arXiv subjects

Evan D. Skillman

Publications and source records attributed to Evan D. Skillman.

At least 217 records · Page 12Linked to original sources

arXiv2004

Probing The Multiphase Interstellar Medium Of The Dwarf Starburst Galaxy NGC 625 With FUSE Spectroscopy

We present new FUSE spectroscopy of the dwarf starburst galaxy NGC 625. These observations probe multiple phases of the interstellar medium, including the coronal, ionized, neutral and molecular gas. This nearby (D = 3.9 +/- 0.2 Mpc) system shows a clear detection of outflowing coronal gas as traced by OVI 1032 Angstrom absorption. The centroid of the OVI profile is blueshifted with respect to the galaxy systemic velocity by ~ 30 km/sec, suggesting a low-velocity outflow. The implied OVI velocity extent is found to be 100 +/- 20 km/sec, which is fully consistent with the detected HI outflow velocity found in radio synthesis observations. We detect multiple lines of diffuse H2 absorption from the ISM of NGC 625; this is one of only a few extragalactic systems with FUSE detections of H2 lines in the Lyman and Werner bands. We find a potential abundance offset between the neutral and nebular gas that exceeds the errors on the derived column densities. Since such an offset has been found in multiple dwarf galaxies, we discuss the implications of a lower-metallicity halo surrounding the central star forming regions of dwarf galaxies. The apparent offset may be due to saturation of the observed OI line, and higher S/N observations are required to resolve this issue.

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arXiv2004

HI Observations of the Local Group Dwarf WLM

We present Australia Telescope Compact Array mosaic HI imaging of the Local Group dwarf irregular galaxy WLM. We find an integrated flux of 149 Jy km/s and a total HI mass of 3.2 x 10^7 solar masses. The major axis of the HI is aligned with the stellar component of the galaxy. The overall HI distribution is relatively smooth at our resolution and has a double peaked central core. One of these peaks is aligned with a region found to have extinction that is internal to WLM and we take this as possible evidence of a large molecular gas complex in the southern half of the galaxy. The other HI peak is in close proximity to the brightest HII regions. WLM's overall velocity field is consistent with rigid body rotation. A rotation curve is derived, and we find a total dynamical mass of 3.00 x 10^8 solar masses. We also performed a wide field search, 38 arcminutes in radius, for HI companions or evidence of recent interactions (e.g., tidal tails), and found no detections to an HI mass limit of M_HI > 8.4 x 10^5 solar masses.

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arXiv2004

Chemical Abundances of H II Regions in the Starburst Galaxy NGC 1705

(Shortened version) We report optical spectroscopy of 16 H II regions in NGC 1705 and [O III] 4363 detections for the first time in five H II regions. The resulting mean oxygen abundance derived directly from measured electron temperatures is 12+log(O/H) = 8.21 +/- 0.05, which corresponds to [O/H] = -0.45, or 35% of the solar value. There are no significant spatial inhomogeneities in [O III] 4363 oxygen abundances from H II regions at a radius approximately 10 arcsec from the super star cluster. In H II regions where [O III] 4363 was not measured, oxygen abundances derived with bright-line methods (accurate only to 0.2 dex) are in agreement with direct values of the oxygen abundance. Faint narrow He II 4686 emission is found in two H II regions, but the implied contribution from O^+3 to the total oxygen abundance is only 0.01 dex. Interestingly, the nitrogen-to-oxygen abundance ratio in the ionized H II gas agrees with the value for the neutral H I, even though the metallicity of the neutral gas may be a factor of six lower than that of the ionized gas. This may be indicative of low-metallicity gas in the halo of the galaxy. Extinction values, A_V, derived from observed Balmer line ratios along lines of sight to H II regions are in the range between zero and 0.9 mag. With respect to the metallicity-luminosity and metallicity-gas fraction diagnostics, the measured oxygen abundance for NGC 1705 is comparable to Local Group dwarf irregulars at a given luminosity and gas fraction.

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arXiv2004

A Realistic Determination of the Error on the Primordial Helium Abundance: Steps Toward Non-Parametric Nebular Helium Abundances

Currently there are two significantly different observational determinations of the primordial helium abundance, and, if only statistical errors in He4 abundance determinations are considered, the discrepancies between the observational determinations and the value favored by the WMAP results are significant. Here we examine in detail some likely sources of systematic uncertainties which may resolve the differences between the two determinations. We conclude that the observational determination of the primordial helium abundance is completely limited by systematic errors and that these systematic errors have not been fully accounted for in any published observational determination of the primordial helium abundance. In principle, the observed metal-poor HII region spectra should be analyzed in a non-parametric way, such that the HII region physical conditions and the helium abundance are derived solely from the relative flux ratios of the helium and hydrogen emission lines. In practice, there are very few HII region spectra with the quality that allow this, so that most analyses depend on assumed ranges or relationships between physical parameters, resulting in parametric solutions with underestimated error bars. A representative result of our analysis yields Y_p = 0.249 \pm 0.009. However, given that most of the spectra analyzed to date do not significantly constrain the primordial helium abundance, we argue in favor a range of allowed values of 0.232 le Y_p le 0.258. [abstract abridged]

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arXiv2004

The Nature of Radio Continuum Emission in the Dwarf Starburst Galaxy NGC 625

We present new multi-frequency radio continuum imaging of the dwarf starburst galaxy NGC 625 obtained with the Very Large Array. Data at 20, 6, and 3.6 cm reveal global continuum emission dominated by free-free emission, with only mild synchrotron components. Each of the major HII regions is detected; the individual spectral indices are thermal for the youngest regions (showing strongest H Alpha emission) and nonthermal for the oldest. We do not detect any sources that appear to be associated with deeply embedded, dense, young clusters, though we have discovered one low-luminosity, obscured source that has no luminous optical counterpart and which resides in the region of highest optical extinction. Since NGC 625 is a Wolf-Rayet galaxy with strong recent star formation, these radio properties suggest that the youngest star formation complexes have not yet evolved to the point where their thermal spectra are significantly contaminated by synchrotron emission. The nonthermal components are associated with regions of older star formation that have smaller ionized gas components. These results imply a range of ages of the HII regions and radio components that agrees with our previous resolved stellar population analysis, where an extended burst of star formation has pervaded the disk of NGC 625 over the last ~ 50 Myr. We compare the nature of radio continuum emission in selected nearby dwarf starburst and Wolf-Rayet galaxies, demonstrating that thermal radio continuum emission appears to be more common in these systems than in typical HII galaxies with less recent star formation and more evolved stellar clusters.

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arXiv2004

The Complex Neutral Gas Dynamics Of The Dwarf Starburst Galaxy NGC 625

We present new multi-configuration HI aperture synthesis imaging of the nearby dwarf starburst galaxy NGC 625 obtained with the Australia Telescope Compact Array. Total HI column density images show gas well-aligned with the optical major axis, and low-column density HI extending to > 6 optical scale lengths. The HI velocity field, on the other hand, is highly disturbed, with neutral gas at nearly all detected velocities within the central region. After considering various interpretations, we find that a blowout scenario most accurately describes the data. Since at our resolution we do not detect any large evacuated holes in the HI disk, we interpret this blowout to be the result of the extended (both spatially and temporally) star formation event which NGC 625 has undergone in the last 100 Myr. This is one of the clearest examples of HI outflow detected in a dwarf galaxy. We find no obvious external trigger for this extended star formation event. We detect strong radio continuum emission from the largest HII regions; comparing to our HST and ground-based H Alpha fluxes suggests either appreciable amounts of extinction toward the star formation regions, or the contribution of non-thermal sources to the radio continuum luminosity.

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arXiv2004

Extended Tidal Structure In Two Lyman Alpha-Emitting Starburst Galaxies

We present new VLA C-configuration HI imaging of the Lyman Alpha-emitting starburst galaxies Tol 1924-416 and IRAS 08339+6517. The effective resolution probes neutral gas structures larger than 4.7 kpc in Tol 1924-416, and larger than 8.1 kpc in IRAS 08339+6517. Both systems are revealed to be tidally interacting: Tol 1924-416 with ESO 338-IG04B (6.6 arcminutes = 72 kpc minimum separation), and IRAS 08339+6517 with 2MASX J08380769+6508579 (2.4 arcminutes = 56 kpc minimum separation). The HI emission is extended in these systems, with tidal tails and debris between the target galaxies and their companions. Since Lyman Alpha emission has been detected from both of these primary systems, these observations suggest that the geometry of the ISM is one of the factors affecting the escape fraction of Lyman Alpha emission from starburst environments. Furthermore, these observations argue for the importance of interactions in triggering massive star formation events.

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arXiv2003

The Recent Evolution of the Dwarf Starburst Galaxy NGC 625 from Hubble Space Telescope Imaging

New HST/WFPC2 imaging of the dwarf starburst galaxy NGC 625 is presented. These data, 80% complete to V and I magnitudes of 26.0 and 25.3, respectively, allow us to study the recent star formation history of NGC 625. We derive a tip of the red giant branch (TRGB) distance modulus of 27.95+/-0.07, corresponding to a distance of 3.89+/-0.22 Mpc, and a location on the far side of the Sculptor Group. NGC 625 has a well-defined radial stellar population gradient, evidenced by a central concentration of young MS stars and an RGB/AGB ratio that increases with galactocentric distance. The prominent AGB is very red, and RGB stars are detected far from the central star forming regions. Using H Alpha and H Beta narrow band imaging and previous optical spectroscopy we identify substantial and varying internal extinction (A_V = 0.0 to 0.6 mag) associated with the central active star formation regions. To better understand the effects of internal extinction on the analysis of young stellar populations, synthetic models are presented which, for the first time, examine and account for this effect. Using the luminous blue helium burning stars, we construct a simple model of the recent (< 100 Myr) star formation in which an elevated but declining star formation rate has been present over this entire period. This is at odds with the presence of spectroscopic W-R features in the major star formation region which imply a short duration (<= 5 Myr) for the recent starburst. This suggests that starbursts displaying W-R features are not necessarily all of a short duration. Finally, we speculate on the possible causes of the present burst of star formation in this apparently isolated galaxy, and compare it to other nearby, well-studied dwarf starburst systems.

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arXiv2003

Deep Hubble Space Telescope Imaging of IC 1613 II. The Star Formation History

We present deep HST WFPC2 imaging of the Local Group dwarf irregular galaxy IC 1613. The photometry is the deepest to date for an isolated dwarf irregular galaxy. The resulting color-magnitude diagram (CMD) is analyzed using three different methods to derive a star formation history (SFH). All three find an enhanced star formation rate (SFR), from 3 to 6 Gyr ago, and similar age-metallicity relationships (AMR). A comparison of the newly observed outer field with an earlier studied central field of IC 1613 shows that the SFR in the outer field has been significantly depressed during the last Gyr. This implies that the optical scale length of the galaxy has been decreasing with time and that comparison of galaxies at intermediate redshift with present day galaxies should take this effect into account. We find strong similarities between IC 1613 and the more distant Milky Way dSph companions in that all are dominated by star formation at intermediate ages. In particular, the SFH and AMR for IC 1613 and Leo I are indistinguishable. This implies that dIrr galaxies cannot be distinguished from dSphs by their intermediate age stellar populations. This type of a SFH may also be evidence for slower or suppressed early star formation in dwarf galaxies due to photoionization after the reionization of the universe by background radiation. Assuming that IC 1613 is typical of a dIrr evolving in isolation, since most of the star formation occurs at intermediate ages, these dwarf systems cannot be responsible for the fast chemical enrichment of the IGM which is seen at high redshift. There is no evidence for any large amplitude bursts of star formation in IC 1613, and we find it highly unlikely that analogs of IC 1613 have contributed to the excess of faint blue galaxies in existing galaxy redshift surveys.

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arXiv2003

Deep HST Imaging of Sextans A. III. The Star Formation History

We present a measurement of the star formation history of Sextans A, based on WFPC2 photometry that is 50% complete to V=27.5 (M_V ~+ 1.9) and I=27.0. The star formation history and chemical enrichment history have been measured through modeling of the CMD. We find evidence for increased reddening in the youngest stellar populations and an intrinsic metallicity spread at all ages. Sextans A has been actively forming stars at a high rate for ~ 2.5 Gyr ago, with an increased rate beginning ~ 0.1 Gyr ago. We find a non-zero number of stars older than 2.5 Gyr, due to the limited depth of the photometry, a detailed star formation history at intermediate and older ages has considerable uncertainties. The mean metallicity was found to be [M/H] ~- 1.4 over the measured history of the galaxy, with most of the enrichment happening at ages of at least 10 Gyr. We also find that an rms metallicity spread of 0.15 dex at all ages allows the best fits to the observed CMD. We revisit our determination of the recent star formation history (age <= 0.7 Gyr) using BHeB stars and find good agreement for all but the last 25 Myr, a discrepancy resulting primarily from different distances used in the two analyses and the differential extinction in the youngest populations. This indicates that star formation histories determined solely from BHeB stars should be confined to CMD regions where no contamination from reddened MS stars is present.

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arXiv2002

Deep HST Imaging of Sextans A. II. Cepheids and Distance

We have identified 82 short-period variable stars in Sextans A from deep WFPC2 observations. All of the periodic variables appear to be short-period Cepheids, with periods as small as 0.8 days for fundamental-mode Cepheids and 0.5 days for first-overtone Cepheids. These objects have been used, along with measurements of the RGB tip and red clump, to measure a true distance modulus to Sextans A of (m-M)_0 = 25.61 +/- 0.07, corresponding to a distance of d = 1.32 +/- 0.04 Mpc. Comparing distances calculated by these techniques, we find that short-period Cepheids (P < 2 days) are accurate distance indicators for objects at or below the metallicity of the SMC. As these objects are quite numerous in low-metallicity star-forming galaxies, they have the potential for providing extremely precise distances throughout the Local Group. We have also compared the relative distances produced by other distance indicators. We conclude that calibrations of RR Lyraes, the RGB tip, and the red clump are self-consistent, but that there appears to be a small dependence of long-period Cepheid distances on metallicity. Finally, we present relative distances of Sextans A, Leo A, IC 1613, and the Magellanic Clouds.

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arXiv2002

Star Formation in Sculptor Group Dwarf Irregular Galaxies and the Nature of "Transition" Galaxies

We present new H-alpha narrow band imaging of the HII regions in eight Sculptor Group dwarf irregular (dI) galaxies. Comparing the Sculptor Group dIs to the Local Group dIs, we find that the Sculptor Group dIs have, on average, lower values of SFR when normalized to either galaxy luminosity or gas mass (although there is considerable overlap between the two samples). The properties of ``transition'' (dSph/dIrr) galaxies in Sculptor and the Local Group are also compared and found to be similar. The transition galaxies are typically among the lowest luminosities of the gas rich dwarf galaxies. Relative to the dwarf irregular galaxies, the transition galaxies are found preferentially nearer to spiral galaxies, and are found nearer to the center of the mass distribution in the local cloud. While most of these systems are consistent with normal dI galaxies which currently exhibit temporarily interrupted star formation, the observed density-morphology relationship (which is weaker than that observed for the dwarf spheroidal galaxies) indicates that environmental processes such as ``tidal stirring'' may play a role in causing their lower SFRs.

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arXiv2002

ISM Abundances in Sculptor Group Dwarf Irregular Galaxies

Using the CTIO 4-m telescope, we have obtained optical spectra of HII regions in five Sculptor Group dwarf irregular galaxies. We derive oxygen, nitrogen, and sulfur abundances from the HII region spectra. Oxygen abundances are derived via three different methods (the ``direct'' method, the empirical method guided by photoionization modeling of McGaugh (1991), and the purely empirical method of Pilyugin (2000)) and are compared. Significant systematic differences are found between the three methods, and we suggest that a recalibration of the empirical abundance scale is required. The N/O ratio for the metal-poor dI ESO 473-G24 of log (N/O) = -1.43 +/- 0.03 lies well above the plateau of log (N/O) = -1.60 +/- 0.02 found by Izotov & Thuan (1999) for a collection of metal-poor blue compact galaxies. This shows that not all galaxies with 12 + log (O/H) < 7.6 have identical elemental abundance ratios, and this implies that the Izotov & Thuan scenario for low metallicity galaxies is not universal. Measurements of the HII regions in NGC 625 yield log (N/O) = -1.25. Assuming N production by intermediate mass stars, this relatively high N/O ratio may be indicative of a long quiescent period prior to the recent active burst of star formation. The oxygen abundances in the Sculptor Group dIs are in good agreement with the relationship between metallicity and luminosity observed in the Local Group dIs. The Sculptor Group dIs, in general, lie closer to the simple closed box model evolutionary path than the Local Group dIs. The higher gas contents, lower average star formation rates, and closer resemblance to closed box evolution could all be indicative of evolution in a relatively low density environment.

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arXiv2002

ISO SWS Observations of H II Regions in NGC 6822 and I ZW 36: Sulfur Abundances and Temperature Fluctuations

We report ISO SWS infrared spectroscopy of the H II region Hubble V in NGC 6822 and the blue compact dwarf galaxy I Zw 36. Observations of Br alpha, [S III] at 18.7 and 33.5 microns, and [S IV] at 10.5 microns are used to determine ionic sulfur abundances in these H II regions. There is relatively good agreement between our observations and predictions of S^+3 abundances based on photoionization calculations, although there is an offset in the sense that the models overpredict the S^+3 abundances. We emphasize a need for more observations of this type in order to place nebular sulfur abundance determinations on firmer ground. The S/O ratios derived using the ISO observations in combination with optical data are consistent with values of S/O, derived from optical measurements of other metal-poor galaxies. We present a new formalism for the simultaneous determination of the temperature, temperature fluctuations, and abundances in a nebula, given a mix of optical and infrared observed line ratios. The uncertainties in our ISO measurements and the lack of observations of [S III] lambda 9532 or lambda 9069 do not allow an accurate determination of the amplitude of temperature fluctuations for Hubble V and I Zw 36. Finally, using synthetic data, we illustrate the diagnostic power and limitations of our new method.

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arXiv2002

Radiative Transfer Effects in He I Emission Lines

We consider the effect of optical depth of the 2 ^{3}S level on the nebular recombination spectrum of He I for a spherically symmetric nebula with no systematic velocity gradients. These calculations, using many improvements in atomic data, can be used in place of the earlier calculations of Robbins. We give representative Case B line fluxes for UV, optical, and IR emission lines over a range of physical conditions: T=5000-20000 K, n_{e}=1-10^{8} cm^{-3}, and tau_{3889}=0-100. A FORTRAN program for calculating emissivities for all lines arising from quantum levels with n < 11 is also available from the authors. We present a special set of fitting formulae for the physical conditions relevant to low metallicity extragalactic H II regions: T=12,000-20,000 K, n_{e}=1-300 cm^{-3}, and tau_{3889} < 2.0. For this range of physical conditions, the Case B line fluxes of the bright optical lines 4471 A, 5876 A, and 6678 A, are changed less than 1%, in agreement with previous studies. However, the 7065 A corrections are much smaller than those calculated by Izotov & Thuan based on the earlier calculations by Robbins. This means that the 7065 A line is a better density diagnostic than previously thought. Two corrections to the fitting functions calculated in our previous work are also given.

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arXiv2001

Dust In I Zw 18 From Hubble Space Telescope Narrow Band Imaging

We present new WFPC2 narrow band imaging of the blue compact dwarf galaxy I Zw 18, which is host to the lowest-metallicity HII regions known. Images at H-alpha and H-beta are combined with archival broad band images to allow the study of the ionized gas distribution and morphology. Analysis of the H-alpha/H-beta flux ratio reveals significant enhancements in some areas of both the ``Northwest'' and ``Southeast'' regions of the galaxy, with ratios elevated to levels as high as 3.4. The H-alpha/H-beta ratio varies considerably with position throughout the galaxy. Comparing this distribution with the stellar distribution indicates that the regions of enhanced H-alpha/H-beta ratio are not due to the effects of either collisional excitation or underlying stellar absorption, and therefore are most likely interpreted as the presence of dust. This dust has an estimated mass of (2-5)x10^3 solar masses, which is consistent with the IRAS far-IR non-detection. Under the assumption that dust traces the presence of molecular gas, these results suggest that the molecular component of the ISM of I Zw 18, which is needed to fuel its active star formation, is also very clumpy. Such a distribution would be consistent with the recent FUSE non-detections of diffuse H_2.

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arXiv2001

Kinematic Constraints on Evolutionary Scenarios for Blue Compact Dwarf Galaxies : I. Neutral Gas Dynamics

We present the results of high spatial resolution HI synthesis observations of six blue compact dwarf (BCD) galaxies. Optically, the selected galaxies have smooth, symmetric isophotes, and thus are the most likely of the BCD class to fade into an object morphologically similar to a dwarf elliptical when the current starburst ends. The neutral gas in all six galaxies appears to be rotationally supported, however, indicating that true morphological transformation from a BCD to a dE will require significant loss of angular momentum. Based on the observed neutral gas dynamics of these and other BCDs, it is unlikely that present-day BCDs will evolve directly into dwarf ellipticals after a starburst phase. We discuss alternative evolutionary scenarios for BCDs and place them within the larger context of galaxy formation and evolution models.

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arXiv2000

Deep HST Imaging of IC 1613. I. Variable Stars and Distance

We present WFPC2 VI photometry of a field in the halo of IC 1613, finding 13 RR Lyraes and 11 Cepheids. Our photometry of the red giant branch tip and red clump is used to derive distances to IC 1613, which are consistent with each other and with distances based on the variable stars. We compare these values with similarly-measured distances for the Magellanic Clouds, and are able to measure metallicity dependencies of the RR Lyrae and Cepheid distances by requiring consistent relative distance measurements from the four techniques. For metallicities of [Fe/H] = -1.3 (RR Lyraes) and -1.0 (Cepheids), we find a relatively steep slope of 0.34 +/- 0.20 magnitudes per dex for the RR Lyraes and a shallow slope of -0.07 +/- 0.16 mag/dex for the Cepheids, both values within the range of theoretical and empirical results in the literature. We find that a dependence of the red clump absolute magnitude on age, in addition to metallicity, is required to produce self-consistent relative distances between IC 1613 and the Magellanic Clouds. Adopting such a red clump calibration and self-consistent calibrations for the other three distance indicators, we find that the distances to all three objects are in excellent agreement. Our best distance modulus to IC 1613 is mu_0 = 24.31 +/- 0.06, corresponding to a distance of 730 +/- 20 kpc. This distance produces an RR Lyrae absolute magnitude of MV = 0.61 +/- 0.08.

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