High-Mass, OB Star Formation in M51: [ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Hα and P[CLC]a[/CLC]α Imaging

Abstract

We have obtained Ha and Paa emission-line images covering the central 3@È4@ of M51 using the WFPC2 and NICMOS instruments on the Hubble Space T elescope to study the high-mass stellar population. The pixels provide 4.6È9 pc resolution in M51, and the Ha/Paa line ratios are used to 0A .1È0A .2 obtain extinction estimates. A sample of 1373 Ha emission regions is cataloged using an automated and uniform measurement algorithm. Their sizes are typically 10È100 pc. The luminosity function for the Ha emission regions is obtained over the range to 2 ] 1039 ergs s~1. The luminosity function is L Ha \ 1036 Ðtted well by a power law with dN/d ln L P L~1.01. The power law is signiÐcantly truncated, and no regions were found with observed above 2 ] 1039 ergs s~1 (uncorrected for extinction). (The L Ha maximum seen in ground-based studies is approximately a factor of 5 higher, very likely because of the blending of multiple regions.) The extinctions derived here increase the maximum intrinsic luminosity to above 1040 ergs s~1. The logarithmically binned luminosity function is also somewhat steeper (a \ [1.01) than that found from ground-based imaging (a \ [0.5 to [0.8)Èprobably also a result of our resolving regions that were blended in the ground-based images. The two-point correlation function for the H II regions exhibits strong clustering on scales ¹2A, or 96 pc.To analyze the variations of H II region properties the galactic structure, the spiral arm areas vis-a`-vis were deÐned independently from millimeter-CO and optical continuum imaging. Although the arms constitute only 25% of the disk surface area, the arms contain 45% of the cataloged H II regions. The luminosity function is somewhat Ñatter in spiral arm regions than in the interarm areas ([0.72 to [0.95) ; however, this is very likely the result of increased blending of individual H II regions in the arms that have higher surface density. No signiÐcant di †erence is seen in the sizes and electron densities of the H II regions in spiral arm and interarm regions. For 209 regions that had º5 p detections in both Paa and Ha, the observed line ratios indicate visual extinctions in the range mag. The mean extinction A V \ 0È6 was mag (weighting each region equally), 2.4 mag (weighting each by the observed Ha A V \ 3.1 luminosity), and 3.0 mag (weighting by the extinction-corrected luminosity). On average, the observed Ha luminosities should be increased by a factor of D10, implying comparable increases in global OB star cluster luminosities and star formation rates. The full range of extinction-corrected Ha luminosities is between 1037 and 2 ] 1040 ergs s~1.The most luminous regions have sizes º100 pc, so it is very likely that they are blends of multiple regions. This is clear based on their sizes, which are much larger than the maximum diameter (¹50 pc) to which an H II region might conceivably expand within the D3 ] 106 yr lifetime of the OB stars. It is also consistent with the observed correlation (L P D2) between the measured luminosities and sizes of th...

High‐excitation OH and H2O Lines in Markarian 231: The Molecular Signatures of Compact Far‐infrared Continuum Sources

“…A scenario with a crowded population of PDRs overlapping along the line of sight is therefore more plausible. If a typical single OB stellar cluster emits up to 10 39 erg s À1 , the maximum value found by Scoville et al (2001) in M51 and also the luminosity of W49, then the number of single clusters to produce the Mrk 231 starburst luminosity of 10 12 L is k4 ; 10 6 . If these clusters are concentrated in a disk of radius 460 pc and thickness 25 pc (DS98), the mean distance between neighbor clusters is only P1.5 pc (see also Keto et al 1992).…”

The nature of the low-frequency emission of M 51

“…This is unlikely at such a low-frequency, because a similar thermal fraction has been found at 4.86 GHz (Fletcher et al 2011), while the thermal fraction must strongly decrease towards lower frequencies. Scoville et al (2001) found 1373 H ii emission regions exist within the central region with sizes up to 100 pc, and therefore thermal free-free absorption is expected. Recently, Adebahr et al (2013) observed a spectral flattening due to free-free absorption at 350 MHz in the core region of M82 where intense star formation is known to be occurring.…”

Star Formation and Asymmetry in the Spiral Arms of M51: Variable Star Formation Caused by More than One Spiral Density Wave

“…(See Scoville et al 2001 for more information about these observations.) The F187N and F190N filters, with effective wavelengths of 1.87 and 1.90 lm, contain Pa line and continuum images, respectively.…”

“…All positions were measured from the center of the galaxy, which was determined from the centroid of an H-band (F160W) image of M51 that was observed simultaneously with the Pa data (Scoville et al 2001). Since the nuclear region of M51 contains little molecular gas (see Fig.…”