Detection of PAH and Far-Infrared Emission from the Cosmic Eye
Total Page：16
File Type：pdf, Size：1020Kb
Accepted for publication in ApJ A Preprint typeset using LTEX style emulateapj v. 08/22/09 DETECTION OF PAH AND FAR-INFRARED EMISSION FROM THE COSMIC EYE: PROBING THE DUST AND STAR FORMATION OF LYMAN BREAK GALAXIES B. Siana1, Ian Smail2, A. M. Swinbank2, J. Richard2, H. I. Teplitz3, K. E. K. Coppin2, R. S. Ellis1, D. P. Stark4, J.-P. Kneib5, A. C. Edge2 Accepted for publication in ApJ ABSTRACT ∗ We report the results of a Spitzer infrared study of the Cosmic Eye, a strongly lensed, LUV Lyman Break Galaxy (LBG) at z =3.074. We obtained Spitzer IRS spectroscopy as well as MIPS 24 and 70 µm photometry. The Eye is detected with high significance at both 24 and 70 µm and, when including +4.7 11 a flux limit at 3.5 mm, we estimate an infrared luminosity of LIR = 8.3−4.4 × 10 L⊙ assuming a magnification of 28±3. This LIR is eight times lower than that predicted from the rest-frame UV properties assuming a Calzetti reddening law. This has also been observed in other young LBGs, and indicates that the dust reddening law may be steeper in these galaxies. The mid-IR spectrum shows strong PAH emission at 6.2 and 7.7 µm, with equivalent widths near the maximum values observed in star-forming galaxies at any redshift. The LP AH -to-LIR ratio lies close to the relation measured in local starbursts. Therefore, LP AH or LMIR may be used to estimate LIR and thus, star formation rate, of LBGs, whose fluxes at longer wavelengths are typically below current confusion limits. We also report the highest redshift detection of the 3.3 µm PAH emission feature. The PAH −4 ratio, L6.2/L3.3 = 5.1 ± 2.7, and the PAH-to-LIR ratio, L3.3/LIR = 8.5 ± 4.7 × 10 , are both in agreement with measurements in local starbursts and ULIRGs, suggesting that this line may serve as a good proxy for LP AH or LIR at z > 3 with the James Webb Space Telescope. Subject headings: galaxies: high-redshift, galaxies: individual (J213512.73-010143, Cosmic Eye), galax- ies: starburst, infrared: galaxies 1. INTRODUCTION wavelengths, several studies have attempted to verify the Star-forming galaxies at high-redshift are often found UV-derived SFRs by comparing with other star forma- by identifying a break in their UV continuum arising tion diagnostics. With near-IR spectroscopy, Hα fluxes from both an intrinsic Lyman Break in their SEDs and have been measured for > 100 LBGs, and give compa- the high opacity of the Lyα forest below rest-frame 1216 rable SFRs to the UV-derived SFRs (Erb et al. 2006). However, these Hα studies indicate that dust geome- A˚ (Steidel et al. 1996). Recent studies estimate that tries may be different in LBGs than in local starbursts more than 25% of all present-day stellar mass was cre- because the ionized gas does not seem to be more ob- ated in these Lyman Break Galaxies (LBGs) at z > scured than the stellar continuum, as is seen in lo- 2 (Reddy & Steidel 2009). Their star formation rates cal starbursts (Calzetti et al. 1994). X-ray stacking of (SFRs) typically have to be determined based on rest- large numbers of LBGs suggests comparable SFRs as frame UV properties alone. This involves using the UV the UV determinations (Reddy & Steidel 2004), but ra- spectral slope to determine the amount of ultraviolet ex- dio continuum stacking analyses have given mixed results tinction in order to derive the intrinsic UV luminosity (Reddy & Steidel 2004; Carilli et al. 2008). No individ- and SFR. However, many uncertainties exist in this pro- ual detections of L∗ LBGs (without AGN) has been de- cedure. Firstly, there are degeneracies between age and tected in the X-ray or radio. dust reddening on the UV spectral slope. Secondly, it Ultimately, the best indicator of the star formation rate arXiv:0904.1742v1 [astro-ph.CO] 10 Apr 2009 is not clear that the obscuration law typically used to is an accurate determination of the bolometric luminos- “unredden” the spectra is appropriate for LBGs. Fur- ity. Because most of the UV light (70-90%) is absorbed thermore, at least in the local Universe, the most lumi- by dust (Adelberger & Steidel 2000; Reddy et al. 2006), nous starbursts contain individual star-forming regions the majority of the starburst’s luminosity is emitted ther- that are so dusty that they effectively emit no UV light mally at infrared wavelengths. With current technology, at all (Goldader et al. 2002), so the IR and UV proper- it is difficult to determine the IR luminosity as the SED ties of these systems are uncorrelated. can not be measured at multiple wavelengths. Typical Though it is more difficult to detect LBGs at other LBGs are below the confusion limit of existing submm 1 California Institute of Technology, MS 105-24, Pasadena, CA telescopes (f850 < 2 mJy, Chapman et al. 2000), and the 91125 Spitzer Space Telescope at 70 and 160 µm. Spitzer can 2 Institute for Computational Cosmology, Durham University, only detect LBGs at 24 µm (rest-frame 6-8 µm) and then South Road, Durham, DH1 3LE, UK only L>L∗ LBGs are detected in the deepest images. 3 Spitzer Science Center, California Institute of Technology, 220- 6, Pasadena, CA 91125 Reddy et al. (2006) conducted a study of 24 µm detected 4 Institute of Astronomy, University of Cambridge, Madingley LBGs (and stacks of non-detections) and concluded that Road, Cambridge CB3 0HA 5 that the average UV-derived SFRs are reliable. However, Laboratoire d’Astrophysique de Marseille, Traverse du Siphon- the MIPS 24 µm band only detects a few percent of the B.P8 13376, Marseille Cedec 12, France 2 Siana et al. total infrared luminosity and the bolometric corrections required to convert from the mid-IR flux to L are large TABLE 1 IR Cosmic Eye Photometry and highly uncertain, as the IR SEDs of LBGs have not been measured. Therefore, in addition to determining Band Flux Density Errora the validity of UV-derived SFRs, it would be useful to [µm] [µJy] [µJy] determine if the L -to-L conversions measured lo- MIR IR 16 90 18 cally are valid in LBGs. 24 281b 65b A few high redshift LBGs have been found that are 70 4100 1300 gravitationally lensed by foreground clusters or individ- ual massive galaxies. Their high magnifications (fac- a Errors are 1σ tors of 10-30) mean that their IR fluxes are above the b From Coppin et al. (2007) current far-IR confusion limits and, in addition, mid-IR We use a ΛCDM cosmology with Ωm =0.3, ΩΛ =0.7, spectroscopy can be performed. These lensed LBGs can −1 −1 therefore be studied in the IR to better determine their and H0 = 70 km s Mpc . All intrinsic luminosities star formation rates and test whether star formation and and star-formation rates are corrected assuming a lensing dust extinction diagnostics measured in local starbursts magnification, µ = 28 ± 3 (Dye et al. 2007). are valid in LBGs. 2. OBSERVATIONS The first detailed IR investigation of such a highly magnified LBG (MS1512-cB58) shows that the PAH Spitzer IRAC and MIPS 24 µm obsevations were taken strengths and the shape of the IR SED are similar to as part of Director’s Discretionary Time in 2006 Novem- starbursts of comparable luminosity in the local universe ber/December and are detailed in Coppin et al. (2007). (Siana et al. 2008). However, the IR luminosity is signif- Additional Spitzer IRS and MIPS observations were icantly lower than expected given the large dust extinc- granted under Program ID 40817. IRS Short-Low first tion implied by cB58’s red UV spectral slope, suggest- order (7.4–14.5 µm) and Long-Low first order (19.5-38.0 ing that the assumed dust extinction law (Calzetti et al. µm) observations were obtained 05 December 2007. The 2000) may not be valid for this galaxy. If this were true of spectra were taken in mapping mode, placing the galaxy other LBGs, it would suggest that the claimed estimates at five different positions along the slits. The Short-Low of their contribution to the star formation rate density in (Long-Low) exposure times were 60 (120) seconds, with the early universe and, consequently, the time-integrated 50 (240) total exposures for a total of 3 (28.8) ks in- stellar mass density are too high. Of course, this is only tegration. The IRS data reduction was performed as one galaxy and there is quite a large dispersion measured specified in Teplitz et al. (2007). First, we remove la- in the UV-IR properties of local starbursts, so IR studies tent charge by fitting the slope of the increase in back- of more LBGs are required. Furthermore, cB58 is not a ground with time, and subtracting this background row typical LBG in that it appears to be far younger than by row. Second, “rogue” pixels were masked using the most LBGs (tage < 30 Myr), has a very red UV spectral IRSCLEAN program provided by the SSC. Finally, the slope, and displays stronger than average interstellar ab- observations at other map positions were used to deter- sorption lines. mine the sky, which was then subtracted. The individ- Several other highly magnified LBGs have re- ual frames were co-added to produce 2D spectra at each cently been found (Allam et al. 2007; Smail et al. 2007; map position. One-dimensional spectra were optimally Belokurov et al. 2007; Lin et al.