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Register a new userSpitzer Observations of Massive Red Galaxies at High Redshift
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Casey Papovich
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We investigate the properties of massive galaxies at z=1-3.5 using HST observations, ground-based near-IR imaging, and Spitzer Space Telescope observations at 3-24 micron. We identify 153 distant red galaxies (DRGs) with J-K > 2.3 mag (Vega) in the southern GOODS field. This sample is approximately complete in stellar mass for passively evolving galaxies above 10^11 solar masses and z < 3. The galaxies identified by this selection are roughly split between objects whose optical and near-IR rest-frame light is dominated by evolved stars combined with ongoing star formation, and galaxies whose light is dominated by heavily reddened starbursts. Very few of the galaxies (< 10%) have no indication of current star formation. Using SFR estimates that include the reradiated IR emission, the DRGs at z=1.5-3 with stellar masses > 10^11 solar masses have specific SFRs (SFRs per unit stellar mass) ranging from 0.2 to 10 Gyr^-1, with a mean value of ~2.4 Gyr^-1. The DRGs with stellar masses > 10^11 solar masses and 1.5 < z < 3 have integrated specific SFRs greater the global value over all galaxies. In contrast, we find that galaxies at z = 0.3-0.75 with these stellar masses have integrated specific SFRs less than the global value, and more than an order of magnitude lower than that for massive DRGs at z = 1.5-3. At z < 1, lower-mass galaxies dominate the overall cosmic mass assembly. This suggests that the bulk of star formation in massive galaxies occurs at early cosmic epochs and is largely complete by z~1.5. [Abridged]
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I discuss constraints on star formation and AGN in massive, red galaxies at z~1-3 using Spitzer observations at 3-24 micron. In particular I focus on a sample of distant red galaxies (DRGs) with J - K > 2.3 in the southern Great Observatories Origins Deep Survey (GOODS-S) field. The DRGs have typical stellar masses >10^11 solar masses. Interestingly, the majority (>50%) of these objects have 24 micron flux densities >50 micro-Jy. At these redshifts massive galaxies undergo intense (and possibly frequent) IR-active phases, which is in constrast to lower-redshift massive galaxies. If the 24 micron emission in these z~1-3 galaxies is attributed to star formation, then it implies star formation rates (SFRs) in excess of ~100 solar masses per year. These galaxies have specific SFRs equal to or exceeding the global average value at that epoch. Thus, this is an active period in their assembly. Based on their X-ray luminosities and near-IR colors, as many as 25% of the massive galaxies at z>1.5 host AGN, suggesting that the growth of supermassive black holes coincides with massive-galaxy assembly.
My colleagues and I identified distant red galaxies (DRGs) with J-K>2.3 mag in the GOODS-S field. These galaxies reside at z~1-3.5, (<z>=2.2) and based on their ACS (0.4-1 micron), ISAAC (1-2.2 micron), and IRAC (3-8 micron) photometry, they typically have inferred stellar masses > 10^11 solar masses. Interestingly, more than 50% of these objects have 24 micron flux densities >50 micro-Jy. Attributing the IR emission to star-formation implies SFRs of ~100-1000 solar masses per year. As a result, galaxies with stellar masses >10^11 solar masses have specific SFRs equal to or exceeding the global value at z~1.5-3. In contrast, galaxies with >10^11 solar masses z~0.3-0.75 have specific SFRs less than the global average, and more than an order of magnitude lower than that for massive DRGs at z~1.5-3. Thus, the bulk of star formation in massive galaxies is largely complete by z~1.5. The red colors and large inferred stellar masses in the DRGs suggest that much of the star formation in these galaxies occurred at redshifts z>5-6. Using model star-formation histories that match the DRG colors and stellar masses at z~2-3, and measurements of the UV luminosity density at z>5-6, we consider what constraints exist on the stellar initial mass function in the progenitors of the massive DRGs at z~2-3.
We present the results of a comprehensive Spitzer survey of 70 radio galaxies across 1<z<5.2. Using IRAC, IRS and MIPS imaging we determine the rest-frame AGN contribution to the stellar emission peak at 1.6um. The stellar luminosities are found to be consistent with that of a giant elliptical with a stellar mass of 10^11-12Msun. The mean stellar mass remains constant at ~10^11.5Msun up to z=3 indicating that the upper end of the mass function is already in place by this redshift. The mid-IR luminosities imply bolometric IR luminosities that would classify all sources as ULIRGs. The mid-IR to radio luminosity generally correlate implying a common origin for these emissions. The ratio is higher than that found for lower redshift, ie z<1, radio galaxies.
We have observed 13 z >= 4.5 QSOs using the Multiband Imaging Photometer for Spitzer, nine of which were also observed with the Infrared Array Camera. The observations probe rest wavelengths ~ 0.6-4.3 micron, bracketing the local minimum in QSO spectral energy distributions (SEDs) between strong optical emission associated directly with accretion processes and thermal emission from hot dust heated by the central engine. The new Spitzer photometry combined with existing measurements at other wavelengths shows that the SEDs of high redshift QSOs (z >= 4.5) do not differ significantly from typical QSOs of similar luminosity at lower redshifts (z <~ 2). This behavior supports other indications that all the emission components and physical structures that characterize QSO activity can be established by z = 6.4. The similarity also suggests that some QSOs at high redshift will be very difficult to identify because they are viewed along dust-obscured sight lines.
We describe a compact cluster of massive red galaxies at z=1.51 discovered in one of the Gemini Deep Deep Survey (GDDS) fields. Deep imaging with the Near Infrared Camera and Multi Object Spectrometer (NICMOS) on the Hubble Space Telescope reveals a high density of galaxies with red optical to near-IR colors surrounding a galaxy with a spectroscopic redshift of 1.51. Mid-IR imaging with Infrared Array Camera (IRAC) on the Spitzer Space telescope shows that these galaxies have spectral energy distributions that peak between 3.6 and 4.5 microns. Fits to 12-band photometry reveal 12 or more galaxies with spectral shapes consistent with z = 1.51. Most are within ~170 co-moving kpc of the GDDS galaxy. Deep F814W images with the Advanced Camera for Surveys (ACS) on HST reveal that these galaxies are a mix of early-type galaxies, disk galaxies and close pairs. The total stellar mass enclosed within a sphere of 170 kpc in radius is > 8E+11 solar masses. The colors of the most massive galaxies are close to those expected from passive evolution of simple stellar populations (SSP) formed at much higher redshifts. We suggest that several of these galaxies will merge to form a single, very massive galaxy by the present day. This system may represent an example of a short-lived dense group or cluster core typical of the progenitors of massive clusters in the present day and suggests the red sequence was in place in over-dense regions at early times.
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