LSB Galaxies :

  • defined as $\mu_o < 23$ B mag arcsecs$^{-2}$

  • usually, but not always, irregular in morphology

  • vary in size from dwarfs to Malin giants

  • single and double horn HI profiles (i.e., some are disks, some have solid body rotation)

  • all have very low SFR (based on H$\alpha$ emission)

  • high gas mass fractions ($f_g > 80$%)

  • low gas metallicities ($-1.5 <$ [Fe/H] $< -0.5$)

  • most very blue in optical and near-IR colors

  • ${M_*} \over {SFR}$ $\approx$ Hubble time

    low SFR, low stellar densities, low metallicities, high gas fractions -> ok, but blue colors?

    --> 20 years of $BVRI$, HI, H$\alpha$, $Spitzer$ 3.5$\mu$m observations of 100 LSB galaxies + HST WPC3 F336W, F555W, F813W images of three LSB's, to resolution their star formation history

  • LSB morphology

    F568-1, V on left, 3.5$\mu$m on right

    Color-magnitude and $L_{H\alpha}$ relations (black = LSB, open = HSB, blue = early-type

    LSB HII regions, V on left, H$\alpha$ on right, note some H$\alpha$ lines up with optical knots, some do not

    HII region cluster mass vs H$\alpha$ flux, IMF/mass models shown for various cluster masses and ages (Myrs) - large star forming regions are found in LSB's

    LSB (blue diamonds) plus S$^4$G (late-type = blue, early-type=red) plus ellipticals (red asterisks) with new population models that include TP-AGBs, models have constant SFR over 12 Gyrs, with modifications of a factor of 4 in SFR for 0.5 Gyrs (weak bursts)

    F608-1 (HST target), V image on left, H$\alpha$ on right

    F608-1 B-V color map on left, WFPC3 F555W on right, H$\alpha$ marked in red

    Close-up of two star clusters, number of bright stars consistent with $L_{H\alpha}$ of HII region

    Close-up of blue region to NE, LSB = dispersed stars