The Nature of High EquivalentWidth Lymana Galaxies

1
The Nature of High Equivalent-Width Lyman-a
Galaxies

• Steven Finkelstein, James Rhoads, Sangeeta
  Malhotra (ASU), Norbert Pirzkal (STScI) Junxian
  Wang (USTC)

2
Outline

• Why do we study Lyman Alpha Emitters (LAEs) ?
• Intro to LALA survey
• MMT/Megacam data acquisition/reduction
• Results
• Discussion
• Future Work

3
Lyman Alpha Emitters - Background

• Lya gives an easy way to spot high-z galaxies
• Lya emission was proposed as a signpost of
  primitive galaxies in formation (Partridge
  Peebles 1967)
• Past optical and X-ray work show the Lya emission
  is not powered by AGN (Malhotra et al. 2003
  Dawson et al. 2004).
• Lya 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

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
  (142557 3532) and Cetus (020520 -0455).
• 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

5
What is causing the emission?

• EW 80 Å for a normal stellar population
• Malhotra and Rhoads (2002) found numerous LAEs
  with EWs gt 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

6
Dusty Scenario

• If the dust is primarily in cold neutral clouds
• Lya 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 Lya
  photons, than the transmitted EW is larger than
  the source EW

7
Dusty Scenario
8
Dusty Scenario
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 Lya 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.

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 3s detection 26.81
• r 3.50 hours 3s detection 26.00
• i 4.78 hours 3s detection 25.60
• z 5.33 hours 3s detection 24.78

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).

12
Lya Galaxy Selection

• We used narrow band images from LALA to find LAEs
  at z 4.5
• 5 filters ??6559,6611,6650,6695,6730
• FWHM 80Å
• Redshift coverage 4.37 lt z lt 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

13
Lya Galaxy Selection

• Lya galaxy detection criteria
• 5s significance detection in the narrowband
• Calculated using a SExtractor aperture flux with
  the associated flux error flux/error 5.
• 4s 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 2s significant flux in the Bw filter.

14
Results

• 98 galaxies met our detection criteria within the
  24 x 24 Megacam field of view
• 22 of those had 2s detections in two of the r,
  i or z bands.
• 14 of the 22 galaxies have Magellan IMACS
  spectra
• 7 of these have confirmed Lya lines putting the
  galaxies at z 4.5
• Other 7 do not show a strong Lya 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)

15
i Band Examples
Cetus 23 z 4.392
Cetus 86 z 4.463
16
Cetus Field

• i band

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 lt EW lt 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?

18
Stellar Population Models

• Bruzual and Charlot (2003) (BC03)
• Model Parameters
• Age 106 109 years
• Metallicity .02Solar Solar
• Lya 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 2109 years (Continuous SF)
• Calzetti Law (1994) dust extinction (0-2 mags at
  1200 Å)
• Applied only to the continuum light (i.e. not the
  Lya emission) in order to model the clumpy dust
  scenario.
• IGM Absorption via Madau (1995)

19
r-i Color Distribution
20
Color-Color Plot

• Model tracks run from Adust 0-2
• Dotted line connects zero dust end of models
• All models are .02solar metallicity

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-i1, with r-nb1-1.5
• Conclusion We see both scenarios, future work
  will determine the likelihood of each scenario in
  a given galaxy.

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

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

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

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

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.

27
LALA Publications

• First Results from the Large Area Lyman Alpha
  Survey Rhoads, J. E. et al. 2000
• Lyman Alpha Emitters at Redshift z5.7 Rhoads,
  J. E. and Malhotra, S. 2001
• Large Equivalent Width Lyman-alpha Line Emission
  at z4.5 Young Galaxies in a Young Universe
  Malhotra, S. Rhoads, J. E. 2002
• Spectroscopic Confirmation of Three Redshift
  z5.7 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 z6.535 Discovery and Spectroscopic
  Confirmation Rhoads, J. E. et al. 2004
• X-ray Nondetection of the Lyman alpha Emitters at
  z4.5 Wang, J. et al. 2004
• Spectroscopic Properties of the z4.5 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 z6.5 and z5.7 Evidence Against
  Reionization at z6 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