Nearby Galaxies Next Generation Sky Survey T. Jarrett (IPAC) April 18, 2000 updated Aug 9, 2001

• see the version for the proposal.

The angular resolution of NGSS allows detailed study of the internal structure of galaxies in the local Universe. There are over 3000 galaxies within 20 Mpc of the Sun, providing a statistically significant sample to study all morphological types, including the Hubble sequence (elipticals, early, late and barred-type spirals), irregular and peculiar, compact and dwarf, radio, Seyfert and interacting galaxies. The mid-infrared bands of NGSS are an effective probe of the underlying stellar population and sites of star-formation in galaxies.

The NGSS survey of nearby galaxies will form a complimentary dataset to the SIRTF Nearby Galaxy Survey Legacy project, expanding their sample of 75 galaxies to many thousands of galaxies, spanning the widest range in morphological type. NGSS will provide the crucial mid-IR information of the SED for large sample multi-wavelength galaxy surveys, spanning the UV (UIT,GALEX), optical (DPOSS,SDSS), near-infrared (2MASS), far-infrared (IRAS, ISO, SIRTF) and the radio (NVSS, FIRST). The figure of NGC 1313 illustrates the power of combining the near and mid-infrared bands. NGC 1313 is a nearby barred-spiral (SBd) whose underlying older stellar population is traced by the 2MASS Ks band (blue tones), and the sites of star-formation (via hot dust and PAH emission) are revealed in the mid-infrared ISOCAM images (green and red tones). The surface brightness (5-sigma) limit for 2MASS is ~35 uJy/arcsec^2, comparable with that of NGSS. The angular resolution is 9", where 2MASS has been degraded to match the ISOCAM data. NGSS will have ~2 times better resolution than ISOCAM, comparable with 2MASS, allowing delination of the individual sites of star formation and resolving large-scale structures.

Point 1: Nearby galaxies are a separate entity from "normal galaxies" in the sense that we resolve internal structure, allowing detailed study as opposed to global (or statistical) study of galaxies. Since there are only a finite number of "nearby" galaxies (i.e., those highly resolved by NGSS), an all-sky approach is essential toward a uniform study of galaxies from all morphological types (including the Hubble T-types, peculiar, irregular, compact, Seyfert & AGNs, and interacters).

Point 2: Sensitivity is important, of course, but the critical component for studying nearby galaxies is angular resolution. The most important constituents of spiral galaxies, for example, are GMCs and star formation regions. Both entities have size scales near the angular resolution limit of most surveys (the exception being HST, which has super-resolution), including NGSS. Improved resolution also effectively increases the sensitivity by decreasing the confusion noise (from the galaxy itself). Hence, this argues for maximizing the NGSS angular resolution: redundancy and optimal dithering lead toward improved image reconstruction (via PSF deconvolution).

The NGSS survey of nearby galaxies will form a complimentary dataset to the SIRTF Nearby Galaxy Survey Legacy project, expanding their sample of 75 galaxies to many thousands of galaxies, spanning the widest range in morphological type. NGSS will provide the crucial mid-IR information of the SED for large sample multi-wavelength galaxy surveys, spanning the UV (e.g., GALEX), optical (DPOSS And the SDSS), near-infrared (2MASS), far-infrared (IRAS, ISO, SIRTF) and the radio (NVSS, FIRST). The surface brightness sensitivity, resolution and all-sky nature of NGSS are naturally matched with that of 2MASS, allowing direct comparison between the near and mid-infrared bands. The figure of NGC 1313 illustrates the power of combining the near and mid-infrared bands. NGC 1313 is a nearby barred-spiral (SBd) whose underlying older stellar population is traced by the 2MASS Ks band (blue tones), and the sites of star-formation (via hot dust and PAH emission) are revealed in the mid-infrared ISOCAM images (green and red tones). The surface brightness (5-sigma) limit for 2MASS is ~35 uJy/arcsec^2, comparable with that of NGSS. The angular resolution is 9", where 2MASS has been degraded to match the ISOCAM data. NGSS will have ~2 times better resolution than ISOCAM, comparable with 2MASS, allowing delination of the individual sites of star formation and resolving large-scale structures.
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Origin of the near and mid-infrared emission

• Normal Galaxy Spectrum (elliptical, spiral, M82)

• The near-infrared (1-5 microns) SED is dominated by photospheric (continuum) emission from the older stellar populations (including the luminous evolved giants). The SED peaks near the H-band (1.6 microns), with a R-J falloff to longer wavelengths. The 2MASS bands (J, H and Ks) are optimized to detect this peak in the SED (towit: normal galaxies with average extinction). Elliptical galaxies show the R-J falloff with their continuum falling to zero at longer wavelength -- ellipticals, in general, do not exhibit a MIR excess (although it is an intriguing possibility that ellipticals harbor merged spiral nuclei, which should be radiating in the MIR).

• The mid-infrared (5-25 microns) SED is dominated by very small grains (VSG; with T > 100 K) and UIB features. The UIBs are usually associated with PAH molecules. The known PAH emission features include the 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 micron bands. For the low-redshift Universe, normal galaxies will be brightest in the 12 (UIBs) and 24 (VSG continuum) micron channels. Due to the complexity of the SED, the K-correction (redshift) is not likely to be intuitive; hence, the K-correction curves require simulation with SED templates. Good general reference (Genzel & Cezarsky 2000).

here is an article I wrote for a SIRTF Legacy proposal

The IRAS 12-micron band first revealed the existence of hot dust grains in the ISM of the Milky Way and in other galaxies. But it was ISO that allowed both the spatial and spectral resolution needed to fully reveal the nature of the mid-infrared (MIR), 4 to 15-micron, radiation. The spectral energy distribution for a normal spiral galaxy, for example, is dominated by continuum emission from small, hot dust grains and strong line emission bands associated with PAHs. The grains are transiently heated by the interstellar radiation field, primarily derived from young hot stars, but also driven by older main-sequence stars. The emission bands are associated with star-formation, with the strongest bands (relative to the continuum) occurring for star-formation "quiescent" galaxies, and the weakest (or non-existent) bands for the most powerful star formation regions. As the continuum rises with heating intensity, the PAHs are probably being destroyed, resulting in weaker PAH bands. ISO has thus shown that the MIR can be used as a direct gauge of the level of star-formation in spiral and active (gas-rich) galaxies (e.g., Helou 1999; Dale et al. 1999; Elbaz et al. 1999; Boiselli et al. 1998). For elliptical (and gas poor) galaxies, the MIR is instead dominated by the Rayleigh-Jeans tail of the cold stellar component.

The sensitivity and spatial resolution of the IRAC bands (3.6, 4.5, 5.8 and 8.0-microns) provide a sharp probe to the underlying (dust-enshrouded) star-formation in nearby galaxies. Individual GMCs, star-formation regions, and morphological "bar" features can be studied in detail with the super-resolution IRAC images in conjunction with optimized PIXON deconstruction. In combination with 2MASS 1-2-micron (NIR) imaging, variations in dust column density can be mapped with exquisite detail, delineating bars, individual GMCs and compact HII regions. The IRAC "PAH- emission bands" at 5.8 and 8.0-microns, compared with the IRAC "continuum bands" at 3.6 and 4.5-microns, indicate the star-formation intensity level. Moreover, colors derived from the IRAC and MIPS bands (in particular, 8.0 vs. 24-microns) provide the highest resolution indicator of embedded star formation at size scales comparable to GMCs.

For gas poor galaxies (including elliptical, S0 and some dwarf galaxies) the NIR-MIR directly traces the dominant stellar populations, providing a more effective measure of the halo/bulge populations and the overall mass-to-light ratio. IRAS showed that gas and dust are not "globally" common in these types of galaxies; however, the resolution of IRAS limits the detailed understanding of any current star formation that may be present. The SIRTF MIR bands provide the necessary sensitivity and resolution to expose any compact star-formation regions in these galaxies. If they exist, hot dust and PAH emission features are seen as a second component (or infrared excess) in the NIR-MIR SED characterized by the R-J blackbody from the old stellar population. Lenticular galaxies are a particularly promising laboratory to study the possible transition between star-forming disk galaxies and (mostly?) dust-free elliptical galaxies.

• NGC 6946
  blue = 2MASS Ks (2.2 microns)
  green = ISOCAM LW2 (5.5 - 8.5 microns)
  red = ISOCAM LW3 (12.2 - 17 microns)
• NGC 1313
  blue = 2MASS Ks (2.2 microns)
  green = ISOCAM LW2 (5.5 - 8.5 microns)
  red = ISOCAM LW3 (12.2 - 17 microns)
• IC 10
  blue = 2MASS Ks (2.2 microns)
  green = ISOCAM LW8 (10.9 - 11.9 microns)
  red = ISOCAM LW3 (12.2 - 17 microns)
• M51
  blue = 2MASS J (1.2 microns)
  green = 2MASS Ks (2.2 microns)
  red = ISOCAM LW2 (5.5 - 8.5 microns)
  
• M83
  blue = 2MASS J (1.2 microns)
  green = 2MASS Ks (2.2 microns)
  red = ISOCAM LW2 (5.5 - 8.5 microns)
  
  
  ** Thanks to Alessandra Contursi for providing the outstanding ISOcam mosaic images **

• Next Generation Sky Survey (Original Proposal) (NGSS)

• HgCdTe and Si:As 1024^2 arrays at 3.5, 4.6, 12 and 25 microns giving 2" pixels, with 5" resolution for the short bands and 10" for 25 microns.
  • L band: 3.2 - 3.8 microns
    
  • M band: 4.2 - 5.2 microns
    
  • N-band: 9 - 15 microns
    
  • IRAS-band2: 20 - 26 microns
  
  
• Normal Galaxy Spectrum (elliptical, spiral, M82)
  see also the M82 spectrum from the SINGS Legacy
  
  
• Extragalactic Results from the Infrared Space Observatory, by Genzel and Cesarsky