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Extended IR Emission from LIRGs. V. Charmandaris, T. Diaz-Santos, (Univ. of Crete) L. Armus & A. Petric (SSC/Caltech). Motivation. A number of the observational properties of SMGs such as: cold infrared colors energy production dominated by star formation ( > 100 Msun/yr)

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extended ir emission from lirgs
Extended IR Emission from LIRGs

V. Charmandaris, T. Diaz-Santos, (Univ. of Crete)

L. Armus & A. Petric (SSC/Caltech)

motivation
Motivation
  • A number of the observational properties of SMGs such as:
    • cold infrared colors
    • energy production dominated by star formation ( > 100 Msun/yr)
    • mid-IR spectral features (ie PAH strength)

… resemble those of local LIRGs rather than ULIRGs.

  • Kinematical evidence from ionized (Hα) and molecular (CO) gas are consistent the presence of extended (~5kpc) star forming disks
  • There is a “broad” connection between mid-IR emission and star formation rates (with some caveats…)
  • We wish to quantify the extended extranuclear emission in local LIRGs in the 5-15μm range to contribute additional evidence in the analogy between the physics of the ISM excitation in LIRGs and the conditions seen in SMGs at z>1.
the sample
The Sample
  • The Sample is drawn from the Great Observatory Allsky LIRG Survey (GOALS; Armus et al. 2009) of 202 systems (181 of which are LIRGs)
  • All systems are observed with all four Spitzer/IRS modules (5-37μm)
  • Apply an upper limit on distance (<85 Mpc) in order for the IRS slit size to correspond to a projected distance < 1.5kpc (best case is 0.26kpc)
  • A total of 93 galaxies are selected.
the method
Use the 2D images of all LIRGs and re-extract total 5-15μm spectrum using the SSC pipeline algorithms
  • Use a standard star (HR7341) as our unresolved point source (PSF)
  • Scale the spatial profile of the standard star along the slit at every wavelength and subtract it from the corresponding profile of each source.
  • Define as fraction of Extended Emission (EE):

Total LIRG flux (λ) - PSF (λ)

Fraction of EE (λ) =

Total LIRG flux (λ)

The Method
extended emission fraction type 1
Extended Emission Fraction Type (1)

Follow the green curve…

No Extended Emission

extended emission fraction type 2
Extended Emission Fraction Type (2)

Constant Extended Emission

Follow the green curve…

extended emission fraction type 3
PAHs

[ArII]

Extended Emission Fraction Type (3)

Wavelength Depended

Extended Emission

mean fraction of mid ir extended emission
Mean Fraction of mid-IR Extended Emission
  • For 90% of the sample, at least 20% of the mid-IR flux is extended
  • For more than 35% of the sample, the EE fraction is larger than 0.5 (= 50%)
fraction of mid ir extended emission
Fraction of mid-IR Extended Emission
  • LIRGs with EE fractions larger than 0.5 are detected up to 80 Mpc!
  • The EE fraction is not correlated with the IR luminosity =>

the most luminous LIRGS are not necessarily the most compact

Limits to identifying a source with intrinsic EE=0.75 due to distance

are pahs more extended than neii 1
Are PAHs more extended than [NeII] ? (1)
  • “Neutral” PAHs such as the 11.3μm are…

… possibly because they do not require a strong ionizing radiation field

are pahs more extended than neii 2
Are PAHs more extended than [NeII] ? (2)
  • “Ionized” PAHs such as the 6.2μm and 7.7μm are not…
agn dependence of mid ir extended emission
AGN dependence of mid-IR Extended Emission

We can calculate the mid-IR AGN fraction using the EW of the 6.2μm PAH (Petric et al. 2010)…

We find no obvious dependence of Extended Emission on mid-IR AGN strength

conclusions perspectives
Conclusions / Perspectives

Even though the spatial resultion of IRS is ~3.6arcsecs..

  • LIRGs display large fraction of extended mid-IR emission in both continuum and 5-15μm features
  • For at least 90% of the sample more than 20% of the mid-IR flux originates outside the nuclear unresolved region.
  • For at least 35% of the sample more than 50% of the mid-IR flux is extranuclear (possibly also their star formation?)
  • The extended emission varies as a function of wavelength.
  • The 11.3μm PAH displays the larger extended emission, consistent with it being a “neutral” PAH.
  • No clear correlation between extended emission and mid-IR AGN classification nor IR luminosity
  • To do: - Explore the correlations with merging stage

- Increase the range of IR luminosity to include some ULIRGs.

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