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Infrared (& Submm) Observa3ons of Brightest Cluster Galaxies (BCGs):
Star-‐forming BCGs as Galaxy Forma5on Laboratories(?)
Eiichi Egami Steward Observatory
University of Arizona
Note: BCGs discussed here live in the most X-‐ray-‐luminous (i.e., most massive) clusters of galaxies.
Collaborators
• A. Fiedler (Spitzer/MIPS) • J. Portouw (Spitzer/IRS) • C. Kuniyoshi, D. Rafferty, M. Wise (Chandra)
• A. Edge, C. O’Dea, S. Baum, A. Quillen
• G. Smith and the LoCuSS team
Brightest cluster galaxies (BCGs) are so dead and boring that they can even be standard candles. Is this really the whole story?
BCGs as Red Dead Galaxies
De Lucia & Blaizot (2007) Star forma3on in small galaxies
Mass assembly via dry mergers
And star-‐forming BCGs are located in strongly cooling cluster cores
Gas 0.6 0.6 1.9 3.0 24.8 29.1 30.9 Gyr cooling 3me
Classical cooling-‐flow clusters
Star-‐forming BCGs as Galaxy Forma3on Laboratories
• Classical cooling-‐flow picture (mass deposi3on rates > 200-‐300 M/yr) is no longer valid. X-‐ray observa3ons do not see this much of cooling gas (ICM must be somehow heated; e.g., AGN).
• However, cooling flows may s3ll exist with a much smaller strength (< 100 M/yr).
• If so, star-‐forming BCGs may allow us to study the process of galaxy forma3on (i.e., cooling gas accre3ng on to a seed mass) as opposed to galaxy transforma3on (galaxy-‐galaxy mergers).
Outline 1. Spitzer/MIPS 24um cluster snapshot survey – Iden3fy IR-‐luminous BCGs – (Iden3fy IR-‐luminous gravita3onally lensed
galaxies)
2. Spitzer/IRS spectroscopic follow-‐up – Examine mid-‐IR spectra (5-‐30um) of IR-‐luminous
BCGs
3. Next steps (Chandra, HST, Gemini, IRAM, PdB, Herschel)
4. Summary
1. Spitzer/MIPS 24um cluster snapshot survey
• Goal: Iden3fy IR-‐luminous BCGs (and strongly lensed galaxies)
• Targets: 103/165 LoCuSS clusters (G. Smith’s talk) – 0.15<z<0.3 – NHI < 7x1020 cm-‐2
– -‐70° < δ < 70°; further trimmed to δ < +30° (ALMA) – Lx > 2 x 1044 erg/s – No previous Spitzer observa3ons – 23 hrs in total
• Sensi3vity: ~100 uJy (5 σ) at 24 um
Spitzer Images of A2218
Background sources dominate the cluster core @24 um -> Cluster works as a transparent lens @24um
2.3'
Z=0.17
Op3cal (DSS) Near-‐IR (2MASS) MIPS 24um F24 = 15 mJy Log(LTIR)=11.6
Type 1: AGN
MMT op3cal spectrum
Type 3: Hot dust heated by evolved stars
Type 4: No 24um Emission
op3cal Near-‐IR MIPS 24um
F24 = 115 uJy Log(LTIR)=9.5
F24 < 100 uJy Log(LTIR) < 9
LTIR distribu3on
No 24um detec3on (35%)
Hot dust heated by evolved stars (42%)
Star forma3on/AGN (23%)
Rieke et al. (2009) F24 -‐> LTIR conversion
2. Spitzer IRS spectroscopic follow-‐up
• Goal: Examine the mid-‐IR spectra (5-‐30 um) of IR-‐bright BCGs with IRS (e.g., SF, AGN?)
• Sample: – IR-‐bright BCGs selected from the MIPS 24um snapshot survey as well as from other Spitzer GTO/GO programs.
– Some number of z<0.15 BCGs included enable us to probe lower-‐luminosity (LTIR<1010 L) BCGs.
– Targets: ~25 BCGs (40 hrs in total)
H2 with SF -‐ Z3146 BCG
1010 M warm H2!! Likely shock-‐heated
Egami et al. (2006)
ROSAT (Edge et al. 1994)
More Strong H2 with SF!
Star forma5on and strong H2 emission triggered by cluster cooling flows?
Egami et al. (2010, in prep)
More detec3ons of H2 monsters (IR-‐luminous BCGs)! Even without PAHs (i.e., SF)!!
No PAHs No PAHs
Egami et al. in prep (2010)
Similarly strong H2 lines are also seen in BCGs in other cool-‐core clusters (de Messieres et al. 2009).
Apparently, strong star-‐forma3on is not required to produce these H2 lines (weak PAH features).
3. Next Steps
• Chandra – Derive the proper3es of parent clusters (e.g., gas cooling 3me, mass deposi3on rates)
• Gemini/NIFS – Examine the spa3al distribu3on of line emiwng regions (e.g., H2, Paα)
• IRAM 30m – Detect CO emission
• Herschel – Determine far-‐IR SEDs; Detect far-‐IR cooling lines
Cooling-‐Flow Cluster BCGs Edge et al. (2010a, b)
PACS (70/100/160um three-‐color)
SPIRE (250/350/500um three-‐color)
[C II] 158 um [O I] 63 um
[N III] 122 um [O Ib] 145 um
Summary
• About 20% of X-‐ray-‐luminous (Lx > 2 x 1044 erg/s) clusters at z=0.15-‐0.3 harbor IR-‐bright (LTIR > 1010 L) BCGs.
• IR luminosi3es of these BCGs are generated by star forma3on and/or AGNs.
• Some of these BCGs show excep3onally strong H2 lines. Are these lines telling something about the ICM cooling process ? High spa3al-‐resolu3on informa3on is needed to understand! (e.g., AO-‐assisted near-‐IR IFU observa3ons).
• Next genera3on of IR space telescopes could detect such H2 lines up to z=6-‐8 (in principle…).