Infrared Luminosity Function of the Coma Cluster

1. Infrared Luminosity Function of the Coma Cluster Lei Bai George Rieke Marcia Rieke Steward Observatory

2. Background Infrared emission from galaxies: late-type galaxies ==> strong early-type galaxies ==> weak Galaxy clusters, dominated by early-type galaxies, are unlikely to be strong sources in the far-infrared The large velocity dispersion in clusters also suggests that very luminous infrared galaxies would be rare. Stripping may remove the ISM so star formation would be reduced (but maybe star formation could be induced by compression provided by the intracluster medium) So is the population of galaxies in a cluster fundamentally different in their infrared properties? Some clusters also contain intracluster gas. Is there any dust associated with this gas? Enough to be detectable in the infrared?

3. IRAS Detected Spirals in the Hercules Cluster Young et al. 1984 ApJL 278 75

4. ISO Observations of Coma Stickel et al. scanned ISOPHOT across the face of Coma at 120mm and at 185mm. They did not claim to detect individual galaxies but interpret their results as showing an extended cool dust component: - ~0.1 MJy/sr = 4x1043 ergs/s Assuming a dust temperature in the range of 25-40K, this corresponds to 6.2x107M < MDust < 1.6x109M  for r<0.2Mpc. Comparison with x-ray implies Mdust/ Mgas between 1.3x10-5 and 3.2x10-4

5. Spitzer Observes Coma Coma was observed in scan map mode with an equivalent of 88 seconds exposure time at 24mm and 40 seconds at 70mm. Photometry of objects was obtained using Sextractor. • 24mm detection LIR>1043.3 erg/s 2 degs NGC 4836 region

6. 7 arc min Coma Core at 24mm 24 mm R (0.7mm) Clusters members are circled.

7. Detection Statistics • Cluster membership was determined by matching 24mm and 70mm sources with galaxies from the spectroscopic surveys of Beijersbergen and van de Hulst (2003) and Mobasher et al. 2001. • MIPS 24mm detected 217 of 498 galaxies in BvdH 70mm detected 58 of 477 galaxies in BvdH • MIPS 24mm detected 123 of 333 galaxies in M et al. 70mm detected 33 of 303 galaxies in M et al. • Because a much larger fraction of cluster members are detected at 24mm, we derived infrared luminosities from this wavelength • SEDs and R/24mm ratios were used to calculate the luminosities.

8. Spectral Energy Distributions This sample of SEDs is weighted towards luminous IR galaxies. There are no galaxies earlier than S0 so a check is needed whether there are biases for early-type galaxies. Soon we can use a Spitzer collection of SEDs! Offset vertically for clarity. SEDs from Devriendt et al. 1999

9. Deriving IR Luminosities If the suite of SEDs represents all types of galaxies, then a correlation between colors and 8-1000mm luminosity can be derived. We use the colors S24/Sr(0.7) and S70/S24derive a relation between S24 and IR luminosity. Note that early-type galaxies have the same behavior as late-type galaxies which are much more common in the SED template set. • SED templates 70mm completeness  early-type late-type solid = 70mm detection

10. Fitting Colors to Get Luminosity Lagache et al. 2003 normal spiral • Coma galaxies  Spirals  Luminous IR galaxy  Ultra luminous IR galaxy Notice small range in LIR/L24: Fitting colors almost unnecessary, no bias introduced by optical selection. Open symbols show values derived from templates; dots show application of fit to Coma galaxies.

11. Cluster Luminosity Function Schecter Function fits: MBC a = 1.52±0.13 log(L*(IR)/L) = 10.72 ± 0.70 BvdHC a = 1.42±0.09 log(L*(IR)/L) = 10.52 ± 0.25 MBC completeness limit MBC = Mosbasher et al. deeper but only on core BvdHC = Beijersbergen and van de Hulstcovers larger area less deeply BvdHC completeness limit 24mm completeness limit Luminosity function shown is for projected area of cluster.

12. Comparison to Field LF Coma: Field* (z=0-0.2): MBC a = 1.52±0.13 a = 1.23±0.0.07 log(L*(IR)/L) = 10.72 ± 0.70 log(L*(IR)/L) = 10.36 ± 0.14 BvdHC a = 1.42±0.09 log(L*(IR)/L) = 10.52 ± 0.25 Comparisons using power-law fits yield similar results: Coma and field LFs very similar with field somewhat flatter. L*virtually the same for field and Coma. f*(L*(IR)) is ~45x higher in Coma than in the field f*(L*(R)) is ~63±15x higher in Coma than in the field => Coma IR LF not markedly different than field LF. * (Perez-Gonzalez et al. 2005); incompleteness may flatten faint end slope

13. LF in Different Cluster Regions LF does not vary strongly with location in the cluster (but beware of small no. statistics). Cluster core does appear to be deficient in both very low and very high luminosity galaxies compared with the outer regions. All galaxies with LIR>1044 erg/sec lie outside the core.

14. LF as Function of Galaxy Type • Spirals dominate bright end, E/S0 the faint end • Spiral a=1.17±0.25 same as Pozzi et al. find for field spirals (1.10 ±0.25) • Early-type galaxies contribute only about 15% of the surface density but are very important for L<1043 ergs/s

15. Extended Dust Emission • Recall possible ISO detection of 0.1 MJy/sr at 120mm • Looking at fluxes after subtraction of galaxies and smoothing to ISO resolution, MIPS sets limits of • 24mm: <.002 MJy/sr • 70mm: <0.1 MJy/sr (need to be careful of foreground cirrus) • Dwek et al. 1990 predictions for dust heated in hot x-ray emitting gas: 25mm ~ 0.5 kJy/sr 60mm ~ .050 MJy/sr So MIPS limits support Dwek’s calculations

16. Conclusions • The Coma LF is similar to the field LF • SFR not strongly dependent on environment • Within the cluster, we find evidence that the SFR is dependent on environment with the lower density portion of the cluster containing the most luminous galaxies • consistent with cluster core galaxies losing some fraction of their ISM • Lowest luminosity galaxies have masses below the threshold suggested for effective stripping (Mori and Burkett) which suggests that some other effect reduces the critical mass for stripping • Global SFR is ~0.8 M /yr • SFR for core 8.5 Mpc2 is 213 M /yr • Extended dust emission neither confirmed nor ruled out

17. More Checks on 24mm to Luminosity Conversion Derived luminosity does not depend on which color is used.