Praktikum

In this Praktikum we will analyze high resolution (5-7 Å ) spectra of the following galaxies: ngc4889, ngc2275, ngc4750, ngc6181, ngc4449, ngc7714.

The spectra, taken from the Kennicutt et al. (1992) Atlas, provide a basic set of continuum energy distributions and emission-line fluxes for a range of galaxy types as shown in Table 1.

  
Table 1: The list of galaxies, with spectral type and B absolute magnitude.

NOTE These spectra have been obtained with a drift scanning technique for galaxies with diameters of 1-14 arcmin. This means that each measurement consisted of one or more integrations, during which the image of the galaxy was trailed across a long slit several times. As a result, the integrated spectrum takes into account the contribution of the light coming from the galaxy nucleus, bulge and disk.

Perform the analysis of each galaxy spectrum following these steps:

• 1) Plot the spectrum with the emission and absorption lines identified. Try to identify at least the lines marked with in Table 2.
• 2) Calculate the redshift of the galaxy considering at least 3 lines.
• 3) Measure the EW and Fluxes for at least the following lines: , , , , , .
• 4) Estimate the dust content and the reddening correction from the ratio.
• 5) Correct for extinction the Fluxes of the lines specified in 3).
• 6) Plot and discuss the position of the galaxy in the 3 diagnostic diagrams showed in Fig. 3, 4, 5.
• 7) Convert the observed (and optionally fluxes to the corresponding luminosities (and ) and calculate the SFR.
• 8) Plot and discuss the following correlation:

  - EW versus spectral type

  - versus spectral type

  - EW versus EW()

NOTE:

• To facilitate the identification of spectral features we have summarized in Table 2 the main lines in emission or in absorption in the integrated galaxy spectra. The symbol in the table indicates the most usual and strongest lines. The redshift of the galaxies is derived from:

   

  where is the measured value and is the rest-frame value given in Table 2. All the galaxies are nearby and 0.001 z 0.021.

• The spectra provided are in F() and therefore a definition of the break slightly different from that given in equation 1 will be used here. Consider

   

  where is the mean flux in the ( wavelength range. While the absolute value of the index will differ from the standard one, its variations with the galaxy spectral types will remain unchanged.

• Measurements of EW and Fluxes are very sensitive to the continuum definition. Therefore, the students should repeat at least some measurements considering different continuum windows and evaluate the scatter in the results.
• For the conversion of the observed flux to luminosity L remember that

   

  where (; 1 Mpc = 3.08 cm)

Commands for the spectral analysis To start the program type: dipso. Then type:

• alaslins 3 2000 To define the lines to be read.
• alasrd name file and push To read one galaxy spectrum and store it in the current buffer.
• sl To see the content of all buffers
• pop buffer number To read a from a buffer position to the current buffer
• dev xwindows To create a graphic window
• pm buffer number To plot the galaxy spectrum in the specified buffer. Without specification the whole wavelength range will be plotted. If one wish to define a type xr x start x end and then pm again.
• erase To erase the plot on the screen.

Important If one wishes a laser plot of the spectra type first dev l, then pm , then dev xwindows. Postscript files with the gks*.ps, gks*.ps.1, gks*.ps.2 etc. names will be created in the working directory. After that, open another window on the screen with the command xterm &. From this window one can send the gks*.ps.* files to the laser with the command lpr gks*.ps.*.

To identify emission and absorption lines :

• xv and click with the cursor. Use Table 2 to identify the absorption or emission lines (at least all those marked with ) and to measure the redshift.

To measure the break of a galaxy spectrum type:

• rxr 3750 3950 to select the appropriate wavelength range and pm
• push to store this wavelength region in a buffer
• pop buffer number
• mean to obtain the mean flux in this spectral region. Do the same for 4050 4250.

To measure EW and Fluxes

• xr xstart xend to select a region around the line to measure
• If the line has not a regular gaussian-like profile but is contaminated by another line, type snip and give with the cursor the limits of the region you want to ignore in the line profiles. A linear interpolation will be done. Then type push to store the ``corrected'' profile in a buffer and pop (buffer number) to read it.
• ew To measure the EW of the lines. Mark one position on each side of the line.
• flux To measure the flux of the lines. Mark one position on each side of the line.

To deconvolve complex profiles (sometime necessary for the line complex )

• rxr x start x end to select only a region around the line to measure
• yv to measure the continuum value and ysub value to subtract it
• push to store the previous spectrum in a buffer
• pop buffer number to read the previous spectrum
• elfinp to define the number of gaussian components, the central wavelengths, and the FWHMs. A guess for the central Wavelength and Full Width at Half Maximum is required. Type c1: number and w1: number. More gaussian components can be given ( c2: number and w2: number).
• qelf to return to dipso
• elfopt to run the optimization program. It gives center, width and line flux of the single gaussian components.
• elfpush [ n1, n2..] to store the fit results in buffers. n1, n2 are the single gaussian components. Without specification only the total fitted profile will be stored
• In order to check the the reliability of the fit type pop original profile then
• nb to avoid to erase the screen
• pop and pm single gaussian components
• pop and pm total fitted profile
• To perform again the profile analysis type elfnewc to clean the input parameters of the previous fit and erase