Spitzer provides substantial advances for studies of external galaxies: 1.) it has the very high sensitivity toward extended, low-surface-brightness sources made possible by a cryogenic telescope in space; 2.) the use of large format arrays and projected pixel scales that sample the telescope image well provide substantial improvements in angular resolution compared with previous space infrared telescopes; and 3.) its spectrograph has wavelength resolution well-suited to extragalactic studies, as well as high performance arrays that provide a major advance in sensitivity. We show how these capabilities are already leading to new possibilities in studying the structure of nearby galaxies. Combined with recent breakthroughs in high quality imaging at other wavelengths, we can anticipate a substantial improvement in our understanding of bow different components of a galaxy relate to each other and bow their interactions influence galaxy evolution. In addition, Spitzer is advancing our understanding of the behavior of the polyaromatic hydrocarbon (PAH) emission features, as a function of environment and metallicity. New features are also being discovered due to the high-sensitivity mapping capabilities of the spectrograph. An additional direction for research with Spitzer arises from its deep survey capabilities and the possibility of testing how the spectral energy distributions of galaxies evolve at large look-back times.

The far-infrared detectors on the Multiband Imaging Photometer for Spitzer (MIPS) represent a significant advancement in both format and sensitivity. We describe some of the operational experience since launch in August 2003. MIPS has three infrared detector arrays, a 128x128 format Si:As impurity band conduction detector operating at 24 μm, a 32x32 format Ge:Ga array operating at 70 μm and a 2x20 format stressed Ge:Ga array operating at 160 μm. Since both germanium detectors utilize conventional bulk photoconductors, they are subject to a number of non-ideal behaviors that are inherent in these types of devices when operated in ultra-low backgrounds. The principal problems are nonlinear time response, changing responsivity in a radiation environment, and flux non-linearities. We describe observing strategies that are used on MIPS to minimize the impact of these effects.

We investigate the luminous X-ray sources in the Lockman Hole (LH) and the Extended Groth Strip (EGS) detected at 24microns using MIPS and also with IRAC on board Spitzer. We assemble optical/infrared spectral energy distributions (SEDs) for 45 X-ray/24micron sources in the EGS and LH. Only about 1/4 of the hard X-ray/24micron sources show pure type 1 AGN SEDs. More than half of the X-ray/24micron sources have stellar-emission-dominated or obscured SEDs, similar to those of local type 2 AGN and spiral/starburst galaxies. One-third of the sources detected in hard X-rays do not have a 24micron counterpart. Two such sources in the LH have SEDs resembling those of S0/elliptical galaxies. The broad variety of SEDs in the optical-to-Spitzer bands of X-ray selected AGN means that AGN selected according to the behavior in the optical/infrared will have to be supplemented by other kinds of data (e.g., X-ray) to produce unbiased samples of AGN.

We present Spitzer observations of the young cluster NGC 2547, obtaining photometry at 3.6, 4.5, 5.8, 8.0, and 24 μm, reaching significantly fainter infrared sensitivities than previous studies. With these observations, we investigate the disk frequency in this cluster. A total of 3770, 2408, 1988, 1238, and 1123 sources were detected in the common region of the five respective bands. The detection limits were 14.9, 14.0, 13.5, 13.3, and 12.0 mag, respectively. The large majority of sources are associated with 2MASS objects. From this large ensemble, we utilize the criteria of Naylor et al. to identify 184 likely members of the cluster. The analyses in this paper are focused on these likely members. Of the 184 candidates, 162 were detected in at least one Infrared Array Camera (IRAC) band. These objects form a well-defined family in the J-K,K-[3.6] color-color diagram. There is no evidence for excess emission at 3.6 μm for cluster candidates with J-K < 0.8, corresponding to spectral types earlier than late K. For later type stars, only 12 have evidence for a 3.6 μm excess. Hence, we derive a 3.6 μm emitting disk fraction of less than 7%. The lack of excess for the more massive stars may indicate a difference in the disk dissipation timescales for different mass stars. At 24 μm, 32 of the sources are detected. Most of them have photospheric K-[3.6] and K-[24] colors, but approximately a quarter show an excess at 24 μm. This observation may be evidence for cool disks with central holes.

Henize 206 is a region of star formation in the Large Magellanic Cloud of the approximate scale of the Orion belt and sword. Our Spitzer Space Telescope infrared images and Cerro Tololo Inter-American Observatory (CTIO) optical images show that the region is experiencing very energetic star formation. The radiation from young stars has excited strong polycyclic aromatic hydrocarbon (PAH) emission throughout Henize 206, except on the side of the nebula with the prominent young supernova remnant. As is also seen in early Spitzer observations of M81, star formation rates calculated from Halpha for Henize 206 may miss the deeply embedded young stars, compared with star formation rates calculated from far infrared emission. For one of the highest surface brightness regions of Henize 206, we obtained snapshot exposures with the Thermal-Region Camera Spectrograph on Gemini South to explore the complex structure. A few percent of the total flux from this brightest region in Henize 206 emanates from infrared peaks of subparsec scale.