The star formation histories of low surface brightness galaxies

Abstract

We have performed deep imaging of a diverse sample of 26 low surface brightness galaxies (LSBGs) in the optical and the near-infrared. Using stellar population synthesis models, we find that it is possible to place constraints on the ratio of young to old stars (which we parameterise in terms of the average age of the galaxy), as well as the metallicity of the galaxy, using optical and near-infrared colours. LSBGs have a wide range of morphologies and stellar populations, ranging from older, high metallicity earlier types to much younger and lower metallicity late type galaxies. Despite this wide range of star formation histories, we find that colour gradients are common in LSBGs. These are most naturally interpreted as gradients in mean stellar age, with the outer regions of LSBGs having younger ages than their inner regions. In an attempt to understand what drives the differences in LSBG stellar populations, we compare LSBG average ages and metallicities with their physical parameters. Strong correlations are seen between a LSBG's star formation history and its K band surface brightness, K band absolute magnitude and gas fraction. These correlations are consistent with a scenario in which the star formation history of a LSBG primarily correlates with its surface density and its metallicity correlates both with its mass and surface density.Comment: 29 pages; 5 figures; re-submitted to MNRAS after replying to referee's repor

Modeling the Interstellar Medium of Low Surface Brightness Galaxies: Constraining Internal Extinction, Disk Color Gradients, and Intrinsic Rotation Curve Shapes

“…However, even then, the amount of reddening due to dust will be insufficient to explain the large radial color gradients now observed in a number of LSB galaxies (de Blok, van der Hulst, & Bothun 1995 ;MGvD99 ;Matthews et al 2000 ;Bell et al 2000). Thus some LSB galaxies must have large intrinsic stellar population and/or metallicity gradients in their disks.…”

“…Nonetheless, using a simple foreground screen model, MGvD99 suggested that in the case of UGC 7321, dust reddening could not explain more than D0.2 mag of the observed B[R color gradient. Using both foreground screen and Triplex dust models (see Disney, Davies, & Phillipps 1989), Bell et al (2000) argued that the amount of dust reddening needed to produce the observed color gradients in their sample of less inclined LSB galaxies was also too large to be consistent with observations.…”

“…A critical assumption in interpreting the physical signiÐ-cance of the large color gradients observed in the UGC 7321 disk, as well as the signiÐcant gradients observed in the disks of other less inclined LSB spirals (e.g., de Blok, van der Hulst, & Bothun 1995 ;Bell et al 2000) is that reddening due to dust is small. For example, in a sample of "" normal ÏÏ edge-on spirals, de Grijs (1998) argued that to within observational errors, the observed color gradients and scale length di †erences in di †erent wave bands could be explained solely from dust.…”

“…For example, estimates of dust reddening, even if small, are needed to correctly interpret the stellar populations inferred from broadband colors and spectroscopic measurements (e.g., Bell et al 2000). It has been found that many of the lowest luminosity LSB galaxies deviate from the standard optical Tully-Fisher relation in the sense that they rotate faster than predicted for their luminosities (e.g., Matthews, van Driel, & Gallagher 1998 ;Stil 1999 ;McGaugh et al 2000), while more luminous LSB galaxies seem to follow the same Tully-Fisher relation as higher surface brightness galaxies (e.g., Zwaan et al 1995 ;Sprayberry et al 1995).…”

“…One difficulty is that these objects tend to be very weak farinfrared and submillimeter sources (e.g., Hoeppe et al 1994 ;Pickering & van der Hulst 1999 ;Bell et al 2000), and hence direct observations of re-emitted thermal radiation from their dust are lacking. Observing the dust in absorption at optical and near-infrared wavelengths can also be problematic, since for LSB systems viewed at moderate or low inclinations, the signatures of dust can be difficult to infer against the patchy and di †use background light of the stellar disk.…”