Extended ir emission from lirgs
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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

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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)

    • 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 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.


  • 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)

Follow the green curve…

No Extended Emission


Extended Emission Fraction Type (2)

Constant Extended Emission

Follow the green curve…


PAHs

[ArII]

Extended Emission Fraction Type (3)

Wavelength Depended

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

  • 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)

  • “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)

  • “Ionized” PAHs such as the 6.2μm and 7.7μm are not…


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

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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