The slowing down of galaxy disks in dissipationless minor mergers

We have investigated the impact of dissipationless minor galaxy mergers on the angular momentum of the remnant. Our simulations cover a range of initial orbital characteristics and the system consists of a massive galaxy with a bulge and disk merging with a much less massive (one-tenth or one-twentieth) gasless companion which has a variety of morphologies (disk- or elliptical-like) and central baryonic mass concentrations. During the process of merging, the orbital angular momentum is redistributed into the internal angular momentum of the final system; the internal angular momentum of the primary galaxy can increase or decrease depending on the relative orientation of the orbital spin vectors (direct or retrograde), while the initially non-rotating dark matter halo always gains angular momentum. The specific angular momentum of the stellar component always decreases independent of the orbital parameters or morphology of the satellite, the decrease in the rotation velocity of the primary galaxy is accompanied by a change in the anisotropy of the orbits, and the ratio of rotation speed to velocity dispersion of the merger remnant is lower than the initial value, not only due to an increase in the dispersion but also to the slowing -down of the disk rotation. We briefly discuss several astrophysical implications of these results, suggesting that minor mergers do not cause a "random walk" process of the angular momentum of the stellar disk component of galaxies, but rather a steady decrease. Minor mergers may play a role in producing the large scatter observed in the Tully-Fisher relation for S0 galaxies, as well as in the increase of the velocity dispersion and the decrease in $v/\sigma$ at large radii as observed in S0 galaxies.Comment: 10 pages, 10 figures, accepted for publication in A&

Characteristics of thick disks formed through minor mergers: stellar excesses and scale lengths

By means of N-body/SPH simulations we investigate the morphological properties of thick disks formed through minor mergers. We show that the vertical surface density profile of the post-merger thick disk follows a sech function and has an excess in the regions far from the disk mid-plane (z>2kpc). This stellar excess also follows a sech function with a larger scale height than the main thick disk component, and it is usually dominated by stars from the primary galaxy. Stars in the excess have a rotational velocity lower than that of stars in the thick disk, and they may thus be confused with stars in the inner galactic halo. The thick disk scale height increases with radius and the rate of its increase is smaller for more gas rich primary galaxies. On the contrary, the scale height of the stellar excess is independent of both radius and gas fraction. We also find that the post-merger thick disk has a radial scale length which is 10-50% larger than that of the thin disk. Two consecutive mergers have basically the same effect on heating the stellar disk as a single merger of the same total mass. To investigate how thick disks produced through secular processes may differ from those produced by minor mergers, we also simulated gravitationally unstable gas-rich disks. These disks do not produce either a stellar excess or a ratio of thick to thin disk scale lengths greater than one. Our simulation results are consistent with observations of the ratio of thick to thin disk scale lengths of the Milky Way and nearby galaxies, and with the Toomre diagram of the Milky Way. We conclude that minor mergers are a viable mechanism for the creation of galactic thick disks and investigating stars at several kpc above the mid-plane of the Milky Way and other galaxies may provide a quantitative method for studying the minor merger history of galaxies.Comment: 13 pages, 13 figures, accepted by A&

Virtual Reality Relaxation for Reducing Perceived Stress of Intensive Care Nurses During the COVID-19 Pandemic

During the COVID-19 pandemic ICU nurses endure high levels of stress. VR relaxation (VRelax, containing 360° immersive environments) provides an easy-to-use and effective means to induce positive affect and reduce perceived stress. We investigated feasibility and immediate effect on perceived stress of VRelax use by ICU nurses during work shifts. ICU nurses working with COVID-19 patients in an academic hospital could use VRelax as a 10-min break during their shift. Primary outcome was the difference between perceived stress immediately before and after VRelax use measured by a single-question VAS-stress scale. Statistically significant difference of the mean VAS-stress before and after use was determined using the paired t student test. A socio-demographic questionnaire, a questionnaire on perceived stress and stress resilience and VRelax user experiences were sent by email. Eighty-six (26%) nurses used VRelax at least once; 77% (N=66) of these filled out the VAS-stress scale before and after use of VRelax. Mean perceived stress lowered with 39.9% after use of VRelax (mean difference=14.0, SD=13.3, p<0.005). Mean score on the perceived stress scale-10 was 11.4 (SD=6.50), mean score on the Connor-Davidson Resilience Scale-10 was 29.0 (SD=5.51). Sixty-two percentage of the ICU nurses thought VRelax was helpful to reduce stress. Main barrier for use was a high workload. It is feasible for nurses to use VRelax in an ICU context. VRelax is an effective intervention to reduce immediate perceived stress and is of added value in stressful situations as during the COVID-19 pandemic, inducing a positive affective state and lowering perceived stress

Spontaneous discontinuation of distressing auditory verbal hallucinations in a school-based sample of adolescents:a longitudinal study

Auditory verbal hallucinations (AVH) can be transiently present in both clinical and healthy adolescent populations. It is not yet fully understood why AVH discontinue in some adolescents and persist in others. The aim of this explorative study is to investigate predictors of spontaneous discontinuation of distressing AVH in a school-based sample of adolescents. 1841 adolescents (mean age 12.4 years, 58% female) completed self-report questionnaires at baseline. The current study included 123 adolescents (7%; 63% female) who reported at least mild distressing AVH at baseline and completed follow-up measurements. LASSO analyses were used to uncover predictors of spontaneous discontinuation of distressing AVH. During follow-up, 43 adolescents (35%) reported having experienced distressing AVH during the last 12 months, while 80 adolescents did not. Spontaneous discontinuation of distressing AVH was predicted by never having used cannabis, parents not being divorced in the past year, never having been scared by seeing a deceased body, less prosocial behaviour, school grade repetition, having the feeling that others have it in for you, having anxiety when meeting new people, having lived through events exactly as if they happened before and having the feeling as if parts of the body have changed. No associations between spontaneous discontinuation of distressing AVH and age or ethnicity were found. Distressing AVH in non-clinical adolescents are mostly transient. Discontinuation was predicted up to a certain extent. However, several predictors were difficult to interpret and do not provide leads for preventive measures, except for discouraging cannabis use.</p

Correction to: Spontaneous discontinuation of distressing auditory verbal hallucinations in a school-based sample of adolescents:A longitudinal study

In the Original Publication Table 2 was incorrectly published. The correct table is given below

De dubbele waterput uit het laat-Romeinse castellum van Oudenburg (prov. West-Vlaanderen): tafonomie, chronologie en interpretatie

This article focuses on a remarkable well structure that was brought to light by the Flemish Heritage Institute during recent archaeological research at the south-western corner of the Saxon Shore fort at Oudenburg (2001-2005). The site of Oudenburg is situated 8 km from the Flemish coastline, in the polder area between Bruges and Ostend. During Roman times however, positioned strategically on an elevated sandy ridge, the site overlooked the coastal plain consisting of mudflats and marshes intersected by natural gullies. The remains of the fort at Oudenburg were discovered in 1956-1957 by J. Mertenslater excavation campaigns in 1960 and 1970 on the western defence area revealed a sequence of three successive forts. The 1960s excavations on two late Roman military cemeteries more than 400 m to the west of the castellum revealed burials of 4th-century fort inhabitants with rich grave goods. During archaeological research within the fort walls in 1976-1977 the first information was collected about the inner organisation of the fort and the remains of a stone building of late 3rd-century date were excavated. It was only in 2001 that new excavations could take place on the fort area. This systematic research resulted in a finer chronology for the occupation of the castellum. A succession of five main fort periods was revealed, dating between ca. 200 and the beginning of the 5th century AD. These excavations yielded insight into the spatial organisation of the south west area of the fort, which had different functions in each successive fort period. The first three phases belonged to wood and earthen fortstemporary installations in times of trouble and Germanic threat. Probably in the later 3rd century AD, a more permanent fort measuring 153 by 176 m was built in stonethis was renovated and reoccupied during the second quarter of the 4th century AD. The characteristics of the ground plan, its topographical position and several finds pointing to a close link with the Saxon Shore forts on the coast of south Britannia, suggest that Oudenburg was probably part of the Litus Saxonicum. In this paper the so-called double well, a context of the fifth fort period (4th centurybeginning 5th century), is analysed. During this period the south-western area of the castellum was dominated by a stone bath building with hypocaust system. Later in the 4th century, long fences were constructed to divide the area into yards, a timber-framed construction with simple plan may be identified as a stable structure, and a large oak basin was probably a reservoir for drinking water. The double well, which received feature number OS 2562, seems to be a key context for this fort period

21-cm synthesis observations of VIRGOHI 21 - a possible dark galaxy in the Virgo Cluster

Many observations indicate that dark matter dominates the extra-galactic Universe, yet no totally dark structure of galactic proportions has ever been convincingly identified. Previously we have suggested that VIRGOHI 21, a 21-cm source we found in the Virgo Cluster using Jodrell Bank, was a possible dark galaxy because of its broad line-width (~200 km/s) unaccompanied by any visible gravitational source to account for it. We have now imaged VIRGOHI 21 in the neutral-hydrogen line and find what could be a dark, edge-on, spinning disk with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has unquestionably been involved in an interaction with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor normally linked with such a feature. Numerical models of NGC 4254 call for a close interaction ~10^8 years ago with a perturber of ~10^11 solar masses. This we take as additional evidence for the massive nature of VIRGOHI 21 as there does not appear to be any other viable candidate. We have also used the Hubble Space Telescope to search for stars associated with the HI and find none down to an I band surface brightness limit of 31.1 +/- 0.2 mag/sq. arcsec.Comment: 8 pages, accepted to ApJ, uses emulateapj.cls. Mpeg animation (Fig. 2) available at ftp://ftp.naic.edu/pub/publications/minchin/video2.mp

A Dark Hydrogen Cloud in the Virgo Cluster

VIRGOHI21 is an HI source detected in the Virgo Cluster survey of Davies et al. (2004) which has a neutral hydrogen mass of 10^8 M_solar and a velocity width of Delta V_20 = 220 km/s. From the Tully-Fisher relation, a galaxy with this velocity width would be expected to be 12th magnitude or brighter; however deep CCD imaging has failed to turn up a counterpart down to a surface-brightness level of 27.5 B mag/sq. arcsec. The HI observations show that it is extended over at least 16 kpc which, if the system is bound, gives it a minimum dynamical mass of ~10^11 M_solar and a mass to light ratio of M_dyn/L_B > 500 M_solar/L_solar. If it is tidal debris then the putative parents have vanished; the remaining viable explanation is that VIRGOHI21 is a dark halo that does not contain the expected bright galaxy. This object was found because of the low column density limit of our survey, a limit much lower than that achieved by all-sky surveys such as HIPASS. Further such sensitive surveys might turn up a significant number of the dark matter halos predicted by Dark Matter models.Comment: Accepted by ApJ