We have obtained multi-object spectroscopy of H ii regions in the spiral galaxy M51 with the Keck i telescope and the Low Resolution Imaging Spectrometer .
For ten objects we have detected the auroral line [ N ii ] \lambda 5755 , while [ S iii ] \lambda 6312 has been measured in seven of these .
This has allowed us to measure the electron temperature of the gas and to derive oxygen , sulfur and nitrogen abundances for the ten H ii regions .
Contrary to the expectations from previous photoionization models of a few H ii regions in M51 and from strong-line abundance indicators , the O/H abundance is below the solar value for most objects , with the most metal-rich H ii regions , P 203 and CCM 72 , having log ( O/H ) = -3.16 [ \sim 1.4 { ( O / H ) } _ { \odot } ] and log ( O/H ) = -3.29 [ \sim 1.0 { ( O / H ) } _ { \odot } ] , respectively .
The reduction of O/H by factors up to two or three with respect to previous indirect determinations has important consequences for the calibration of empirical abundance indicators , such as R _ { 23 } , in the abundance and excitation range found in the central regions of spiral galaxies .
The abundance gradients in these galaxies can therefore be considerably flatter than those determined by making use of such empirical calibrations .
The H ii regions with a measured electron temperature span the 0.19–1.04 R _ { 0 } range in galactocentric radius , and indicate a shallow abundance gradient for M51 : -0.02 \pm 0.01 dex kpc ^ { -1 } .
The S/O abundance ratio is found to be similar to previous determinations of its value in other spiral galaxies , log ( S/O ) \approx -1.6 .
Therefore , we find no evidence for a variation in massive star initial mass function or nucleosynthesis at high oxygen abundance .
An overabundance of nitrogen is measured , with log ( N/O ) \simeq - 0.6 .
Based on our new abundances we revise the effective yield for M51 , now found to be almost four times lower than previous estimates , and we discuss this result within the context of chemical evolution in galactic disks .
Features from Wolf-Rayet stars ( the blue bump at 4660 Å and the C iii line at 5696 Å ) are detected in a large number of H ii regions in M51 , with the C iii \lambda 5696 line found preferentially in the central , most metal-rich objects .