infrared properties of interacting galaxies: from spirals to (u)lirgs

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Infrared properties of Interacting Galaxies: from Spirals to (U)LIRGs Vassilis Charmandaris Univ. of Crete, Greece IAG team: P. Appleton, C. Struck, B. Smith Paper: Smith et al. 2007, AJ, 133, 791 (U)LIRG team: L. Armus & V. Desai (Caltech), T. Diaz-Santos (Crete), H. Spoon (Cornell) Papers: Armus et al. 2007, ApJ 656, 148 ; Desai et al. 2007, ApJ 669, 810 ; Diaz-Santos et al. 2010 ApJ in press (@arXiv:1009.0038)

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Infrared properties of Interacting Galaxies: from Spirals to (U)LIRGs. Vassilis Charmandaris Univ. of Crete, Greece. IAG team: P. Appleton, C. Struck, B. Smith Paper: Smith et al. 2007, AJ, 133, 791 (U)LIRG team: L. Armus & V. Desai (Caltech), T. Diaz-Santos (Crete), H. Spoon (Cornell) - PowerPoint PPT Presentation

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Page 1: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

Infrared properties of Interacting Galaxies: from Spirals to (U)LIRGs

Infrared properties of Interacting Galaxies: from Spirals to (U)LIRGs

Vassilis CharmandarisUniv. of Crete, Greece

IAG team: P. Appleton, C. Struck, B. SmithPaper: Smith et al. 2007, AJ, 133, 791

(U)LIRG team: L. Armus & V. Desai (Caltech), T. Diaz-Santos (Crete), H. Spoon (Cornell)Papers: Armus et al. 2007, ApJ 656, 148 ; Desai et al. 2007, ApJ 669, 810 ;

Diaz-Santos et al. 2010 ApJ in press (@arXiv:1009.0038)

Page 2: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

PMO - Oct 2010VC

Outline of the talkOutline of the talkOutline of the talkOutline of the talk

Why perform extragalactic studies at infrared wavelengths

Why study Interacting galaxies and major results we have learned?

Where is most of the energy produced in Interacting galaxies

Discuss some IR diagnostics for the presence of an active nucleus

Global mid-IR properties (U)LIRGs based on Spitzer/IRS samples.

Page 3: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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History of Star-Formation in the UniverseHistory of Star-Formation in the UniverseHistory of Star-Formation in the UniverseHistory of Star-Formation in the Universe

Optical surveys indicate that the mean SFR in the Universe was much greater at z > 1 (e.g. Madau et al. 1996)

COBE revealed a cosmic far-IR background with energy > the integrated UV/optical light dust extinction is important in the early Universe!

IR/sub-mm surveys indicate even greater rates of star formation than seen in optical.

To accurately determine the “SFR”

requires both optical and far-IR/sub-mm surveys.

Blain et al. 2002

Photospheric light from stars

Photospheric lightreprocessed by dustMicrowave

Background

Page 4: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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Extragalactic IR from 1971 until today Extragalactic IR from 1971 until today Extragalactic IR from 1971 until today Extragalactic IR from 1971 until today

Number of extragalactic sources detected at ~10μm : 1971 13 (Neugebauer et al. ARA&A )1978 ~100 (Rieke & Lebofsky ARA&A)1988 IRAS PSC ~250,000 entries (~75,000 CGQ)1995: ISO is launched first mid-IR spectroscopy2003: Spitzer is launched. New era of IR begins2009: Herschel is launched! Far-IR possibilities

Page 5: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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10/00

IRS

MIPS

IRAC

Spitzer Instruments – BALL Oct. 2000Spitzer Instruments – BALL Oct. 2000

Price ~$30 millionPrice ~$30 million

Page 6: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

PMO - Oct 2010VC

Spitzer Cape Canaveral Launch Aug. 25, 2003Spitzer Cape Canaveral Launch Aug. 25, 2003Spitzer Cape Canaveral Launch Aug. 25, 2003Spitzer Cape Canaveral Launch Aug. 25, 2003

Page 7: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

PMO - Oct 2010VC

The Advantages of SpaceThe Advantages of Space......

The Advantages of Space:

100% Transmission and aOne-Million Fold Decreasein Sky Brightness.

The outer Space is cold!

Page 8: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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Why study Interacting GalaxiesWhy study Interacting GalaxiesWhy study Interacting GalaxiesWhy study Interacting Galaxies

Most galaxies are not isolated (Baade 1920)

Interactions determine the morphology and evolution of galaxies.

Our own Galaxy is interacting with the LMC and SMC

The galaxy merging rate increases with redshift ~(1+z)m, m>2 (Carlberg et al.

1990, Lavery et al ’96, ‘04)

=> Cosmological implications (a must in order to attract attention and funding!)

Massive starbursts are found in regions with high dust content

they are often hidden in the optical -> IRAS (Soifer et al. 1984, Houck et al. 1984,

Bushouse 87)

most of the energy is emitted in the infrared wavelengths

Nearly all Luminous IR galaxies (LIGs) are mergers (ie. Sanders 1988)

The optical/near-IR morphology is misleading (Bushouse & Werner 1990, Mirabel

et al. 1998)

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Bushouse 1987

Page 10: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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HST/WFPC2

Vigroux et al 1996

NGC 4038/39: The Antennae

Whitmore & Schweizer, AJ,1995

Page 11: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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NGC4038/39 – Mid-IR spectroscopyNGC4038/39 – Mid-IR spectroscopyNGC4038/39 – Mid-IR spectroscopyNGC4038/39 – Mid-IR spectroscopy

Mirabel et al., A&A, 1998

L(IR)~5x1010 Lsun

Page 12: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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Dust and Star FormationDust and Star FormationDust and Star FormationDust and Star Formation

Dust grains act as catalyst for the formation of molecular gas

Dust grains are responsible for the heating of the gas

A far-UV photon hits a dust grain and ejects an electron The ejected photoelectron heats the gas (very inefficiently ~0.1 - 1 %)

50% of gas heating is due to grains of sizes < 15 Å

Subsequently the gas cools via far-IR emission lines ( [OI] 63 um , [CII]

158 um) Emission from Polycyclic Aromatic Hydrocarbons (PAHs), dominate

the mid-IR (5-20 um) flux in normal galaxies and quiescent star forming regions

One can use mid- / far-IR prescriptions to estimate star formation rates (Far-IR: Kennicutt 1998, Mid-IR/ISO: Rousell et al. 2002, Forster-Schreiber et al 2004, Mid-IR/Spitzer: Calzetti et al. 2005, Wu et al. 2005, Relano et al. 2006)

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A day in the life of a dust grainA day in the life of a dust grainA day in the life of a dust grainA day in the life of a dust grain

τabs: time between two photon hits a: grain size

Draine 2002

Page 14: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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PAH Normal ModesPAH Normal Modes• 10-20% of the total IR luminosity of a galaxy• Tens - hundreds of C atoms• Bending, stretching modes 3.3,6.2,7.7,8.6,11.2,12.7 m• PAH ratios ionized or neutral, sizes, radiation field, etc.

Leger & Puget (1984)Sellgren (1984)Desert, et al. (1990)Draine & Li, (2001)Peeters, et al. (2004)

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Mid-IR Emission as star formation tracer (ISO)Mid-IR Emission as star formation tracer (ISO)Mid-IR Emission as star formation tracer (ISO)Mid-IR Emission as star formation tracer (ISO)

Forster-Schreiber et al., A&A, 2004

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Mid-IR Emission as star formation tracer (Spitzer)Mid-IR Emission as star formation tracer (Spitzer)Mid-IR Emission as star formation tracer (Spitzer)Mid-IR Emission as star formation tracer (Spitzer)

Calzetti et al., ApJ, 2007

LP = (6.2μm + 11.2μm) PAHFarrah et al. 2007

Page 17: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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Energy Balance in GalaxiesEnergy Balance in GalaxiesEnergy Balance in GalaxiesEnergy Balance in Galaxies

Helou et al., ApJ, 2001

Page 18: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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The SED of LIGsThe SED of LIGsThe SED of LIGsThe SED of LIGs

In the Mid-IR:

• we are less affected by absorption than in optical Av = 70*A (15μm)• better spatial resolution than Far-IR

BUT…

•Only a fraction of the bolometric luminosity is emitted in the mid-IR.

Can we still say something about

the global energy production

using the mid-IR?

Yes! or maybe...Sanders & Mirabel, ARA&A, 1996

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SED Decomposition of a GalaxySED Decomposition of a GalaxySED Decomposition of a GalaxySED Decomposition of a Galaxy

Credit: F. Galliano

Page 20: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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Toomre’s SequenceToomre’s SequenceToomre’s SequenceToomre’s Sequence

Page 21: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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Mid-IR Far-IR Correlation in Mid-IR Far-IR Correlation in Toomre’s SequenceToomre’s Sequence

Mid-IR Far-IR Correlation in Mid-IR Far-IR Correlation in Toomre’s SequenceToomre’s Sequence

ISO: 15μm / 7μm

IRAS 60μm / IRAS 100μm

IRAS 25μm / IRAS 12μm

Mid-IR imaging does reveal the intensity/age of a starburst

Charmandaris et al., 2001

Page 22: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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Interacting Galaxies with SpitzerInteracting Galaxies with SpitzerInteracting Galaxies with SpitzerInteracting Galaxies with Spitzer

A Sample of 35 binary interacting systems has been imaged with Spitzer (IRAC/MIPS : GO-1 Struck et al.)

Systems are nearby (<150Mpc), disturbed in the optical, with extended tidal features (>3arcmin), well separated, with 8.9<log[L(IR)]<11.2

Their mid-IR properties were analyzed and compared to a control sample of normal galaxies (Spirals, Es, Irr) (Smith et al. 2007)

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Interacting Galaxies with Spitzer (2)Interacting Galaxies with Spitzer (2)Interacting Galaxies with Spitzer (2)Interacting Galaxies with Spitzer (2)

Page 24: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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Interacting Galaxies: mid-IR LuminositiesInteracting Galaxies: mid-IR LuminositiesInteracting Galaxies: mid-IR LuminositiesInteracting Galaxies: mid-IR Luminosities

Smith, et, al 2007

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Interacting Galaxies: mid-IR LuminositiesInteracting Galaxies: mid-IR LuminositiesInteracting Galaxies: mid-IR LuminositiesInteracting Galaxies: mid-IR Luminosities

7% of Arp disks

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Interacting Galaxies: mid-IR LuminositiesInteracting Galaxies: mid-IR LuminositiesInteracting Galaxies: mid-IR LuminositiesInteracting Galaxies: mid-IR Luminosities

<10% of Arp disks

Page 27: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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Interacting Galaxies: mid-IR LuminositiesInteracting Galaxies: mid-IR LuminositiesInteracting Galaxies: mid-IR LuminositiesInteracting Galaxies: mid-IR Luminosities

<10% of Arp disks

Page 28: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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Star Formation Rate & Star Formation Rate & mid-IR nuclear concentrationmid-IR nuclear concentration

Star Formation Rate & Star Formation Rate & mid-IR nuclear concentrationmid-IR nuclear concentration

Smith, et, al 2007

The SFR for isolated spirals (SINGS) and well separated interacting Arp systems is similar ~ few Msun/yr. However, in the Arp systems the mid-IR flux is 2-4 times more centrally concentrated.

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Moving to higher Luminosities: GOALSMoving to higher Luminosities: GOALS 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) Additional Spitzer data with IRAC/MIPS, as well as HST, GALEX, VLA, CO

Page 30: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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(U)LIRG Basics (U)LIRG Basics

Properties LIR 1012 L ; Lbol ~ LIR ; Lopt < 0.1 LIR ; LIRGs 1012 L LIR 1011 L ; 90 – 95% are interacting, or in merging systems (50% in LIRGs) very strong OIR emission lines (H+, [OI,] [OIII], [NII], [SII], [FeII], etc.) NIR CO absorption bands from young stars large, compact reservoirs of cold molecular gas ( > 109 - 1010 M ) in their nuclei

(R 1 kpc). drive “superwinds” of hot, enriched gas into the IGM relatively rare in the local Universe - only ~3% of galaxies in IRAS BGS are

ULIRGs, but much more important at high-z

Questions What are the dominant power sources (50-500 Myr -1 SB or AGN ?) Do we see any mid-IR spectral “evolution” indicative of a change in SF properties

with redshift or luminosity (e.g. PAH strength, H2 fraction, etc.) ?

Samples IRS/GTO: 110 ULIRGs out to z=0.9 GOALS Legacy program: 202 systems (181 LIRGs)

Page 31: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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ULIRGs dominate the high luminosityULIRGs dominate the high luminositygalaxy population in the local Universegalaxy population in the local UniverseULIRGs dominate the high luminosityULIRGs dominate the high luminosity

galaxy population in the local Universegalaxy population in the local Universe

Sanders & Mirabel 1996

LIRGs and ULIRGs are responsible for ~3% of of the L(IR) at z~0 but for >75% at z>~1 (Elbaz et al. 2003)

Page 32: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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ULIRGs (ULIRGs (LLIRIR>10>101212) are Interacting Systems) are Interacting Systems ULIRGs (ULIRGs (LLIRIR>10>101212) are Interacting Systems) are Interacting Systems

Sanders et al. 1988Surace & Sanders 2001

Page 33: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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The first 18

low-resolution

IRS spectra

of ULIRGs

Diversity!

is the name of the game…

Page 34: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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ISO PHT-S spectra of BGS Sources ISO PHT-S spectra of BGS Sources IRS Spectra of BGS Sources IRS Spectra of BGS Sources

Charmandaris et al. 2005

Page 35: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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Spoon et al. 2004, A&A, 414,873

Arp220 Arp220

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Extended Emission: The MethodExtended Emission: 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)

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Types of MIR spatial profilesTypes of MIR spatial profiles

Three spatial profiles are identified: Constant: no variation as a function of λ (~50% of sample),

PAH/line extended: 20-70% of PAH flux is extended (~17% of sample),

Si “extended”: Si at 9.7μm appears extended (~24% of sample) -> suggests that integrated spectrum underestimates nuclear extinction.

Diaz-Santos et al. 2010

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Extended Emission and L(IR)Extended Emission and L(IR)

The median fraction of extended emission decreases when L(IR) increases.

Similarly for the 13μm continuum emission

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Extended Emission and Interaction stageExtended Emission and Interaction stage

Use Merger optical/near-IR classification from Petric et al. 2010 (submitted) 0: non interacting -> 5: mergers More advanced mergers are more luminous and also more compact in their mid-IR continuum

(Similar to what has been shown in other wavelengths)

Diaz-Santos et al. 2010

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Extended Emission and AGNExtended Emission and AGN

Use mid-IR AGN classification from Petric et al. 2010 (ApJ submitted) Using “Laurent diagram” (Laurent et al. 2000) AGN dominated sources also more compact in their mid-IR continuum

(Note that we refer to mid-IR dominant AGN)

Diaz-Santos et al. 2010

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Extended Emission and FIR ColorsExtended Emission and FIR Colors

Use IRAS colors as probe of the global ISM “temperature”

Sources which are more compact in their mid-IR continuum have warmer far-IR colors

To be tested with Herschel (Key Project: Hercules ; PI P. van der Werf)

Diaz-Santos et al. 2010

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UV/mid-IR comparison of two LIRGsUV/mid-IR comparison of two LIRGsUV/mid-IR comparison of two LIRGsUV/mid-IR comparison of two LIRGs

UV data: Goldader et al. 2002Charmandaris, et, al 2004

The spatial resolution of ground & Spitzer/MIPS24 surveys of LIRGs at z~2 will result in blending of the emission from the unresolved interacting components leading to a

systematic underestimation of their dust content.

7μm/UV ~ 800:10:35 7μm/UV ~ 330:160:190

Images: HST/STIS UV - Contours: ISO/CAM 7um

To be explored better with GOALS:

Preliminary GALEX/MIPS results (Howell et al. 2010) indicate that:

Among GOALs interacting systems 32% have one galaxy dominate the UV emission and the other the 24μm emission Based on number counts 15-30% of z~2 systems are unresolved as VV114

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Averaged ULIRG SEDsAveraged ULIRG SEDsAveraged ULIRG SEDsAveraged ULIRG SEDs

Desai et al. 2007

Page 44: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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PAH EQW vs. LIRPAH EQW vs. LIRPAH EQW vs. LIRPAH EQW vs. LIR

Armus et al. 2007

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Desai et al. 2007

Local ULIRGs & high-z ULIRGs/sub-mmLocal ULIRGs & high-z ULIRGs/sub-mm

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Probing the dominant energy source of galaxiesProbing the dominant energy source of galaxies

Many methods developed to quantify the fraction of star formation or emission from an accretion disk (AGN) in galaxies.

(Mushotzsky @astro-ph/0405144)

The presence of an AGN can be detected most ambiguously via:• Hard X-rays (>10keV)• Multi frequency radio observations (thermal/synchrotron fraction)

Difficulties:• Very few hard X-ray photons• Problems of self absorption in the interpretation of radio

For practical reasons most work has been performed via:• Optical spectroscopy (i.e. Kim et al. 1998, Hao et al. 2005)• Near-IR spectroscopy (i.e. Veilleux et al. 1999)

Advantages of IR:• Most galaxies emit a larger fraction of their energy in the IR.• Less affected by extinction than optical/near-IR

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IR diagnostics of AGNIR diagnostics of AGNIR diagnostics of AGNIR diagnostics of AGN

IRAS Era:IRAS Era:using the differences in IRAS colors - warm/cold sources (i.e. de Grijp 1985)Difficulty: Broadband colors only.

ISO Era:ISO Era:

• Detect High Ionization lines (Genzel et al 1998, Sturm et al. 2002)• [NeV] at 14.3μm / 23.2μm (Ep~97eV)• [OIV] at 25.9μm (Ep~55eV)Difficulty: the lines are faint

• Detect changes in continuum / broad features• Presence of 7.7μm PAH (Lutz et al. 1999)• Relative strength of 7.7μm PAH with respect to the 5.5μm

and 15μm continuum (Laurent et al. 2000)Difficulties:

- The 7.7PAH is affected by the 9.7μm silicate feature - The mid-IR continuum was not well defined with

ISO PHOT-S (~11.8μm) & ISOCAM/CVF (~16μm)

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IR diagnostics of AGN (2)IR diagnostics of AGN (2)IR diagnostics of AGN (2)IR diagnostics of AGN (2)Spitzer Era:Spitzer Era: On-going efforts

Large sample of galaxies, both in the local universe and at high-z

1. Detect ISO High Ionization lines (Armus et al 2007, Farrah et al. 2007, Veilleux et al. 2009)• [NeV] at 14.3μm / 23.2μm (Ep~97eV)• [OIV] at 25.9μm (Ep~55eV)

2. Extend to other Ionization lines (Dale et al. 2009 & Hao et al. 2009)• [SiII] at 34.8μm & [S III] at 33.8μm • [FeII] at 26.0μm

• Detect changes in continuum / broad features1. Presence of 6.2μm PAH (Desai et al. 2007)2. Measurements of 9.7μm Si extinction (Spoon et al. 2007)

1. Combine with L,M, band slope• PAH at 3.3μm (Imanishi et al. 2006)• Slope at 2-4μm (Nardini et al. 2008)

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MIR Diagnostic DiagramMIR Diagnostic Diagram

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• Rising slope of thermal emission from the AGN torus from 2-5μm due to grains radiating in near-equilibrium

• Excess over starlight at ~4-5μm

Alonso-Herrero et al. 2003

f(5μm)~10Jyz~0.004

D~14Mpc

f(5μm)~7mJyz~0.9

D~5.8Gpc

FSC15307+3252

AGN thermal emission in mid-IR continuum AGN thermal emission in mid-IR continuum

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Variation in IRS low resolution spectraVariation in IRS low resolution spectraVariation in IRS low resolution spectraVariation in IRS low resolution spectra

UGC 5101[Armus et al. 2004]

NGC 4151[Weedman et al. 2005]

NGC 7714[Brandl et al. 2004]

6.2 PAH

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M 17[ISO-SWS/1000.]

NGC 7023[Werner et al. 2004]

3C 273[Hao et al. 2005]

Selection of the three extreme spectra Selection of the three extreme spectra

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Modified “Laurent” Diagram

Starburst/HII-like

PAH/PDR-like

AGN-like

75%

75%

75%

The mid-IR [The mid-IR [AGNAGN//SBSB//PDRPDR] parameter space] parameter space

Page 54: Infrared properties of  Interacting Galaxies:  from Spirals to (U)LIRGs

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Populating the [Populating the [AGNAGN//SBSB//PDRPDR] diagram] diagram

PAH/PDR-like

AGN-like

Starburst/HII-like

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Infrared Continuum Fitting

Another approach to quantitatively access the contribution of the various components in the observed IR emission from a source is to fit the IR spectrum using theoretical models.

Full radiative transfer modeling of the source (ie. Siebenmorgen & Grugel 1992,1993; Nenkova & Elitzur 2001,2002; Dopita et al. 2005) is challenging since in addition to dust it requires major assumptions on the geometry of the source. Typically applicable to detailed studies of individual sources.

For studies of larger samples, semi-empirical methods using linear composition of known IR templates with different extinctions have proven very useful (ie. Tran et al. 2001)

Another fitting approach relying on the Spitzer/IRS 5-38μm spectra, using constrains from near-/far-IR observations has been developed (Marshall et al. 2007)…

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The case of NGC7714: a starburstThe case of NGC7714: a starburst

T*~3100 K Tcold~30 KTwarm~85 Kτwarm~0.6

Ldust~4.5x1010 L Lcold/Ldust~0.67Lwarm/Ldust~0.33LPAH/Ldust~0.05

Mcold~5.7x106 M

Mwarm~ 8.3x103 M

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The case of PG0804-761: a QSO/AGNThe case of PG0804-761: a QSO/AGN

T*~4500 K Tcold~43 KTwarm~215 KThot~817 K

τwarm~0.7τhot~0.7

Ldust~8.4x1011 L Lcold/Ldust~0.07Lwarm/Ldust~0.61Lhot/Ldust~0.48

Mcold~1.4x106 M

Mwarm~ 3.4x103 M

Mhot~ 30 M

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The case of Mrk1014: a ULIRGThe case of Mrk1014: a ULIRG

T*~3300 K Tcold~38 KTwarm~121 KThot~454 K

τwarm~1.7τice~0.3

Ldust~4.2x1012 L

Lcold/Ldust~0.57Lwarm/Ldust~0.32Lhot/Ldust~0.1

Mcold~1.5x108 M

Mwarm~ 2.6x105 M

Mhot~ 83 M

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Conclusions/PerspectivesConclusions/PerspectivesConclusions/PerspectivesConclusions/Perspectives

Spatially resolved mid-IR imaging of interacting systems in now easily possible with Spitzer.

Directly probes deeply and moderately enshrouded star formation with minimal need (if at all) correction for extinction.

Wealth of data available in Spitzer Comparison and cross calibration with other star formation tacers UV (Galex) and Hα (where available) will provide unique tools in probing the properties and distribution of the population.

There are a wide range of MIR spectra shapes among ULIRGs due to PAH emission, silicate and H2O / hydrocarbon absorptions, and hot dust.

The combination of fine structure line ratios, PAH strengths, and continuum fitting (hot dust) provides an accurate assessment of the AGN/SB fraction.

The high-luminosity ULIRGs are much more AGN-like in the MIR.

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谢谢谢谢

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