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Magellanic Cloud Star Formation: From the Milky Way to Distant Galaxies
“What can 30 Doradus tell us about Galactic Star Formation?”
Abstract: 30 Doradus is a key object as the most luminous HII region in the Local Group. At its center is R136, the most massive young cluster known within the Milky Way and the Magellanic Clouds. The 30 Doradus region is therefore a key object in our understanding of massive star forming region, a crucial stepping stone to super star clusters and an opportunity to probe star formation detail in an environment different than the Milky Way.
I will briefly outline some of the areas where 30 Doradus can shine light on the star formation process in ways that are difficult, if not impossible, in the Galaxy. I will in particular touch on the Initial Mass Function and what 30 Doradus can tell us in already now and in the future. I will further discuss some prospects on the possibility to observe star formation on large scales in 30 Doradus, a thing that is extremely difficult in the Galaxy.
“Contextualizing Magellanic Star Formation: From the Milky Way to Distant Galaxies”
I review measurements of star formation rates as a function of wavelength, and the assumptions that enter in their calibration. I highlight the potential pitfalls of using `whole-galaxy' star formation rates for regions within galaxies, and what `corrections' need to be implemented to generalize the use of these indicators.
“Star formation rates and patterns in HII regions at different metallicity”
HII regions are the most prominent sites observable in galaxies at great distance.
As such, they have been used to estimate star formation rates (SFRs) in galaxies throughout the universe. As the environments in the present and early universe have drastically different physical conditions and may affect the underlying star formation, it is important to critically examine HII regions at a range of metallicity and galactic environment. We have used resolved stellar and gas contents in selected HII regions in the Galaxy and Magellanic Clouds to determine SFRs and examine star formation patterns, in order to investigate the environmental effect on star formation.
"Star formation history in the Small Magellanic Cloud: the HST perspective"
"We derive the star formation history of the Small Magellanic Cloud using the deepest optical color-magnitude diagram (CMD) to date. Our observations cover six fields, imaged with the Advanced Camera for Surveys onboard the Hubble Space Telescope and located in different regions of the main body, Wing and Halo of the galaxy. To give a more robust solutions we applied two independent synthetic CMD procedures.
A feature which is systematically present in all our solutions is a low star formation pace until 5-7 Gyr ago, followed by a two-three times higher activity from then on. This is remarkable since at that epoch, dynamical models predict a negligible influence of either the Large Magellanic Cloud or the Milky Way. The age-metallicity relations we infer from our best fitting models are monotonically increasing with time, or constant, with no evidence of dips. This may argue against the major merger scenario proposed by Tsujimoto & Bekki 2009, although a minor merger cannot be ruled out.
The intermediate to recent activity varies from field to field, with the most peripheral ones also being the oldest. The only exception is the Wing field SFH9: its unusually strong current activity is likely driven by the interaction with the LMC. "
"Feedback regulated star formation: variations accross the Local Group galaxies"
Abstract: I will present results from a semi-analytical model that follows star formation in protocluster forming clumps. In this model, dense cores from in the clump following its gravoturbulent fragmentation and then collapse to form stars. The cumulative mechanical energy from the newly formed massive stars expells the gas from the clump and terminates the star formation process. I will discuss some of the main outcomes of this model, namely the star formation efficiencies, the age spread of stars in the newborn clusters, the shape of the age distribution of stars, the relationship between the cluster mass function and the clump mass function, and the implications for the galactic star formation laws, and discuss their potential variations in galaxies of different metallicities.
“Star Formation Rates from Galactic Molecular Clouds to Galaxies”
The current picture of star formation has mostly been assembled from detailed observations of nearby star-forming regions. It is, however, unclear to what degree conclusions from such observations are universal and representative for the entire Galaxy. In this presentation, we discuss an extension of the current picture of star formation, as derived from observations in the Galactic neighborhood, to galaxy-wide scales when compared to observations of external galaxies. In particular, we investigate scaling relations between star formation rates and measurements of both dense and total molecular gas masses. We find that the total star formation rate in a molecular cloud or galaxy is linearly proportional to the mass of dense gas within the cloud or galaxy. This simple relation, first documented in previous studies, holds over a span of mass covering nearly nine orders of magnitude and indicates that the rate of star formation is directly controlled by the amount of dense molecular gas that can be assembled within a star formation complex. We further show that the star formation rates and total molecular masses, characterizing both local clouds and galaxies, are correlated over similarly large scales of mass and can be described by a linear star formation scaling law, parametrized by the fraction of dense gas contained within the clouds or galaxies. That is, the underlying star formation scaling law is always linear for clouds and galaxies with the same dense gas fraction. These considerations provide a single unified framework for understanding the relation between the standard (nonlinear) extragalactic Schmidt-Kennicutt scaling law, that is typically derived from CO observations of the gas, and the linear star formation scaling law derived from HCN observations of the dense gas.
Jay S. Gallagher
Magellanic Star Clusters Through Time
Star cluster record a number of conditions from their time and circumstances of formation. In this talk I will briefly review some of these features and their potential implications for the evolution of the Magellanic Clouds.
“The N158-N159-N160 LMC star-forming complex observed by Spitzer, Herschel and LABOCA”
We present a study of the infrared/submillimeter emission of the massive star forming complex N158-N159-N160 located in the Large Magellanic Cloud. Combining observations from the Spitzer Space Telescope (3.6-70 μm), the Herschel Space Observatory (100-500 μm) and LABOCA (on APEX, 870 μm) allows us to work at the best angular resolution available now for an extragalactic source. We observe a remarkably good correlation between the Herschel SPIRE and LABOCA emission and resolve the low surface brightnesses emission. We use the Spitzer and Herschel data to perform a resolved Spectral Energy Distribution (SED) modelling of the complex (Galliano et al. (2011) SED modelling technique) and derive maps of the star formation rate, the grain temperature, the mean starlight intensity, the fraction of Polycyclic Aromatic Hydrocarbons (PAH) or the dust mass surface density of the region. We also investigate the drivers of the Herschel/PACS and SPIRE submm colours and find that the submm ratios correlate strongly with the radiation field intensity and with the near and mid-IR surface brightnesses equally well. Comparing our dust map to HI and CO observations in N159, we then investigate variations in the gas-to-dust mass ratio (G/D) or the CO-to-H2 conversion factor XCO. We finally model individual regions to analyse variations in the SED shape across the complex and the 870 μm emission in more details. No measurable submm excess
emission at 870 μm seems to be detected in these regions.
“Stellar clustering in Star-Forming Regions of the Magellanic Clouds”
I discuss the clustered nature of star formation, as it is revealed from significant numbers of low-mass Pre--Main-Sequence (PMS) stars, identified with Hubble Space Telescope in the vicinity of star-forming regions in the Magellanic Clouds. Our findings suggest that at 100-pc scales, star formation occurs in several, apparently independent, centers of gravity, i.e., clusters and associations, which all seem to share more or less the same types of stars and stellar IMFs and appear to be gravitationally related to each other. Whether these objects are the products of a single "fragmented" star formation event, or are they formed by independent "sporadic" events is an open question. I will address this question using the paradigm of the HII complex NGC 346 (N66) in the Small Magellanic Cloud, in an attempt to draw an observational portrait of the formation of multiple stellar systems, and interpret the hierarchical clustering of their PMS stars.
"Star Formation in Different Galaxy Types"
Galaxies cover a wide range of masses and star formation histories. In this review, I summarize some of the evolutionary key features of common galaxy types and compare them to Magellanic-type irregulars. In very massive, early-type galaxies, very rapid, efficient early star formation is observed, accompanied by strong enrichment and later quiescence, which is well-described by downsizing scenarios. In the intermediate-mass regime, early-type galaxies may still show activity in low-mass environments or when being rejuvenated by wet mergers. In late-type galaxies including Magellanic dwarfs, we find continuous, though variable star formation over a Hubble time. In the low-mass dwarf regime, a wide range of properties from bursty activity to quiescence is observed. Generally, stochasticity dominates here, and star formation rates and efficiencies tend to be low. To first order, morphological types and their star formation properties correlate with environment.
"The feedback of the Wolf-Rayet population in the LMC"
Massive stars are key players in the cosmic evolution. They have a strong influence on the chemical and the dynamical evolution of their host clusters and galaxies. Their input of chemical enriched matter, momentum and ionizing radiation into the interstellar as well as intergalactic media by their stellar wind and supernovae explosion is of fundamental importance. We present a detailed and comprehensive study of WN stars in the LMC, using optical spectra as well as UV spectra if available. Stellar and atmospheric parameters have been derived for 101 WN LMC stars, covering almost the whole WN population in the LMC. Due to the given LMC membership, these results are free from uncertainties inferred from the distance. The observations are fitted with theoretical spectra, using the Potsdam Wolf-Rayet model atmosphere code (PoWR). For this purpose, large grids of line-blanket models with different hydrogen abundances have been calculated for the range of 25kK to about 180kK. We briefly review the effects that those massive stars have on their surroundings by the two feedback agents, photoionization and stellar winds.
“Hiding Star-Formation in Disks.”
Interstellar dust reprocesses the light from recently formed stars and re-emits it at longer wavelengths. A central unknown in the reprocessing efficiency of galaxies is the relative distribution of dusty ISM and stars in the disk, especially in low-mass galaxies.
In the edge-on perspective on spiral galaxy disks, we can identify the vertical distribution of both stars and dust absorption. The latter presents as a thin dark stripe mid-plane, the dust lane.
I will present results from the GalaxyZoo citizen science project on the prevalence of dust lanes in edge-on galaxies as a function of mass and star-formation, as well as results from SED models of low-mass disks.
“Embedded Clusters in the Large Magellanic Cloud”
In our Galaxy, the majority of stars form in embedded clusters (Lada & Lada 2003), which makes them the fundamental unit of star formation. Despite their importance, our knowledge of cluster formation remains primitive. For example, we don't have a clear idea of how varying physical environments affects the cluster formation process. In order to address such issues we have undertaken a comprehensive, systematic search for embedded stellar clusters in the Large Magellanic Cloud (LMC) using near-infrared data from the VISTA Magellanic Clouds Survey (Cioni et al. 2011). To date, we have searched for clusters in ~5% of the LMC and have identified 83 young embedded cluster candidates, 68 of which are newly discovered as clusters. We are determining the physical properties of these cluster candidates and comparing them to the physical properties of their natal molecular clouds. I will present current results from this work as well as present preliminary NIR IFU spectral data for 7 cluster candidates from SINFONI on the VLT.
“Photodissociation regions in the LMC”
I will present preliminary results from several Herschel key programs (SHINING, HERITAGE, Dwarf Galaxy Survey) targeting several HII regions in the Magellanic Clouds. For the first time we are able to spatially resolve the emission of [CII] 157um, [OI] 63um, 145um, [NII] 122um, and [OIII] 88um in these sources with the PACS instrument.
I will show that [CII] emission follows closely the emission from polycyclic aromatic hydrocarbons (PAHs). The heating of the gas in which [CII] arises from is better described by PAH emission than by total infrared emission. We show that this is a consequence of warm dust located in HII regions, in which [CII] emission is negligible.
We demonstrate that [CII] emission is spatially extended, and that most of the [CII] in a star-forming region originates from relatively diffuse photodissociation regions located far (>10-50pc) from the massive stellar clusters. As a consequence, [CII] is the main coolant before [OI] 63um, the latter dominating the cooling only in relatively denser clouds with high UV field. Finally, I will discuss how these results will help us to understand and constrain [CII] as a star-formation tracer in unresolved galaxies.
“Spectroscopy of Young Stellar Objects in the Magellanic Clouds”
I will describe spectroscopic observations of YSOs in the Magellanic Clouds. For the SMC, I will describe work done with IRS spectra of a sample of 33 YSOs in a variety of evolutionary stages. We have found that the PAH emission is dominated by small neutral PAHs, as recently proposed in the literature, and describe how a metallicity effect may not be enough to explain these observations. From measurements of molecular hydrogen line intensities we construct excitation diagrams; these reveal that the emission originates from gas with temperature consistent with Galactic YSOs of similar luminosity, but with lower average column density; the same diagrams also indicate the presence of a large reservoir of colder gas. Finally I describe the comparative analysis of the ice properties in the Galaxy, LMC and SMC. We find that ice ratios are different for the 3 galaxies and no CO ice was detected in SMC YSOs. I describea scenario that may explain these observations. I will end by introducing our follow-up far-IR spectroscopic programme with Herschel.
Linda J. Smith and the SHUCS Team
“The Snapshot Hubble U-band Cluster Survey (SHUCS): characterising the star cluster populations of nearby star-forming galaxies”
I will present the first results from the Snapshot Hubble U-band Cluster Survey (SHUCS), a project aimed at characterizing the star cluster populations of ten nearby galaxies through new U band imaging with Wide Field Camera 3, and archival BVI data from the Hubble Space Telescope. Completing the UBVI baseline reduces the age-extinction degeneracy of optical colours, and enables the measurement of reliable ages and masses for the thousands of clusters covered in the survey. I will discuss our results on the starburst galaxy NGC 2146 and, in particular, the discovery of a ring-like cluster complex with an H-alpha luminosity similar to that of the 30 Doradus region in the Large Magellanic Cloud. I will also discuss the cluster populations of the spiral galaxies NGC 4041 and 2997.
“Studies on Individual H II Regions: the Young Stellar Content”
The Spitzer Space Telescope was the first instrument that enabled studies of the resolved (proto)stellar populations in the Magellanic Clouds (MCs). The Spitzer’s sensitivity and resolution, combined with the proximity of the MCs, allowed observations of the protostars and small clusters at sub-parsec resolution. The Spitzer's galaxy-wide studies identified ~1800 young stellar objects (YSOs) in the MCs; however, due to their selection criteria, they missed less embedded massive YSOs (e.g., Stage II and Stage III/PMS stars) and low-luminosity (lower-mass) YSOs. The census of YSOs detected by Spitzer can only be completed on the scale of individual H II regions where the conservative source selection criteria used in the galaxy-wide surveys can be relaxed, allowing the identification of low-luminosity YSOs. In H II regions, each source can be studied individually, which minimizes the risk of mis-classification, particularly when high-resolution data are available. The Stage III (PMS stars) in the MCs can only be detected with the Hubble Space Telescope (HST). In the last few years, a rich population of PMS stars was identified as the low-mass stellar content of star-forming regions in the MCs. The youngest Stage 0/I YSOs can be identified with the Herschel Space Observatory. The challenges of the identification and characterization of YSO candidates will be discussed.
The interstellar medium (ISM) in H II regions has been surveyed in great detail, allowing us to determine the relationship between star formation and the physical properties of the ISM. The spatial and temporal distribution of young stellar populations at different evolutionary stages (from the youngest Stage 0 protostars to more evolved Stage III / PMS stars) and a comparison to the ISM allow a reconstruction of star formation histories of H II regions.
“The Star Formation Histories of the Magellanic Clouds”
I will discuss our current knowledge of the star formation histories (SFHs) of the Magellanic Clouds as determined by analysis of resolved stellar populations. I will review the method of measuring SFHs from stellar color-magnitude diagrams CMDs and discuss how uncertainties in model parameters (e.g., extinction) and systematics offsets between stellar evolution models can affect the derived SFHs. In this context, I will review select past and current efforts to measure the SFHs of the SMC and LMC, and assess where our knowledge is strong and where it can still be improved.
Lynn Redding Carlson
“Nine HII Regions in the Large Magellanic Cloud: Young Stellar Objects, Star Formation Rates, and the ISM”
We present a detailed examination of nine LMC star-forming regions with diverse environments, sizes, and morphologies. We identify 1045 Young Stellar Object (YSO) candidates (918 not previously known) and derive star-formation rates for each region via two distinct methods. We combine Spitzer infrared data from SAGE-LMC (Surveying the Agents of Galaxy Evolution; Meixner et al. 2006), optical photometry from the Magellanic Clouds Photometric Survey (MCPS; Zaritsky et al. 1997), and near-infrared photometry from the InfraRed Survey Facility (IRSF; Kato et al. 2007) to identify and characterize individual YSO candidates. From these, we derive bottoms-up star formation rates for each region, which we compare to top-down rates estimated from 70-micron flux. Regions are LHA 120-N 11, N 44, N 51, N 105, N 113, N 120, N 144, N 160, and N 206. -- From Carlson, et al. 2012 A&A 542, 66
Guido De Marchi
“Recent star formation in the Galaxy and Magellanic Clouds”
Using the Hubble Space Telescope, we have undertaken a systematic study of pre-main-sequence (PMS) stars spanning a wide range of masses (0.5 - 4 M_sun), metallicities (0.1 - 1 Z_sun) and ages (0.5 - 30 Myr). These PMS objects are located in very different environments in the Milky Way and Magellanic Clouds. Thanks to a novel method that we have developed to combine broad-band (V,I) photometry with narrow-band Halpha imaging, we have determined the physical parameters (temperature, luminosity, age, mass and mass accretion rate) of more than 3000 bona-fide PMS stars still undergoing active mass accretion. This is presently the largest and most homogeneous sample of PMS objects with known physical properties and it includes not only very young objects, but also PMS stars older than 10-20 Myr that are approaching the main sequence. We present here some of the main results of this research, including the fact that mass accretion rate appears to scale with the first power of the stellar mass, with the square root of the age, and approximately with the inverse of metallicity.
“The Magellanic Mopra Asessment - SMC: 12CO(1-0) Data release”
The Magellanic Mopra Assessment (MAGMA) explores the molecular cloud population throughout the LMC and SMC. These observations of the SMC are the highest-resolution and most complete 12CO data available, spanning approximately 0.5 square degrees in 5-6 contiguous regions. As of October 2012, they are available for community use.
The nearby (~63 kpc; Cioni et al, 2000) SMC hosts a low metallicity, a high gas to dust ratio, and has a notoriously low CO luminosity (Israel et al, 1993). The basic properties of molecular clouds, their evolutionary conditions, their relationships to star-formation and star-formation tracers is poorly understood in these conditions.
The MAGMA-SMC data of the CO transitions throughout the SMC are the highest resolution sampling of the SMC to date, representing also the most comprehensive coverage of the SMC. While not completely sampled, this survey focuses on the northern part of the SMC, with slightly lower sensitivity observations towards the south-west (time granted during 2013 will improve the sensitivity). MAGMA-SMC reveals and confirms that the CO clouds in the SMC a) appear to poorly trace the extended dust distribution, b) generally clump on scales apparently equivalent (or less than) the 33 arcsecond beam. Later data releases will include improved senstivity in the south-west SMC, sampling of regions as-yet unsampled, as well as 13CO(1-0).
Ian Stephens (2x posters)
“Probing Isolated Massive Star Formation in the LMC”
Whether massive stars can form in isolation is one of the most debated questions in star formation. Observations of main sequence O-stars indicate that 5-10% of them form in isolation, but models of massive star formation suggest that massive stars should form in cluster environments. Isolated massive young stellar objects (YSOs) are better suited to address whether or not massive stars truly form in isolation since YSOs have had less time to disrupt their natal environment or move away from their stellar siblings. We have developed a unique sample of 7 candidates for isolated massive YSOs in the LMC. Within 80 pc, these objects are not associated with 1) other massive and intermediate-mass YSOs, 2) OB associations, and 3) giant molecular clouds (GMCs). In all cases ground-based H-alpha observations show that they are affiliated with non-elongated, small HII regions and therefore are unlikely to be part of a runaway population. With the Hubble Space Telescope we have observed these 7 candidates with WFC3/UVIS and IR observations in five bands (VIJH and H-alpha) to examine the interstellar environment and determine the main sequence and pre-main sequence (PMS) populations down to ~0.7 solar masses. In addition, coordinated parallel ACS/WFC observations will be used to assess the nearby control-field populations. From these observations we can search for lower-mass PMS stars, infer the local star formation history, and determine whether evidence exists for remnants of a disrupted GMC. With this statistically significant sample, we will have the ability to assess the possibility of massive stars forming in isolation.
“Evolution of HII Regions around Massive YSOs”
We survey HII free-free emission around ∼60 spectroscopically confirmed young stellar objects (YSOs) in the LMC using the Australia Telescope Compact Array (ATCA) at 3.3 and 5.5 cm. From each YSOs’ infrared spectrum, we: a) quantify how embedded/evolved the YSO is through principle component analysis (PCA) of the silicate absorption (Seale et al. 2009); and b) estimate the mass from SED models (Robitaille et al. 2007). We have four main results: (1) based on mass estimates from SED models and ATCA detection limits, we find that most massive YSOs are in HII regions regardless of age, (2) older massive YSOs (as indicated by silicate PCA index) are much more likely to be resolved than younger YSOs, indicating evolving HII regions, (3) resolved (typically older) sources usually have lower densities; thus we see a transition from ultra-compact HII to HII regions, and (4) we find that accretion about the massive YSO is likely non-spherical, resulting in HII regions in the shape of prolate spheroids.
“LMC LBV Nebulae as Stepping Stones to Stellar Evolution and Feedback at Low Z”
Within their lives massive stars may pass a short but very active phase, while entering the LBV state. LBV stars are close to the Eddington limit and are therefore prone to instabilities and have a strong massloss. A visible consequence of instability and winds are small circumstellar nebulae, LBV nebulae. A comparison of galactic LBV nebulae with those in the LMC--- therefore a region of lower metalicity---shows significant differences in the nebulae's parameters as size, morphologies and expansion velocities. One totally unexpected result is that LMC LBV nebulae are on average much larger compared to those in our Galaxy. If this is a general trend, LBV nebulae could provide an ideal case study for stellar evolution and feedback in galaxies with much lower Z, as these nebulae could be detectable at larger distanes. An overview of the LMC LBV nebulae will be given, as well as an outlook for even lower Z LBV populations.