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Register a new userDoes Stellar Feedback Create HI Holes? An HST/VLA Study of Holmberg II
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We use deep HST/ACS F555W and F814W photometry of resolved stars in the M81 Group dwarf irregular galaxy Ho II to study the hypothesis that the holes identified in the neutral ISM (HI) are created by stellar feedback. From the deep photometry, we construct color-magnitude diagrams (CMDs) and measure the star formation histories (SFHs) for stars contained in HI holes from two independent holes catalogs, as well as select control fields, i.e., similar sized regions that span a range of HI column densities. Converting the recent SFHs into stellar feedback energies, we find that enough energy has been generated to have created all holes. However, the required energy is not always produced over a time scale that is less than the estimated kinematic age of the hole. The combination of the CMDs, recent SFHs, and locations of young stars shows that the stellar populations inside HI holes are not coherent, single-aged, stellar clusters, as previously suggested, but rather multi-age populations distributed across each hole. From a comparison of the modeled and observed integrated magnitudes, and the locations and energetics of stars inside of HI holes, we propose a potential new model: a viable mechanism for creating the observed HI holes in Ho II is stellar feedback from multiple generations of SF spread out over tens or hundreds of Myr, and thus, the concept of an age for an HI hole is intrinsically ambiguous. We further find that halpha and 24 micron emission, tracers of the most recent star formation, do not correlate well with the positions of the HI holes. However, UV emission, which traces star formation over roughly the last 100 Myr, shows a much better correlation with the locations of the HI holes.
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We study protostellar envelope and outflow evolution using Hubble Space Telescope NICMOS or WFC3 images of 304 protostars in the Orion Molecular clouds. These near-IR images resolve structures in the envelopes delineated by the scattered light of the central protostars with 80 AU resolution and they complement the 1.2-870 micron spectral energy distributions obtained with the Herschel Orion Protostar Survey program (HOPS). Based on their 1.60 micron morphologies, we classify the protostars into five categories: non-detections, point sources without nebulosity, bipolar cavity sources, unipolar cavity sources, and irregulars. We find point sources without associated nebulosity are the most numerous, and show through monochromatic Monte Carlo radiative transfer modeling that this morphology occurs when protostars are observed at low inclinations or have low envelope densities. We also find that the morphology is correlated with the SED-determined evolutionary class with Class 0 protostars more likely to be non-detections, Class I protostars to show cavities and flat-spectrum protostars to be point sources. Using an edge detection algorithm to trace the projected edges of the cavities, we fit power-laws to the resulting cavity shapes, thereby measuring the cavity half-opening angles and power-law exponents. We find no evidence for the growth of outflow cavities as protostars evolve through the Class I protostar phase, in contradiction with previous studies of smaller samples. We conclude that the decline of mass infall with time cannot be explained by the progressive clearing of envelopes by growing outflow cavities. Furthermore, the low star formation efficiency inferred for molecular cores cannot be explained by envelope clearing alone.
The nearby irregular galaxy Holmberg II has been extensively mapped in HI using the Very Large Array (VLA), revealing intricate structure in its interstellar gas component (Puche et al. 1992). An analysis of these structures shows the neutral gas to contain a number of expanding HI holes. The formation of the HI holes has been attributed to multiple supernova events occurring within wind-blown shells around young, massive star clusters, with as many as 10-200 supernovae required to produce many of the holes. From the sizes and expansion velocities of the holes, Puche et al. assigned ages of ~10^7 to 10^8 years. If the supernova scenario for the formation of the HI holes is correct, it implies the existence of star clusters with a substantial population of late-B, A and F main sequence stars at the centers of the holes. Many of these clusters should be detectable in deep ground-based CCD images of the galaxy. In order to test the supernova hypothesis for the formation of the HI holes, we have obtained and analyzed deep broad-band BVR and narrow-band H-alpha images of Ho II. We compare the optical and HI data and search for evidence of the expected star clusters in and around the HI holes. We also use the HI data to constrain models of the expected remnant stellar population. We show that in several of the holes the observed upper limits for the remnant cluster brightness are strongly inconsistent with the SNe hypothesis described in Puche et al. Moreover, many of the HI holes are located in regions of very low optical surface brightness which show no indication of recent star formation. Here we present our findings and explore possible alternative explanations for the existence of the HI holes in Ho II, including the suggestion that some of the holes were produced by Gamma-ray burst events.
The question, whether the stellar populations in the Milky Way take part in flaring of the scale heights as observed for the HI gas is a matter of debate. Standard mass models for the Milky Way assume a constant scale height for each of the different stellar distributions. However, there is mounting evidence that at least some of the stellar distributions reach at large galactocentric distances high altitudes that are incompatible with a constant scale height. We discuss recent observational evidence for stellar flaring and compare it with HI data from the Leiden/Argentine/Bonn (LAB) survey. Within the systemic and statistical uncertainties we find a good agreement between both.
It has been suggested that the locality of information transfer in quantum entanglement indicates that reality is subjective, meaning that there is an innate inseparability between the physical system and the conscious mind of the observer. This paper attempts to outline the relation between macroscopic and microscopic worlds in the measurement process in regards to whether observation creates reality. Indeed, the Maxwells demon thought experiment suggests a correlation between a microscopic (quantum) system and a macroscopic (classical) apparatus, which leads to an energy transfer from the quantum vacuum to the physical world, similar to particle creation from a vacuum. This explanation shows that observation in quantum theory conserves, rather than creates, energy.
A detailed photometric study of star-forming regions (SFRs) in the galaxy Holmberg II has been carried out using archival observational data from the far infrared to ultraviolet obtained with the GALEX, Spitzer, and Herschel telescopes. Spectroscopic observations with the 6-m telescope of Special Astrophysical Observatory of the Russian Academy of Sciences are used to estimate ages and metallicities of SFRs. For the first time, the ages of SFRs have been related to their emission parameters in a wide spectral range and with the physical parameters determined by fitting the observed spectra. It is shown that fluxes at 8 and 24 micron characterizing the emission of polycyclic aromatic hydrocarbons (PAHs) and hot dust grains decrease with age, but their ratio increases. This implies that the relative PAH contribution to the total infrared flux increases with age. It is suggested that the detected increase in the ratio of the fluxes at 8 and 24 micron is related to the growth in the PAH mass due to destruction of larger grains.
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