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The Nature of High Equivalent-Width Lyman- α Galaxies

The Nature of High Equivalent-Width Lyman- α Galaxies. Steven Finkelstein, James Rhoads, Sangeeta Malhotra (ASU), Norbert Pirzkal (STScI) & Junxian Wang (USTC). Outline. Why do we study Lyman Alpha Emitters (LAEs) ? Intro to LALA survey MMT/Megacam data acquisition/reduction Results

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The Nature of High Equivalent-Width Lyman- α Galaxies

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  1. The Nature of High Equivalent-Width Lyman-α Galaxies Steven Finkelstein, James Rhoads, Sangeeta Malhotra (ASU), Norbert Pirzkal (STScI) & Junxian Wang (USTC) MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  2. Outline • Why do we study Lyman Alpha Emitters (LAEs) ? • Intro to LALA survey • MMT/Megacam data acquisition/reduction • Results • Discussion • Future Work MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  3. Lyman Alpha Emitters - Background • Lyα gives an easy way to spot high-z galaxies • Lyα emission was proposed as a signpost of primitive galaxies in formation (Partridge & Peebles 1967) • Past optical and X-ray work show the Lyα emission is not powered by AGN (Malhotra et al. 2003; Dawson et al. 2004). • Lyα too narrow to be Type I (broad-lined) AGN • Chandra data show no X-ray detections of 49 LAEs, so Type II AGN ruled out MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  4. Large Area Lyman Alpha (LALA) Survey • LALA began in 1998 at KPNO (Rhoads et al. 2000) • Final area of 0.72 deg2 in two fields, Boötes (14:25:57 +35:32) and Cetus (02:05:20 -04:55). • This survey includes broad and narrow band imaging, as well as spectroscopic data on LAEs. • Spectroscopic success rate of up to 70% • This survey represents ~ 40 nights on the Kitt Peak 4m, 8 nights on Keck, 7 nights (so far) on the MMT, and ongoing programs at the Magellan Observatory. • 12+ published papers MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  5. What is causing the emission? • EW ~ 80 Å for a normal stellar population • Malhotra and Rhoads (2002) found numerous LAEs with EWs > 200 Å • Large EW could be produced via star formation if the stellar photospheres were hotter than normal • Could be true in: • Low metallicity galaxies • Galaxies with an extreme IMF • Both scenarios possible in primitive galaxies, which contain young stars and little dust • EW could be enhanced from the geometry of the ISM • Dusty clouds embedded in a tenuous inter-cloud medium MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  6. Dusty Scenario • If the dust is primarily in cold neutral clouds: • Lyα photons scatter of the clouds and spent most of their time in the inter-cloud medium • ICM hot, mainly ionized • Continuum photons penetrate deep into the clouds and suffer greater extinction (Neufeld 1991; Hansen & Oh 2006) • ISM of our Galaxy is known to be clumpy down to small scales • If the continuum is more absorbed than the Lyα photons, than the transmitted EW is larger than the source EW MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  7. To Observer Continuum Photons (Neufeld 1991) Dusty Scenario MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  8. To Observer Lyman α Photons (Neufeld 1991) Dusty Scenario MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  9. MMT Goals • The colors of these galaxies might distinguish the cause of the large EW • Blue colors would indicate young stars with hot photospheres • Red colors would indicate dust quenching of the continuum, enhancing the Lyα EW • Our goal was to obtain deep broadband imaging in the g’, r’, i’ and z’ bands in order to distinguish between the two scenarios for the cause of the large EW. • Observations were of the LALA Cetus Field • z = 4.5 • We used this data to study the continuum properties of individual LAEs at this redshift for the first time. MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  10. Observations • We observed with Megacam at the MMT on three whole nights from 11/3/05 – 11/5/05, and on four 1/4 nights from 1/1/06 – 1/4/06. • Total exposure times were: • g’: 4.33 hours 3σ detection = 26.81 • r’: 3.50 hours 3σ detection = 26.00 • i’: 4.78 hours 3σ detection = 25.60 • z’: 5.33 hours 3σ detection = 24.78 MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  11. Data Reduction • We developed a pipeline using the MSCRED (Valdes & Tody 1998; Valdes 1998) and MEGARED (McLeod) packages in IRAF (Tody 1986, 1993) , along with the image combining software SWarp (Bertin). • We gave SWarp an input weight map which flagged the MEGARED bad pixels and cosmic rays • Photometric zeropoints calculated in each band using standard stars from Landolt (1992). • Used transforms to SDSS bands from Fukugita et al. (1996). MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  12. Lyα Galaxy Selection • We used narrow band images from LALA to find LAEs at z ~ 4.5 • 5 filters: λλ6559,6611,6650,6695,6730 • FWHM = 80Å • Redshift coverage 4.37 < z < 4.57 • Also used broadband data from the NOAO Deep Wide-Field Survey (Jannuzi & Dey 1999). • Bw, R & I • Used SExtractor (Bertin & Arnouts 1996) to identify the objects and perform photometry • The Megacam data was registered and remapped onto the same grid as the narrow band data (0.26 “/pixel). • SExtractor used a nine pixel (2.32”) aperture MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  13. Lyα Galaxy Selection • Lyα galaxy detection criteria: • 5σ significance detection in the narrowband • Calculated using a SExtractor aperture flux with the associated flux error: flux/error ≤ 5. • 4σ significant excess of narrowband flux • Calculated via (narrow flux – broad flux)/sqrt(errorn2 + errorb2) ≥ 4. • Factor of ≥ 2 ratio between narrow and broad band flux densities. • No more than 2σ significant flux in the Bw filter. MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  14. Results • 98 galaxies met our detection criteria within the 24’ x 24’ Megacam field of view • 22 of those had ≥ 2σ detections in two of the r’, i’ or z’ bands. • 14 of the 22 galaxies have Magellan – IMACS spectra • 7 of these have confirmed Lyα lines putting the galaxies at z ~ 4.5 • Other 7 do not show a strong Lyα line, but 3-4 of these may with deeper spectra • 12 of the 22 galaxies detected in the H0 (6559 Å) filter, 6 in H8 (6650 Å) and 4 in H16 (6730 Å) MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  15. i’ Band Examples Cetus 23 z = 4.392 Cetus 86 z = 4.463 MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  16. Cetus Field i’ band MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  17. EW Distribution • All EW reported are rest-frame • These are photometric EW using the narrowband from KPNO and the broad band from the MMT • EW calculated as the ratio of the line flux to the continuum flux density • Normal stellar population: EW ~ 80 Å • Data range 106 < EW < 202 Å • Object with EW = 23 Å is likely interloper • Data shown is only the 22 high-quality galaxies, the other 76 we detected will tend to lie at higher EWs • Implies something is causing EW to be higher than “normal” • Massive stars or dust enhancement? MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  18. Stellar Population Models • Bruzual and Charlot (2003) (BC03) • Model Parameters: • Age: 106 – 109 years • Metallicity: .02*Solar – Solar • Lyα emission: Calculated from the number of ionizing photons produced from a given stellar population • SFR: Exponential Decay • Tau = 103 years (Instantaneous burst) • Tau = 107 years • Tau = 108 years • Tau = 2*109 years (Continuous SF) • Calzetti Law (1994) dust extinction (0-2 mags at 1200 Å) • Applied only to the continuum light (i.e. not the Lyα emission) in order to model the clumpy dust scenario. • IGM Absorption via Madau (1995) MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  19. r’-i’ Color Distribution MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  20. Color-Color Plot • Model tracks run from Adust = 0-2 • Dotted line connects zero dust end of models • All models are .02*solar metallicity MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  21. Color – Color Plot • Three groups identified: • Group of intrinsically blue objects • Located below dotted line • Young hot stars, homogeneous dust suppresses both line and continuum • Group of objects with EW enhanced via dust • Upper-right 4-6 objects • High EW + red colors • A few objects which are older with lower EW • Objects at r’-i’~1, with r’-nb~1-1.5 • Conclusion: We see both scenarios, future work will determine the likelihood of each scenario in a given galaxy. MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  22. Work In Progress • We plan to fit the 22 individual LAEs to BC03 models to determine: • Age • Mass • Metallicity • Star formation history • Dust content • Stacking analysis of 76 other galaxies MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  23. SWIRC • Three full nights, May 15-17, 2006 • Goal: Obtain NIR broadband photometry of LAEs in the LALA Boötes Field. • So far: 3 objects detected in J band, 1 in H band MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  24. Future Work • We plan to do a similar analysis with broadband data for the Boötes Field • Using broadband photometry from the KPNO 4m and the NIR data from SWIRC • Larger wavelength baseline, more accurately determine existence of dusty scenario • Continue work out to higher redshifts • z = 5.7 • z = 6.5 MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  25. The End • This work was supported by the ASU/NASA Space Grant and the ASU Dept. of Physics and Astronomy. • Abbreviated References • Bruzual, G. & Charlot, S. 2003, MNRAS, 344, 1000 • Dawson, S. et al. 2004, ApJ, 617, 707 • Hansen, M., & Oh, S. P. 2006, MNRAS, 367, 979 • Malhotra, S. et al. 2003, ApJ, 585, L25 • Malhotra, S. & Rhoads, J.E. 2002, ApJ, 565, L71 • Neufeld, D.A. 1991, ApJ, 370, L85 • Papovich, C., Dickinson, M. & Ferguson, H. C. 2001, ApJ, 559, 620 • Partridge, R.B. & Peebles, P.J.E. 1967, ApJ, 147, 868 • Rhoads, J.E. et al. 2000, ApJ, 545, L85 MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  26. Abstract • Lyman alpha emission lines from high redshift galaxies often show equivalent widths beyond those expected for normal stellar populations. Spectroscopy and x-ray photometry of these galaxies demonstrates that the Lyman alpha emission is predominantly powered by star formation rather than by accretion power. To explain the largest equivalent widths then requires either (a) unusually hot stellar populations, as might be expected from a top heavy initial mass function or from low stellar metallicity; or (b) differences in the radiative transfer of Lyman alpha and continuum photons, which could allow Lyman alpha photons to escape even while continuum light is suppressed by dust. • Broad band colors can be used to distinguish between these two scenarios, because hot photospheres will result in a blue continuum while dust absorption will redden the escaping continuum light. We have used Megacam on the MMT to obtain broad band colors of about 100 Lyman alpha galaxies. We will present our photometry and compare it with model colors expected under these two alternative scenarios. MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

  27. LALA Publications • First Results from the Large Area Lyman Alpha Survey; Rhoads, J. E. et al. 2000 • Lyman Alpha Emitters at Redshift z=5.7; Rhoads, J. E. and Malhotra, S. 2001 • Large Equivalent Width Lyman-alpha Line Emission at z=4.5: Young Galaxies in a Young Universe; Malhotra, S. & Rhoads, J. E. 2002 • Spectroscopic Confirmation of Three Redshift z~5.7 Lyman-Alpha Emitters from the Large-Area Lyman Alpha Survey; Rhoads, J. E. et al. 2003 • No X-Ray-bright Type II Quasars among the Lyman-Alpha Emitters; Malhotra, S. et al. 2003 • A Luminous Lyman-alpha Emitting Galaxy at Redshift z=6.535: Discovery and Spectroscopic Confirmation; Rhoads, J. E. et al. 2004 • X-ray Nondetection of the Lyman alpha Emitters at z~4.5; Wang, J. et al. 2004 • Spectroscopic Properties of the z=4.5 Lyman Alpha Emitters; Dawson, S. et al. 2004 • 172 ks Chandra Exposure of the LALA Boötes Field: X-ray Source Catalog; Wang, J. et al 2004 • Luminosity Functions of Lyman alpha Emitters at Redshift z=6.5 and z=5.7: Evidence Against Reionization at z=6; Malhotra, S. & Rhoads, J. E. 2004 • A Luminous Lyman alpha Emitting Galaxy at Redshift z = 6.535: Discovery and Spectroscopic Confirmation; Rhoads, J. E. et al. 2004 • The Volume Fraction of Ionized Intergalactic Gas at Redshift z = 6.5; Malhotra, S. & Rhoads, J. E. 2006 • Physical Properties of Lyman Alpha Emitting Galaxies at z ~ 4.5; Finkelstein, S. L. et al. in preparation MMT Symposium June 14th, 2006 Steven Finkelstein (ASU)

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