MID-INFRARED SPECTROSCOPY OF TWO LENSED STAR-FORMING GALAXIES

We present low-resolution, rest-frame ∼5–12 μm Spitzer/IRS spectra of two lensed z ∼ 2 UV-bright star-forming galaxies, SDSS J120602.09+514229.5 and SDSS J090122.37+181432.3. Using the magnification boost from lensing, we are able to study the physical properties of these objects in greater detail than is possible for unlensed systems. In both targets, we detect strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2, 7.7, and 11.3 μm, indicating the presence of vigorous star formation. For J1206, we find a steeply rising continuum and significant [S iv] emission, suggesting that a moderately hard radiation field is powering continuum emission from small dust grains. The strength of the [S iv] emission also implies a sub-solar metallicity of ∼0.5 Z☉, confirming published rest-frame optical measurements. In J0901, the PAH lines have large rest-frame equivalent widths (>1 μm) and the continuum rises slowly with wavelength, suggesting that any active galactic nucleus (AGN) contribution to LIR is insignificant, in contrast to the implications of optical emission-line diagnostics. Using [O iii] line flux as a proxy for AGN strength, we estimate that the AGN in J0901 provides only a small fraction of its mid-infrared continuum flux. By combining the detection of [Ar ii] with an upper limit on [Ar iii] emission, we infer a metallicity of ≳1.3 Z☉. This work highlights the importance of combining rest-frame optical and mid-IR spectroscopy in order to understand the detailed properties of star-forming galaxies at high redshift.

[1]  M. Sauvage,et al.  THE SPITZER VIEW OF LOW-METALLICITY STAR FORMATION. III. FINE-STRUCTURE LINES, AROMATIC FEATURES, AND MOLECULES , 2010, 1002.0991.

[2]  C. Steidel,et al.  DUST OBSCURATION AND METALLICITY AT HIGH REDSHIFT: NEW INFERENCES FROM UV, Hα, AND 8 μm OBSERVATIONS OF z ∼ 2 STAR-FORMING GALAXIES , 2010, 1002.0837.

[3]  J. Jakoboski,et al.  INFRARED SPECTRAL ENERGY DISTRIBUTIONS OF SEYFERT GALAXIES: SPITZER SPACE TELESCOPE OBSERVATIONS OF THE 12 μm SAMPLE OF ACTIVE GALAXIES , 2010, 1001.4974.

[4]  M. Elitzur,et al.  INFRARED DIAGNOSTICS FOR THE EXTENDED 12 μm SAMPLE OF SEYFERTS , 2009, 0912.3545.

[5]  J. Frieman,et al.  THE SLOAN BRIGHT ARCS SURVEY: FOUR STRONGLY LENSED GALAXIES WITH REDSHIFT > 2 , 2009, 0910.4600.

[6]  J. Bernard-Salas,et al.  PROBING THE EXCITATION OF EXTREME STARBURSTS: HIGH-RESOLUTION MID-INFRARED SPECTROSCOPY OF BLUE COMPACT DWARFS , 2009, 0908.4084.

[7]  J. Bernard-Salas,et al.  A SPITZER HIGH-RESOLUTION MID-INFRARED SPECTRAL ATLAS OF STARBURST GALAXIES , 2009, 0908.2812.

[8]  D. Tucker,et al.  REST-FRAME OPTICAL SPECTRA OF THREE STRONGLY LENSED GALAXIES AT z ∼ 2 , 2009, 0906.2197.

[9]  S. Veilleux,et al.  SPITZER QUASAR AND ULIRG EVOLUTION STUDY (QUEST). IV. COMPARISON OF 1 Jy ULTRALUMINOUS INFRARED GALAXIES WITH PALOMAR-GREEN QUASARS , 2009, 0905.1577.

[10]  Durham,et al.  DETECTION OF FAR-INFRARED AND POLYCYCLIC AROMATIC HYDROCARBON EMISSION FROM THE COSMIC EYE: PROBING THE DUST AND STAR FORMATION OF LYMAN BREAK GALAXIES , 2009, 0904.1742.

[11]  I. Smail,et al.  MID-INFRARED SPECTROSCOPY OF SUBMILLIMETER GALAXIES: EXTENDED STAR FORMATION IN MASSIVE HIGH-REDSHIFT GALAXIES , 2009, 0903.4017.

[12]  D. Tucker,et al.  THE SLOAN BRIGHT ARCS SURVEY: SIX STRONGLY LENSED GALAXIES AT z = 0.4–1.4 , 2008, 0812.3934.

[13]  Jeffrey M. Kubo,et al.  DISCOVERY OF A VERY BRIGHT, STRONGLY LENSED z = 2 GALAXY IN THE SDSS DR5 , 2008, 0809.4475.

[14]  R. Chary,et al.  Spitzer Observations of the z = 2.73 Lensed Lyman Break Galaxy: MS 1512–cB58 , 2008, 0808.2465.

[15]  D. Elbaz,et al.  Mid-Infrared Spectral Diagnosis of Submillimeter Galaxies , 2007, 0711.1553.

[16]  S. Veilleux,et al.  The Oxygen Abundances of Luminous and Ultraluminous Infrared Galaxies , 2007, 0708.1766.

[17]  D. Schneider,et al.  A new survey for giant arcs , 2006, astro-ph/0610061.

[18]  Sergey E. Koposov,et al.  The Cosmic Horseshoe: Discovery of an Einstein Ring around a Giant Luminous Red Galaxy , 2007, 0706.2326.

[19]  B. Brandl,et al.  High-Resolution Mid-Infrared Spectroscopy of Ultraluminous Infrared Galaxies , 2007, 0706.0513.

[20]  Takao Nakagawa,et al.  A Spitzer IRS Low-Resolution Spectroscopic Search for Buried AGNs in Nearby Ultraluminous Infrared Galaxies: A Constraint on Geometry between Energy Sources and Dust , 2007, astro-ph/0702136.

[21]  R. Genzel,et al.  A Mid-Infrared Spectroscopic Study of Submillimeter Galaxies: Luminous Starbursts at High Redshift , 2007, astro-ph/0701816.

[22]  A. Pope,et al.  Measuring PAH Emission in Ultradeep Spitzer IRS Spectroscopy of High-Redshift IR-Luminous Galaxies , 2007, astro-ph/0701409.

[23]  J. Surace,et al.  Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z ~ 2. I. The Spectra , 2006, astro-ph/0612297.

[24]  J. Frieman,et al.  The 8 O’Clock Arc: A Serendipitous Discovery of a Strongly Lensed Lyman Break Galaxy in the SDSS DR4 Imaging Data , 2006, astro-ph/0611138.

[25]  B. Draine,et al.  Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era , 2006, astro-ph/0608003.

[26]  J. Bernard-Salas,et al.  The Mid-Infrared Properties of Starburst Galaxies from Spitzer-IRS Spectroscopy , 2006 .

[27]  A. M. Swinbank,et al.  A Very Bright, Highly Magnified Lyman Break Galaxy at z = 3.07 , 2006, astro-ph/0611486.

[28]  Max Pettini,et al.  A Spectroscopic Survey of Redshift 1.4 ≲ z ≲ 3.0 Galaxies in the GOODS-North Field: Survey Description, Catalogs, and Properties , 2006, astro-ph/0609296.

[29]  Dario Fadda,et al.  Star Formation and Extinction in Redshift z~2 Galaxies: Inferences from Spitzer MIPS Observations , 2006, astro-ph/0602596.

[30]  J. Moustakas,et al.  An Integrated Spectrophotometric Survey of Nearby Star-forming Galaxies , 2005, astro-ph/0511729.

[31]  J. Bernard-Salas,et al.  Mid-Infrared Properties of Low-Metallicity Blue Compact Dwarf Galaxies from the Spitzer Infrared Spectrograph , 2005, astro-ph/0510856.

[32]  E. Floc’h,et al.  Redshifts from Spitzer Spectra for Optically Faint Radio-selected Infrared Sources , 2005, astro-ph/0510621.

[33]  A. Sternberg,et al.  Mid-Infrared Spectroscopy of Two Luminous Submillimeter Galaxies at z ~ 2.8 , 2005, astro-ph/0504431.

[34]  Alexander G. Gray,et al.  EFFICIENT PHOTOMETRIC SELECTION OF QUASARS FROM THE SLOAN DIGITAL SKY SURVEY. II. ∼1, 000, 000 QUASARS FROM DATA RELEASE 6 , 2004, The Astrophysical Journal Supplement Series.

[35]  C. Steidel,et al.  X-Ray and Radio Emission from Ultraviolet-selected Star-forming Galaxies at Redshifts 1.5 ≲ z ≲ 3.0 in the GOODS-North Field , 2004, astro-ph/0401432.

[36]  Max Pettini,et al.  Optical Selection of Star-forming Galaxies at Redshifts 1 < z < 3 , 2004, astro-ph/0401445.

[37]  M. Pettini,et al.  [O III] / [N II] as an abundance indicator at high redshift , 2004, astro-ph/0401128.

[38]  J. Surace,et al.  The IRAS Revised Bright Galaxy Sample , 2003, astro-ph/0306263.

[39]  M. Giavalisco,et al.  Lyman Break Galaxies at Redshift z ~ 3: Survey Description and Full Data Set , 2003, astro-ph/0305378.

[40]  W. Vacca,et al.  A mid-infrared spectroscopic survey of starburst galaxies: Excitation and abundances , 2003, astro-ph/0301201.

[41]  Department of Physics,et al.  accepted for publication in the Astrophysical Journal Luminous Lyman Break Galaxies at z>5 and the Source of , 2003 .

[42]  E. Oliva,et al.  Mid-Infrared line diagnostics of active galaxies - A spectroscopic AGN survey with ISO-SWS , 2002, astro-ph/0207381.

[43]  A. Sternberg,et al.  Massive Star Formation and Evolution in Starburst Galaxies: Mid-infrared Spectroscopy with the ISO Short Wavelength Spectrometer , 2000, astro-ph/0003334.

[44]  Cambridge,et al.  ∼ 4 and the Evolution of the Uv Luminosity Density at High Redshift , 2022 .

[45]  H. Ford,et al.  Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant , 1998, astro-ph/9801080.

[46]  D. Kunze,et al.  What Powers Ultraluminous IRAS Galaxies? , 1997, astro-ph/9711255.

[47]  M. Dickinson,et al.  Spectroscopy of Lyman Break Galaxies in the Hubble Deep Field , 1996, astro-ph/9604140.

[48]  R. Carlberg,et al.  A Proto-Galaxy Candidate at Z = 2.7 , 1996, astro-ph/9602121.

[49]  R. Kennicutt,et al.  Spectrophotometric Properties of Merging Galaxies , 1995 .

[50]  P. Roche,et al.  An atlas of mid-infrared spectra of galaxy nuclei , 1991 .