Perspectives.

LARGE SUNSPOTS.—The naked-eye sunspot recently noted in these columns was followed shortly afterwards by the appearance of another which was seen as a naked-eye object from June 26 until July i. In a small telescope, the spot appeared elongated with a double umbra; there were several small companions near it and others in a cluster following at some distance. Details of position, etc., are given below in the usual tabular form. This latest spot brings the total number of naked-eye spots to eight for the first six months of 1926, as compared with eight for the whole of the preceding year.

The detection of the Fermi Bubbles suggests that spiral galaxies such as the Milky Way can undergo active periods. Using gamma-ray observations, we can investigate the possibility that such structures are present in other nearby galaxies. We have analyzed the region around the Andromeda Galaxy (Messier Catalog M31) for signs of bubble-like emission using TeV gamma ray data recorded by the High-Altitude Water Cherenkov Observatory. We fit a model consisting of two 6 kpc bubbles symmetric about and perpendicular to the M31 galactic plane and assume a power-law distribution for the gamma-ray flux. We compare the emission from these bubble regions to that expected from structures similar to the Fermi Bubbles found in the Milky Way. No significant emission was observed. We report upper limits on the TeV flux from Fermi Bubble structures in M31.

The rapid suppression of star formation, or quenching, is thought to be an important process in the evolution of the most massive galaxies, but the mechanisms involved are still hotly debated. Here, we consider two agents that control star formation and can ultimately lead to its suppression: AGN feedback and galaxy mergers.
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\nIn the first part of the thesis, we study the interplay between stellar structure, nuclear activity, and molecular gas in the context of AGN feeding and feedback. We start presenting our catalogue of stellar mass maps for more than 1500 nearby galaxies, which has been publicly released. Using the stellar mass map of the spiral galaxy M51, we show that there is sufficient molecular gas inflow to feed the AGN (~1 Msun/yr), and feedback effects which include a nuclear molecular outflow (at a rate comparable to the inflow), as well as a large-scale radio jet which pushes molecular gas laterally, inducing shocks and turbulence.
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\nIn the second part, we address the role of galaxy mergers in the buildup of a passive population of lenticular galaxies (or S0s). While lenticulars are the most common early-type galaxies in the Universe, their formation channels remain elusive. We use numerical simulations to show that even major mergers of spiral galaxies can result in lenticulars, with a bulge-disc coupling in agreement with observations. We also find that major mergers can simultaneously account for the difference in angular momentum and concentration between spirals and S0s, as recently found by the CALIFA team.
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\nGlobally, our results show that both internal processes (transport of gas and AGN feedback) and external mechanisms (mergers) have the ability to regulate and eventually suppress star formation in galaxies. Current and future instrumental advancements (ALMA, NOEMA, JWST) will permit to
\nconfirm our findings with other galaxies and further assess their relative importance.

There are three spiral galaxies in our Local Group: the Milky Way Galaxy (MWG), the Andromeda Galaxy (M31, NGC 224) and M33 (NGC 598). These three are the largest and brightest of the 50 or so members of the group. The others are all low-luminosity elliptical galaxies, irregular galaxies and dwarf spheroidal galaxies.

view Abstract Citations (6) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dust-to-gas ratio in disk galaxies Jura, M. Abstract Optical polarization in disk galaxies can be produced by scattering off grains. By comparing the amount of polarization with the column density of gas derived from 21 cm measurements, it is possible to estimate a dust-to-gas ratio. We argue that observations of some inner portions of M31 and M81 suggest that the dust-to-gas ratio in the observed regions is probably at least 0.005 by mass and therefore comparable to the value in the solar neighborhood. Publication: The Astrophysical Journal Pub Date: April 1979 DOI: 10.1086/156979 Bibcode: 1979ApJ...229..485J Keywords: Abundance; Cosmic Dust; Galaxies; Interstellar Gas; Interstellar Matter; Optical Polarization; Andromeda Galaxy; Centimeter Waves; Polarization Characteristics; Radio Galaxies; Spiral Galaxies; Astrophysics full text sources ADS | data products NED (2)

view Abstract Citations (11) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Maps of high-velocity hydrogen clouds. Wright, M. C. H. Abstract Maps are presented for four high-velocity hydrogen clouds toward the center of the local group, and of two clouds within the Magellanic Stream. The four clouds have diameters 20-40 arc min, surface densities 2 x 10 to the 19th atoms/sq cm, and half-widths of about 20 km/s, similar to concentrations within previous high-velocity clouds. They are quite different from the clouds within the Magellanic Stream which have diameters of about 1 deg, surface densities not greater than 10 to to the 19th/sq cm, and half-widths of about 40 km/s. The clouds could have resulted from a close encounter between M33 and M31 or may be part of a local supernovae shell. Publication: The Astrophysical Journal Pub Date: November 1979 DOI: 10.1086/157469 Bibcode: 1979ApJ...234...27W Keywords: Astronomical Maps; Cosmic Gases; Hydrogen Clouds; Intergalactic Media; Magellanic Clouds; Andromeda Galaxy; High Speed; Radio Spectra; Spiral Galaxies; Supernova Remnants; Astrophysics; H I Clouds:Intergalactic Matter; H I Clouds:Local Group; H I Clouds:Magellanic Stream; H I Clouds:Radio Maps full text sources ADS | data products NED (2)

view Abstract Citations (65) References (57) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The H II Region of the Galaxy M101 Scowen, P. A. ; Dufour, R. J. ; Hester, J. J. Abstract Spectrophotometrically calibrated CCD imagery of two overlapping 16 arcmin fields in the SAB (rs)cdI galaxy M101 in the emission of Hα, [O III] λ5007, Hβ, and [S II] λ6723 are used to study various physical properties of the H II region population. Individual H II regions are identified and mapped using an automated algorithm thus eliminating personal bias in the measurements. Characteristics of the population studied include the H II region luminosity function and radial variations in extinction, [O III]/Hβ, O/H, [S II]/Hα, ionization parameter and numbers of ionizing photons. In addition, radial and azimuthal variations in the H II surface density are studied in comparison to that of H I and H_2_ inferred from radio 21 cm and CO observationS. It is demonstrated that the magnitude of trends and "gradients" found from such analyses depend on the surface-brightness threshold set in defining the H II region boundaries; so results are presented for both low (log F (Hα) = - 15.7 ergs s^-1^ cm^-2^ pixel^-2^; 625 H II regions) and high (-15.0; 248 H II regions) thresholds. Radial gradients in both extinction, C(Hβ)7 and O/H are seen in the H II region population- with a distinct flattening in the O/H gradient seen in the outer disk beginning at about 10 kpc. The luminosity function of the H II region population found is similar to previous studies, though both the high and low ends are sensitive to the threshold levels set. Other parameters such as the ionization parameters luminosities, and size distribution, show no evidence for systematic variations with galactocentric distance. Current star-formation processes active in the disk of M101 are analyzed two dimensionally by comparison of the distribution of ionized and neutral hydrogen. The star-formation efficiency shows considerable variation across the disk, suggesting significant spatial variation in the critical density for cloud collapse across the disk of M101 and thus departures from the simple Toomre model. Publication: The Astronomical Journal Pub Date: July 1992 DOI: 10.1086/116223 Bibcode: 1992AJ....104...92S Keywords: Astronomical Photometry; Charge Coupled Devices; H Ii Regions; Luminous Intensity; Spiral Galaxies; Star Formation; Active Galactic Nuclei; Carbon Monoxide; H Alpha Line; H Beta Line; Line Of Sight; Oxygen; Sulfur; Astrophysics; GALAXIES: SPIRAL; GALAXIES: INDIVIDUAL: M101; H II REGIONS full text sources ADS | data products SIMBAD (253) NED (251) CDS (1) Related Materials (1) Erratum: 1994AJ....107.1203S

A deep (R_lim_ ~ 25) color-magnitude diagram of a 1'x 2' field in the halo of M31 9 kpc from the nucleus has been constructed, reaching just to the level of the horizontal branch. The color spread of the red giant branch is significantly larger than expected from observational uncertainties, and is (as concluded by previous works) likely due to a spread in [Fe/H] of the halo stars. Comparison of the red giant branch with color-calibrated isochrones shows that its mean metallicity is high (<[Fe/H]>~ -0.6), with an intrinsic dispersion of ~0.2-0.3 dex. Modeling of the CMD with artificial star experiments suggests that the M31 halo has only a very weak blue HB component, with most HB stars even redder than in 47 Tucanae. These results further demonstrate that the halo stars of M31 in this field are more metal-rich than the M31 globular clusters, as has been observed for many large galaxies. However, this situation contrasts strongly with the Milky Way, where the halo clusters and field stars have fairly similar metallicity distribution functions (MDFs); in addition, it suggests that M31 underwent considerably more pre-enrichment (by 1 dex in (Fe/H]) before the main epoch of halo formation.

M51, the Whirlpool Galaxy, at a distance of ≈ 9,6 Mpc and a systemic velocity of 465 km s−1, is the closest “Grand Design” spiral galaxy. Its low inclination (20°) makes it an excellent target for structural studies, e.g. the formation of arms in response to a spiral density wave causing gas streaming motions. We have obtained a high resolution, sensitive map of the inner 2.5′ of M51 using the Caltech six-element OVRO array. The map consists of a 19-field mosaic, taken using three different telescope configurations. The resolution is 2.5″, (corresponding to 115 pc linear size) and 3.5” for the robustly and naturally weigthed maps, respectively.

We measure the correlation between the arrival directions of the highest energy cosmic rays detected by the Pierre Auger Observatory with the position of the galaxies in the H i Parkes All-Sky Survey (HIPASS) catalogue, weighted for their H i flux and Auger exposure. The use of this absorption-free catalogue, complete also along the Galactic plane, allows us to use all the Auger events. The correlation is significant, being 86.2 per cent for the entire sample of H i galaxies, and becoming 99 per cent when considering the richest galaxies in H i content or 98 per cent with those lying between 40 and 55 Mpc. We interpret this result as the evidence that spiral galaxies are the hosts of the producers of ultra-high energy cosmic rays and we briefly discuss the classical (i.e. energetic and distant) long gamma-ray burst (GRBs), short GRBs, as well as newly born or late-flaring magnetars as possible sources of the Auger events. With the caveat that these events are still very few, and the theoretical uncertainties are conspicuous, we found that newly born magnetars are the best candidates. If so, they could also be associated with sub-energetic, spectrally soft, nearby, long GRBs. We finally discuss why there is a clustering of Auger events in the direction on the radio galaxy Cen A and an absence of events in the direction of the radio galaxy M87.

Context: We introduce the Dwarf Galaxy Survey with Amateur Telescopes (DGSAT) project and report the discovery of eleven low surface brightness (LSB) galaxies in the fields of the nearby galaxies NGC 2683, NGC 3628, NGC 4594 (M104), NGC 4631, NGC 5457 (M101), and NGC 7814. Aims: The DGSAT project aims to use the potential of small-sized telescopes to probe LSB features around large galaxies and to increase the sample size of the dwarf satellite galaxies in the Local Volume. Methods: Using long exposure images, fields of the target spiral galaxies are explored for extended low surface brightness objects. After identifying dwarf galaxy candidates, their observed properties are extracted by fitting models to their light profiles. Results: We find three, one, three, one, one, and two new LSB galaxies in the fields of NGC 2683, 3628, 4594, 4631, 5457, and 7814, respectively. In addition to the newly found galaxies, we analyse the structural properties of nine already known galaxies. All of these 20 dwarf galaxy candidates have effective surface brightnesses in the range $25.3\lesssim\mu_{e}\lesssim28.8$ mag.arcsec$^{-2}$ and are fit with Sersic profiles with indices $n\lesssim 1$. Assuming that they are in the vicinity of the above mentioned massive galaxies, their $r$-band absolute magnitudes, their effective radii, and their luminosities are in the ranges $-15.6 \lesssim M_r \lesssim -7.8$, 160 pc $\lesssim R_e \lesssim$ 4.1 kpc, and $0.1\times 10^6 \lesssim\left(\frac{L}{L_{\odot}}\right)_r\lesssim127 \times 10^6$, respectively. To determine whether these LSB galaxies are indeed satellites of the above mentioned massive galaxies, their distances need to be determined via further observations. Conclusions: Using small telescopes, we are readily able to detect LSB galaxies with similar properties to the known dwarf galaxies of the Local Group.

On May 31, 2011 UT a supernova (SN) exploded in the nearby galaxy M51 (the Whirlpool Galaxy). We discovered this event using small telescopes equipped with CCD cameras, as well as by the Palomar Transient Factory (PTF) survey, and rapidly confirmed it to be a Type II supernova. Our early light curve and spectroscopy indicates that PTF11eon resulted from the explosion of a relatively compact progenitor star as evidenced by the rapid shock-breakout cooling seen in the light curve, the relatively low temperature in early-time spectra and the prompt appearance of low-ionization spectral features. The spectra of PTF11eon are dominated by H lines out to day 10 after explosion, but initial signs of He appear to be present. Assuming that He lines continue to develop in the near future, this SN is likely a member of the cIIb (compact IIb; Chevalier and Soderberg 2010) class, with progenitor radius larger than that of SN 2008ax and smaller than the eIIb (extended IIb) SN 1993J progenitor. Our data imply that the object identified in pre-explosion Hubble Space Telescope images at the SN location is possibly a companion to the progenitor or a blended source, and not the progenitor star itself, as its radius (~10^13 cm) would be highly inconsistent with constraints from our post-explosion photometric and spectroscopic data.

The unusual morphology of the Andromeda galaxy (Messier 31, the closest spiral galaxy to the Milky Way) has long been an enigma. Although regarded for decades as showing little evidence of a violent history, M31 has a well-known outer ring of star formation at a radius of ten kiloparsecs whose centre is offset from the galaxy nucleus. In addition, the outer galaxy disk is warped, as seen at both optical and radio wavelengths. The halo contains numerous loops and ripples. Here we report the presence of a second, inner dust ring with projected dimensions of 1.5 x 1 kiloparsecs and offset by about half a kiloparsec from the centre of the galaxy (based upon an analysis of previously-obtained data). The two rings appear to be density waves propagating in the disk. Numerical simulations indicate that both rings result from a companion galaxy plunging through the centre of the disk of M31. The most likely interloper is M32. Head-on collisions between galaxies are rare, but it appears nonetheless that one took place 210 million years ago in our Local Group of galaxies.

Andromeda galaxy (M31,NGC224) is the biggest spiral in the Local Group. By studying the star formation history(SFH) and chemical evolution of M31, and comparing with the Milky Way Galaxy, we are able to understand more about the formation and evolution of spiral galaxies.

Deep-imaging observations of the Andromeda galaxy and its surroundings have revealed a wide but thin planar structure of satellite galaxies that all orbit their host in the same rotational direction. See Letter p.62 Giant spiral galaxies are assembled from smaller systems through a process known as hierarchical clustering. In orbit around these giants are dwarf galaxies, which are presumably remnants of the galactic progenitors. Recent studies of the dwarf galaxies of the Milky Way have led some astronomers to suspect that their orbits are not randomly distributed. This suspicion, which challenges current theories of galaxy formation, is now bolstered by the discovery of a plane of dwarf galaxies corotating as a coherent pancake-like structure around the Andromeda galaxy, the Milky Way's close neighbour and in many respects its 'twin'. The structure is extremely thin yet contains about half of the dwarf galaxies in the Andromeda system. The authors report that 13 of the 15 satellites in the plane share the same sense of rotation.