Far ultraviolet to far infrared images of the nearby galaxy NGC 5194 ( M51a ) , from a combination of space–based ( Spitzer , GALEX , and Hubble Space Telescope ) and ground–based data , are used to investigate local and global star formation , and the impact of dust extinction .
The Spitzer data provide unprecedented spatial detail in the infrared , down to sizes \sim 500 pc at the distance of NGC 5194 .
The multiwavelength set is used to trace the relatively young stellar populations , the ionized gas , and the dust absorption and emission in HII–emitting knots , over 3 orders of magnitude in wavelength range .
As is common in spirals , dust extinction is high in the center of the galaxy ( A _ { V } \sim 3.5 mag ) , but its mean value decreases steadily as a function of galactocentric distance , as derived from both gas emission and stellar continuum properties .
In the IR/UV–UV color plane , the NGC 5194 HII knots show the same trend observed for normal star–forming galaxies , having a much larger dispersion ( \sim 1 dex peak–to–peak ) than starburst galaxies .
We identify the dispersion as due to the UV emission predominantly tracing the evolved , non–ionizing stellar population , up to ages \sim 50–100 Myr .
While in starbursts the UV light traces the current SFR , in NGC 5194 it traces a combination of current and recent–past SFR .
Possibly , mechanical feedback from supernovae is less effective at removing dust and gas from the star formation volume in normal star forming galaxies than in starbursts , because of the typically lower star formation rate ( SFR ) densities in the former .
The application of the starburst opacity curve for recovering the intrinsic UV emission ( and deriving SFRs ) in local and distant galaxies appears therefore appropriate only for SFR densities \gtrsim 1 M _ { \sun } yr ^ { -1 } kpc ^ { -2 } .
Unlike the UV emission , the monochromatic 24 \mu m luminosity is an accurate local SFR tracer for the HII knots in NGC 5194 , with a peak–to–peak dispersion of less than a factor of 3 relative to hydrogen emission line tracers ; this suggests that the 24 \mu emission carriers are mainly heated by the young , ionizing stars .
However , preliminary results show that the ratio of the 24 \mu m emission to the SFR varies by a factor of a few from galaxy to galaxy ; this variation needs to be understood and carefully quantified before the 24 \mu m luminosity can be used as a SFR tracer for galaxy populations .
While also correlated with star formation , the 8 \mu m emission is not directly proportional to the number of ionizing photons ; it is overluminous , by up to a factor \sim 2 , relative to the galaxy ’ s average in weakly ionized regions and is underluminous , by up to a factor \sim 3 , in strongly ionized regions .
This confirms earlier suggestions that the carriers of the 8 \mu m emission are heated by more than one mechanism .