1 point We can detect 21-cm emission from other galaxies as well as from our own Galaxy. However, 21-cm emission from our own Galaxy fills most of the sky, so we usually see both at once. How can we distinguish the extragalactic 21-cm emission from that arising in our own Galaxy? O External galaxies are moving relative to ours, and thus, their emission is shifted to a slightly different wavelength and can be observed in the spectrum by using the doppler method. O There is no difference in the spectrums between external galaxies and the 21-cmismission from our own Galaxy, this there is no way to distinguish between 21-cm . emission from extragalactic sources and our own Galaxy. External galaxies are filled with more molecular clouds with more dust and heavier elements and thus, their emission shows up as thicker lines on the spectrums and this difference can be observed by looking at the molecular lines. External galaxies are moving relative to ours, and thus, their emission is shifted to a slightly different wavelength and can be observed in the spectrum by looking at the molecular lines. O External galaxies are filled with more molecular clouds with more dust and heavier elements and thus, their emission shows up as thinner lines on the spectrums and this difference can be observed by looking at the molecular lines