Astro 7B Research Project



Joey Cheung
Dylan Nelson
Donald Van Ness
Lauren Anderson


Introduction and Data Collection:
The DEEP2 Redshift Survey examines several thousand galaxies at redshifts of z > 0.7. Our task is to sift through this data to look for binary galaxies.

In order to obtain the necessary data, we downloaded a portion of the high-resolution 2-D DEIMOS spectra from the DEEP2 Data Release One (DR1) (1), specifically masks 4248-4253. In addition, we obtained the redshift catalog (zcat) with all 8,015 entries covering the entire 2-D specetra. As noted on the DEEP2 webpage, this particular catalog may contain multiple entries for the same object, and our sorting procedure takes into account the possibility of redundancy.

We focused on the first 200 spectra of our data set. We used the viewer.pro procedure in order to load each 'spatial distance vs. wavelength' spectrum in to ATV in order to visually identify objects of interest. An example of a potential binary object, visualized in 2D and 3D is given below:


Fig. 1: Example visualization of a potential binary galaxy within an emission spectra.

We next systematically examined the galaxy emission spectra, specifically looking for binary galaxies emitting at different wavelengths (which, after correcting for the cosmological redshift, would give us a peculiar orbital velocity of the given galaxies). The indication of a binary, as can be seen in our data, is a spatial seperation between two close objects. We also recorded data of galaxies that we saw that had interesting and/or strange emission characteristics.

For each object of interest we recorded the mask, the element containing the object, and the x and y pixel coordinates of the object(s). We also captured an image of the object.

All of our collected data is available on this spreadsheet.


Data Analysis Method:
Our principal analysis of the spectra data was carried out with IDL. During this process we used, created, and/or modified a variety of programs which are noted below. Some of the original concept code was completed by Professor Marc Davis, GSI Brian Gerke, or Carmen Anderson.

A listing of relevant IDL procedures and functions:
NameDescriptionRevision History
Viewer.pro Prepares a selected range of slit images and loads them in ATV. Brian Gerke, Carmen Anderson, revised by group
Lambda.pro Extracts the z-value and computes the relevant emission wavelengths. Written by group
Zcat2.pro Loads the zcat fits structure containing the underlying information about the galaxies we are observing. Written by Brian Gerke, Carmen Anderson
Locgal.pro Incomplete proof of concept program: Attempts to automatically locate and isolate galaxies within spectra. Written by group
Cosmo_Init.pro Preforms the integral to convert from redshift to distance, given matter density omega_m and dark energy state w. Written by Marc Davis, Brian Gerke
Z2R.pro After cosmo_init, converts a redshift z-value to a distance r in units of c/H_0. Written by Marc Davis

To begin, we used lambda.pro to extract the redshift z-value, observed emission wavelength (given x,y), and then calculate the rest emission wavelength using the formula:

(1)

The results of this calculation are recorded on our data spreadsheet.

We then hoped to compare our calculated wavelength values (in Angstroms) to known values for certain atomic transitions, and thus determine the elements whose emissions we were observing. Below is a table of emission wavelengths of various elements that we use for initial comparsion of galaxy emission spectra that we observed:

Element Wavelength (Å) Element Wavelength (Å)
[C III] 1909 [Fe VII] 5721.1
[O II] 3726.1 [N II] 5754.6
[O II] 3728.8 [Fe VII] 6086.9
[Fe VII] 3760.3 [O I] 6300.3
[Ne III] 3868.8 [S III] 6312.1
[Ne III] 3967.5 [Fe X] 6374.6
[S II] 4068.6 [N II] 6583.4
[O III] 4363.2 [S II] 6716.4
[Ar IV] 4711.3 [S II] 6730.8
[Ar IV] 4740.0 [Ar III] 7135.4
[O III] 5006.9 [O II] 7319.9
[Fe VIII] 5159.0 [O II] 7330.2
[Fe VI] 5176.4 [N I] 5197.9
Fig. 2: A table of common atomic transitions and their associated wavelengths. (2)


However, more accuracy was needed and we consulted the NIST Handbook of Basic Atomic Spectroscopic Data (3), which contains approximately 12,000 lines for all elements with proton number Z 1-99.

As this is not the primarily focus of our research project, all the individual results are not recorded here. It is interesting to note, however, that the O II 3727 doublet occurs quite frequently within our data set.


Further Data Analysis:
We selected obvious candidates for binary galaxies as well as rotating galaxies in order to preform a more in-depth analysis. For both cases, we calculated the difference in wavelength in order to obtain the peculiar velocity of the system.

For the binary case, we approached the problem as follows:
Fig. 3: Relative velocities in original frame.Fig. 4: Relative velocity in transformed frame.

In order to calculate the relative velocity, we switched to the frame of one of the galaxies wherein one is stationary and the other has velocity 2v, which we can calculate via:

v/c ~ Δ λ / λ0

(2)


Then, using the cosmo_init and Z2R IDL programs, distance to the galaxy was calculated from the cosmological redshift. The angular seperation between the two binary galaxy candidates was calculated from the pixel to arcsec correspondence, where 1 pixel = 0.113 arcsec. Thereafter, the following equation gives the radial seperation between the galaxies:

    (3)

Finally, given the radial seperation r and the radial velocity v we can make an estimate for the enclosed mass within this seperation. Making the assumption that the two galaxies have roughly equal masses, this enclosed mass is equal to the mass of one galaxy. Additionally, the binary system is assumed to be viewed edge-on (that is, sin(i)=1).

(4)

The results of these calculations are given below:

Galaxy ImageGalaxy #Δ λ (Å)λ0,av (Å)velocity(*107 cm/s)zdistance (Mpc)#pixelsθ (*10-6 rad)r (*1022 cm)Mass (*1011 solar masses)
6 5.47 4990.04 3.28 0.706 1765 6 3.28 1.79 1.44
23 2.90 4596.29 6.29 0.861 2065 13 7.12 4.54 5.88


In the case of a single rotating galaxy, we used the same process.

Galaxy ImageGalaxy #Δ λλ0,av (Å)velocity(*107 cm/s)zdistance (Mpc)#pixelsθ (*10-6 rad)r (*1022 cm)Mass (*1011 solar masses)
3 4.32 4056.54 3.195 0.847 2039 13 7.12 4.48 3.44
5 4.17 3898.54 3.21 0.858 2060 11 6.02 3.83 2.97
22 6.49 4694.46 4.146 0.861 2065 13 7.12 4.54 5.88
35 4.90 3890.93 3.77 0.862 2067 12 6.57 4.19 4.48

Conclusion:
Our calculations for the mass of the observed galaxies were on the order of 1011 Solar Masses. This is characteristic of many galaxies that we know of (Carroll and Ostlie site the characteristics of early spirals as having masses of 109 - 1012 and late spirals with masses 108-1010)(4). Thus, the assumptions we made seem to be reasonable, and the method of estimating galactic mass through the relative change in wavelenghts between the binary galaxies or edges of the rotating galaxy appears to offer a viable option at large distances (z~1 and farther). This is a viable estimate because we can assume that the binaries or rotating edges of the galaxy are at about the same distance from us. Therefore, the spacially separated wavelengths are both equally affected by cosmological expansion and the relative velocity between the two points can be calculated using non-relativistic approximations.


References:
(1): http://deep.berkeley.edu/DR1/dr1.primer.html
(2): http://www.astr.ua.edu/keel/galaxies/emission.html
(3): http://physics.nist.gov/PhysRefData/Handbook/index.html
(4): Carroll, Bradley W. and Dale A. Ostlie. An Introduction to Modern Astrophysics. 1996.
Site Created: April 26, 2006