<p>Caveat this with the fact that I am not familiar with this code.</p>

<p>The input asks you for the flux through the filter, but a filter has a bandpass, so if you have a 10 nm bandpass filter and you measure $1 W/m^2$, then you measured $0.1 W/m^2/nm$ (sorry, I'm a physicist so I never got used to measuring wavelengths in Hz).</p>

<p>As for your second question, the factor of 23 is because in cgs units a Jansky is $10^{23} erg/s/Hz/cm^{-1}$ and that TOPCAT equation is in cgs (the equation is $m_\text{AB} = -\frac{5}{2}\,log_{10}\,f_v - 48.6$) and if you are normalizing it to Janskys, you need to divide by $10^{-23}$.</p>

CC BY-SA 3.0
2015-11-19T04:04:55.033Z
2015-11-20T11:57:56.813Z
2015-11-20T11:57:56.813Z
Dave
<p>I'm a bit puzzled and not an observer, so please bear with me if I'm being stupid here. </p>

<p>The code <a href="http://www.iap.fr/magphys/magphys/MAGPHYS.html" rel="nofollow">MAGPHYS</a> specifies Jansky (<code>Jy</code>) as the input unit for flux through a filter (see <code>Section 3.2.3</code> in the <a href="http://www.iap.fr/magphys/magphys/MAGPHYS_files/readme.pdf" rel="nofollow">documentation</a>). Meaning MAGPHYS wants me to use <code>Jy</code> as unit, when specify the total light of a whole galaxy in e.g. the SDSS-g band. But <code>Jy</code> is a unit of spectral <em>density</em>, i.e. <code>W/m²/Hz</code>. I would expect a flux as input unit when talking about the amont of light coming through a filter, i.e. in <code>W/m²</code> or in $L_\odot$ (solar luminosity), so integrated over all frequencies that go through the filter and weighted by the filter response function.</p>

<p>If I have the absolute AB magnitude of a galaxy in the SDSS-g band, how do I convert it to <code>Jy</code> so that MAGPHYS will be happy?</p>

<p>I'm aware of:</p>

<p>$m_{\text{AB}\nu} = -2.5 \log_{10} \left(\frac{f_{\nu}}{3631\text{Jy}}\right) \qquad \text{or} \qquad \left(\frac{f_{\nu}}{\text{Jy}}\right) = 10^{-0.4\  (m_{\text{AB}\nu} - 8.9)}$</p>

<p>But that is in one frequency, not a whole band. </p>

<p>Additionally, I have found the formula below in <a href="http://www.star.bris.ac.uk/~mbt/topcat/" rel="nofollow">TOPCAT</a>:</p>

<p>$\left(\frac{F}{\text{Jy}}\right) = 10^{\left(23-0.4 \ (m_\text{AB}+48.6)\right)}$</p>

<p>Where does the <code>23</code> in the exponent come from?</p>

<luminosity><magnitude>
Conversion of magnitudes to Jansky and MAGPHYS?
con-f-use
# Conversion of magnitudes to Jansky and MAGPHYS?

(Asked by: username_0 on 2015-11-18)

I'm a bit puzzled and not an observer, so please bear with me if I'm being stupid here.

The code [MAGPHYS](http://www.iap.fr/magphys/magphys/MAGPHYS.html) specifies Jansky (`Jy`) as the input unit for flux through a filter (see `Section 3.2.3` in the [documentation](http://www.iap.fr/magphys/magphys/MAGPHYS_files/readme.pdf)). Meaning MAGPHYS wants me to use `Jy` as unit, when specify the total light of a whole galaxy in e.g. the SDSS-g band. But `Jy` is a unit of spectral *density*, i.e. `W/m²/Hz`. I would expect a flux as input unit when talking about the amont of light coming through a filter, i.e. in `W/m²` or in $L\_\odot$ (solar luminosity), so integrated over all frequencies that go through the filter and weighted by the filter response function.

If I have the absolute AB magnitude of a galaxy in the SDSS-g band, how do I convert it to `Jy` so that MAGPHYS will be happy?

I'm aware of:

$m\_{\text{AB}\nu} = -2.5 \log\_{10} \left(\frac{f\_{\nu}}{3631\text{Jy}}\right) \qquad \text{or} \qquad \left(\frac{f\_{\nu}}{\text{Jy}}\right) = 10^{-0.4\ (m\_{\text{AB}\nu} - 8.9)}$

But that is in one frequency, not a whole band.

Additionally, I have found the formula below in [TOPCAT](http://www.star.bris.ac.uk/~mbt/topcat/):

$\left(\frac{F}{\text{Jy}}\right) = 10^{\left(23-0.4 \ (m\_\text{AB}+48.6)\right)}$

Where does the `23` in the exponent come from?

**Tags:** luminosity, magnitude

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## 1 answers

### Answer by username_1 on 2015-11-19. Score: 4

Caveat this with the fact that I am not familiar with this code.

The input asks you for the flux through the filter, but a filter has a bandpass, so if you have a 10 nm bandpass filter and you measure $1 W/m^2$, then you measured $0.1 W/m^2/nm$ (sorry, I'm a physicist so I never got used to measuring wavelengths in Hz).

As for your second question, the factor of 23 is because in cgs units a Jansky is $10^{23} erg/s/Hz/cm^{-1}$ and that TOPCAT equation is in cgs (the equation is $m\_\text{AB} = -\frac{5}{2}\,log\_{10}\,f\_v - 48.6$) and if you are normalizing it to Janskys, you need to divide by $10^{-23}$.


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