Dust Abundance and Properties in the Nearby Dwarf Galaxies NGC 147 and NGC 185

Abstract

We present new mid-to far-infrared images of the two dwarf compact elliptical galaxies that are satellites of M31, NGC 185 and NGC 147, obtained with the Spitzer Space Telescope. Spitzer's high sensitivity and spatial resolution enable us for the first time to look directly into the detailed spatial structure and properties of the dust in these systems. The images of NGC 185 at 8 and 24 micron display a mixed morphology characterized by a shell-like diffuse emission region surrounding a central concentration of more intense infrared emission. The lower resolution images at longer wavelengths show the same spatial distribution within the central 50 ′′ but beyond this radius, the 160µm emission is more extended than that at 24 and 70µm. On the other hand, the dwarf galaxy NGC 147 located only a small distance away from NGC 185 shows no significant infrared emission beyond 24 micron and therefore its diffuse infrared emission is mainly stellar in origin. For NGC 185, the derived dust mass based on the best fit to the spectral energy distribution is 1.9 × 10 3 M ⊙ , implying a gas mass of 3.0 × 10 5 M ⊙ . These values are in agreement with those previously estimated from infrared as well as CO and HI observations and are consistent with the predicted mass return from dying stars based on the last burst of star formation 1 × 10 9 yr ago. Based on the 70 to 160µm flux density ratio, we estimate a temperature for the dust of ∼ 17K. For NGC 147, we obtain an upper limit for the dust mass of 4.5 × 10 2 M ⊙ at 160µm (assuming a temperature of ∼ 20K), a value consistent with the previous upper limit derived using ISO observations of this galaxy. In the case of NGC 185, we also present full 5 − 38µm low-resolution

DUSTiNGS. III. DISTRIBUTION OF INTERMEDIATE-AGE AND OLD STELLAR POPULATIONS IN DISKS AND OUTER EXTREMITIES OF DWARF GALAXIES

“…The galaxy has little gas content with the last star-formation event occurring between ∼300 Myr (Golshan et al 2016) and ∼1 Gyr (e.g., Sage et al 1998;Marleau et al 2010). Crnojević et al (2014) reported evidence of emerging symmetric tidal tails in the NW and SE directions, but at larger radii than covered by our observations.…”

“…5 M e (Marleau et al 2010)and hosts a population of stars younger than ∼1 Gyr (Martínez- Delgado et al 1999;Davidge 2005)and possibly as young as 100 Myr (Davidge 2005, ;Golshan et al 2016). Stellar metallicity gradients have been reported based on observations of RGB stars both spectroscopically (Vargas et al 2014;Ho et al 2015) and photometrically (Crnojević et al 2014).…”

The ionization mechanism of NGC 185: how to fake a Seyfert galaxy?★

“…Marleau, Noriega‐Crespo & Misselt (2010) took Infrared Spectrograph (IRS, onboard the Spitzer Space Telescope ) spectra of NGC 185, and that shows strong polycyclic aromatic hydrocarbon emission, deep silicate absorption features and H 2 pure rotational line ratios consistent with having the dust and molecular gas inside the dust cloud being impinged by the far‐ultraviolet radiation field of a relatively young stellar population. This young stellar population was never really detected, possibly due to the large extinction caused by the large amount of dust in this galaxy.…”

“…NGC 185 is a dwarf spheroidal (dSph) and, together with NGC 205 and 147, is one of the third brightest dwarf companions of M31. Unlike the other two however, NGC 185 has an important content of gas and dust (Marleau, Noriega-Crespo & Misselt 2010). At first, the dwarf ellipticals and the dSphs have been regarded as simple, old stellar systems, with a stellar content resembling that of Galactic globular clusters.…”

TheHerschelVirgo Cluster Survey

“…In particular, the ISM in the immediate surroundings of M87 can be traced by dust emission, as strong radio continuum emission considerably reduces the ALFALFA Hi detection sensitivity within 1 • of M87 (Giovanelli et al 2007). To date, the Andromeda satellites NGC 205 and NGC 185 are the only dEs that have been detected in the far-infrared (Haas 1998;Marleau et al 2006Marleau et al , 2009. In particular, no dust emission has been detected from cluster dEs.…”