Young Super Star Clusters in the Starburst of M82: The Catalogue

Recent results from Hubble Space Telescope (HST) have resolved starbursts as collections of compact young stellar clusters. Here we present a photometric catalogue of the young stellar clusters in the nuclear starburst of M82, observed with the HST WFPC2 in Halpha (F656N) and in four optical broad-band filters. We identify 197 young super stellar clusters. The compactness and high density of the sources led us to develop specific techniques to measure their sizes. Strong extinction lanes divide the starburst into five different zones and we provide a catalogue of young super star clusters for each of these. In the catalogue we include relative coordinates, radii, fluxes, luminosities, masses, equivalent widths, extinctions, and other parameters. Extinction values have been derived from the broad-band images. The radii range between 3 and 9 pc, with a mean value of 5.7 +/- 1.4pc, and a stellar mass between 10e4 and 10e6 Mo. The inferred masses and mean separation, comparable to the size of super star clusters, together with their high volume density, provides strong evidence for the key ingredients postulated by Tenorio et al. (2003) as required for the development of a supergalactic wind.Comment: 45 pages, 5 figures, 12 tables. Accepted for publication in ApJ. Added Erratu

On the ongoing multiple blowout in NGC 604

Several facts regarding the structure of NGC 604 are examined here. The three main cavities, produced by the mechanical energy from massive stars which in NGC 604 are spread over a volume of 10$^6$ pc$^3$, are shown here to be undergoing blowout into the halo of M33. High resolution long slit spectroscopy is used to track the impact from massive stars while HST archive data is used to display the asymmetry of the nebula. NGC 604 is found to be a collection of photoionized filaments and sections of shells in direct contact with the thermalized matter ejected by massive stars. The multiple blowout events presently drain the energy injected by massive stars and thus the densest photoionized gas is found almost at rest and is expected to suffer a slow evolution.Comment: 15 pages (11 text), 4 figures. To be published in Ap

The young stellar population of NGC 4214 as observed with HST. I. Data and methods

We present the data and methods that we have used to perform a detailed UV-optical study of the nearby dwarf starburst galaxy NGC 4214 using multifilter HST/WFPC2+STIS photometry. We explain the process followed to obtain high-quality photometry and astrometry of the stellar and cluster populations of this galaxy. We describe the procedure used to transform magnitudes and colors into physical parameters using spectral energy distributions. The data show the existence of both young and old stellar populations that can be resolved at the distance of NGC 4214 (2.94 Mpc) and we perform a general description of those populations.Comment: 33 pages, 9 figures, and 8 table

Tip of the Red Giant Branch Distances to NGC 4214, UGC 685, and UGC 5456

We have used WFPC2 VRI observations to calculate the distances to three nearby galaxies, NGC 4214, UGC 685, and UGC 5456 using the tip of the red giant branch method. Our values for NGC 4214 (2.94 +/- 0.18 Mpc) and UGC 685 (4.79 +/- 0.30 Mpc) are the most precise measurementes of the distances to these objects ever made. For UGC 5456 the data do not allow us to reach a decisive conclusion since there are two possible solutions, one leading towards a short distance around 3.8 Mpc and another one towards a long distance of 5.6 Mpc or more.Comment: To appear in the March 2002 issue of the Astronomical Journal. 19 pages, including 4 tables and 8 figures. Due to the size limitations of the astro-ph service, a reduced resolution version of figures 1-3 is provided here. For a full resolution version, please go to http://www.stsci.edu/~jmai

The young stellar population of NGC 4214 as observed with HST. II. Results

We present the results of a detailed UV-optical study of the nearby dwarf starburst galaxy NGC 4214 using multifilter HST/WFPC2+STIS photometry. The stellar extinction is found to be quite patchy, with some areas having values of E(4405-5495)< 0.1 mag and others, associated with star forming regions, much more heavily obscured, a result which is consistent with previous studies of the nebular extinction. We determined the ratio of blue-to-red supergiants and found it to be consistent with theoretical models for the metallicity of the SMC. The stellar IMF of the field in the range 20-100 solar masses is found to be steeper than Salpeter. A number of massive clusters and associations with ages between a few and 200 million years are detected and their properties are discussed.Comment: 49 pages, 12 figures, 6 table

On the multiplicity of the zero-age main-sequence O star Herschel 36

We present the analysis of high-resolution optical spectroscopic observations of the zero-age main-sequence O star Herschel 36 spanning six years. This star is definitely a multiple system, with at least three components detected in its spectrum. Based on our radial-velocity (RV) study, we propose a picture of a close massive binary and a more distant companion, most probably in wide orbit about each other. The orbital solution for the binary, whose components we identify as O9 V and B0.5 V, is characterized by a period of 1.5415 +/- 0.0006 days. With a spectral type O7.5 V, the third body is the most luminous component of the system and also presents RV variations with a period close to 498 days. Some possible hypotheses to explain the variability are briefly addressed and further observations are suggested.Comment: 6 pages, 2 figure

Spectral Variations of Of?p Oblique Magnetic Rotator Candidates in the Magellanic Clouds

Optical spectroscopic monitoring has been conducted of two O stars in the Small and one in the Large Magellanic Cloud, the spectral characteristics of which place them in the Of?p category, which has been established in the Galaxy to consist of oblique magnetic rotators. All of these Magellanic stars show systematic spectral variations typical of the Of?p class, further strengthening their magnetic candidacy to the point of virtual certainty. The spectral variations are related to photometric variations derived from OGLE data by Naze et al. (2015) in a parallel study, which yields rotational periods for two of them. Now circular spectropolarimetry is required to measure their fields, and ultraviolet spectroscopy to further characterize their low-metallicity, magnetically confined winds, in support of hydrodynamical analyses.Comment: 18 pages, 6 figures, accepted for publication by A

The Interaction between the ISM and Star Formation in the Dwarf Starburst Galaxy NGC 4214

We present the first interferometric study of the molecular gas in the metal-poor dwarf starburst galaxy NGC 4214. Our map of the 12CO(1-0) emission, obtained at the OVRO millimeter array, reveals an unexpected structural wealth. We detected three regions of molecular emission in the north-west (NW), south-east (SE) and centre of NGC 4214 which are in very different and distinct evolutionary stages (total molecular mass: 5.1 x 10^6 M_sun). These differences are apparent most dramatically when the CO morphologies are compared to optical ground based and HST imaging: massive star formation has not started yet in the NW region; the well-known starburst in the centre is the most evolved and star formation in the SE complex started more recently. We derive a star formation efficiency of 8% for the SE complex. Using high--resolution VLA observations of neutral hydrogen HI and our CO data we generated a total gas column density map for NGC 4214 (HI + H_2). No clear correlation is seen between the peaks of HI, CO and the sites of ongoing star formation. This emphasizes the irregular nature of dwarf galaxies. The HI and CO velocities agree well, so do the H-alpha velocities. In total, we cataloged 14 molecular clumps in NGC 4214. Our results from a virial mass analysis are compatible with a Galactic CO-to-H_2 conversion factor for NGC 4214 (lower than what is usually found in metal-poor dwarf galaxies).Comment: accepted for publication in the AJ (February 2001), full ps file at: ftp://ftp.astro.caltech.edu/users/fw/ngc4214/walter_prep.p