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Ha and the redshift of distant galaxies

Rafael Sampaio
in General
Is it useful to capture Ha of distant galaxies (like NGC 5426 and NGC 6769) with a narrowband filter? I was wondering if the redshift could make the captured data useless. What to do you think?    

Comments

  • BlockHeadBlockHead
    edited June 2024
    You are correct that when the redshift is great enough the Ha emission line would be shifted out of the bandpass for your "local reference" wavelength. It is for this reason that astronomers use filters that accommodate this shift (different bandpass). 

    I suspect that for galaxies that we are interested in imaging...the line would not be shifted entirely out- but it would be diminished by increasing redshift. I suspect most things less than 100million light years away are good.

    If you did the math... you can figure out the exact answer... 
    So...I asked Chat GPT... its answer (probably correct... and this looks good to me.. but I did not check it) is 70 million or so.

    See below:

    To determine how far you can observe Hα (Hydrogen-alpha) emission in another galaxy using a filter with a 3nm bandpass before the emission line is shifted out of the filter's bandpass due to redshift, we need to consider the cosmological redshift effect. 

    ### Understanding Redshift

    The redshift \( z \) is given by:

    \[ z = \frac{\Delta \lambda}{\lambda_0} \]

    where:
    - \( \Delta \lambda \) is the change in wavelength (redshift),
    - \( \lambda_0 \) is the original wavelength of the Hα line (656.28 nm).

    ### Filter Bandpass and Redshift

    Your filter has a bandpass of 3 nm centered around the Hα line. This means the filter allows wavelengths from \( 656.28 \) nm to \( 659.28 \) nm.

    ### Calculating Maximum Redshift

    To determine the maximum redshift that can be observed with this filter, we need to find the redshift \( z \) where the Hα line is shifted to 659.28 nm:

    \[ z = \frac{659.28 - 656.28}{656.28} = \frac{3}{656.28} \approx 0.00457 \]

    ### Converting Redshift to Distance

    To convert the redshift to distance, we can use the Hubble's law for nearby galaxies:

    \[ v = H_0 \times d \]

    where:
    - \( v \) is the recession velocity,
    - \( H_0 \) is the Hubble constant, approximately 70 km/s/Mpc,
    - \( d \) is the distance in megaparsecs (Mpc).

    The relationship between redshift and velocity for small redshifts is:

    \[ v \approx c \times z \]

    where \( c \) is the speed of light (approximately 300,000 km/s).

    Thus, for \( z = 0.00457 \):

    \[ v \approx 300,000 \times 0.00457 \approx 1371 \, \text{km/s} \]

    Now, using Hubble's law to find the distance:

    \[ d = \frac{v}{H_0} = \frac{1371}{70} \approx 19.59 \, \text{Mpc} \]

    ### Converting to Light Years

    1 Mpc is approximately 3.26 million light years, so:

    \[ d \approx 19.59 \times 3.26 \approx 63.90 \, \text{million light years} \]

    ### Conclusion

    With a 3nm bandpass Hα filter, you can observe Hα emission in galaxies up to approximately 63.90 million light years away before the emission line is redshifted out of the filter's bandpass.
  • Rafael Sampaio
    Interesting, thanks Adam! Looks right to me. I noticed that when shooting Ha of NGC 6769 and NGC 5426, two very distant galaxies (190 and 130 million light years away) in Obstech. Seems to me I can’t get the isolated Ha emission of them
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