Spectral charts, for Newbies Part 5 Emission and Absorption Spectra
The orbits of electrons of atoms and molecules can emit and absorb light
They use the same wavelengths of light (the same colour) whether they are emitting or absorbing.
If you can determine the line or wavelength, you can find what atom or molecule is involved.
Emission lines make peaks, like this "^" on a spectral chart.
Absorption lines make valleys, like this "V" on a spectral chart.
Emitted light makes bright lines in a spectrum, which are drawn as peaks "^" in a spectral chart.
Compare a spectrum of hot hydrogen with a spectral chart of a galaxy with hot hydrogen in it.
See how 4 of the bright lines of the spectrum correspond to some of the peaks on the spectral chart from the galaxy.http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=587729386083582038
We can conclude that there is hot hydrogen in this galaxy.
For the quantum theory of light emission see
OOTD Saturday 17th October 2009 Hot hydrogen.. a billion light-years away
See this OOTD Thursday 2nd April 2009
for examples of emission spectrum.Normal stars are not hot enough to have emission lines!!!
Galaxies which consist of only normal stars will not have emission lines either!!!
From NGC3314: "Normal stars have either no emission lines or emission lines so weak that they do not show up in data of this spectral sampling.
Emission lines generally come from excited gas
gas illuminated by hot stars,
gas outflowing from stars,
gas shocked by supernova explosions, or
gas lit up by the hot surroundings of a black hole."
gas heated by merging of galaxies
Picture of galaxy with gas temperature of millions of degrees
The most common gas is hydrogen gas, so hydrogen will produce the main peaks.
They are the hydrogen alpha (Hα), the hydrogen beta (Hβ) and then hydrogen gamma (Hγ) peaks.587738568703017016 UV.violet.blue.green.yellow.red........far red.............near IR
Here is an elliptical galaxy with no excited gas. There are no emission peaks.
All colours are present in equal amounts, and the galaxy is white. Posted by Lanky Yankee. 587738195572949357 UV.violet.blue.green.yellow.red........far red.............near IR
Here is a galaxy with some activity. There is an emission peak at 6800Å.
Hydrogen gas is falling into the black hole in the nucleus, getting very hot.
The galaxy is white, but the nucleus is red. The hydrogen alpha emission in the red part of the spectrum gives the nucleus its red colour.
Posted by AlexandredOr 587739158720544838 UV.violet.blue.green.yellow.red........far red.............near IR
This is a quasar, which is a galaxy with a very active nucleus. It has a large hydrogen alpha peak, a hydrogen beta peak, and a peak of excited oxygen.
Large amount of matter (stars?) are falling into the black hole. Some matter is ejected out of the top and bottom of the quasars, forming jets.
Explanation of the width of quasar emission lines by EigenState
Posted by zeta2008 588848899912040466 UV.violet.blue.green.yellow.red........far red.............near IR
The ion jet is pointing right towards us
from "Method identifies blazar candidates" http://spie.org/x18204.xml?ArticleID=x18204
spectrum goes straight across, (with values high above zero) and no apparent emission peaks
This is apparently how to recognize a blazar
[.... ]from here) [......]The stronger the blazar component is, the less there is to see in an optical spectrum - it was a frustrating struggle for the discoverers to even get their redshifts and find out what kind of objects these are.587736809916399664 UV.violet.blue.green.yellow.red........far red.............near IR
A Wolf-Rayet galaxy
with excited hydrogen and oxygen gas. The hydrogen alpha emission is represented by red light, and the oxygen emission is represented by blue light, so the colour of the gas is magenta (blue-red).
Matter is being forced away from the nucleus, forming jets.
Look at the magnitude of the units on the y-axis.
Posted by Titanium Dragon
Exploring that nice magenta colour.
This colour is a blend of red and blue, called magenta.
The blue colour comes from the OIII emission line, a green wavelength, presented to us as blue.
The red colour comes the hydrogen alpha line, a far red emission, presented to us as red. If the hydrogen alpha line is redshifted to 7000Å or greater, it appears as red in the galaxy photo, and the magenta colour appears. 587735346962497616 UV.violet.blue.green.yellow.red........far red.............near IR
The OIII peak makes a blue colour.
The hydrogen peak makes a red colour.
blue + red = magenta (also called purple or even "pink")
posted by sdrew123 587745244701655168 UV.violet.blue.green.yellow.red........far red.............near IR
posted by Mukund Vedapudi
This is a Compact Blue galaxy.
There is much blue light between 4000-5000Å, so there are many young blue stars.
It has emission lines of oxygen and hydrogen.
The oxygen peaks makes blue colour.
The hydrogen peaks makes green colour.
OIII galaxies:Some galaxies with active nuclei have emission peaks coming from hot oxygen gas.
OIII emission peaks are light emitted by oxygen atoms when two electrons are torn away from it.
This is known as doubly ionized oxygen
The lines are at 5007Å and 4959. This is green light, but SDSS presents green light as blue, so galaxies emitting this hot ionized gas are blue when close by (z>0.1) 88010359621353529 UV.violet.blue.green.yellow.red........far red.............near IR
This is a blue OIII object, a "blue pea". The big emission peak of OIII is near 5000Å.
posted by Galaxy Hunters Inc 587738410863493299 UV.violet.blue.green.yellow.red........far red.............near IR
This is a similar looking chart, but the galaxy is farther away (z=0.14) and the OIII emission line is redshifted past 5500Å into the part of the chart where red light is presented as green, so the object looks green. These objects are nicknamed "peas", because they look like the green garden vegetable.
posted by Rick Nowell
Research is still on-going in this newly discovered type of galaxy. More info on: OOTD Friday 21 May 2010 Spectra of Peas! Blue, purple, white, green and redOOTD Tuesday 14th October, 2008 topic:OIII Objects
and The story of the peas: writing a scientific-paperGalaxy Zoo Green Peas: Discovery of A Class of Compact
Extremely Star-Forming GalaxiesOOTD Saturday 19th September, 2009
See the thread Give peas a chanceHere
is a list of emission lines used by SDSS.
If there is gas or dust is in between the galaxy and us, the gas or dust will absorb some of the light.
Different atoms and molecules absorb different wavelengths of light.
This absorption makes black gaps in a spectrum, which are drawn as valleys "V" in a spectral chart.
Cooler stars and galaxies have many valleys, and from these, you can determine what atoms or molecules are present. 587734623784075325 UV.violet.blue.green.yellow.red........far red.............near IR
Here is an elliptical galaxy with little excited gas. All colours are present in equal amounts, and the galaxy is white.
There are no emission peaks here. The "peaks" that you see are gaps between large absoption valleys.
The spectrum of our sun has 2 black lines at "D", where sodium (Na+) absorbs light.
The spectral chart of this galaxy shows absorption by Na+
The computer labels the important wavelengths for us.
The green lines mark wavelengths where absorption commonly occurs.
Common atoms are Na+, K+ and Mg++, also N
G in the spectra of stars and galaxies doesn't denote a specific element, but a specific Sun-like spectral feature. In this case it comes from many weaker things blended together - the CH molecule, Fe lines, and more.
The black lines mark weaker or uncommon features.
The magenta dot-dash lines are the wavelengths of strong airglow emission features from our atmosphere (such as [OI] 5577 Å). These are the most likely wavelengths for upward or downward artifacts in a faint-object spectrum in case of imperfect sky subtraction, so any peaks or valleys at these locations may be artifacts.
The other dashed lines show wavelengths of expected (emission or absorption) spectral features at the estimated redshift of the object.
The green spectrum at the bottom shows the blank-sky spectrum which has been subtracted, so you can see whether that was a large correction or not (and whether there was some problem with the automatic processing). 587735665854841008 UV.violet.blue.green.yellow.red........far red.............near IR
In this spiral galaxy, the dust lane blocks the light from the nucleus.
Explanation from NGC3314 “That's the kind of spectrum you see when a lot of the starlight from a galaxy has been filtered through interstellar dust. Since many of the dust particles are smaller than the wavelength of visible light, it blocks bluer light more effectively than red light (just like the particles in our atmosphere - starlight behind dust is reddened the same way our local star appears reddened at sunrise and sunset). This kind of spectrum - sloping strongly to the red but with the absorption line ratios typical of bluer populations - shows up in a lot of galaxies with strong dust lanes. It also shows up in some red spirals even when we see them face-on, but that's another thread.”
The spectrograph can only target an area 3" across.
See OOTD Tuesday 3rd Feb 2009 for a picture of the spectrographic plate
The arms of a spiral galaxy have more young blue stars, so there is more blue in the spectral chart from the arm.
spiral arm 587739407876554999
spiral galaxy nucleus 587739407876620430
posted by jip26
Spectroscopy becomes very complicated
Topic: Zooite Guide to Spectra by Budgieye http://www.galaxyzooforum.org/index.php?topic=275839.0
For more information:
SDSS has examples of spectra of 8 diverse objects at http://www.sdss.org/gallery/gal_spectra.html
Here are two OOTDs on Thurday 19th March 2009 (with pictures!) on the invention of the spectroscope and spectra
and another OOTD on Thursday 2nd April 2009 absoption spectra
Another OOTD Thursday 8th October 2009 The spectrographic plates at Apache Point
Objects with a high OIII peak OOTD Friday 16th January, 2009
and OOTD Tuesday 14th October, 2008
I Zwicky 18 is studied in OOTD Tuesday 2nd December, 2008
. This galaxy shows a sudden increase in star-forming activity, a "starburst" galaxy.
There are massive hydrogen alpha lines within NGC 3690 (ARP 299) OOTD Saturday 2nd Febrary, 2008
The importance of dust in the evolution of galaxies in Wanted! Galaxy pairs which overlap but are not merging
OOTD Wednesday 3rd June 2008 Metals and star populations
Blue ellipticals and red spirals OOTD Tuesday 9th December, 2009
Blue galaxies on OOTD Wednesday 14 November 2008
SDSS has information on introductory emission and absorption
and advanced emision and absorption
Wikipedia has a good description of the Emission spectrum
This is a simple guide to colour and the spectra of galaxies.
There are more comprehensive guides on galaxy spectra on the forum:
by NGC3314 on Tutorial bits on galaxy spectra
by Eigenstate in Introduction to Electronic Spectroscopy
and Introduction to Spectral Line Profiles
SDSS has tutorials and projects on DR 7 projects