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Register a new userStar Formation in Massive LSB Galaxies
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K. ONeil
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Massive low surface brightness galaxies have disk central surface brightnesses at least one magnitude fainter than the night sky, but total magnitudes and masses that show they are among the largest galaxies known. Like all low surface brightness (LSB) galaxies, massive LSB galaxies are often in the midst of star formation yet their stellar light has remained diffuse, raising the question of how star formation is proceeding within these systems. HI observations have played a crucial role in studying LSB galaxies as they are typically extremely gas rich. In the past few years we have more than quadrupled the total number of massive LSB galaxies,primarily through HI surveys. To clarify their structural parameters and stellar and gas content, we have undertaken a multi-wavelength study of these enigmatic systems. The results of this study, which includes HI, CO, optical, near UV, and far UV images of the galaxies, will provide the most in depth study done to date of how, when, and where star formation proceeds within this unique subset of the galaxy population.
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Surface photometry at 3.6$mu$m is presented for 61 low surface brightness (LSB) galaxies ($mu_o < 19$ 3.6$mu$m mag arcsecs$^{-2}$). The sample covers a range of luminosity from $-$11 to $-$22 in $M_{3.6}$ and size from 1 to 25 kpc. The morphologies in the mid-IR are comparable to those in the optical with 3.6$mu$m imaging reaches similar surface brightness depth as ground-based optical imaging. A majority of the resulting surface brightness profiles are single exponential in shape with very few displaying upward or downward breaks. The mean $V-3.6$ color of LSB is 2.3 with a standard deviation of 0.5. Color-magnitude and two color diagrams are well matched to models of constant star formation, where the spread in color is due to small changes in the star formation rate (SFR) over the last 0.5 Gyrs as also suggested by the specific star formation rate measured by H$alpha$.
The luminosities, colors and Halpha emission for 429 HII regions in 54 LSB galaxies are presented. While the number of HII regions per galaxy is lower in LSB galaxies compared to star-forming irregulars and spirals, there is no indication that the size or luminosity function of HII regions differs from other galaxy types. The lower number of HII regions per galaxy is consistent with their lower total star formation rates. The fraction of total $L_{Halpha}$ contributed by HII regions varies from 10 to 90% in LSB galaxies (the rest of the H$alpha$ emission being associated with a diffuse component) with no correlation with galaxy stellar or gas mass. Bright HII regions have bluer colors, similar to the trend in spirals; their number and luminosities are consistent with the hypothesis that they are produced by the same HII luminosity function as spirals. Comparison with stellar population models indicates that the brightest HII regions in LSB galaxies range in cluster mass from a few $10^3 M_{sun}$ (e.g., $rho$ Oph) to globular cluster sized systems (e.g., 30 Dor) and that their ages are consistent with clusters from 2 to 15 Myrs old. The faintest HII regions are comparable to those in the LMC powered by a single O or B star. Thus, star formation in LSB galaxies covers the full range of stellar cluster mass.
The bimodality in observed present-day galaxy colours has long been a challenge for hierarchical galaxy formation models, as it requires some physical process to quench (and keep quenched) star formation in massive galaxies. Here we examine phenomenological models of quenching by post-processing the star formation histories of galaxies from cosmological hydrodynamic simulations that reproduce observations of star-forming galaxies reasonably well. We consider recipes for quenching based on major mergers, halo mass thresholds, gas temperature thresholds, and variants thereof. We compare the resulting simulated star formation histories to observed g-r colour-magnitude diagrams and red and blue luminosity functions from SDSS. The merger and halo mass quenching scenarios each yield a distinct red sequence and blue cloud of galaxies that are in broad agreement with data, albeit only under rather extreme assumptions. In detail, however, the simulated red sequence slope and amplitude in each scenario is somewhat discrepant, perhaps traceable to low metallicities in simulated galaxies. Merger quenching produces more massive blue galaxies, earlier quenching, and more frosting of young stars; comparing to relevant data tends to favor merger over halo mass quenching. Although physically-motivated quenching models can produce a red sequence, interesting generic discrepancies remain that indicate that additional physics is required to reproduce the star formation and enrichment histories of red and dead galaxies.
Massive low surface brightness galaxies have disk central surface brightnesses at least one magnitude fainter than the night sky, but total magnitudes and masses that show they are among the largest galaxies known. Like all low surface brightness (LSB) galaxies, massive LSB galaxies are often in the midst of star formation yet their stellar light has remained diffuse, raising the question of how star formation is proceeding within these galaxies. We have undertaken a multi-wavelength study to clarify the structural parameters and stellar and gas content of these enigmatic systems. The results of these studies, which include HI, CO, optical, near UV, and far UV images of the galaxies will provide the most in depth study done to date of how, when, and where star formation proceeds within this unique subset of the galaxy population.
A series of population models are designed to explore the star formation history of gas-rich, low surface brightness (LSB) galaxies. LSB galaxies are unique in having properties of very blue colors, low H$alpha$ emission and high gas fractions that indicated a history of constant star formation (versus the declining star formation models used for most spirals and irregulars). The model simulations use an evolving multi-metallicity composite population that follows a chemical enrichment scheme based on Milky Way observations. Color and time sensitive stellar evolution components (i.e., BHB, TP-AGB and blue straggler stars) are included, and model colors are extended into the Spitzer wavelength regions for comparison to new observations. In general, LSB galaxies are well matched to the constant star formation scenario with the variation in color explained by a fourfold increase/decrease in star formation over the last 0.5 Gyrs (i.e., weak bursts). Early-type spirals, from the S$^4$G sample, are better fit by a declining star formation model where star formation has decreased by 40% in the last 12 Gyrs.
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