Units

From CoolWiki
Revision as of 00:40, 31 July 2020 by Rebull (talk | contribs)
Jump to navigationJump to search

General IR astronomy units

Wavelengths in infrared astronomy are commonly expressed in microns = micrometers = µm (or um if you don't have a µ).

  • 5000 Å =500 nm =0.5 µm =Visible light
  • ~0.9 to 5 µm =Near-infrared (~smoke particles)
  • 5 µm to ~30 µm = Mid-infrared (~hair)
  • 30 µm to ~350 µm = Far-infrared (~salt grain)

Brightnesses or fluxes are most likely to be given in Janskys (Jy) or mJy (milli Jy) or µJy (micro Jy). 1 Jansky = Watts/m^2/Hz.

Jy can be converted to magnitudes which have historically been relatively rarely used in the mid- or far-infrared.

Because the unit is named for Karl Jansky, the plural of the unit is really Janskys, not Janskies.

Fluxes and flux densities

Astronomically, it can be important to understand the difference between luminosity, flux, and flux density. In practice for this stuff, you probably don't need to know the gritty details of this until you are more familiar with the numbers and the jargon.

Colloquially, flux means the rate of something through something else, such as water through a pipe, or traffic on a highway. In physics and astronomy, it means the same thing.

Flux is a measurement of energy per unit area per unit time. Using our analogies above, this would be the number of cars per lane per second that pass under a bridge on a highway (or grams of water through the cross-sectional area of the pipe per second). In measuring energy from celestial objects, the units of flux are Joules per second per meter squared if you like mks (meters-kilograms-seconds) units, or ergs per second per centimeter squared if you like cgs (centimeters-grams-seconds) units.

Luminosity is a measurement of energy per unit of time, such as Joules per second if you like mks units, or ergs per second if you like cgs units. This would be, in our analogy, the total number of cars on the highway passing under the bridge per second. (The flux of cars is the luminosity per lane.)

Flux density is a measurement essentially of energy per unit area per unit time "per photon". In our analogy, this would be the number of RED cars per lane per second that pass under the bridge on the highway. In this analogy, the "per photon" is seen in the red cars. In astronomy, the "per photon" manifests itself as a "per Hz" (unit of frequency) or "per cm" (unit of wavelength). A Jansky is proportional to Watts/m^2/Hz. Recall that Watts are energy per second. So this is energy per second per square meter per Hertz.

Now, just to further confuse things, the units of Spitzer mosaics are not just Janskys, but Janskys per area! To make the numbers easier, they are in MJy/sr, but they could also be in uJy/square arcsecond. Read on for more, including definitions and scale factors!

For completeness, we note here that magnitudes are proportional to the log of the ratio of two fluxes. Most magnitudes with which you are most likely familiar are tied to the magnitude of Vega, so a magnitude of 0 means that the object has the same flux as Vega. There's more on this below, too.

Units of Spitzer Images

Optical data with which you are familiar may be in counts or photons, or possibly (like Hubble data) calibrated to be energies. That, combined with the exposure time of the image, gives you flux units. Spitzer data comes in flux (density) per unit (pixel) area instead, MegaJanskys per steradian (MJy/sr). 1 MJy = Jy, and a sr is a solid angle.

If you've done photometry before, and expect to do it exactly the same way again here, it won't work, because this matters.

1 square arcsec is sr. (1 degree = 60 arcmin = 3600 arcsec.)

If you want to convert the image from MJy/sr to uJy/square arcsec, multiply the image by 23.5045. The units of this number are (uJy/arcsec)/(MJy/sr).

If you want to take a Spitzer image and use your previous routines on it, the most efficient way to do this is probably to take the image in MJy/sr and multiply out the "per sr" part of it so that it is instead in MJy/px. The subtlety in this step is that each Spitzer array has slightly different pixel sizes, and the mosaics that we create have different sizes yet again from the original images. You can make mosaics with whatever size pixels you want, so if you get Spitzer mosaics from more than one astronomer, or more than one Spitzer wavelength, chances are excellent that the pixels will be slightly different sizes. The information on the pixel sizes are in the FITS header of each image.

The following paragraphs are a high-level summary of what to do for any Spitzer image data you may encounter; see below for a cookbook of the process for one mosaic.

Look in the FITS header of the mosaics for the keywords "CDELT1" and "CDELT2". These keywords are set to be the scale of the rows and columns in degrees per pixel. Using the values of these keywords, and the conversions above, you can figure out the number of square degrees per pixel, the number of square arcsec per pixel, and finally the number of steradians per pixel. Multiply the whole image in MJy/sr by the number of sr/px to get MJy/px.

If you are instead working with the individual BCDs (read this as: the individual little images that went into the big mosaic), you should look for keywords "PXSCAL1" and "PXSCAL2". NOTE that these pixels ARE NOT SQUARE, and this is more important for MIPS data. From here, you now have the same information as the "CDELT1" and "CDELT2" above, so you can follow the same procedure.

Units of Spitzer Photometry

See this page for a brief introduction to photometry and magnitudes.

Introduction

The photometry software that people at the SSC wrote for use with Spitzer data, called APEX, produces flux densities in microJanskys. The final bandmerged catalog you can get has listed flux densities in microJanskys, as well as magnitudes.

Astronomers use magnitudes in color-color or color-magnitude plots. Astronomers use a variant on flux densities in spectral energy distribution (SED) plots.

Magnitudes

A magnitude is really a flux ratio. It is defined as follows, where M's are magnitudes and F's are fluxes:

      (eqn 1)

The magnitude system (in the optical) was defined to be referenced to Vega. In other words, Vega is defined to be zero magnitude, and you would then define magnitudes of anything else as follows:

         (eqn 2)

When they looked at Vega with IRAS, they discovered that it did NOT look like they expected, and in fact it has a large infrared excess! Therefore, infrared magnitudes are defined with respect to what Vega would be, if it did not have an excess.

Generally, the zero points (e.g., the "Vega flux") are published for most of the bandpasses you might encounter. They are consolidated on the Central wavelengths and zero points page. Therefore, in order to convert the uJy that apex returns into magnitudes, use the equation 2 above, substituting these so-called "zero-point fluxes" in for "Fvega." Note that the zero-point fluxes are in Janskys and the fluxes returned by APEX are in microJanskys. You can find the zeropoints for many other bands on the web as well, such that you can freely convert between mags and flux densities.

Note that plain magnitudes get fainter (the number gets larger) as the distance of the object increases. This happens because Vega, your reference object, stays the same while such an example object moves farther away. BUT, colors (which are differences in magnitudes) are ratios of fluxes of the same object, and therefore independent of distance. This is powerful when you are studying objects whose distances you don't know, or comparing objects at a variety of distances.

To convert magnitudes back into fluxes (e.g., if you have optical or 2MASS magnitudes and need to get fluxes), you need to invert the equation above. Recall that to invert a logarithm (base 10), you have to raise both sides to the power of 10, e.g., if log x = y then x = 10^y.

Aside on AB mags

BE CAREFUL to keep track of whether you are working with Vega-based magnitudes or AB mags. Vega magnitudes define things with respect to a Vega spectrum (as above), but some folks (largely extragalactic folks) define things with respect to a flat spectrum source instead, and those are AB mags. Most Sloan folks (even those folks working with stars, and even those working with Sloan filters but not necessarily SDSS archival data) work in AB mags instead. For AB mags, you always use a flat reference spectrum, so the zero point is 3631 Jy for all bands.

Spectral Energy Distributions (SEDs)

SEDs are energy plotted against some measure of the photon -- frequency or wavelength. The reason astronomers do this is to see how much energy is produced by the object as a function of frequency or wavelength. Now it's really going to get a little hairy! Steel your nerves and plunge onwards... it really all comes down to unit conversion.

The units that are used in Spitzer data are Janskys. 1 Jy = erg/s/cm^2/Hz (in cgs units rather than mks units, sorry -- and just to be clear, I mean erg / (s*cm^2*Hz), it's just easier to read in the way I wrote it above). A Jansky is technically a unit of "flux density," represented by . We want to plot "energy density", so one way to do this (DON'T DO THIS TO YOUR DATA YET) is to get rid of the "per Hz", e.g., multiply the Jy by the frequency () of the bandpass center, or . You could just stop here, and plot vs. to get something that is technically a spectral energy distribution. BUT, now it starts to get a little hairy, because there are some cultural influences here. Do you know off the top of your head what the frequency of the IRAC-1 band is? I don't either, but I do know its wavelength -- 3.6 microns. Astronomers coming from the longer wavelengths (mm, radio, etc.), because they think in units of frequencies, will tend to plot up against (nu). The units of really are Janskys. BUT, astronomers coming from the shorter wavelengths (optical, etc.), because they think in units of wavelengths, will tend to plot instead , where (lambda) is the wavelength of the light. This is what we want to do here (because we have been thinking of the wavelengths of the Spitzer bands but not the frequencies). The catch here is that the units of are NOT Janskys.

, the speed of light. In order to convert the Janskys into units of , you need to take into account the differentials (ah-HA, calculus being used here!), e.g., the fact that

    and 

So you need to multiply the by to convert it into . But we are not done yet! Recalling from above, the units of are not an energy density. You need to get another factor of in there to make the units work out to be energy density: calculate to get units of ergs/s/cm^2.

SO, IN SUMMARY: Take your measurements that are in Jy. (Ensure they are in Jy! If they're in magnitudes, convert them to Jy first; see 'magnitude' discussion above.) Multiply by to get them into cgs units. Multiply these values by to get them into . Multiply them by to get them into . WATCH YOUR UNITS. NB: c = 2.997924d10 cm/sec

In other words, in order to convert our data from photometry to SEDs, we need to do the following:

  1. Read in the catalogs you have.
  2. Convert any magnitudes (and errors) into flux densities (if necessary).
  3. Make SED plots for individual objects, but converting numbers first into the right units:
    1. Create an array of the wavelengths of each measurement, keeps a copy of the version in microns, and convert to cm.
    2. For any real measurements, convert the flux densities (probably in microJanskys) into cgs units.
    3. For any real measurements, convert into by multiplying the values by the corresponding to the wavelength of each bandpass.
    4. For any real measurements, multiply by the lambda corresponding to the wavelength of each bandpass to get .
  4. For any real measurements, plot the log of the data points (in cgs units) against the log of the lambda data points (in microns, only because that makes it easier to read). Label the axes (with units)! Plot the error bars on top of the data points (also converted from uJy).


Notes on plotting

Why are we plotting vs. instead of ? Well, only because I think in wavelength, not frequency. I don't know off the top of my head the frequencies of the Spitzer bandpasses, but I do know their wavelengths.

Why are we plotting instead of ? Well, only for internal consistency. Since one axis is in wavelength units, it makes sense to have the other axis also in wavelength units.

If you have gotten this far using real data, you will find (if you have done the calculations correctly) that you have numbers that are very small, like 9.77237e-12, 1.99526e-11, etc. Any time you find yourself with these kinds of numbers, you should automatically plot in log space (or log/log space), NOT linear space. You want to actually plot log() vs. log().

The next step

IF you are highly motivated and ready to go on to the next step... It can be useful, in the course of analysis of the Spitzer data, to pretend that the contribution from the star is a blackbody. It's not really, but it's awful close, especially in the infrared. A blackbody's flux density is given by (where T is temperature, and other constants are given below)

         (eqn 3)

but of course we want to plot :

         (eqn 4)

Values of these constants all in cgs units:

  • h = 6.6260755d-27 erg*sec
  • c = 2.997924d10 cm/sec
  • k = 1.380658d-16 erg/deg

So, in summary, for the list of things to do above, add this one:

  1. For any real measurements, for any star with at least 2 fluxes, fit a model -- one very simple way to do that is to fit a blackbody to the energies derived from the three 2MASS and first 2 IRAC bands. There are two free parameters in this fit -- the temperature of the blackbody and an additive (in the log) offset related to the distance of the object. If we know the temperature of the star (via a spectral type) and the distance to the object, then we know the values for the temperature and the offset.