The Stellar Contents of Intermediate-mass Disk Galaxies in the Virgo Cluster. I. GMOS Spectra^{* † ‡}

Abstract

The stellar contents of six intermediate mass early-type disk galaxies in the Virgo cluster are examined using deep long slit spectra. The isophotal and photometric properties of the galaxies at near-and mid-infrared wavelengths are also examined. Characteristic ages and metallicities are found by making comparisons with the strengths of lines measured from model spectra. The light from the central regions of these galaxies at visible/red wavelengths is not dominated by old populations. Rather, the central regions of four galaxies (NGC 4305, NGC 4306, NGC 4497, and NGC 4620) are dominated by populations with ages ∼ 1.5 − 3 Gyr. Centrally-concentrated line emission is found in two of the galaxies (NGC 4491 and NGC 4584), and the relative strengths of Hα and [SII]6746 are consistent with this emission originating in star-forming regions. The disks of these galaxies are dominated by populations that are at least 1 Gyr older than those near the centers, indicating that the quenching of star formation progressed from large radii inwards, and did not occur over a short timescale. NGC 4497 has the oldest disk, with a luminosity-weighted age of 10 Gyr. The metallicities of the galaxies in this sample are consistent with their integrated brightnesses, suggesting that they have not been subjected to large-scale stellar stripping. [Mg/Fe] is roughly solar, suggesting that these systems retained and enriched gas over time scales of at least 1 Gyr. The progenitors of these galaxies were likely late-type galaxies that were accreted by Virgo during intermediate or early epochs, and have since been depleted of gas and dust.

Intermediate-mass Early-type Disk Galaxies in the Virgo Cluster. II. Near-Infrared Spectra and Evidence for Differences in Evolution* † ‡ † ‡

“…Most disk galaxies have radial spectrophotometric gradients that can be characterized in terms of metallicity and age. A metallicity gradient, in the sense of progressively lower luminosity-weighted metallicities towards larger radii, was found by Davidge (2018a) in five of the galaxies that they studied, and similar trends are common in disk galaxies (e.g. Goddard et al 2017;Li et al 2018).…”

“…Recalling the age sensitivity of the C 2 feature demonstrated in Figure 4, the behaviour of the C 2 band in NGC 4497 is consistent with what might be expected from the GMOS spectra of that galaxy. In particular, Davidge (2018a) found evidence for radial age gradients in NGC 4497, in the sense of older luminosity-weighted ages at larger radii. Figures 16 and 17 of Davidge (2018a) indicate that the luminosityweighted age of the center of NGC 4497 is 2 -3 Gyr.…”

“…In particular, Davidge (2018a) found evidence for radial age gradients in NGC 4497, in the sense of older luminosity-weighted ages at larger radii. Figures 16 and 17 of Davidge (2018a) indicate that the luminosityweighted age of the center of NGC 4497 is 2 -3 Gyr. The contribution made by C stars to integrated NIR light at sub-solar metallicities peaks in populations with similar ages (Maraston 2005), and so C stars might be expected near the center of NGC 4497, with a diminishing C star frequency at progressively larger radii as mean age increases.…”

“…The Hβ and Mg 2 indices measured by Davidge (2018a) indicate that the luminosityweighted ages in the central regions of these galaxies fall between 1.5 and 3 Gyr. These same data also suggest that the luminosity-weighted [M/H] range from -0.3 to just above solar.…”

“…The characteristic age and metallicity estimates from Figures 16 and 17 of Davidge (2018a) are such that some of these galaxies might be expected to have high C star frequencies. The Ballick-Ramsey C 2 band at 1.76µm can be a prominent feature in the spectra of late-type C stars, and has been detected in the integrated light of nearby galaxies with large intermediate age populations (Miner et al 2011;Davidge 2015bDavidge , 2016.…”