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Register a new userA weak lensing view of the downsizing of star-forming galaxies
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We describe a weak lensing view of the downsizing of star forming galaxies based on cross correlating a weak lensing ($kappa$) map with a predicted map constructed from a redshift survey. Moderately deep and high resolution images with Subaru/Hyper Suprime-Cam covering the 4 deg^2 DLS F2 field provide a $kappa$ map with 1 arcmin resolution. A dense complete redshift survey of the F2 field including 12,705 galaxies with $Rleq20.6$ is the basis for construction of the predicted map. The zero-lag cross-correlation between the kappa and predicted maps is significant at the $30sigma$ level. The width of the cross-correlation peak is comparable with the angular scale of rich cluster at $zsim0.3$, the median depth of the redshift survey. Slices of the predicted map in $delta{z} = 0.05$ redshift bins enable exploration of the impact of structure as a function of redshift. The zero-lag normalised cross-correlation has significant local maxima at redshifts coinciding with known massive X-ray clusters. Even in slices where there are no known massive clusters, there is significant signal in the cross-correlation originating from lower mass groups that trace the large-scale of the universe. Spectroscopic $D_n4000$ measurements enable division of the sample into star-forming and quiescent populations. The significance of the cross-correlation with structure containing star-forming galaxies increases with redshift from $5sigma$ at $z = 0.3$ to $7 sigma$ at $z = 0.5$. The weak lensing results are consistent with the downsizing view of galaxy evolution established on the basis of many other independent studies.
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Observations of molecular gas in high-z star-forming galaxies typically rely on emission from CO lines arising from states with rotational quantum numbers J > 1. Converting these observations to an estimate of the CO J=1-0 intensity, and thus inferring H2 gas masses, requires knowledge of the CO excitation ladder, or spectral line energy distribution (SLED). The few available multi-J CO observations of galaxies show a very broad range of SLEDs, even at fixed galaxy mass and star formation rate, making the conversion to J=1-0 emission and hence molecular gas mass highly uncertain. Here, we combine numerical simulations of disk galaxies and galaxy mergers with molecular line radiative transfer calculations to develop a model for the physical parameters that drive variations in CO SLEDs in galaxies. An essential feature of our model is a fully self-consistent computation of the molecular gas temperature and excitation structure. We find that, while the shape of the SLED is ultimately determined by difficult-to-observe quantities such as the gas density, temperature, and optical depth distributions, all of these quantities are well-correlated with the galaxys mean star formation rate surface density (Sigma_SFR), which is observable. We use this result to develop a model for the CO SLED in terms of Sigma_SFR, and show that this model quantitatively reproduces the SLEDs of galaxies over a dynamic range of ~200 in SFR surface density, at redshifts from z=0-6. This model should make it possible to significantly reduce the uncertainty in deducing molecular gas masses from observations of high-J CO emission.
We present a study of a large, statistically complete sample of star-forming dwarf galaxies using mid-infrared observations from the {it Spitzer Space Telescope}. The relationships between metallicity, star formation rate (SFR) and mid-infrared color in these systems show that the galaxies span a wide range of properties. However, the galaxies do show a deficit of 8.0 um polycyclic aromatic hydrocarbon emission as is apparent from the median 8.0 um luminosity which is only 0.004 lstarf while the median $B$-band luminosity is 0.05 lstarb. Despite many of the galaxies being 8.0 um deficient, there is about a factor of 4 more extremely red galaxies in the [3.6] $-$ [8.0] color than for a sample of normal galaxies with similar optical colors. We show correlations between the [3.6] $-$ [8.0] color and luminosity, metallicity, and to a lesser extent SFRs that were not evident in the original, smaller sample studied previously. The luminosity--metallicity relation has a flatter slope for dwarf galaxies as has been indicated by previous work. We also show a relationship between the 8.0 um luminosity and the metallicity of the galaxy which is not expected given the competing effects (stellar mass, stellar population age, and the hardness of the radiation field) that influence the 8.0 um emission. This larger sample plus a well-defined selection function also allows us to compute the 8.0 um luminosity function and compare it with the one for the local galaxy population. Our results show that below 10$^{9}$ $L$solar, nearly all the 8.0 um luminosity density of the local universe arises from dwarf galaxies that exhibit strong ha emission -- i.e., 8.0 um and ha selection identify similar galaxy populations despite the deficit of 8.0 um emission observed in these dwarfs.
The infrared spectral energy distributions (SEDs) of main-sequence galaxies in the early universe (z > 4) is currently unconstrained as infrared continuum observations are time consuming and not feasible for large samples. We present Atacama Large Millimetre Array (ALMA) Band 8 observations of four main-sequence galaxies at z ~ 5.5 to study their infrared SED shape in detail. Our continuum data (rest-frame 110$rm mu m$, close to the peak of infrared emission) allows us to constrain luminosity weighted dust temperatures and total infrared luminosities. With data at longer wavelengths, we measure for the first time the emissivity index at these redshifts to provide more robust estimates of molecular gas masses based on dust continuum. The Band 8 observations of three out of four galaxies can only be reconciled with optically thin emission redward of rest-frame 100$rm mu m$. The derived dust peak temperatures at z ~ 5.5 (38$pm$8K) are elevated compared to average local galaxies, however, 5-10K below what would be predicted from an extrapolation of the trend at $z<4$. This behaviour can be explained by decreasing dust abundance (or density) towards high redshifts, which would cause the infrared SED at the peak to be more optically thin, making hot dust more visible to the external observer. From the 850$rm mu m$ dust continuum, we derive molecular gas masses between $10^{10}$ and $10^{11},{rm M_{odot}}$ and gas fractions (gas over total mass) of 30-80% (gas depletion times of 100-220Myrs). All in all, our results provide a first measured benchmark SED to interpret future millimetre observations of normal, main-sequence galaxies in the early Universe.
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