans A) observed by ULLYSES. Central Panel: The same as the left panel, but for the logarithm of the SFR. Right Panel: SFR, on logarithmic scale, as a function of the morphological types. SFRs are derived from GALEX UV luminosity Lee2009, scaled to the area covered by the F275W filter.
• Figure 4: Footprints of the GULP observations, except NGC 4449, already shown in Fig. \ref{['f:GULPvsGALEX']}. The ACS/SBC mosaics, in blackbody Asinh color-map, are superimposed over the GALEX color-composite images (the FUV chanel is in blue and the NUV chanel in red). The $2\farcs7 \times 2\farcs7$ footprint of the archival UVIS observations in the F275W filter, used to target the brightest UV regions that do not saturate the SBC detector, is shown in green.
• Figure 5: NGC 4449 DOLPHOT photometry errors (upper-left panel), $\chi^2$ (upper-right panel), sharpness (middle-left panel), PSF-roundness (middle-right panel), and crowding (lower-left panel) plotted as a function of $\rm{m}_{F275W}$.