How do I determine the surface magnitude of a galaxy from a SED?

from synphot import SourceSpectrum
from synphot.models import BlackBodyNorm1D
sp = SourceSpectrum(BlackBodyNorm1D, temperature=6170.4796)

wave = sp.waveset
flux = sp(wave) #Photlam
flux_flam = flux.to(u.erg / u.s / u.angstrom/u.cm**2,equivalencies=u.spectral_density(wave))

 
plt.plot(wave, flux_flam)
plt.xlim(1000, 30000) 
plt.xlabel('Wavelength (Angstrom)')
plt.ylabel('Flux [FLAM]')

import numpy as np

# Assuming a circular aperture with a diameter in arcseconds
aperture_diameter_arcsec = 1.0  # Example: 1 arcsecond diameter aperture

# Calculate the area of the circular aperture in square arcseconds
aperture_area_arcsec2 = np.pi * (aperture_diameter_arcsec / 2)**2

# Calculate the surface brightness in FLAM per square arcsecond
surface_brightness_flam_arcsec2 = flux_flame / aperture_area_arcsec2

# Plot the surface brightness
plt.plot(wave, surface_brightness_flam_arcsec2)
plt.xlim(1000, 30000)
plt.xlabel('Wavelength (Angstrom)')
plt.ylabel('Surface Brightness [FLAM/arcsec^2]')