arXiv subjects

Jason Harris

Publications and source records attributed to Jason Harris.

33 records · Page 2Linked to original sources

arXiv2006

Spectroscopic Survey of Red Giants in the SMC. I: Kinematics

We present a spectroscopic survey of 2046 red giant stars, distributed over the central 4x2 kpc of the Small Magellanic Cloud (SMC). After fitting and removing a small velocity gradient across the SMC (7.9 km/s/deg oriented at 10 deg E of N), we measure an rms velocity scatter of 27.5+-0.5 km/s. The line of sight velocity distribution is well-characterized by a Gaussian and the velocity dispersion profile is nearly constant as a function of radius. We find no kinematic evidence of tidal disturbances. Without a high-precision measurement of the SMC's proper motion, it is not possible to constrain the SMC's true rotation speed from our measured radial-velocity gradient. However, even with conservative assumptions, we find that v < sigma and hence that the SMC is primarily supported by its velocity dispersion. We find that the shape of the SMC, as measured from the analysis of the spatial distribution of its red giant stars, is consistent with the degree of rotational flattening expected for the range of allowed v/sigma values. As such, the properties of the SMC are consistent with similar low luminosity spheroidal systems. We conclude that the SMC is primarily a low luminosity spheroid whose irregular visual appearance is dominated by recent star formation. A simple virial analysis using the measured kinematics implies an enclosed mass within 1.6 kpc of between 1.4 and 1.9x10^9 Mo, and a less well constrained mass within 3 kpc of between 2.7 and 5.1x10^9 Mo.

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arXiv2005

The Nature of the Density Clump in the Fornax Dwarf Spheroidal Galaxy

We have imaged the recently discovered stellar overdensity located approximately one core radius from the center of the Fornax dwarf spheroidal galaxy using the Magellan Clay 6.5m telescope with the Magellan Instant Camera (MagIC). Superb seeing conditions allowed us to probe the stellar populations of this overdensity and of a control field within Fornax to a limiting magnitude of R=26. The color-magnitude diagram of the overdensity field is virtually identical to that of the control field with the exception of the presence of a population arising from a very short (less than 300 Myr in duration) burst of star formation 1.4 Gyr ago. Coleman et al. have argued that this overdensity might be related to a shell structure in Fornax that was created when Fornax captured a smaller galaxy. Our results are consistent with this model, but we argue that the metallicity of this young component favors a scenario in which the gas was part of Fornax itself.

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arXiv2004

The Magellanic Clouds Photometric Survey: The Large Magellanic Cloud Stellar Catalog and Extinction Map

We present our catalog of U, B, V, and I stellar photometry of the central 64 sq. deg. area of the Large Magellanic Cloud. Internal and external astrometric and photometric tests using existing optical photometry (U, B, and V from Massey's bright star catalog and I from the near-infrared sky survey DENIS) are used to confirm our observational uncertainty estimates. We fit stellar atmosphere models to the optical data to check the consistency of the photometry for individual stars across the passbands and to estimate the line-of-sight extinction. Finally, we use the estimated line-of-sight extinctions to produce an extinction map across the Large Magellanic Cloud, confirm the variation of extinction as a function of stellar population, and produce a simple geometrical model for the extinction as a function of stellar population.

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arXiv2004

Variations in Star Formation History and the Red Giant Branch Tip

We examine the reliability of the tip of the red giant branch (TRGB) as a distance indicator for stellar populations with different star formation histories (SFHs) when photometric errors and completeness corrections at the TRGB are small. In general, the TRGB-distance method is insensitive to the shape of the SFH except when it produces a stellar population with a significant component undergoing the red giant branch phase transition. The I-band absolute magnitude of the TRGB for the middle and late stages of this transition (~1.3-1.7 Gyr) is several tenths of a magnitude fainter than the canonical value of M_I ~ -4.0. If more than 30% of all stars formed over the lifetime of the Universe are formed at these ages, then the distance could be overestimated by 10-25%. Similarly, the TRGB-distance method is insensitive to the metallicity distribution of stars formed except when the average metallicity is greater than <[Fe/H]> = -0.3. If more than ~70% of all stars formed have [Fe/H] > -0.3, the distance could be overestimated by ~10-45%. We find that two observable quantities, the height of the discontinuity in the luminosity function at the TRGB and the median (V-I)_0 at M_I = -3.5 can be used to test if the aforementioned age and metallicity conditions are met.

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arXiv2003

The Ionized Gas in Local Starburst Galaxies: Global and Small--Scale Feedback from Star Formation

Abridged: The small-- and intermediate--scale structure and the fraction of the ISM ionized by non--radiative processes is investigated in a small sample of four local starburst galaxies, imaged with the HST WFPC2. The sample comprises three dwarf galaxies, NGC3077, NGC4214, and NGC5253, and one giant spiral, NGC5236 (M83). The galaxies span a range in metallicity, luminosity, and environment, enabling the investigation of non--radiative ionization processes in a variety of galactic conditions. For this purpose, the four galaxies were imaged in the lines of H-beta(4861 A), [OIII](5007 A), H-alpha(6563 A), and [SII](6717,6731 A). The non--photoionized gas in these galaxies has been traced on scales ranging from a few tens of pc to a few hundred pc, providing a full budget for this ionized gas component. Using the `maximum starburst line' of Kewley et al. (2001) to discriminate between photoionized and non--photoionized gas, we find that in all four galaxies non--photoionization processes are responsible for a small fraction of the total H-alpha emission, at the level of 3%--4%. The starbursts in the three dwarf galaxies deposit a significant fraction, 70%-100%, of their mechanical energy into the surrounding interstellar medium and require time--extended (a few x 10^7 yr to ~10^8 yr) star formation, in order to account for the observed luminosity of the non--photoionized gas. In the massive spiral, the non--photoionized gas is concentrated in localized areas surrounded by active star--formation, with no evidence for extended structures (as those in the dwarfs). This confirms the picture that starbursts remain confined events in massive galaxies, likely due to the deep potential well.

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arXiv2003

Quantifying the Drivers of Star Formation on Galactic Scales. I. The Small Magellanic Cloud

We use the star formation history of the Small Magellanic Cloud (SMC) to place quantitative limits on the effect of tidal interactions and gas infall on the star formation and chemical enrichment history of the SMC. The coincident timing of two recent (< 4 Gyr) increases in the star formation rate and SMC/Milky Way(MW) pericenter passages suggests that global star formation in the SMC is driven at least in part by tidal forces due to the MW. The Large Magellanic Cloud (LMC) is the other potential driver of star formation, but is only near the SMC during the most recent burst. The poorly constrained LMC-SMC orbit is our principal uncertainty. To explore the correspondence between bursts and MW pericenter passages further, we model star formation in the SMC using a combination of continuous and tidally-triggered star formation. The behavior of the tidally-triggered mode is a strong inverse function of the SMC-MW separation (preferred behavior ~ r^-5, resulting in a factor of ~100 difference in the rate of tidally-triggered star formation at pericenter and apocenter). Despite the success of these closed-box evolutionary models in reproducing the recent SMC star formation history and current chemical abundance, they have some systematic shortcomings that are remedied by postulating that a sizable infall event (~ 50% of the total gas mass) occured about 4 Gyr ago. Regardless of whether this infall event is included, the fraction of stars in the SMC that formed via a tidally triggered mode is > 10% and could be as large as 70%.

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arXiv2001

Star Formation in the Local Universe

We present preliminary results of a long-term study aimed at answering a number of open questions on the evolution of starbursts in local galaxies. The project employes mainly HST data from the ultraviolet to the red of the stellar continuum and of the nebular emission from the galaxies. Here we concentrate on NGC5253 and NGC5236 (M83), that form a dwarf--massive galaxy pair at about 4 Mpc distance. The recent star formation history of the centers of the two galaxies is investigated in order to identify similarities and differences in the evolution of their central starbursts.

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arXiv2001

The Magellanic Clouds Photometric Survey: The Small Magellanic Cloud Stellar Catalog and Extinction Map

We present our catalog of U, B, V, and I stellar photometry of the central 18 sq. degree area of the Small Magellanic Cloud. We combine our data with the 2MASS and DENIS catalogs to provide, when available, U through K_S data for stars. Internal and external astrometric and photometric tests using existing optical photometry (U, B, and V from Massey's bright star catalog; B, V, and I from the microlensing database of OGLE and I from the near-infrared sky survey DENIS) are used to determine the observational uncertainties and identify systematic errors. We fit stellar atmosphere models to the optical data to check the consistency of the photometry for individual stars across the passbands and to estimate the line-of-sight extinction. Finally, we use the estimated line-of-sight extinctions to produce an extinction map across the Small Magellanic Cloud and investigate the nature of extinction as a function of stellar population.

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arXiv2001

A Method for Determining the Star Formation History of a Mixed Stellar Population

We present a method to determine the star formation history of a mixed stellar population from its photometry. We perform a chi-squared minimization between the observed photometric distribution and a model photometric distribution, based on theoretical isochrones. The initial mass function, distance modulus, interstellar reddening, binary fraction and photometric errors are incorporated into the model, making it directly comparable to the data. The model is a linear combination of individual synthetic color-magnitude diagrams (CMDs), each of which represents the predicted photometric distribution of a stellar population of a given age and metallicity. We present extensive tests of the algorithm, using both simulated and actual photometric data. From a preliminary application of the algorithm to a subregion of the Large Magellanic Cloud (LMC), we find that the that the lull in star formation observed among the LMC's cluster population between 3 and 8 Gyr ago is also present in the field population. The method was designed with flexibility and generality in mind, and we make the code available for use by the astronomical community.

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arXiv2000

Ultraviolet and Optical Observations of OB Associations and Field Stars in the Southwest Region of the Large Magellanic Cloud

Using photometry from the Ultraviolet Imaging Telescope (UIT) and photometry and spectroscopy from three ground-based optical datasets we have analyzed the stellar content of OB associations and field areas in and around the regions N 79, N 81, N 83, and N 94 in the LMC. We compare data for the OB association Lucke-Hodge 2 (LH 2) to determine how strongly the initial mass function (IMF) may depend on different photometric reductions and calibrations. We also correct for the background contribution of field stars, showing the importance of correcting for field star contamination in determinations of the IMF of star formation regions. It is possible that even in the case of an universal IMF, the variability of the density of background stars could be the dominant factor creating the differences between calculated IMFs for OB associations. We have also combined the UIT data with the Magellanic Cloud Photometric Survey to study the distribution of the candidate O-type stars in the field. We find a significant fraction, roughly half, of the candidate O-type stars are found in field regions, far from any obvious OB associations. These stars are greater than 2 arcmin (30 pc) from the boundaries of existing OB associations in the region, which is a distance greater than most O-type stars with typical dispersion velocities will travel in their lifetimes. The origin of these massive field stars (either as runaways, members of low-density star-forming regions, or examples of isolated massive star formation) will have to be determined by further observations and analysis.

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arXiv2000

The Morphologies of the Small Magellanic Cloud

We compare the distribution of stars of different spectral types, and hence mean age, within the central SMC and find that the asymmetric structures are almost exclusively composed of young main sequence stars. Because of the relative lack of older stars in these features, and the extremely regular distribution of red giant and clump stars in the SMC central body, we conclude that tides alone are not responsible for the irregular appearance of the central SMC. The dominant physical mechanism in determining the current-day appearance of the SMC must be star formation triggered by a hydrodynamic interaction between gaseous components. These results extend the results of population studies (cf. Gardiner and Hatzidimitriou) inward in radius and also confirm the suggestion of the spheroidal nature of the central SMC based on kinematic arguments (Dopita et al; Hardy, Suntzeff & Azzopardi). Finally, we find no evidence in the underlying older stellar population for a ``bar'' or ``outer arm'', again supporting our classification of the central SMC as a spheroidal body with highly irregular recent star formation.

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arXiv1999

On the Spatial Distribution of Stellar Populations in the Large Magellanic Cloud

We measure the angular correlation function of stars in a region of the Large Magellanic Cloud (LMC) that spans 2 degrees by 1.5 degrees. We find that the correlation functions of stellar populations are represented well by exponential functions of the angular separation for separations between 2 and 40 arcmin (corresponding to ~ 30 pc and 550 pc for an LMC distance of 50 kpc). The inner boundary is set by the presence of distinct, highly correlated structures, which are the more familiar stellar clusters, and the outer boundary is set by the observed region's size and the presence of two principal centers of star formation within the region. We also find that the normalization and scale length of the correlation function changes systematically with the mean age of the stellar population. The existence of positive correlation at large separations (~300 pc), even in the youngest population, argues for large-scale hierarchical structure in current star formation. The evolution of the angular correlation toward lower normalizations and longer scale lengths with stellar age argues for the dispersion of stars with time. We show that a simple, stochastic, self-propagating star formation model is qualitatively consistent with this behavior of the correlation function.

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arXiv1999

Constraints on Intervening Stellar Populations Toward the Large Magellanic Cloud

The suggestion by Zaritsky & Lin that a vertical extension of the red clump feature in color-magnitude diagrams of the Large Magellanic Cloud (LMC) is consistent with a significant population of foreground stars to the LMC that could account for the observed microlensing optical depth has been challenged by various investigators. We respond by (1) examining each of the challenges presented and (2) presenting new photometric and spectroscopic data. We conclude that although the CMD data do not mandate the existence of a foreground population, they are entirely consistent with a foreground population associated with the LMC that contributes significantly (~ 50%) to the observed microlensing optical depth. From our new data, we conclude that <~ 40% of the VRC stars are young, massive red clump stars because (1) synthetic color-magnitude diagrams created using the star formation history derived indepdently from HST data suggest that < 50% of the VRC stars are young, massive red clump stars, (2) the angular distribution of the VRC stars is more uniform than that of the young (age < 1 Gyr) main sequence stars, and (3) the velocity dispersion of the VRC stars in the region of the LMC examined by ZL is inconsistent with the expectation for a young disk population. Each of these arguments is predicated on assumptions and the conclusions are uncertain. Therefore, an exact determination of the contribution to the microlensing optical depth by the various hypothesized foreground populations, and the subsequent conclusions regarding the existence of halo MACHOs, requires a detailed knowledge of many complex astrophysical issues, such as the IMF, star formation history, and post-main sequence stellar evolution. (abridged)

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arXiv1997

On the Distribution of Dust in the Large Magellanic Cloud

We present a detailed map of the reddening in a 1.9 x 1.5 degree section of the Large Magellanic Cloud, constructed from UBVI photometry of 2069 O and B main sequence stars. We use two reddening-free photometric parameters to determine the line-of-sight reddening to these stars. We find =0.20 mag, with a non-Gaussian tail to high values. When the reddening is corrected for foreground Galactic extinction, we find =0.13 mag. The line-of- sight values are then interpolated onto a uniform grid with a local least- squares plane fitting routine to construct a reddening map of the region. We use the distribution of reddening values to constrain the line-of-sight geometry of stars and dust in the LMC, and to test and normalize a standard extinction correction for galaxy photometry. We attempt to distinguish between line-of-sight depth effects and structure in the dust distribution as possible causes for the observed differential reddening through this region. We conclude that our data are consistent with a vertical exponential distribution of stars and dust in the LMC, for which the dust scale height is twice that of the OB stars; that the dust distribution must be non- uniform (clumpy) to account for the full distribution of measured reddening values; and that the B-band optical depth through the observed region of the LMC is 0.69 < tau < 0.82.

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arXiv1997

A Digital Photometric Survey of the Magellanic Clouds: First Results From One Million Stars

We present the first results from, and a complete description of, our ongoing UBVI digital photometric survey of the Magellanic Clouds. In particular, we discuss the photometric quality and automated reduction of a CCD survey (magnitude limits, completeness, and astrometric accuracy) that covers the central 8 by 8 degrees of the Large Magellanic Cloud (LMC) and 4 by 4 degrees of the Small Magellanic Cloud (SMC). We discuss photometry of over 1 million stars from the initial survey observations (an area northwest of the LMC bar covering 2 by 1.5 degrees) and present a deep stellar cluster catalog that contains about 45% more clusters than previously identified within this region. Of the 68 clusters found, only 12 are also identified as concentrations of ``old'', red clump stars. Furthermore, only three clusters are identified solely on the basis of a concentration of red clump stars, rather than as a concentration of luminous (V < 21) main sequence stars. Extrapolating from the current data, we expect to obtain B and V photometry for 25 million stars, and U and I photometry for 10 and 20 million stars, respectively, over the entire survey area.

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