Deidre A. Hunter

Star Formation in R136: A Cluster of O3 Stars Revealed by Hubble Space Telescope Spectroscopy*

The R136 cluster in 30 Doradus is the prototype super star cluster, and the only example sufficiently close that its massive star content can be studied directly. We have used the Hubble Space Telescope to obtain spectra of 65 of the bluest, most luminous stars in R136 and find that the majority of these stars are of type O3, the hottest, most luminous, and most massive stars known. The total number of O3 stars in this one cluster exceeds the total number known elsewhere in the Milky Way or Magellanic Clouds. The highest luminosity stars found are O3 If*, O4 If+, O3 If/WN6-A, and H-rich WN stars, with masses in excess of 120 ?, the highest masses for which appropriate evolutionary tracks are currently available. In accord with de Koter, Heap, & Hubeny, we conclude that these WN stars must be core H-burning stars whose spectra are WR-like because of high luminosity, and we find that their individual luminosities are a factor of 10 higher than is normal for WN stars of similar type but are like those found in the Galactic cluster NGC 3603, which they also resemble spectroscopically. Our spectroscopy does include stars as late as B0 V and samples most stars in the core of the R136 cluster with masses >50 M?. The spectroscopy has been combined with HST photometry to study the star formation history and initial mass function of the R136 cluster. The young age (<1-2 Myr) for the highest mass stars, combined with what was previously known for the intermediate-mass populations, suggests that the lower mass stars began forming 4-5 Myr ago and continued forming until the high mass stars formed, consistent with the paradigm in which the formation of massive stars shuts down further star formation in the molecular cloud. Despite the unique preponderance of the highest mass and luminosity stars ever seen, the initial mass function (IMF) is found to be completely normal, with a slope ? = -1.3 to -1.4. The number of high-mass stars is in good accord with that predicted by the IMF of the intermediate-mass stars, suggesting that a Salpeter-like IMF holds over the mass range 2.8-120 ? within the R136 cluster. The fact that the IMF slope in R136 is indistinguishable from those of Galactic and Magellanic Cloud OB associations suggests that star formation produces the same distribution of masses over a range of ~200 times in stellar density, from that of sparse OB associations to that typical of globular clusters. The large number of O3 stars in R136 is then simply a consequence of its youth (<1-2 Myr) and its richness, suggesting that the upper mass cutoff to the IMF seen in OB associations may simply be the result of their sparcity.

Far-infrared spectroscopy of normal galaxies: Physical conditions in the interstellar medium

The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/blue ratios (indicating star formation activity and optical depth). In two-thirds of the galaxies in this sample, the [C II] line flux is proportional to FIR dust continuum. The other one-third show a smooth decline in L[C II]/LFIR with increasing Fν(60 μm)/Fν(100 μm) and LFIR/LB, spanning a range of a factor of more than 50. Two galaxies at the warm and active extreme of the range have L[C II]/LFIR < 2 × 10-4 (3 σ upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains. The ratio of the two principal photodissociation region (PDR) cooling lines L[O I]/L[C II] shows a tight correlation with Fν(60 μm)/Fν(100 μm), indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [N II] (122 μm) and [C II] from ionized gas and use it to separate [C II] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. with observed ratios of (1) L[O I]/L[C II] and (L[C II] + L[O I])/LFIR and (2) L[O I]/LFIR and Fν(60 μm)/Fν(100 μm) yields far-UV flux G0 and gas density n. The G0 and n values estimated from the two methods agree to better than a factor of 2 and 1.5, respectively, in more than half the sources. The derived G0 and n correlate with each other, and G0 increases with n as G0 ∝ nα, where α ≈ 1.4 . We interpret this correlation as arising from Stromgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The high values of PDR surface temperature (270-900 K) and pressure (6 × 104-1.5 × 107 K cm-3) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions. The differences in G0 and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds. Galaxies with higher G0 and n have larger and/or denser star-forming clouds.

Planetary Camera observations of NGC 1275 - Discovery of a central population of compact massive blue star clusters

We have discovered a population of bright blue pointlike sources within 5 kpc of the nucleus of NGC 1275 using HST Planetary Camera observations. The typical object has M_v~- 12 to - 14 (H_0 = 75 km s^(-1) Mpc^(-1); the brightest has M_v~-16. They are all blue, with V- R≾0.3. The color distribution and lack of excess Ha emission are consistent with nearly all being continuum sources. Many of the sources are unresolved even with the HST and consequently have sizes of ≾ 15 pc. We suggest that these are young star clusters that will evolve to look like globular clusters. They are bluer than any clusters seen in the Milky Way or M87, and brighter than the blue clusters seen in the LMC. We derive ages of several hundred million years or less and corresponding masses of 10^5-10^8 M_☉. The existence of these young clusters may be connected with a current or previous interaction with another galaxy, with the cooling flow in NGC 1275, or with some combination. Structure is detected in the underlying galaxy light that is suggestive of a merge between NGC 1275 and a second galaxy some 10^8 yr ago. If this merger triggered star formation, it would naturally account for the observed uniformity of cluster colors. Steady-state star formation in the x-ray cooling flow would imply a wider range in cluster age and color than is seen, unless the clusters disrupt. An interaction with the projected high-velocity, infalling system cannot explain the observations because this system has not yet reached the center of NGC 1275 where the clusters are concentrated, and because it has a total interaction time that is far too short for either the observed cluster lifetimes or the dynamical lifetime of structure in the galaxy. If the presence of recently formed protoglobulars around NGC 1275 is related to a previous merger, this would remove an important objection to the merger hypothesis for elliptical galaxy origins, provided that adequate gas is available in the merger for their formation.

STAR FORMATION PROPERTIES OF A LARGE SAMPLE OF IRREGULAR GALAXIES

We present the results of Hα imaging of a large sample of irregular galaxies. Our sample includes 94 galaxies with morphological classifications of Im, 26 blue compact dwarfs (BCDs), and 20 Sm systems. The sample spans a large range in galactic parameters, including integrated absolute magnitude (MV of -9 to -19), average surface brightness (20–27 mag arcsec-2), current star formation activity (0–1.3 M⊙ yr-1 kpc-2), and relative gas content (0.02–5 M⊙/LB). The Hα images were used to measure the integrated star formation rates, determine the extents of star formation in the disks, and compare azimuthally averaged radial profiles of current star formation to older starlight. The integrated star formation rates of Im galaxies normalized to the physical size of the galaxy span a range of a factor of 104 with 10% Im galaxies and one Sm system having no measurable star formation at the present time. The BCDs fall, on average, at the high star formation rate end of the range. We find no correlation between star formation activity and proximity to other cataloged galaxies. Two galaxies located in voids are similar in properties to the Sm group in our sample. The H II regions in these galaxies are most often found within the Holmberg radius RH, although in a few systems H II regions are traced as far as 1.7RH. Similarly, most of the star formation is found within three disk scale lengths RD, but in some galaxies H II regions are traced as far as 6RD. A comparison of Hα surface photometry with V-band surface photometry shows that the two approximately follow each other with radius in Sm galaxies, but in most BCDs there is an excess of Hα emission in the centers that drops with radius. In approximately half of the Im galaxies Hα and V correspond well, and in the rest there are small to large differences in the relative rate of falloff with radius. The cases with strong gradients in the LHα/LV ratios and with high central star formation rate densities, which include most of the BCDs, require a significant fraction of their gas to migrate to the center in the last gigayear. We discuss possible torques that could have caused this without leaving an obvious signature, including dark matter bars and past interactions or mergers with small galaxies or H I clouds. There is now a substantial amount of evidence for these processes among many surveys of BCDs. We note that such gas migration will also increase the local pressure and possibly enhance the formation of massive dense clusters but conclude that the star formation process itself does not appear to differ much among BCD, Im, and Sm types. In particular, there is evidence in the distribution function for Hα surface brightness that the turbulent Mach numbers are all about the same in these systems. This follows from the Hα distribution functions corrected for exponential disk gradients, which are log-normal with a nearly constant dispersion. Thus, the influence of shock-triggered star formation is apparently no greater in BCDs than in Im and Sm types.

SN 1992A : ultraviolet and optical studies based on HST, IUE, and CTIO observations

The Type Ia supernova SN 1992A in the SO galaxy NGC 1380 was observed as a target of opportunity by the International Ultrauiolet Explorer (IUE) and with great alacrity by the Hubble Space Telescope (HST). Here we present the HST and IUE spectra and photometry that we obtained, as well as optical spectra obtained at the Cerro Tololo Inter-American Observatory (CTIO). The HST Faint Object Spectrograph (FOS) spectra, from 5 and 45 days past maximum light, are the best UV spectra of a Type Ia supernova and reveal for the first time with good signal-to-noise ratio the Type Ia spectral region blueward of ∼2650 A

The Relationship between Gas, Stars, and Star Formation in Irregular Galaxies: A Test of Simple Models

Irregular galaxies are a unique test of models for the physical laws regulating star formation because of their lack of spiral density waves and rotational shear. Here we explore various instability models for the onset of star formation in irregular galaxies. If the gas is unstable, clouds and eventually stars can form, and so these models should predict where star formation occurs. Critical gas densities were calculated for gravitational instabilities in two models, one with a thin, pure-gas disk (Σc) and another with a thick disk composed of gas and a starlike fluid (Σc,2f). We also calculated the stability properties of three-dimensional systems including dark matter, considered the thermal state of the gas, and used a modified threshold column density written in terms of the local rate of shear instead of the epicyclic frequency. The model predictions were compared to the azimuthally averaged present-day star formation activity traced by the Hα surface brightness and to the 1 Gyr integrated star formation activity represented by the stellar surface brightness. We find that the ratio of the observed gas density to the critical gas density, Σg/Σc, is lower by a factor of ~2 in most of the Im galaxies than it is in spiral galaxies, both at the intermediate radii where Σg/Σc is highest and at the outer radii where star formation ends. We also find that although star formation in irregulars usually occurs at intermediate radii where Σg/Σc is highest, this activity often ends before Σg/Σc drops significantly in the outer regions, and it remains high in the inner regions where Σg/Σc is often low. There are also no correlations between the peak, average, or edge values of Σg/Σc and the integrated star formation rates in irregulars. These results suggest that Σg/Σc does not trace star formation with the same detail in irregular galaxies as it appears to trace it in giant spiral galaxies. The low value of α also implies that either the gas in irregulars is more stable than it is in spirals, or Σc is not a good threshold for star-forming instabilities. Dark matter in the disks of irregulars makes the gas more unstable, but stars do the same for the disks of spirals, which leaves the ratio of the two α-values about the same. Moreover, the instability parameter with dark matter still does not follow the star formation activity in irregulars. The thermal model suggests that irregulars have difficulty in sustaining a cool, dense gas phase, and it also fails to predict where star formation occurs. An alternative model in which cloud formation involves a competition between self-gravity and shear, rather than an instability in the usual sense, is more successful in defining the threshold for star formation, but it does not predict where star formation ends either. The failure of these models suggests that processes other than spontaneous instabilities are important for star formation in irregular galaxies. The role of Σg/Σc in spiral galaxies is also questioned. The observed sensitivity of the star formation rate to Σg/Σc may be strongly dependent on instabilities specific to spiral arms and not on general instabilities of the type for which Σg/Σc was originally derived. In that case, large-scale star formation may end in the outer disks of spirals because the stellar density waves end there, at the outer Lindblad resonance. The only azimuthally averaged quantity that correlates with the current star formation activity in irregulars is the stellar surface density. A causal connection is possible if stellar energy input to the interstellar medium acts as a feedback process to star formation. If this process played a key role in initiating star formation in irregulars from the beginning, then it could explain why irregular galaxies began their evolution slowly compared to larger disk systems with spiral arms.

CLUSTER MASS FUNCTIONS IN THE LARGE AND SMALL MAGELLANIC CLOUDS: FADING AND SIZE-OF-SAMPLE EFFECTS

The properties of ~939 star clusters in the Large and Small Magellanic Clouds were determined from ground-based CCD images in UBVR passbands. The areal coverage was extensive, corresponding to 11.0 kpc2 in the LMC and 8.3 kpc2 in the SMC. After corrections for reddening, the colors and magnitudes of the clusters were converted to ages and masses, and the resulting mass distributions were searched for the effects of fading, evaporation, and size-of-sample bias. The data show a clear signature of cluster fading below the detection threshold. The initial cluster mass function (ICMF) was determined by fitting the mass and age distributions with cluster population models. These models suggest a new method to determine the ICMF that is nearly independent of fading or disruption and is based on the slope of a correlation between age and the maximum cluster mass in equally spaced intervals of log age. For a nearly uniform star formation rate, this correlation has a slope equal to 1/(α - 1) for an ICMF of dn(M)/dM ∝ M-α. We determine that α is between 2 and 2.4 for the LMC and SMC using this method plus another method in which models are fitted to the mass distribution integrated over age and to the age distribution integrated over mass. The maximum mass method also suggests that the cluster formation rate in the LMC age gap between 3 and 13 Gyr is about a factor of 10 below that in the period from 0.1 to 1 Gyr. The oldest clusters correspond in age and mass to halo globular clusters in the Milky Way. They do not fit the trends for lower mass clusters but appear to be a separate population that either had a very high star formation rate and became depleted by evaporation or formed with only high masses.

Hubble Space Telescope imaging of super-star clusters in NGC 1569 and NGC 1705

We examine the structural properties of three super-star clusters in the nearby, H I-rich galaxies NGC 1569 and NGC 1705. The clusters, which have total absolute V magnitudes between -13.3 and -14.1, appear to be point sources on ground-based images but are partially resolved in new images obtained with the Hubble Space Telescope (HST) Planetary Camera. From deconvolved V- and I-band images we find that the three clusters have very compact cores with extended halos that are partially resolved into individual stars. Using new distances to the galaxies derived from color-magnitude diagrams for field stars, we find that the half-light radii are 2.2-3.4 pc. The cluster in NGC 1705 is barely resolved in the HST images. The clusters in NGC 1569, on the other hand, show significant substructure in their cores and ellipticities that are comparable to the flattenings seen in young clusters in the Large Magellanic Cloud (LMC). The clusters show internal (V-I) color gradients. The properties of these clusters are similar to R136, the core of the luminous star-forming complex 30 Doradus in the LMC, except that R136 has a lower luminosity and central surface brightness. The half-light surface brightness of the brightest cluster (NGC 1569 A) is 1.3 x 10(exp 6) L(sub v) solar/ sq cm, which is over 65 times higher than R136 and 1200 times higher than the mean rich LMC star cluster other than R136 after allowing for aging effects. The next brightest clusters in each of these galaxies are greater than or = 2 mag fainter. Thus, the super-star clusters represent an extreme but uncommon mode of star formation. In terms of luminosity and size, they appear to be good analogs of young globular clusters.

Broadband Imaging of a Large Sample of Irregular Galaxies

We present the results of UBV imaging of a large sample of irregular galaxies: 94 Im systems, 24 blue compact dwarfs (BCDs), and 18 Sm galaxies. We also include JHK imaging of 42 of these galaxies. The sample spans a large range in galactic parameters. Ellipse fit axial ratios, inclinations, and position angles are derived, integrated photometry and azimuthally averaged surface photometry profiles are determined, and exponential fits give the central surface brightnesses, scale lengths, and isophotal and half-power radii. These data are used to address the shapes of Im galaxies, look for clues to past interactions in large-scale peculiarities, examine the nature and consequences of bars, study color gradients and large-scale color variations, and compare the exponential disk profiles of the young and old stellar components. For example, color gradients exhibit a great variety and not all passbands are correlated. Bars are associated with higher star formation rates. Many irregulars show a double-exponential radial light profile that is steeper in the outer parts, and these are reproduced by a new model of star formation that is discussed in a companion paper. Some galaxies, primarily BCDs, have double exponentials that are steeper (and bluer) in the inner parts, presumably from centralized star formation. Im-type galaxies have thicker, less prominent dust layers than spiral galaxies because of their lower average surface densities and midplane extinctions.

A survey for extra-H II region ionized gas structures in irregular galaxies

We have conducted a deep Hα imaging survey of 51 irregular and amorphous galaxies with the objective of searching for large ionized gas structures outside of normal H II regions. In this sample 12% of the galaxies contain at least one ionized supershell (radius≥3OO pc), 24% contain supergiant ionized filaments which are not obviously connected with a particular star forming region, and 27% contain one or more of these types of structures. For the most part, large ionized gas structures are found in galaxies that are engaging in intense star formation or that contain at least one unusually large concentration of massive stars. Thus, in most galaxies these structures are likely to have been produced by massive stars