We investigate the nature of the diffuse intra-cluster ultraviolet light seen in twelve local starburst galaxies , using long-slit ultraviolet spectroscopy obtained with the Space Telescope Imaging Spectrograph ( STIS ) aboard the Hubble Space Telescope ( HST ) .
We take this faint intra-cluster light to be the field in each galaxy , and compare its spectroscopic signature with STARBURST99 evolutionary synthesis models and with neighboring star clusters .
Our main result is that the diffuse ultraviolet light in eleven of the twelve starbursts lacks the strong O-star wind features that are clearly visible in spectra of luminous clusters in the same galaxies .
The difference in stellar features dominating cluster and field spectra indicate that the field light originates primarily from a different stellar population , and not from scattering of UV photons leaking out of the massive clusters .
A cut along the spatial direction of the UV spectra establishes that the field light is not smooth , but rather shows numerous “ bumps and wiggles. ” Roughly 30–60 % of these faint peaks seen in field regions of the closest ( < 4 Mpc ) starbursts appear to be resolved , suggesting a contribution from superpositions of stars and/or faint star clusters .
Complementary WFPC2 UVI imaging for the three nearest target galaxies , NGC 4214 , NGC 4449 , and NGC 5253 are used to obtain a broader picture , and establish that all three galaxies have a dispersed population of unresolved , luminous blue sources .
Because the field spectra are dominated by B stars , we suggest that the individual sources observed in the WFPC2 images are individual B stars ( rather than O stars ) , or small star clusters .
We consider several scenarios to understand the lack of observed massive stars in the field , and their implications for the origin of the field stellar population .
If the field stellar populations formed in situ , the field must either have an IMF which is steeper than Salpeter ( \alpha \sim - 3.0 ~ { } \mbox { to } -3.5 ) , or a Salpeter slope with an upper mass cutoff of 30– 50 ~ { } M _ { \odot } .
If star formation occurs primarily in star clusters , the field could be composed of older , faded clusters , and/or a population which is coeval with the luminous clusters but lower in mass .
We use these benchmark populations to place constraints on the field stellar population origin .
Although the field probably includes stars of different ages , the UV light is dominated by the youngest stellar populations in the field .
If the field is composed of older , dissolving clusters , we estimate that star clusters ( regardless of mass ) need to dissolve on timescales 7–10 Myr to create the field .
If the field is composed of young clusters which fall below the detection limit of individual sources in our spectroscopy , they would have to be several hundred solar masses or less , in order to be deficient in O stars , despite their extreme youth .