SF in satellites

Star Formation in Satellite Galaxies : 

Star Formation in Satellite Galaxies J. G. Funes, S.J1, C. Gutiérrez2, F. Prada2, M. Azzaro3, M.B. Ribeiro4 1. Vatican Observatory, Tucson, AZ 2. Instituto de Astrofísica de Canarias, Spain 3. Isaac Newton Group of Telescopes, Spain 4. Universidade Federal do Río de Janeiro, Brazil

Introduction: 

Introduction The hierarchical galaxy formation scenario implies that galaxies are formed by merging and accretion of previously collapsed systems. This framework has been successful in predicting the clustering of galaxies as a function of redshift (Silk 2000); however semianalytic models (Kauffmann, White, andamp; Guiderdoni 1993) and numerical simulation (Klypin et al.1999; Moore et al.1999) of hierarchical galaxy formation predict satellite number counts an order of magnitude larger than the observed counts in the Local Group. The study of isolated giant galaxies and their satellites outside the Local Group is a key test of the different formation scenarios of both the parent and satellite galaxies.

Slide3: 

The formation and evolution of satellite galaxies poses questions that are still open: What are the relationships between the morphologies of the parent galaxies and their satellite populations? How do the kinematic, dynamical, and chemical properties of the satellites compare to those of the parent galaxies? How do the properties of the satellites vary with projected distance from the primary galaxy? What are the star formation histories of the satellite galaxies? Are they related to the star formation history of the primary galaxies? What is the role of interactions among satellites or between satellites and their parent galaxies in the evolution of the system?

Observational Program: 

Observational Program To address these questions, we are carrying out a project that includes broadband photometry of optical and infrared wavelengths for satellites of external giant spiral galaxies (Gutiérrez et al 2002). In this work we present the preliminary results of an Ha imaging program aimed at studing the star formation properties of satellites. The Sample. The galaxies have been selected from the catalog by Zaritsky et al (1997). This catalogue is the most complete compilation of satellite galaxies in the literature and contains 115 galaxies orbiting 69 primary isolated spiral galaxies. The limiting magnitude for this catalog is MB ~ –15.5. They include satellites: - that are 2.2 magnitude fainter than the parent galaxy - with a projected distances DD ≤ 500 kpc from their primaries - with a different recessional velocity Dv ≤ 500 kms

Morphology and Structural Parameters: 

Morphology and Structural Parameters Gutiérrez et al (2002) have observed a subsample of 60 satellites taken from the compilation of Zaritsky et al.(1997).This sample includes most of the objects situated at decl. ≥ - 22°. The satellites span a large range of morphologies and surface brightness profiles. Hubble types vary from elliptical to irregular with 35 objects were classified as spirals. For each galaxy the effective radius, the scale radius and the B/D ratio are tabulated. The broadband photometry will be published in a forthcoming paper (Gutiérrez et al., in preparation)

Ha Imaging: 

Ha Imaging We have obtained Ha images for a subsample of 37 spiral and irregular satellite galaxies. The narrow-band images of the sample were acquired during three runs in December 2001, May 2002, and December 2002 with the 1.8-m Vatican Advanced Technology Telescope (VATT) at the Mt. Graham International Observatory. A back illuminated 2048 x 2048 Loral CCD was used as the detector at the aplanatic Gregorian focus, f/9. It yielded a field of view was 6.4’ x 6.4’ with an image scale of 0.4' pixel-1 after 2 x 2 pixel binning.The seeing varied between 0'.9 - 1'.8 with a typical value of 1'.2.

Slide7: 

For each galaxy we have obtained 3 x 1800 second narrow-band images using a set of interference filters with 70 A widths that isolate the spectral region characterized by the redshifted Ha and [NII] (6548, 6583 A) emission lines. To subtract the stellar continuum we subtracted a 1200-second R-band image, suitably scaled, from the narrow-band image. The mean scale factor for the continuum image was estimated by comparing the intensity of the field stars in the two bandpasses. The data from the two first runs have been reduced. The Ha images will be flux-calibrated using the spectrophometric standard stars observed during the different runs. As examples of the CCD images we show the R-band and continuum-free Ha images of NGC 1517a (Figure 1). In Figure 2 we also display the Ha images of 21 galaxies.

Slide8: 

R Band Ha NGC 1517a N E 6.4’ Figure 1

Slide9: 

NGC 1517a NGC 1620ab NGC 1961a NGC 1961b NGC 2424b NGC 488c NGC 772b NGC 772c Figure 2

Slide10: 

NGC 2775b NGC 2775c NGC 2916a NGC 3043a NGC 3735a NGC 4725a Figure 2 Cont.

Slide11: 

NGC 5248a NGC 5248b NGC 5899a NGC 5962d NGC 5965a NGC 6181a NGC 7678a Figure 2 Cont.

Interaction Among Satellites: 

Interaction Among Satellites Gutierrez et al. (2002) pointed out two cases of interaction in pairs of satellites. NGC 2718a and NGC 2718b: In the R-band image (Figure 3), a bridge connecting the satellites is evident. In the Ha image, line emission is only observed in NGC 2718a, with no evidence for emission along the bridge. NGC 4541b and NGC 4541e: Two tidal tails emerging from NGC4541b are seen in the R-band image (Figure 4). One tail is pointing to NGC 4541e and the other one is in the opposite direction. In the Ha image there is no line emission seen in NGC 4541b. Preliminary results indicate that interacting satellite galaxies exhibit a higher level of star formation than satellites with no signs of interaction. Our work extends to satellite galaxies the idea - established previously for luminous galaxies- that interactions trigger star formation

References: 

References Gutierrez, C.M., Azzaro, M., andamp; Prada,F., 2002, ApJS, 141, 61 Kauffmann, G., White, S.D.M., andamp; Guiderdoni, B., 1993, MNRAS, 264, 201 Klypin, A., Kravtsov, A.V., Valenzuela, O., andamp; Prada, F., 1999, ApJ, 522, 82 Moore, B., Ghigna, S., Governato, F., Lake, G., Quinn, T., Stadel, J., andamp; Tozzi, P., 1999, ApJ, 524, L19 Silk, J., 2000, PASP, 112, 1003 Zaritsky, D., Smith, R., Frenk, C., andamp; White, S. D. M. 1997, ApJ, 478, 39

NGC 2718ab: 

NGC 2718ab NGC 2718b NGC 2718a NGC 2718b Figure 3 R Band Ha

NGC 4541abe: 

NGC 4541abe NGC 4541e NGC 4541b NGC 4541a NGC 4541a NGC 4541e Figure 4 R Band Ha