The Lockman Hole is a well-studied extragalactic field with extensive multi-band ancillary data covering a wide range in frequency , essential for characterising the physical and evolutionary properties of the various source populations detected in deep radio fields ( mainly star-forming galaxies and AGNs ) .
In this paper we present new 150-MHz observations carried out with the LOw Frequency ARray ( LOFAR ) , allowing us to explore a new spectral window for the faint radio source population .
This 150-MHz image covers an area of 34.7 square degrees with a resolution of 18.6 \times 14.7 arcsec and reaches an rms of 160 \mu Jy beam ^ { -1 } at the centre of the field .

As expected for a low-frequency selected sample , the vast majority of sources exhibit steep spectra , with a median spectral index of \alpha _ { 150 } ^ { 1400 } = -0.78 \pm 0.015 .
The median spectral index becomes slightly flatter ( increasing from \alpha _ { 150 } ^ { 1400 } = -0.84 to \alpha _ { 150 } ^ { 1400 } = -0.75 ) with decreasing flux density down to S _ { 150 } \sim 10 mJy before flattening out and remaining constant below this flux level .
For a bright subset of the 150-MHz selected sample we can trace the spectral properties down to lower frequencies using 60-MHz LOFAR observations , finding tentative evidence for sources to become flatter in spectrum between 60 and 150 MHz .
Using the deep , multi-frequency data available in the Lockman Hole , we identify a sample of 100 Ultra-steep spectrum ( USS ) sources and 13 peaked spectrum sources .
We estimate that up to 21 per cent of these could have z > 4 and are candidate high- z radio galaxies , but further follow-up observations are required to confirm the physical nature of these objects .