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Register a new userThe DIRECT Project: Catalogs of Stellar Objects in Nearby Galaxies. I. The Central Part of M33
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The DIRECT project aims to determine direct distances to two important galaxies in the cosmological distance ladder -- M31 and M33 -- using detached eclipsing binaries (DEBs) and Cepheids. The search for these variables requires time-series photometry of large areas of the target galaxies and yields magnitudes and positions for tens of thousands of stellar objects, which may be of use to the astronomical community at large. During the first phase of the project, between September 1996 and October 1997, we were awarded 95 nights on the F. L. Whipple Observatory 1.2 m telescope and 36 nights on the Michigan-Dartmouth-MIT 1.3 m telescope to search for DEBs and Cepheids in the M31 and M33 galaxies. This paper, the first in our series of stellar catalogs, lists the positions, three-color photometry, and variability indices of 57,581 stars with 14.4 < V < 23.6 in the central part of M33. The catalog is available from our FTP site at ftp://cfa-ftp.harvard.edu/pub/kstanek/DIRECT/star_catalog/M33ABC/
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The DIRECT project aims to determine direct distances to two important galaxies in the cosmological distance ladder - M31 and M33 - using detached eclipsing binaries (DEBs) and Cepheids. We present the results of the first large-scale CCD-based search for variables in M33. We have observed two fields located in the central region of M33 for a total of 95 nights on the F. L. Whipple Observatory 1.2-m telescope and 36 nights on the Michigan-Dartmouth-MIT 1.3-m telescope. We have found a total of 544 variables, including 251 Cepheids and 47 eclipsing binaries. The catalog of variables is available online, along with finding charts and BVI light curve data (consisting of 8.2x10^4 individual measurements). The complete set of CCD frames is available upon request.
Using Gemini QuickStart infrared observations of the central 22 of M33, we analyze the stellar populations in this controversial region. Based on the slope of the giant branch we estimate the mean metallicity to be -0.26+/-0.27, and from the luminosities of the most luminous stars, we estimate that there were two bursts of star formation ~2 and ~0.5 Gyr ago. We show that the stellar luminosity function not only has a different bright end cutoff, but also a significantly different slope than that of the Galactic bulge, and suggest that this difference is due to the young stellar component in M33. We combine our infrared Gemini data with optical HST-WFPC2 measurements revealing a CMD populated with young, intermediate, and old age stellar populations. Using surface brightness profiles from 0.1 to 18, we perform simple decompositions and show that the data are best fit by a three-component, core + bulge + disk model. Finally, we find no evidence for radial variations of the stellar populations in the inner 3-10 of M33 based on a spatial analysis of the color-magnitude diagrams and luminosity functions.
The relation between tidal interactions, starbursts, and the onset and/or fueling of active galactic nuclei (AGN) is a matter of debate. I Zw 1 is considered as the prototypical narrow-line Seyfert 1 galaxy (NLS1) and as one of the closest quasi-stellar objects (QSOs). With a clear spiral host and a small companion galaxy to the west, I Zw 1 is a possible example of minor-merger-related nuclear activity. This study investigates possible signs of a relation between merger process, star formation activity, and AGN properties in the case of I Zw 1. The morphology of I Zw 1 and nearby sources is investigated via high-resolution NIR images. Color trends in the host galaxy of I Zw 1 are discussed by means of optical-to-NIR color composites. Long-slit spectra of the QSO nucleus of I Zw 1 and of the two nearby sources to the north and the west of the I Zw 1 disk are analyzed. The data support the scenario of a tidal interaction between I Zw 1 and the small companion galaxy to the west. A concentration of blue color in the western part of the I Zw 1 host galaxy might be the manifestation of merger-induced star formation activity. Previous findings that the likely companion has an old evolved stellar population are substantiated by the new data. An extension to the west of the putative companion emerges as a separate source. The source to the north of the I Zw 1 disk is reconfirmed as a late-type foreground star. Lines in the nuclear K-band spectrum of I Zw 1 are discussed in comparison to data prior to this article and line fluxes are reported.
We present a new determination of the metallicity gradient in M33, based on Keck/LRIS measurements of oxygen abundances using the temperature-sensitive emission line [OIII] 4363 A in 61 HII regions. These data approximately triple the sample of direct oxygen abundances in M33. We find a central abundance of 12 + log(O/H) = 8.36+/-0.04 and a slope of -0.027+/-0.012 dex/kpc, in agreement with infrared measurements of the neon abundance gradient but much shallower than most previous oxygen gradient measurements. There is substantial intrinsic scatter of 0.11 dex in the metallicity at any given radius in M33, which imposes a fundamental limit on the accuracy of gradient measurements that rely on small samples of objects. We also show that the ionization state of neon does not follow the ionization state of oxygen as is commonly assumed, suggesting that neon abundance measurements from optical emission lines require careful treatment of the ionization corrections.
We present Spitzer IRAC $3.6-8$um and MIPS $24$um point-source catalogs for seven galaxies: NGC$6822$, M$33$, NGC$300$, NGC$2403$, M$81$, NGC$0247$, and NGC$7793$. The catalogs contain a total of $sim300,000$ sources and were created by dual-band selection of sources with $>3sigma$ detections at both $3.6$um and $4.5$um. The source lists become significantly incomplete near $m_{3.6}=m_{4.5}simeq18$. We complement the $3.6$um and $4.5$um fluxes with $5.8$um, $8.0$um and $24$um fluxes or $3sigma$ upper limits using a combination of PSF and aperture photometry. This catalog is a resource as an archive for studying mid-infrared transients and for planning observations with the James Webb Space Telescope.
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