A close relationship at z2: SMGs and BzKs

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A close relationship at z2SMGs and BzKs

• Toshi Takagi
• (JSPS fellow, ISAS/JAXA)

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BzK-selection of galaxies

• A simple two color selection for culling
  galaxies at 1.4ltzlt2.5
• Classify galaxies as star-forming or passive
  systems
• The selection is independent of reddening
• SMGs may be an extreme subset of star-forming BzKs

BzK(z-K)-(B-z)gt-0.2
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Redshift distributions
BzK/SF
SMGs
Reddy et al. (2005)
Chapman et al. (2005)
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BzK colours of SMGs
Radio-detected SMGs from SHADES/SXDF
Almost all of K-faint radio-detected SMGs are BzKs
Takagi et al. in prep
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Direct optical ID of SMGs

• Since K-faint SMGs are BzKs, we could identify
  some of SMGs using BzK colours of associated
  galaxies.
• We adopt the same method as in the radio ID, but
  for NIR-selected galaxies i.e. we calculated the
  corrected Poisson probability (PNIR) of random
  association by using the number counts of BzKs
  and also BzK-selected EROs.
• We regard the association with PNIR lt0.05 as a
  robust identification

Surface density FLUX BzKs
7.1 arcmin-2 (S850 1mJy) BzK/SMGs
0.07 arcmin-2 (S850 gt 5mJy) 1 of BzKs are
SMGs
What is the difference between BzKs and SMGs?
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Kinematic properties at z2

• The size-velocity of BzKs are similar to local
  spiral
• BzKs Disk
• SMGs are compact and have large velocity
• SMGs Merger

SMGs
SMGs are merging BzKs?
Bouche et al. (2007)
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The SFR-M? relation at z1
????????????????????SFR??????????? ????????????
????????ULIRG?????????????? z1???z0???????????S
FR???????????????ULIRG??????????? ???SMG???z2???
??????
z0
z1
Elbaz et al. (2007)

• ULIRGs are strong outlier

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Morphology of LIRGs at z1
Elbaz et al. (2007)

• Only 1/3 are major merger
• About 1/2 are grand-design spirals

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The SFR-M? relation
z2????????????????ULIRG??? ???BzK?SMG???????????
BzK-selected galaxies

• At M?1011M?,
• SFR 200 M?/yr
• SFR 60 M?/yr
• SFR 9 M?/yr

ULIRGs
z2
z1
z0
Daddi et al. (2007)
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SFR enhancement (1)

• Local ULIRGs (Genzel et al. 2001)
• ltSFRgt 250 M?/yr
• ltM?gt 81010 M?
• SFR enhancement 30
• SMG sample (with BzK colour)
• ltSFRgt 600 M?/yr
• ltM?gt 11011M?
• SFR enhancement 3

SMGs
Local ULIRGs
Takagi et al. in prep
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SFR enhancement (2)
105 star-forming galaxies from SDSS
Enhancement in log (SFR/M?)
Projected separation
Li et al. (2008)

• Typical enhancement is 23 for rlt20 kpc
• Similar results are obtained at z1 using DEEP2
  (Lin et al. 2008)

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• SFR Enhancement

Direct merger
Gas
Star
DM
Retrograde merger
Di Matteo et al. (2007)
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SFR enhancement (3)
SPH numerical simulation
Di Matteo et al. (2007)

• Typical enhancement is lt5
• Strong bursts have shorter lifetime (lt200 Myr)
• Local ULIRGs are very rare, SMGs are popular?

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Number density comparison

• It seems that a typical interaction between BzKs
  with rlt20 kpc can generate SMGs.
• The fraction of merger would be gt 10 (at least
  higher than local). We could expect gt10 of SMGs
  in BzKs.
• But only 1 of BzKs are SMGs. Why?
• What makes SMGs special in kinematic sense?
  Multiple mergers?

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Why are SMGs rare?

• The high specific SFR of BzKs may be the result
  of galaxy interaction to some extent.
• In this case, there would be almost no net SFR
  enhancement from the SFR-M? relation in normal
  interactions.
• If so, the apparently moderate SFR enhancement of
  SMGs may indeed significant as galaxies at z2
• SMGs might be rare mergers, owing to some
  particular orbit parameters, an equal mass ratio,
  or multiple mergers.

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In the ALMA era...

• BzKs have an average 850µm flux of 1 mJy (Daddi
  et al. 2005). Thus, BzKs are only a factor of 5
  fainter than SMGs. However, the kinematical
  properties (e.g. size and velocity) are clearly
  different each other.
• We need to address why SMGs are so special and
  rare even though the enhancement of SFR is
  moderate. Are they experiencing special merger?
• It is not likely that strong dynamical
  disturbance is found only in bright (gt 5-mJy)
  submm sources, since less massive systems would
  also experience major merger. ALMA will be used
  to identify major mergers or dynamically hot
  galaxy population within submm-faint galaxies.
• To do this, ALAM (sub)mm spectroscopic survey of
  star-forming galaxies at z2 would be very
  important to study the physics of major mergers
  from their kinematical properties, and their role
  in galaxy evolution.

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The END