Star Formation in the Dark Disk of NGC2915

1
Star Formation in the Dark Disk of NGC2915

• Gerhardt R. Meurer (JHU),
• K.C. Freeman (MSSSO),
• J. Bland-Hawthorn (AAO),
• P. Knezek (JHU STScI) R.J. Allen (STScI)

2
Abstract

• In the optical continuum, NGC 2915 is a
  nondescript Blue Compact Dwarf galaxy with an
  optical radius of only 3 kpc. HI observations,
  however, show that NGC 2915 is spectacularly dark
  matter dominated, with an HI disk and HI spiral
  arms extending out to a radius of 15 kpc, far
  beyond the optical radius. We present deep Ha
  imaging with the 3.9m AAT and the Taurus Tunable
  Filter, revealing for the first time faint HII
  regions out in the HI disk, at projected radii of
  3 to 6 kpc. Deep spectroscopy with the ANU 2.3m
  telescope confirm that two of the HII regions are
  indeed within NGC 2915. The integrated surface
  brightness and mass density of the associated
  young stars are estimated. We also present high
  resolution spectra of a background galaxy
  projected near the ridge of one of the outer HI
  spiral arms, which are used to constrain the Na I
  and Ca II column density within NGC 2915.

3
Introduction

• NGC2915 is one of the darkest known galaxies.
  Fig. 1 is a combined optical and HI image of
  NGC2915, showing its extended HI disk (Meurer et
  al. 1996). Up until now no stars have been
  detected beyond the Holmberg radius (see Table
  1). However, features of this disk suggest stars
  may be present at large radii.
• Nearly constant velocity dispersion (8 km/s) and
  Toomre (1964) Q parameter. Could energy input
  from stars be keeping the disk warm?
• HI spiral arms suggest gravitational instability
  in the disk despite it having Q7 (too high for
  the formation of massive HII regions Kennicutt,
  1989). Could there be sufficient stellar mass in
  the disk to account for the spiral arms?
• We have begun a campaign to probe the contents of
  NGC2915 in as many ways as possible in order to
  constrain the mass budget of this enigmatic
  galaxy. Here we report on deep Ha imaging and
  deep spectroscopy which we use to probe the
  stellar and chemical content of the disk.

4
Fig. 1. Combined optical HI image
3 4.6 Kpc
5
Table 1. Properties of NGC2915
Source Meurer et al. (1994 1996)
6
Deep Ha imaging

• Simultaneous deep Ha continuum images where
  obtained with the Taurus Tunable Filter on the
  Anglo-Australian Telescope.
• Two fields were observed the center, and an
  outer HI arm.
• Fig. 2 shows the location of the fields.
• Fig. 3 shows combined continuum subtracted Ha
  images of the fields.
• The diffuse Ha detection limit is 0.3 Rayleighs
  (1.7x10-18 erg cm-2 s-1 arcsec-2), and is set by
  artifacts of sky field and continuum subtraction.
• To this limit, no diffuse Ha was observed for R
  gt 90 (2.3 kpc).

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Fig. 2. Location of fields and sources
Center field
D
Arm field
A
B
A0924-767
C
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Fig 3a. Ha - center field
9
Fig 3b. Ha - arm field
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Deep Ha spectroscopy

• Ha spectra were obtained with the ANU 2.3m
  telescope.
• PA 130o (HI major axis)
• texposure(total) 4.4 hr (NGC2915), 2.2 hr (sky)
• Resolution 60 km/s
• Slit length 6.7
• Fig. 4 shows the reduced 2D spectrum in the
  vicinity of Ha.
• Ha can be traced to R 93 (2.3 kpc) on SE side,
  consistent with the Ha imaging.
• NII can be traced to R 46 (1.2 kpc) on SE
  side.
• SII can be traced to R 65 (1.7 kpc) on SE
  side.

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Figure 4. Deep red spectrum

• Center of NGC2915

Scale 20 0.51 kpc
Lines shown from left to right are NII6548Å,
Ha, NII6584Å, HeI6678Å, SII6716Å, SII6731Å.

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HII region candidates

• The Ha frames were visually inspected for HII
  region candidates at R gt 1.5 kpc.
• Three candidates were found in the central field,
  one in the arm field.
• Finder charts are shown in Fig. 3.
• Estimated fluxes are given in Table 2.
• Follow up spectroscopy of three of these with the
  ANU 2.3m confirms that two of the central HII
  region candidates are real.
• The candidate C, in the arm field, was not
  detected.
• Probably a blue background galaxy.
• Fig. 5 shows the confirmation spectrum for HII
  regions A D.

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Fig. 5. HII region AD Confirmation spectra.
HII region D
NGC2915 center
HII region A
Ha NII spectra are 2x2000s, obtained with SSO
2.3m, sky and continuum subtracted.
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Table 2. Candidate HII regions
15
HII region implications

• The HII regions are very faint they could be
  ionized by a single late O or early B star.
• E.g. Log QH0 48.87, 48.55 for O9V, B0III
  (Vacca et al 1996).
• Over R 2.5 to 5 kpc, with IMF Salpeter or Scalo
  (0.1 - 100 M)
• SFR per area 1.1 or 2.4 x 10-6 M kpc-2
• Lmech density 1 or 3 x 1043 erg Myr-1 pc-2
• Replenish Ek of disk in 630 or 220 e-1 Myr
  (similar to tdyn 130 Myr)
• Stellar mass density 0.01 or 0.02 M pc-2
  after 10 Gyr.
• 0.5 or 1.0 of gas disk density (tiny
  contribution).
• 0.01 or 0.02 of projected Dark Matter density
  (negligible contribution)
• Above calculations based on the Starburst99
  models (Leitherer et al. 1999)

16
Search for metals in the outer disk

• The background galaxy A0924-767 (see Fig. 2) was
  observed with the ANU2.3m telescope DBS
  spectrograph
• Resolution 80 km/s
• texposure(total) 5.6 hr
• CaII H K observed with the blue arm
• Na I D observed with the red arm.
• Integrated spectra shown in Fig. 6.
• Table 3 lists measurements of these spectra.
• Only upper limits can be placed on the Ca and Na
  Column density.

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Table 3. Interstellar lines towards A0924-767
18
Fig. 6. Spectrum towards A0924-767
Expected wavelength of interstellar absorption
from NGC2915 shown with error-bars.
19
Conclusions

• We have discovered HII regions in the outer disk
  of NGC2915.
• They have such low luminosities that they can
  each be ionized by a single late O or early B
  main sequence star.
• The stellar population they trace may make a tiny
  (1) contribution to the mass of the disk.
• It makes a negligible contribution to the total
  mass of the system.
• These HII regions may contribute significantly to
  the heating of the gas disk.
• Implications
• Extended HI disks contain trace stellar
  populations.
• Star formation at densities below the critical
  density (Kennicutt 1989) may stabilize extended
  gas disks.

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References

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