R. Walterbos

Diffuse Ionized Gas in M51/NGC 5195 and M81

We present deep Hα and [S II] emission-line images of three galaxies, the Sab spiral M81, the Sc spiral M51, and its companion, NGC 5195, in our ongoing study of the diffuse ionized gas (DIG) in spiral galaxies. M81 and M51 show widespread DIG with the characteristic enhanced [S II]/Hα line intensities also seen in other galaxies. A large fraction of the Hα emission in these systems, 0.40-0.50, is contributed by DIG, which is in agreement with results obtained for other disk galaxies. In spite of a drastic difference in the star formation rate per unit disk area and the difference in Hubble types, the overall properties of the DIG appear similar in both galaxies. In detail, however, differences do appear. There is some indication that the [S II]/Hα ratio is systematically lower in M51 than it is in M81. There is also a radial dependence on the [S II]/Hα ratio in M51 that is not seen in M81. The Hα emission from the general bulge area of M81, while diffuse in nature, has a distinctly higher [S II]/Hα intensity ratio than disk DIG elsewhere in M81. This ionized gas in the bulge is most likely not photoionized by massive stars, the dominant source of ionization for DIG in galactic disks. Overall, our results reinforce the important role of DIG in the ISM of spiral galaxies in general. We detect significant emission from the companion to M51, NGC 5195. We find clumps of Hα emission in an 800 × 800 pc region offset from the nucleus and spectacular kiloparsec-long filaments reminiscent of the outflow cones in starburst galaxies. We attribute these structures to the recent passage of NGC 5195 through the disk plane of M51, which led to a burst of star formation some 70 million years ago.

Past and present star formation in the SMC: NGC 346 and its neighborhood

In the quest to understand how star formation occurs and propagates in the low-metallicity environment of the Small Magellanic Cloud (SMC), we acquired deep F555W (~V) and F814W (~I) Hubble Space Telescope ACS images of the young and massive star-forming region NGC 346. These images and their photometric analysis provide us with a snapshot of the star formation history of the region. We find evidence for star formation extending from ?10 Gyr in the past until ?150?Myr in the field of the SMC. The youngest stellar population (~3 ? 1?Myr) is associated with the NGC 346 cluster. It includes a rich component of low-mass pre-main-sequence stars mainly concentrated in a number of subclusters spatially colocated with CO clumps previously detected by Rubio and coworkers. Within our analysis uncertainties, these subclusters appear coeval with each other. The most massive stars appear concentrated in the central subclusters, indicating possible mass segregation. A number of embedded clusters are also observed. This finding, combined with the overall wealth of dust and gas, could imply that star formation is still active. An intermediate-age star cluster, BS 90, formed ~4.3 ? 0.1?Gyr ago, is also present in the region. Thus, this region of the SMC has supported star formation with varying levels of intensity over much of the cosmic time.

The stellar mass distribution in the giant star forming region NGC 346

Deep F555W and F814W Hubble Space Telescope (HST ) Advanced Camera for Survey (ACS) images are the basis for a study of the present-day mass function (PDMF) of NGC 346, the largest active star-forming region in the Small Magellanic Cloud (SMC). We find a PDMF slope of Γ = −1.43 ± 0.18 in the mass range 0.8-60 M☉, in excellent agreement with the Salpeter initial mass function (IMF) in the solar neighborhood. Caveats on the conversion of the PDMF to the IMF are discussed. The PDMF slope changes, as a function of the radial distance from the center of the NGC 346 star cluster, indicating a segregation of the most massive stars. This segregation is likely primordial considering the young age (~ 3 Myr) of NGC 346, and its clumpy structure which suggests that the cluster has likely not had sufficient time to relax. Comparing our results for NGC 346 with those derived for other star clusters in the SMC and the Milky Way (MW), we conclude that, while the star formation process might depend on the local cloud conditions, the IMF does not seem to be affected by general environmental effects such as galaxy type, metallicity, and dust content.

Discovery of a population of pre-main-sequence stars in NGC 346 from deep Hubble space telescope acs images

We report the discovery of a rich population of low mass stars in the young, massive star forming region N66/NGC346 in the Small Magellanic Cloud, from deep V, I and H alpha images taken with the HST/ACS. These stars have likely formed together with the NGC346 cluster, ~3-5 Myr ago. Their magnitude and colors are those of pre-main sequence stars in the mass range 0.6-3 Mo, mostly concentrated in the main cluster, but with secondary subclusters spread over a region across ~45 pc. These subclusters appear to be spatially coincident with previously known knots of molecular gas identified in ground based and ISO observations. We show that N66/NGC346 is a complex region, being shaped by its massive stars, and the observations presented here represent a key step towards the understanding of how star formation occurred and has progressed in this low metallicity environment.

Optical Spectroscopy of Diffuse Ionized Gas in M31

We have obtained sensitive long-slit spectra of diffuse ionized gas (DIG) in the Andromeda galaxy, M31, covering the wavelength range of 3550-6850 A. By co-adding extracted DIG spectra, we reached a 1 σ uncertainty of 9.3 × 10-19 ergs s-1 cm-2 arcsec-2 corresponding to 0.46 pc cm-6 in emission measure. We present average spectra of DIG at four brightness levels with emission measures ranging from 9 to 59 pc cm-6. We present the first measurements of [O II] λ3727 and [O III] λ5007 of the truly diffuse ionized medium in the disk of an external spiral galaxy. We find that I[O II]/IHα = 0.9-1.4. The [O III] line is weak (I[O III]/IHβ = 0.5), but it is stronger than found for the Galactic DIG. Measurements of [N II] λ6583 and [S II] (λ6717 + λ6731) are also presented. The [S II] lines are clearly stronger than typical H II regions (I[S II]/IHα = 0.5 compared to 0.2), confirming various imaging studies of spiral galaxies. Overall, the line ratios are in agreement with predictions of photoionization models for diffuse gas exposed to a dilute stellar radiation field, but the line ratios of the DIG in M31 are somewhat different than observed for Galactic DIG. The differences indicate a less diluted radiation field in the DIG of M31s spiral arms compared to DIG in the Solar Neighborhood of the Milky Way. Turbulent mixing layers can contribute at most 20% of the ionization budget of the DIG, with lower percentages producing better fits to the observed line ratios. We have also detected He I λ5876 emission from the brightest DIG in M31. The He I line appears to be stronger than in the Galactic DIG, possibly indicating that most of the Helium in the bright DIG in M31 is fully ionized. However, this result is somewhat tentative since bright night sky lines hamper an accurate measurement of the He I line strength.

LEGACY EXTRAGALACTIC UV SURVEY (LEGUS) WITH THE HUBBLE SPACE TELESCOPE. I. SURVEY DESCRIPTION

The Legacy ExtraGalactic UV Survey (LEGUS) is a Cycle 21 Treasury program on the Hubble Space Telescope aimed at the investigation of star formation and its relation with galactic environment in nearby galaxies, from the scales of individual stars to those of ~kiloparsec-size clustered structures. Five-band imaging from the near-ultraviolet to the I band with the Wide-Field Camera 3 (WFC3), plus parallel optical imaging with the Advanced Camera for Surveys (ACS), is being collected for selected pointings of 50 galaxies within the local 12 Mpc. The filters used for the observations with the WFC3 are F275W(λ2704 A), F336W(λ3355 A), F438W(λ4325 A), F555W(λ5308 A), and F814W(λ8024 A); the parallel observations with the ACS use the filters F435W(λ4328 A), F606W(λ5921 A), and F814W(λ8057 A). The multiband images are yielding accurate recent (lesssim50 Myr) star formation histories from resolved massive stars and the extinction-corrected ages and masses of star clusters and associations. The extensive inventories of massive stars and clustered systems will be used to investigate the spatial and temporal evolution of star formation within galaxies. This will, in turn, inform theories of galaxy evolution and improve the understanding of the physical underpinning of the gas-star formation relation and the nature of star formation at high redshift. This paper describes the survey, its goals and observational strategy, and the initial scientific results. Because LEGUS will provide a reference survey and a foundation for future observations with the James Webb Space Telescope and with ALMA, a large number of data products are planned for delivery to the community.

Progressive Star Formation in the Young SMC Cluster NGC 602

NGC 602 is a young stellar cluster located in a peripheral region of the Small Magellanic Cloud (SMC) known as the wing. Far from the main body of the galaxy and abutting the Magellanic Bridge, the SMCs wing is characterized by low gas and stellar content. With deep optical imaging from the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST), we have discovered an extensive pre-main-sequence (PMS) population, with stellar masses in the range 0.6-3 M☉. These low-mass PMS stars formed coevally with the central cluster about 4 Myr ago. Spitzer IRAC images of the same region also reveal a population of young stellar objects, some of which are still embedded in nebular material and most of which likely formed even more recently than the young stars detected with HST ACS imaging. We infer that star formation started in this region ~ 4 Myr ago with the formation of the central cluster and gradually propagated toward the outskirts where star formation is presently ongoing.

The Brightest Young Star Clusters in NGC 5253

The nearby dwarf starburst galaxy NGC 5253 hosts a number of young, massive star clusters, the two youngest of which are centrally concentrated and surrounded by thermal radio emission (the radio nebula). To investigate the role of these clusters in the starburst energetics, we combine new and archival Hubble Space Telescope images of NGC 5253 with wavelength coverage from 1500 A to 1.9 μm in 13 filters. These include Hα, Pβ, and Pα, and the imaging from the Hubble Treasury Program LEGUS (Legacy Extragalactic UV Survey). The extraordinarily well-sampled spectral energy distributions enable modeling with unprecedented accuracy the ages, masses, and extinctions of the nine optically brightest clusters (M_V < −8.8) and the two young radio nebula clusters. The clusters have ages ~1–15 Myr and masses ~1 × 10^4–2.5 × 10^5 M_⊙. The clusters spatial location and ages indicate that star formation has become more concentrated toward the radio nebula over the last ~15 Myr. The most massive cluster is in the radio nebula; with a mass ~2.5 × 10^5 M_⊙ and an age ~1 Myr, it is 2–4 times less massive and younger than previously estimated. It is within a dust cloud with A_V ~ 50 mag, and shows a clear near-IR excess, likely from hot dust. The second radio nebula cluster is also ~1 Myr old, confirming the extreme youth of the starburst region. These two clusters account for about half of the ionizing photon rate in the radio nebula, and will eventually supply about 2/3 of the mechanical energy in present-day shocks. Additional sources are required to supply the remaining ionizing radiation, and may include very massive stars.

Detection of [O I] λ6300 and Other Diagnostic Emission Lines in the Diffuse Ionized Gas of M33 with Gemini-North

We present spectroscopic observations of diffuse ionized gas (DIG) in M33 near the H II region NGC 604. We present the first detection of [O I] λ6300 in the DIG of M33, one of the critical lines for distinguishing photoionization from shock ionization models. We measure [O I]/Hα in the range of 0.04-0.10 and an increase in this ratio with decreasing emission measure. Our measurements of [S II]/Hα and [N II]/Hα also rise with decreasing emission measure, while our [O III]/Hβ measurements remain fairly constant. We have one tentative detection of He I in the region of brightest emission measure, with a ratio of He I/Hα = 0.033 ± 0.019, indicating that the helium is at least partially ionized. We compare our observed emission-line ratios to photoionization models and find that field star ionization models do not fit our data well. Leaky H II region models are consistent with our data, without the need to invoke additional ionization mechanisms to fit our [O I] or [O III] measurements. The closest large H II region is NGC 604 and is therefore a likely candidate for the source of the ionizing photons for the gas in this region.

Discovery of Candidate Luminous Blue Variables in M31

Luminous blue variables (LBVs) constitute a short-lived, eruptive phase in the evolution of some of the most massive stars. Only a handful have yet been identified in the Galaxy and in each of the nearby galaxies; there are four known LBVs in M31. We have found an efficient method to identify candidate LBV stars in nearby galaxies. The candidates are identified in a pair of deep, continuum-subtracted narrowband H? and [S II] images as objects with extremely low [S II] to H? ratios, and with coincident stellar objects in continuum images. Five of the most promising new candidates identified by these criteria in the northeastern half of M31 were subsequently confirmed by optical spectroscopy to show spectra similar to the previously identified M31 LBV, HS var 15. These five candidates also have much in common with B[e] stars, of which none were known to exist in M31. They are bright H? sources, (120 L? < LH? < 1300 L?) with no detectable [S II] emission, large H? equivalent widths (-60 to -400 ?), and broad wings on the H? profiles (FWZI = 1200-2000 km s-1). Most candidates have Fe II emission. We discuss the environments of the candidates and find that many objects are likely older than several million years because they tend not to be located inside bright H II regions. We predict, based on the current results, that at least 20-25 LBV/B[e] candidates may be present in M31.