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Register a new userProbing the Star Formation History and Initial Mass Function of the z~2.5 Lensed Galaxy SMM J163554.2+661225 with Herschel
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We present the analysis of Herschel SPIRE far-infrared (FIR) observations of the z = 2.515 lensed galaxy SMM J163554.2+661225. Combining new 250, 350, and 500 micron observations with existing data, we make an improved fit to the FIR spectral energy distribution (SED) of this galaxy. We find a total infrared (IR) luminosity of L(8--1000 micron) = 6.9 +/- 0.6x10^11 Lsol; a factor of 3 more precise over previous L_IR estimates for this galaxy, and one of the most accurate measurements for any galaxy at these redshifts. This FIR luminosity implies an unlensed star formation rate (SFR) for this galaxy of 119 +/- 10 Msol per yr, which is a factor of 1.9 +/- 0.35 lower than the SFR derived from the nebular Pa-alpha emission line (a 2.5-sigma discrepancy). Both SFR indicators assume identical Salpeter initial mass functions (IMF) with slope Gamma=2.35 over a mass range of 0.1 - 100 Msol, thus this discrepancy suggests that more ionizing photons may be necessary to account for the higher Pa-alpha-derived SFR. We examine a number of scenarios and find that the observations can be explained with a varying star formation history (SFH) due to an increasing star formation rate (SFR), paired with a slight flattening of the IMF. If the SFR is constant in time, then larger changes need to be made to the IMF by either increasing the upper-mass cutoff to ~ 200 Msol, or a flattening of the IMF slope to 1.9 +/- 0.15, or a combination of the two. These scenarios result in up to double the number of stars with masses above 20 Msol, which produce the requisite increase in ionizing photons over a Salpeter IMF with a constant SFH.
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We report the detection of the Paschen-alpha emission line in the z=2.515 galaxy SMM J163554.2+661225 using Spitzer spectroscopy. SMM J163554.2+661225 is a sub-millimeter-selected infrared (IR)-luminous galaxy maintaining a high star-formation rate (SFR), with no evidence of an AGN from optical or infrared spectroscopy, nor X-ray emission. This galaxy is lensed gravitationally by the cluster Abell 2218, making it accessible to Spitzer spectroscopy. Correcting for nebular extinction derived from the H-alpha and Pa-alpha lines, the dust-corrected luminosity is L(Pa-alpha) = (2.57+/-0.43) x 10^43 erg s^-1, which corresponds to an ionization rate, Q = (1.6+/-0.3) x 10^55 photons s^-1. The instantaneous SFR is 171+/-28 solar masses per year, assuming a Salpeter-like initial mass function. The total IR luminosity derived using 70, 450, and 850 micron data is L(IR) = (5-10) x 10^11 solar luminosities, corrected for gravitational lensing. This corresponds to a SFR=90-180 solar masses per year, where the upper range is consistent with that derived from the Paschen-alpha luminosity. While the L(8 micron) / L(Pa-alpha) ratio is consistent with the extrapolated relation observed in local galaxies and star-forming regions, the rest-frame 24 micron luminosity is significantly lower with respect to local galaxies of comparable Paschen-alpha luminosity. Thus, SMM J163554.2+661225 arguably lacks a warmer dust component (T ~ 70 K), which is associated with deeply embedded star formation, and which contrasts with local galaxies with comparable SFRs. Rather, the starburst is consistent with star-forming local galaxies with intrinsic luminosities, L(IR) ~ 10^10 solar luminosities, but scaled-up by a factor of 10-100.
The first supernovae will soon be visible at the edge of the observable universe, revealing the birthplaces of Population III stars. With upcoming near-infrared missions, a broad analysis of the detectability of high-$z$ supernovae is paramount. We combine cosmological and radiation transport simulations, instrument specifications, and survey strategies to create synthetic observations of primeval core-collapse, Type IIn and pair-instability supernovae with the James Webb Space Telescope ($JWST$). We show that a dedicated observational campaign with the $JWST$ can detect up to $sim 15$ pair-instability explosions, $sim 300$ core-collapse supernovae, but less than one Type IIn explosion per year, depending on the Population III star formation history. Our synthetic survey also shows that $approx 1-2 times10^2$ supernovae detections, depending on the accuracy of the classification, are sufficient to discriminate between a Salpeter and flat mass distribution for high redshift stars with a confidence level greater than 99.5 per cent. We discuss how the purity of the sample affects our results and how supervised learning methods may help to discriminate between CC and PI SNe.
We study the star formation rate (SFR) - stellar mass (M*) relation in a self-consistent manner from 0 < z < 2.5 with a sample of galaxies selected from the NEWFIRM Medium-Band Survey. We find a significant non-linear slope of the relation, SFR propto M*^0.6, and a constant observed scatter of 0.34 dex, independent of redshift and M*. However, if we select only blue galaxies we find a linear relation SFR propto M*, similar to previous results at z = 0 by Peng et al. (2010). This selection excludes red, dusty, star-forming galaxies with higher masses, which brings down the slope. By selecting on L_IR/L_UV (a proxy for dust obscuration) and the rest-frame U-V colors, we show that star-forming galaxies fall in three distinct regions of the log(SFR)-log(M*) plane: 1) actively star-forming galaxies with normal dust obscuration and associated colors (54% for log(M*) > 10 at 1 < z < 1.5), 2) red star-forming galaxies with low levels of dust obscuration and low specific SFRs (11%), and 3) dusty, blue star-forming galaxies with high specific SFRs (7%). The remaining 28% comprises quiescent galaxies. Galaxies on the normal star formation sequence show strong trends of increasing dust attenuation with stellar mass and a decreasing specific SFR, with an observed scatter of 0.25 dex (0.17 dex intrinsic scatter). The dusty, blue galaxies reside in the upper envelope of the star formation sequence with remarkably similar spectral shapes at all masses, suggesting that the same physical process is dominating the stellar light. The red, low-dust star-forming galaxies may be in the process of shutting off and migrating to the quiescent population.
The star formation rate (SFR) is a key parameter in the study of galaxy evolution. The accuracy of SFR measurements at z~2 has been questioned following a disagreement between observations and theoretical models. The latter predict SFRs at this redshift that are typically a factor 4 or more lower than the measurements. We present star-formation rates based on calorimetric measurements of the far-infrared (FIR) luminosities for massive 1.5<z<2.5, normal star-forming galaxies (SFGs), which do not depend on extinction corrections and/or extrapolations of spectral energy distributions. The measurements are based on observations in GOODS-N with the Photodetector Array Camera & Spectrometer (PACS) onboard Herschel, as part of the PACS Evolutionary Probe (PEP) project, that resolve for the first time individual SFGs at these redshifts at FIR wavelengths. We compare FIR-based SFRs to the more commonly used 24 micron and UV SFRs. We find that SFRs from 24 micron alone are higher by a factor of ~4-7.5 than the true SFRs. This overestimation depends on luminosity: gradually increasing for log L(24um)>12.2 L_sun. The SFGs and AGNs tend to exhibit the same 24 micron excess. The UV SFRs are in closer agreement with the FIR-based SFRs. Using a Calzetti UV extinction correction results in a mean excess of up to 0.3 dex and a scatter of 0.35 dex from the FIR SFRs. The previous UV SFRs are thus confirmed and the mean excess, while narrowing the gap, is insufficient to explain the discrepancy between the observed SFRs and simulation predictions.
We analyze the dependence of galaxy structure (size and Sersic index) and mode of star formation (Sigma_SFR and SFR_IR/SFR_UV) on the position of galaxies in the SFR versus Mass diagram. Our sample comprises roughly 640000 galaxies at z~0.1, 130000 galaxies at z~1, and 36000 galaxies at z~2. Structural measurements for all but the z~0.1 galaxies were based on HST imaging, and SFRs are derived using a Herschel-calibrated ladder of SFR indicators. We find that a correlation between the structure and stellar population of galaxies (i.e., a Hubble sequence) is already in place since at least z~2.5. At all epochs, typical star-forming galaxies on the main sequence are well approximated by exponential disks, while the profiles of quiescent galaxies are better described by de Vaucouleurs profiles. In the upper envelope of the main sequence, the relation between the SFR and Sersic index reverses, suggesting a rapid build-up of the central mass concentration in these starbursting outliers. We observe quiescent, moderately and highly star-forming systems to co-exist over an order of magnitude or more in stellar mass. At each mass and redshift, galaxies on the main sequence have the largest size. The rate of size growth correlates with specific SFR, and so does Sigma_SFR at each redshift. A simple model using an empirically determined SF law and metallicity scaling, in combination with an assumed geometry for dust and stars is able to relate the observed Sigma_SFR and SFR_IR/SFR_UV, provided a more patchy dust geometry is assumed for high-redshift galaxies.
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