Next week’s night sky:
The moon will officially reach its full phase on Thursday, June 24 at 2:39 p.m. EDT (1839 GMT). The June full moon, colloquially known as the Strawberry Moon, Mead Moon, Rose Moon, or Hot Moon, always shines in or near the stars of southern Ophiuchus, the Serpent-Bearer. The indigenous Ojibwe people of the Great Lakes region call this moon Ode’miin Giizis, the Strawberry Moon. For the Cree Nation it’s Opiniyawiwipisim, the Egg Laying Moon (referring to the activities of wild water-fowl). The Mohawks call it Ohiarí:Ha, the Fruits are Small Moon. The Cherokee call it Tihaluhiyi, the “the Green Corn Moon”, when crops are growing. Because the moon is full when it is opposite the sun in the sky, full moons always rise in the east as the sun is setting, and set in the west at sunrise. Since sunlight is hitting the moon vertically at that time, no shadows are cast; all of the variations in brightness you see arise from differences in the reflectivity, or albedo, of the lunar surface rocks.
Issues with Hubble
Engineers are troubleshooting an apparent glitch with an aging memory module aboard the Hubble Space Telescope that triggered protective “safe mode” software, putting the observatory in a sort of electronic hibernation until the problem is resolved.
The shutdown occurred just after 20:00 GMT on 13 June when Hubble’s payload computer, which controls and coordinates observations by Hubble’s science instruments, stopped sending routine “keep-alive” signals to the telescope’s primary computer. That, in turn, activated software that automatically halted instrument operations, shutting down science observations.
The payload computer has two independent data processing units, or “strings,” either one of which can carry out all necessary functions. Both strings are connected to four 64K CMOS memory chips, only one of which is used at a time. The problem occurred when the active memory module apparently misbehaved, possibly due to age-related degradation.
“The operations team at NASA’s Goddard Space Flight Center is currently in the process of switching memory modules onboard the spacecraft,” a NASA spokesman said in a 17 June email. “There is no definitive timeline yet as to when this will be completed, tested, and brought back to operational status.”
The computer system was built in the 1980s. It is part of the telescope’s Science Instrument Command and Data Handling system, which was replaced during the final space shuttle servicing mission in 2009.
Hubble sees galaxies without dark matter
The most accurate distance measurement yet of ultra-diffuse galaxy (UDG) NGC1052-DF2 (DF2) confirms beyond any shadow of a doubt that it is lacking in dark matter. The newly measured distance of 22.1 +/-1.2 megaparsecs was obtained by an international team of researchers led by Zili Shen and Pieter van Dokkum of Yale University and Shany Danieli, a NASA Hubble Fellow at the Institute for Advanced Study.
“Determining an accurate distance to DF2 has been key in supporting our earlier results,” stated Danieli. “The new measurement reported in this study has crucial implications for estimating the physical properties of the galaxy, thus confirming its lack of dark matter.”
The results, published in Astrophysical Journal Letters on June 9, 2021, are based on 40 orbits of NASA’s Hubble Space Telescope, with imaging by the Advanced Camera for Surveys and a ‘tip of the red giant branch’ (TRGB) analysis, the gold standard for such refined measurements. In 2019, the team published results measuring the distance to neighboring UDG NGC1052-DF4 (DF4) based on 12 Hubble orbits and TRGB analysis, which provided compelling evidence of missing dark matter. This preferred method expands on the team’s 2018 studies that relied on “surface brightness fluctuations” to gage distance. Both galaxies were discovered with the Dragonfly Telephoto Array at the New Mexico Skies observatory.
In addition to confirming earlier distance findings, the Hubble results indicated that the galaxies were located slightly farther away than previously thought, strengthening the case that they contain little to no dark matter. If DF2 were closer to Earth, as some astronomers claim, it would be intrinsically fainter and less massive, and the galaxy would need dark matter to account for the observed effects of the total mass.
Dark matter is widely considered to be an essential ingredient of galaxies, but this study lends further evidence that its presence may not be inevitable. While dark matter has yet to be directly observed, its gravitational influence is like a glue that holds galaxies together and governs the motion of visible matter. In the case of DF2 and DF4, researchers were able to account for the motion of stars based on stellar mass alone, suggesting a lack or absence of dark matter. Ironically, the detection of galaxies deficient in dark matter will likely help to reveal its puzzling nature and provide new insights into galactic evolution.
While DF2 and DF4 are both comparable in size to the Milky Way galaxy, their total masses are only about one percent of the Milky Way’s mass. These ultra-diffuse galaxies were also found to have a large population of especially luminous globular clusters.
This research has generated a great deal of scholarly interest, as well as energetic debate among proponents of alternative theories to dark matter, such as modified newtonian dynamics (MOND). However, with the team’s most recent findings—including the relative distances of the two UDGs to NGC1052—such alternative theories seem less likely. Additionally, there is now little uncertainty in the team’s distance measurements given the use of the TRGB method. Based on fundamental physics, this method depends on the observation of red giant stars that emit a flash after burning through their helium supply that always happens at the same brightness.
Moving forward, researchers will continue to hunt for more of these oddball galaxies, while considering a number of questions such as: How are UDGs formed? What do they tell us about standard cosmological models? How common are these galaxies, and what other unique properties do they have? It will take uncovering many more dark-matter-lacking galaxies to resolve these mysteries and the ultimate question of what dark matter really is.
To learn more, go here: https://iopscience.iop.org/article/10.3847/2041-8213/ac0335