Evolution of dIrr galaxies in the cluster environment.

Talk presented at The Outer Edges of Dwarf Irregular Galaxies:
Stars and Gas,, 10 & 11 October 2002 Lowell Observatory, Flagstaff AZ, USA

Conference website:http://www2.lowell.edu/workshops/Lowell02/

Summary of the 2002 Lowell Observatory Workshop, to appear in PASP Conference Highlights http://arxiv.org/abs/astro-ph/0212246

Transcript

1. Evolution of dIrr galaxies in the cluster environment. Arna Karick Dr. Michael Drinkwater (UQ) Dr. Rachel Webster Fornax Cluster: with members from the SW sub-cluster region indicated by red circles.
2. What are the dominant evolutionary mechanisms affecting dIrr galaxies in clusters? External Influences: - Gas Stripping: Ram-pressure – interaction with hot cluster gas (~108 K)  gas compression Tidal interaction – cluster potential/galaxy-galaxy  tidal disruption - Mergers & direct collisions: - Accretion: e.g. gas clouds onto spiral galaxies Internal Influences: -SN effects: outflows/feedback, shocks
3. What are the dominant evolutionary mechanisms affecting dIrr galaxies in clusters? External Influences: - Gas Stripping: Ram-pressure – interaction with hot cluster gas (~108 K)  gas compression Tidal interaction – cluster potential/galaxy-galaxy  tidal disruption - Mergers & direct collisions: - Accretion: e.g. gas clouds onto spiral galaxies Internal Influences: -SN effects: outflows/feedback, shocks
4. Observables: - Density–morphology relation (Dressler 1979): e.g. dIrrs in infall regions  dEs in cores? - X-ray halos  feedback into ICM. - HI deficiency in cluster cores: evidence for ICM-ISM stripping OR.. fully evolved SF? - HЋ-emission from galaxies  recent SF
5. Observables: - Density–morphology relation (Dressler 1979): e.g. dIrrs in infall regions  dEs in cores? - X-ray halos  feedback into ICM. - HI deficiency in cluster cores: evidence for ICM-ISM stripping OR.. fully evolved SF? HЋ-emission from galaxies  recent SF
6. The Fornax Cluster: - 340 “likely” members:  186 morphologically classified as dwarf galaxies (Ferguson 1989). - velocity dispersions: giants: 308 ± 30 kms-1, dwarfs: 429 ± 41 kms-1 (infalling?) - evidence for cluster sub-structure (Drinkwater, Gregg & Colless. 2001). (radial) infall region: Galaxies unlikely to be interacting Gas-rich dwarfs dominate central core: Hot gas and high densities. dEs and Es dominate. Galaxies likely to interact
7. Spectroscopic Observations: - 62 bright cluster dwarfs 16.5 < bj < 18 (Drinkwater et al. 2001). - Flair-II fibre spectrograph (UKST/AAO). - HЋ EWs were measured using 6.7” fibres. - EWs over-estimate SF if it is centrally concentrated. - No information about the spatial distribution of SF.
8. Results: HЋ emission indicates more SF than implied by morphology. 35% star-forming from Ha emission 19% classified as late-types in FCC Dwarfs with the highest SFR occupy the SW sub-cluster. Fornax main cluster & SW sub-cluster: star-forming non-detection triangles: scaled by the log of their SFRs: 103 – 0.1 MSun/yr
9. - What triggers star-formation in infalling and sub-cluster dwarfs? - Can we attribute the high SFRs to environmental influences? - Can the spatial distribution of star-formation give us any clues?
10. HЋ imaging: - qualitatively determine positions and morphologies of HII regions. - illustrates the limitations of fibre spectroscopy. Spiral galaxy: FCC 285: EW – 22.8, Bmag – 14.2 Left Right: UKST R-band, HЋ-emission images highlighting HII regions
11. early-type late-type (Ha-emission)
12. early-type late-type (Ha-emission) Ha-imaging
13. FCC 113: Sc dwarf EW – 10.1 Bmag – 15.5 - 5 main HII region - asymmetric distribution - SF on leading edge FCC 282: BCD/? EW – 8.2 Bmag - 15 - multiple HII regions - asymmetric distribution - SF on leading edge 3’ across 3’ across 15”
14. FCC 306: Sm EW – 34.7 Bmag – 16.1 - 2 sites of SF - SF is centralised and on the edge away from cluster centre FCC 76: BCD/? EW – 6.1 Bmag – 16.3 - SF is centralised - possibly SF at leading edge? 3’ across 3’ across
15. early-type late-type (Ha-emission) Ha-imaging far edge leading edge
16. Conclusions & Future Work: - Initial spectroscopic measurements suggest high SFR for cluster dIrrs. - Velocity dispersions suggest they form a dynamically separate infalling population. - Ha imaging indicates multiple HII regions which are spatially asymmetric.  this may be related to a galaxies passage through the ICM.... - Currently analysing high resolution multicolour imaging of the cluster: - CTIO 4m Blanco Mosaic telescope: g’ u’ r’ i’ z’ and Ha (0.27”/pix)  quantitative analysis of individual HII regions: Ha fluxes and SFRs  Colours of individual regions: metallicities. - Imaging of galaxies in the SW sub-cluster region.