This week on HST

HST Programs: June 29 - July 5, 2015

Selected highlights

GO 13472: The Hubble Constant to 1%? STAGE 4: Calibrating the RR Lyrae PL relation at H-Band using HST and Gaia Parallax Stars

RR Lyrae's light curve at visible wavelengths

The classical cosmic distance scale rests on a series of distance indicators that step outwards from the Milky Way, establishing reliable measurements to ever more distant galaxies. Cepheids have long been the prime calibrators in this process, but other pulsating variables, notably Mira AGB long-period variables and RR Lyrae variables, also make significant contributions. RR Lyrae variables are evolved, near-solar-mass stars that are passing through the instability strip where it crosses the horizontal branch. With periods of 0.5 to 1.5 days, they have long served as distance indicators for old stellar populations (Baade's Population II). They have been known in the Galactic field and in Galactic globular clusters for over 150 years, and they are also present in the older stellar populations of the dwarf spheroidal Galactic satellites. Cluster (or dsph) RR Lyraes are particularly interesting, since their metallicities and ages can be deduced from analysis of the colour-magnitude diagrams for those systems. They are significantly less luminous than Cepheids, nonetheless, near-infrared photometric monitoring has demonstrated that these stars delineate a period-luminosity relation at those wavelengths that has the potential to establish distances to better than 1.5% accuracy. The absolute calibration of that relationship, however, rests on only 4 nearby RR Lyraes with trigonometric parallax measurements. The present program aims to add to the sample of astrometricall well-observed RR Lyraes by using spatial scanning on WFC3 to determine accurate parallaxes for a sample of Galactic variables lying at distances up to several kpc from the Sun. Spatial scanning enables astrometry to an acuracy of ~40 microarcseconds, offering the prospect of distances accurate to 4% for individual stars, and an overall distance scale calibration accurate to better than 3%%.

GO 13804: Important Nearby Galaxies without Accurate Distances

M104, the Sombrero galaxy, one of the systems targeted in this program

The Spitzer Infrared Nearby Galaxies Survey (SINGS) was a highly successful Spitzer Legacy program that compiled near- and mid-infrared imaging and spectroscopy for 75 galaxies within 75 Mpc of the Milky Way. SINGS' success stimulated a series of follow-up programs, including THINGS (HI gas), HERACLES (CO gas), and KINGFISH (Herschel mid- and far-IR data), that have resulted in exhaustive multi-wavelength observations that characterise the star-formation properties in many of the SINGS galaxies. However, a surprising number of these systems still lack accurate distance determinations. HST is well suited to determining distances to the closer SINGS galaxies : the high sensitivity of the Advanced Camera for Surveys and Wide-Field Camera 3 combined with the unparalleled angular resolution enables resolution of the most luminous stars; constructing the colour-magnitude provides access to a number of distance indicators, including the tip of the first red giant branch (RGB). Red giants have completed the core hydrogen-burning main-sequence stage of evolution and have moved to burning hydrogen in an inner shell. The maximum luminosity in this phase, and hence the location of the tip of the RGB, is set when the core reaches a sufficiently high temperature to ignite helium burning, the so-called helium flash. At that point, hydrogen shell-burning is extinguished, the star contracts and moves onto the horizontal branch. The present program is using HST to target several SINGS spiral galaxies within 10 Mpc, including M74, M104 (the Sombrero), NGC 1291, M51, NGC 4559, NGC 4625 and NGC 5398.

GO 13862: Measuring the Impact of Starbursts on the Circum-Galactic Medium

ALMA image of the gaseous outflows surrounding the nearby galaxy, NGC 253

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution of galaxy assembly therefore is tied closely to how gas is accreted, recycled, circulated through the halo and disk, and, perhaps, ejected back into the intergalactic medium. In particular, starburst-driven outflows may represent an important means of removing gas from and galaxy and quenching star formation, producing the systems that populate the "red and dead" sequence. Tracing the overall history is difficult, since gas passes through many different phases, some of which are easier to detect than others. During accretion and, probably, subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting such gas is through ultraviolet spectroscopy, where gas within nearby systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. The present program is using the Cosmic Origins Spectrograph to probe gas in fifteen starburst galaxies at redshifts between z~0.02 and z~0.15. The galaxy-quasar pairs are drawn from the Sloan Digital Sky Survey. The galaxies have starburst ages spanning the range <100 to ~600 Myrs and the attendant quasars have angular separations corresponding to <250 kpc at the galaxy redshift. The COS observations are capable of detecting highly ionised species such as CIV, OVI and SiIV, as well as Lyman-alpha, OI, CII, SiII and SiIII.

GO 14064: A New Disturbance in Saturn's North Sub-Polar Region

The evolution of the 2010 Great White Spot on Saturn

The atmospheric structure of the gas giants in the solar system is characterised by a banded appearance, reflecting the different circulation cells due to the coriolis force generated by the rapid rotation, and smaller scale spots due to local disturbances i.e. storms. In the latter case, Jupiter has produced several long-lived storms, notably the Great Red Spot, first reported in 1635 but apparently now shrinking significantly in size. Saturn's atmosphere, twice as far from the Sun as Jupiter and receiving correspondingly less insolation, is less subject to large-scale storms, but periodically (roughly every 30 years), such systems have developed. These large-scale storms are now believed to be generated through the upper atmosphere cooling and increasing ind ensity to the extent that downflows develop, circulating warmer, water-rich gases into the upper atmosphere. The last major disturbance was in 2010, but in late 2014, amateur astronomers noted a large dark spot near Saturn's north pole (latitude ~+58.5), an unusually high latitude for such a disturbance. That spot has persisted, and recent observations suggest that further distrubances may be developing in its vicinity. The process is being monitored by the Cassini probe, but its orbit is less than optimum, so Hubble observations with the WFC3 UVIS camera will be used to supplement the in situ data.