* Astronomy

M51: Chandra Captures Galaxy Sparkling in X-rays

Nearly a million seconds of observing time with NASA's Chandra X-ray Observatory has revealed a spiral galaxy similar to the Milky Way glittering with hundreds of X-ray points of light.
The galaxy is officially named Messier 51 (M51) or NGC 5194, but often goes by its nickname of the "Whirlpool Galaxy." Like the Milky Way, the Whirlpool is a spiral galaxy with spectacular arms of stars and dust. M51 is located about 30 million light years from Earth, and its face-on orientation to Earth gives us a perspective that we can never get of our own spiral galactic home.
Read more

Title: A CO J=3-2 map of M51 with HARP-B: Radial properties of the spiral structure
Authors: C. Vlahakis, P. van der Werf, F. P. Israel, R. P. J. Tilanus

We present the first complete CO J=3-2 map of the nearby grand-design spiral galaxy M51 (NGC 5194), at a spatial resolution of ~600 pc, obtained with the HARP-B instrument on the James Clerk Maxwell Telescope. The map covers the entire optical galaxy disk and out to the companion NGC 5195, with CO J=3-2 emission detected over an area of ~9'x6' (~21x14 kpc). We describe the CO J=3-2 integrated intensity map and combine our results with maps of CO J=2-1, CO J=1-0 and other data from the literature to investigate the variation of the molecular gas, atomic gas and polycyclic aromatic hydrocarbon (PAH) properties of M51 as a function of distance along the spiral structure on sub-kpc scales. We find that for the CO J=3-2 and CO J=2-1 transitions there is a clear difference between the variation of arm and inter-arm emission with galactocentric radius, with the inter-arm emission relatively constant with radius and the contrast between arm and inter-arm emission decreasing with radius. For CO J=1-0 and HI the variation with radius shows a similar trend for the arm and inter-arm regions, and the arm-inter-arm contrast appears relatively constant with radius. We investigate the variation of CO line ratios (J=3-2/2-1, J=2-1/1-0 and J=3-2/1-0) as a function of distance along the spiral structure. Line ratios are consistent with the range of typical values for other nearby galaxies in the literature. The highest CO J=3-2/2-1 line ratios are found in the central ~1 kpc and in the spiral arms and the lowest line ratios in the inter-arm regions. We find no clear evidence of a trend with radius for the spiral arms but for the inter-arm regions there appears to be a trend for all CO line ratios to increase with radius. We find a strong relationship between the ratio of CO J=3-2 intensity to stellar continuum-subtracted 8mu PAH surface brightness and the CO J=3-2 intensity that appears to vary with radius.

Read more (11742kb, PDF)

Title: Geometric Offsets Across Spiral Arms in M51: Nature of Gas and Star Formation Tracers
Authors: M. Louie (1), J. Koda (1), F. Egusa (2) ((1) Stony Brook University, (2) JAXA)

We report measurements of geometric offsets between gas spiral arms and associated star forming regions in the grand-design spiral galaxy M51. These offsets are a suggested measure of the star formation timescale after the compression of gas at spiral arm entry. A surprising discrepancy, by an order of magnitude, has been reported in recent offset measurements in nearby spiral galaxies. Measurements using CO and H-alpha emission find large and ordered offsets in M51. On the contrary, small or non-ordered offsets have been found using the HI 21cm and 24 micron emissions, possible evidence against gas flow through spiral arms, and thus against the conventional density-wave theory with a stationary spiral pattern. The goal of this paper is to understand the cause of this discrepancy. We investigate potential causes by repeating those previous measurements using equivalent data, methods, and parameters. We find offsets consistent with the previous measurements and conclude that the difference of gas tracers, i.e., HI versus CO, is the primary cause. The HI emission is contaminated significantly by the gas photo-dissociated by recently-formed stars and does not necessarily trace the compressed gas, the precursor of star formation. The HI gas and star forming regions coincide spatially and tend to show small offsets. We find mostly positive offsets with substantial scatter between CO and H-alpha, suggesting gas flow through spiral arms (i.e., density-wave) though the spiral pattern may not necessarily be stationary.

Read more (752kb, PDF)

Title: Hubble Space Telescope Pixel Analysis of the Interacting S0 Galaxy NGC 5195 (M51B)
Authors: Joon Hyeop Lee, Sang Chul Kim, Chang Hee Ree, Minjin Kim, Hyunjin Jeong, Jong Chul Lee, Jaemann Kyeong (Korea Astronomy and Space Science Institute)

We report the properties of the interacting S0 galaxy NGC 5195 (M51B), revealed in a pixel analysis using the HST/ACS images in the F435W, F555W and F814W (BVI) bands. We analyse the pixel colour-magnitude diagram (pCMD) of NGC 5195, focusing on the properties of its red and blue pixel sequences and the difference from the pCMD of NGC 5194 (M51A; the spiral galaxy interacting with NGC 5195). The red pixel sequence of NGC 5195 is redder than that of NGC 5194, which corresponds to the difference in the dust optical depth of 2<\Delta\tau_V<4 at fixed age and metallicity. The blue pixel sequence of NGC 5195 is very weak and spatially corresponds to the tidal bridge between the two interacting galaxies. This implies that the blue pixel sequence is not an ordinary feature in the pCMD of an early-type galaxy, but that it is a transient feature of star formation caused by the galaxy-galaxy interaction. We also find a difference in the shapes of the red pixel sequences on the pixel colour-colour diagrams (pCCDs) of NGC 5194 and NGC 5195. We investigate the spatial distributions of the pCCD-based pixel stellar populations. The young population fraction in the tidal bridge area is larger than that in other areas by a factor > 15. Along the tidal bridge, young populations seem to be clumped particularly at the middle point of the bridge. On the other hand, the dusty population shows a relatively wide distribution between the tidal bridge and the NGC 5195 center.

Read more (3522kb, PDF)