A bright, high rotation-measure FRB that skewers the M33 halo

ABSTRACT We report the detection of a bright fast radio burst, FRB 191108, with Apertif on the Westerbork Synthesis Radio Telescope. The interferometer allows us to localize the FRB to a narrow 5 arcsec × 7 arcmin ellipse by employing both multibeam information within the Apertif phased-array feed beam pattern, and across different tied-array beams. The resulting sightline passes close to Local Group galaxy M33, with an impact parameter of only 18 kpc with respect to the core. It also traverses the much larger circumgalactic medium (CGM) of M31, the Andromeda Galaxy. We find that the shared plasma of the Local Group galaxies could contribute ∼10 per cent of its dispersion measure of 588 pc cm−3. FRB 191108 has a Faraday rotation measure (RM) of +474 $\pm \, 3$ rad m−2, which is too large to be explained by either the Milky Way or the intergalactic medium. Based on the more moderate RMs of other extragalactic sources that traverse the halo of M33, we conclude that the dense magnetized plasma resides in the host galaxy. The FRB exhibits frequency structure on two scales, one that is consistent with quenched Galactic scintillation and broader spectral structure with Δν ≈ 40 MHz. If the latter is due to scattering in the shared M33/M31 CGM, our results constrain the Local Group plasma environment. We found no accompanying persistent radio sources in the Apertif imaging survey data.

Candidate fossil groups in the CFHTLS: a probabilistic approach

Context. Fossil groups (FGs) have been discovered 25 years ago, and are now defined as galaxy groups with an X-ray luminosity higher than $ 10^{42}\,h_{50}^{-2} $ erg s−1 and a brightest group galaxy brighter than the other group members by at least two magnitudes. However, the scenario of their formation remains controversial. Aims. We propose here a probabilistic analysis of FGs, extracted from the large catalog of candidate groups and clusters previously detected in the CFHTLS survey based on photometric redshifts to investigate their position in the cosmic web and probe their environment. Methods. Based on spectroscopic and photometric redshifts, we estimated the probability of galaxies to belong to a galaxy structure, and by imposing the condition that the brightest group galaxy is at least brighter than the others by two magnitudes, we computed the probability for a given galaxy structure to be a FG. We analyzed the mass distribution of these candidate FGs, and estimated their distance to the filaments and nodes of the cosmic web in which they are embedded. Results. We find that structures with masses lower than 2.4 × 1014 M⊙ have the highest probabilities of being fossil groups (PFG). Overall, structures with PFG ≥ 50% are located close to the cosmic web filaments (87% are located closer than 1 Mpc to their nearest filament). They are preferentially four times more distant from their nearest node than from their nearest filament. Conclusions. We confirm that FGs have low masses and are rare. They seem to reside closely to cosmic filaments and do not survive in nodes. Being in a poor environment might therefore be the driver of FG formation because the number of nearby galaxies is not sufficient to compensate for the cannibalism of the central group galaxy.

The H i mass function of group galaxies in the ALFALFA survey

ABSTRACT We estimate the H i mass function (HIMF) of galaxies in groups based on thousands of ALFALFA (Arecibo Legacy Fast ALFA survey) H i detections within the galaxy groups of four widely used SDSS (Sloan Digital Sky Survey) group catalogues. Although differences between the catalogues mean that there is no one definitive group galaxy HIMF, in general we find that the low-mass slope is flat, in agreement with studies based on small samples of individual groups, and that the ‘knee’ mass is slightly higher than that of the global HIMF of the full ALFALFA sample. We find that the observed fraction of ALFALFA galaxies in groups is approximately 22 per cent. These group galaxies were removed from the full ALFALFA source catalogue to calculate the field HIMF using the remaining galaxies. Comparison between the field and group HIMFs reveals that group galaxies make only a small contribution to the global HIMF as most ALFALFA galaxies are in the field, but beyond the HIMF ‘knee’ group galaxies dominate. Finally, we attempt to separate the group galaxy HIMF into bins of group halo mass, but find that too few low-mass galaxies are detected in the most massive groups to tightly constrain the slope, owing to the rarity of such groups in the nearby Universe where low-mass galaxies are detectable with existing H i surveys.

B-supergiants in IC1613: testing low-Z massive star physics and evolution

The physical processes taking place in massive stars during their life and death are highly dependent on the metallicity (Z) of their parent cloud. Observations of these stars in low-Z nearby galaxies are crucial to understand these processes. IC1613 is the nearest Local Group galaxy with ongoing star formation and O-abundance lower than the SMC, although UV spectroscopy suggests it is not so metal poor. We performed a spectral analysis of early B-type stars in the galaxy, obtaining physical parameters and abundances. Our results confirm the low O-abundance of IC1613.