We present a new census of the stellar and substellar members of the young cluster IC 348. We have obtained images at I and Z for a 42'x28' field encompassing the cluster and have combined these measurements with previous optical and near-infrared photometry. From spectroscopy of candidate cluster members appearing in these data, we have identified 122 new members, 15 of which have spectral types of M6.5-M9, corresponding to masses of 0.08-0.015 M_sun by recent evolutionary models. The latest census for IC 348 now contains a total of 288 members, 23 of which are later than M6 and thus are likely to be brown dwarfs. From an extinction-limited sample of members (A_V<=4) for a 16'x14' field centered on the cluster, we construct an IMF that is unbiased in mass and nearly complete for M/M_sun>=0.03 (<=M8). In logarithmic units where the Salpeter slope is 1.35, the mass function for IC 348 rises from high masses down to a solar mass, rises more slowly down to a maximum at 0.1-0.2 M_sun, and then declines into the substellar regime. In comparison, the similarly-derived IMF for Taurus from Briceno et al. and Luhman et al. rises quickly to a peak near 0.8 M_sun and steadily declines to lower masses. The distinctive shapes of the IMFs in IC 348 and Taurus are reflected in the distributions of spectral types, which peak at M5 and K7, respectively. These data provide compelling, model-independent evidence for a significant variation of the IMF with star-forming conditions.Comment: 47 pages, 14 figures, 3rd para of 4.5.3 has been added, this is final version in press at ApJ, also found at http://cfa-www.harvard.edu/sfgroup/preprints.htm

Aims. We quantify the contributions of 24 µm galaxies to the Far-Infrared (FIR) Background at 70 and 160 µm. We provide new estimates of the Cosmic Infrared Background (CIB), and compare it with the Cosmic Optical Background (COB). Methods. Using Spitzer data at 24, 70 and 160 µm in three deep fields, we stacked more than 19000 MIPS 24 µm sources with S 24 ≥ 60 µJy at 70 and 160 µm, and measured the resulting FIR flux densities.Results. This method allows a gain up to one order of magnitude in depth in the FIR. We find that the Mid-Infrared (MIR) 24 µm selected sources contribute to more than 70% of the Cosmic Infrared Background (CIB) at 70 and 160 µm. This is the first direct measurement of the contribution of MIR-selected galaxies to the FIR CIB. Galaxies contributing the most to the total CIB are thus z ∼ 1 luminous infrared galaxies, which have intermediate stellar masses. We estimate that the CIB will be resolved at 0.9 mJy at 70 and 3 mJy at 160 µm. By combining the extrapolation of the 24 µm source counts below analysis, we obtain lower limits of 7.1 ± 1.0 and 13.4 ± 1.7 nW m −2 sr −1 for the CIB at 70 and 160 µm, respectively. Conclusions. The MIPS surveys have resolved more than three quarters of the MIR and FIR CIB. By carefully integrating the Extragalactic Background Light (EBL) SED, we also find that the CIB has the same brightness as the COB, around 24 nW m −2 sr −1 . The EBL is produced on average by 115 infrared photons for one visible photon. Finally, the galaxy formation and evolution processes emitted a brightness equivalent to 5% of the primordial electromagnetic background (CMB).

We report new Spitzer 24 m photometry of 76 main-sequence A-type stars. We combine these results with previously reported Spitzer 24 m data and 24 and 25 m photometry from the Infrared Space Observatory and the Infrared Astronomy Satellite. The result is a sample of 266 stars with mass close to 2.5 M , all detected to at least the $7 level relative to their photospheric emission. We culled ages for the entire sample from the literature and/or estimated them using the H-R diagram and isochrones; they range from 5 to 850 Myr. We identified excess thermal emission using an internally derived K À 24 (or 25) m photospheric color and then compared all stars in the sample to that color. Because we have excluded stars with strong emission lines or extended emission (associated with nearby interstellar gas), these excesses are likely to be generated by debris disks. Younger stars in the sample exhibit excess thermal emission more frequently and with higher fractional excess than do the older stars. However, as many as 50% of the younger stars do not show excess emission. The decline in the magnitude of excess emission, for those stars that show it, has a roughly t 0 /time dependence, with t 0 $ 150 Myr. If anything, stars in binary systems (including Algoltype stars) and k Boo stars show less excess emission than the other members of the sample. Our results indicate that (1) there is substantial variety among debris disks, including that a significant number of stars emerge from the protoplanetary stage of evolution with little remaining disk in the 10-60 AU region and (2) in addition, it is likely that much of the dust we detect is generated episodically by collisions of large planetesimals during the planet accretion end game, and that individual events often dominate the radiometric properties of a debris system. This latter behavior agrees generally with what we know about the evolution of the solar system, and also with theoretical models of planetary system formation.

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We present deep K s < 21:5 (Vega) identifications, redshifts, and stellar masses for most of the sources composing the bulk of the 24 m background in the GOODS/CDFS. Our identified sample consists of 747 Spitzer MIPS 24 m objects and includes $94% of all the 24 m sources in the GOODS-South field that have fluxes S (24 m) > 83 Jy (the $80% completeness limit of the Spitzer/GTO 24 m catalog); 36% of our galaxies have spectroscopic redshifts (mostly at z < 1:5), and the remaining ones have photometric redshifts of very good quality, with a median of jdzj ¼ jz spec À z phot j/(1 þ z spec ) ¼ 0:02. We find that MIPS 24 m galaxies span the redshift range z $ 0 4 and that a substantial fraction (28%) lie at high redshifts z k 1:5. We determine the existence of a bump in the redshift distribution at z $ 1:9, indicating the presence of a significant population of galaxies with PAH emission at these redshifts. The 24 m galaxy population ranges from sources with intermediate luminosities (10 10 L < L IR < 10 11 L ) and low-to-intermediate assembled stellar masses (10 9 M P M P 10 11 M ) at z P 0:8, to massive (M k 10 11 M ) hyperluminous galaxies (L IR > 10 12 L ) at redshifts z $ 2 3. Massive star-forming galaxies at redshifts 2 P z P 3 are characterized by very high star formation rates (SFR > 500 M yr À1 ), and some of them are able to construct a mass of %10 10 -10 11 M in a single burst lifetime ($0.01-0.1 Gyr). At lower redshifts z P 2, massive star-forming galaxies are also present but appear to be building their stars on long timescales, either quiescently or in multiple modest burstlike episodes. At redshifts z $ 1 2, the ability of the burstlike mode to produce entire galaxies in a single event is limited to some lower (M P 7 ; 10 10 M ) mass systems, and it is basically negligible at z P 1. Our results support a scenario in which star formation activity is differential with assembled stellar mass and redshift, and where the relative importance of the burstlike mode proceeds in a downsizing way from high to low redshifts.

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