We present the results of a survey for star clusters in M33 using HST WFPC2 archive images. We have found 104 star clusters, including 32 new ones, in the images of 24 fields that were not included in previous studies. Combining these with previous data in the literature, we increase the number of M33 star clusters found in the HST images to 242. We have derived BVI integrated photometry of these star clusters from the CCD images taken with the CFH12k mosaic camera at the CFHT. Integrated color-magnitude diagrams of the M33 star clusters are found to be similar in general to those of star clusters in the Large Magellanic Cloud, except that M33 has a much lower fraction of blue star clusters. We find 29 red star clusters with 0:5 (B À V ) 0 1:1 and 0:7 (V À I ) 0 1:2, making them old globular cluster candidates. We divide the cluster sample into three groups according to their (B À V ) 0 color: blue star clusters with (B À V ) 0 0:3, intermediate-color star clusters with 0:3 < (B À V ) 0 < 0:5, and red star clusters with (B À V ) 0 ! 0:5. Most of the new clusters in M33 are located along a sequence that is consistent with the theoretical evolutionary path for Z ¼ 0:004, Y ¼ 0:24 in the (B À V ) 0 -(V À I ) 0 diagram, while a few of them are on the redder side in the (V À I ) 0 color. Relatively more red clusters are found in the outer region of M33 than blue and intermediate-color clusters, and many of the blue stars are located in H ii regions. The luminosity function for the blue star clusters shows a peak at M V % À7:3 mag, while that for the intermediate-color star clusters shows a peak at the fainter magnitude M V % À6:3 mag. The luminosity function for the red star clusters also shows a peak at M V % À6:8 mag, although the number of clusters is small.
Recent studies suggest that faint active galactic nuclei may be responsible for the reionization of the universe. Confirmation of this scenario requires spectroscopic identification of faint quasars (M 1450 > −24 mag) at z 6, but only a very small number of such quasars have been spectroscopically identified so far. Here, we report the discovery of a faint quasar IMS J220417.92+011144.8 at z ∼ 6 in a 12.5 deg 2 region of the SA22 field of the Infrared Medium-deep Survey (IMS). The spectrum of the quasar shows a sharp break at ∼ 8443Å, with emission lines redshifted to z = 5.944 ± 0.002 and rest-frame ultraviolet continuum magnitude M 1450 = −23.59 ± 0.10 AB mag. The discovery of IMS J220417.92+011144.8 is consistent with the expected number of quasars at z ∼ 6 estimated from quasar luminosity functions based on previous observations of spectroscopically identified lowluminosity quasars . This suggests that the number of M 1450 ∼ −23 mag quasars at z ∼ 6 may not be high enough to fully account for the reionization of the universe. In addition, our study demonstrates that faint quasars in the early universe can be identified effectively with a moderately wide and deep near-infrared survey such as the IMS.
We present a study on four new star clusters discovered in the halo of the intriguing dwarf irregular galaxy NGC 6822 from a wide field survey covering 3 • × 3 • area carried out with MegaCam at Canada-France-Hawaii Telescope (CFHT). The star clusters have extended structures with half-light radii R h ≈ 7.5-14.0 pc, larger than typical Galactic globular clusters and other known globular clusters in NGC 6822. The integrated colors and color magnitude diagrams (CMD) of resolved stars suggest that the new star clusters are 2 -10 Gyr old and relatively metal poor with Z=0.0001-0.004 based on the comparison with theoretical models. The projected distance of each star cluster from the galaxy center ranges from 10.7 ′ (≈ 1.5 kpc) to 77 ′ (≈ 11 kpc), far beyond the optical body of the galaxy. Interestingly, the new star clusters are aligned along the elongated old stellar halo of NGC 6822, which is almost perpendicular to the HI gas distribution where young stellar populations exist. We also find that the colors and half-light radii of the new clusters are correlated with the galactocentric distance: clusters farther from the galaxy center are larger and bluer than those closer to the galaxy center. We discuss the stellar structure and evolution of NGC 6822 implied by these new extended star clusters in the halo. We also discuss the current status of observational and theoretical understandings regarding the origin of extended star clusters in NGC 6822 and other galaxies.
The intergalactic medium (IGM) at z ∼ 5 to 6 is largely ionized, and yet the main source for the IGM ionization in the early universe is uncertain. Of the possible contributors are faint quasars with , but their number density is poorly constrained at z ∼ 5. In this paper, we present our survey of faint quasars at z ∼ 5 in the European Large-Area Infrared Space Observatory Survey-North 1 (ELAIS-N1) field over a survey area of 6.51 deg2 and examine if such quasars can be the dominant source of the IGM ionization. We use the deep optical/near-infrared data of the ELAIS-N1 field as well as the additional medium-band observations to find z ∼ 5 quasars through a two-step approach using the broadband color selection, and spectral energy distribution fitting with the medium-band information included. Adopting Bayesian information criterion, we identify 10 promising quasar candidates. Spectra of three of the candidates are obtained, confirming all of them to be quasars at z ∼ 5 and supporting the reliability of the quasar selection. Using the promising candidates, we derive the z ∼ 5 quasar luminosity function at −26 ≲ M 1450 ≲ −23. The number density of faint z ∼ 5 quasars in the ELAIS-N1 field is consistent with several previous results that quasars are not the main contributors to the IGM-ionizing photons at z ∼ 5.
We describe the overall characteristics and the performance of an optical CCD camera system, Camera for QUasars in EArly uNiverse (CQUEAN), which is being used at the 2.1 m Otto Struve Telescope of the McDonald Observatory since 2010 August. CQUEAN was developed for followup imaging observations of red sources such as high redshift quasar candidates (z 5), Gamma Ray Bursts, brown dwarfs, and young stellar objects. For efficient observations of the red objects, CQUEAN has a science camera with a deep depletion CCD chip which boasts a higher quantum efficiency at 0.7 − 1.1 µm than conventional CCD chips. The camera was developed in a short time scale (∼one year), and has been working reliably. By employing an auto-guiding system and a focal reducer to enhance the field of view on the classical Cassegrain focus, we achieve a stable guiding in 20 minute exposures, an imaging quality with FWHM ≥ 0.6 ′′ over the whole field (4.8 ′ × 4.8 ′ ), and a limiting magnitude of z = 23.4 AB mag at 5-σ with one hour total integration time.
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