Observations of submillimeter galaxies and of the Sunyaev-Zeldovich effect toward clusters of galaxies

Doktorsavhandling, 2011

This thesis describes observations of a population of high-redshift, dusty star-forming galaxies: the submillimeter (submm) galaxies. The submm galaxies detected so far are very massive and lumi- nous systems, with huge star-formation-rates ( SFR > 100 M yr − 1 ), large reservoirs of molecular gas ( M gas ∼ 1 × 10 10 M ) and large dynamical masses ( M dyn > 10 10 M ). The median redshift of detected submm galaxies is z ∼ 2 . 2 , which corresponds to a look-back time of 10 Gyr. We have used the LABOCA bolometer camera mounted on the APEX telescope in Chile to search for gravitationally lensed submm galaxies behind massive galaxy clusters (with a total mass M clust > 5 × 10 14 M ). The gravitational magnification induced by the galaxy clusters boosts the observed flux densities of the background submm galaxies and allows us to probe deeper into the faint galaxy population. The number counts of submm galaxies show that there are many more faint galaxies than bright ones, and thus the properties of the bright submm galaxies mainly studied so far may not be representative of the bulk of the population. In two papers we present our results of the observations toward galaxy clusters. In paper I, we discuss a merging galaxy cluster: the Bullet Cluster at redshift z ∼ 0 . 3 . The huge mass concentration provides large magnification factors, and one of the background galaxies detected with LABOCA is gravitationally magnified ∼ 100 times. The number of galaxies detected toward the Bullet Cluster is consistent with previously published number counts, and we probed the number counts to some of the lowest flux density levels so far. Paper I also includes a study of the multi-wavelength-properties of the LABOCA galaxies, where we identified counterparts in infrared Spitzer maps. In paper II the study of submm galaxies lensed by clusters was extended with four additional clusters. The number of detected galaxies was more than doubled compared to paper I. A more so- phisticated method was used to determine the magnification factors for each galaxy, and to correct the number counts for the effect of magnification differences across the map. The number counts agree with previous work but may indicate a flattening towards low flux densities. A study of the faint, unresolved background population was also included, via the stacking technique. We used the positions of all sources detected in Spitzer 24 μ m maps across the observed LABOCA fields. The LABOCA flux was extracted at each 24 μ m position and when added together this yielded a 14 . 5 σ detection, and a signal 5 to 10 times lower than the adopted flux limit for extraction of significant sources in the maps. In combination with gravitational magnification, stacking makes it possible to probe deeper into the submm population, although only mean properties of the faint galaxies can be derived. Having studied the observational properties of a sample of submm galaxies in paper III we focused on one particular galaxy behind the Bullet Cluster, SMMJ0658, gravitationally magnified up to 100 times in total. The strong lensing caused by the total cluster potential and an elliptical galaxy within the cluster gives rise to three images of the same background galaxy. In our i observations with a radio/mm interferometer, the Australia Telescope Compact Array (ATCA), we detected two rotational transitions of the carbon monoxide (CO) molecule in SMMJ0658. CO is commonly used as a tracer of the more abundant molecule H 2 , which constitutes a large fraction of the molecular gas in galaxies. Because of the large magnification factor of this galaxy it is possible to probe the physical conditions in a system with ten times less molecular gas and a lower star formation rate than the bright submm galaxy population studied so far. Finally, I have participated in the APEX-SZ project. APEX-SZ is a bolometer camera oper- ating at 2 mm wavelength, designed with the aim of detecting the Sunyaev–Zeldovich decrement in clusters of galaxies. The Sunyaev–Zeldovich effect is a secondary anisotropy to the Cosmic Mi- crowave Background (CMB) radiation. It is due to inverse Compton scattering of CMB photons by hot electrons in the cluster gas. At 2 mm the SZ effect gives rise to a lower temperature when compared to the 2.7 K radiation. I participated in six observing runs in Chile between 2008 and 2010. The scientific studies with APEX-SZ focus on the physical state of the intra-cluster gas in galaxy clusters. In two of the papers summarized in this thesis the APEX-SZ detection was used together with XMM-Newton X-ray observations to constrain non-parametric models for the distribution and temperature of the hot gas. The existence of dusty galaxies within high-redshift galaxy clusters is a possible explanation as to why several high-redshift galaxy clusters have eluded detection in Sunyaev–Zeldovich experiments.