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

Abstract

We discuss near-infrared (NIR) slit spectra of six early-type disk galaxies in the Virgo Cluster that were examined previously at visible/red wavelengths by Davidge (2018a, AJ, 156, 233). Despite having similar intrinsic luminosities, colors, and morphologies, the NIR spectrophotometric properties of these galaxies indicate that they are not a homogeneous ensemble differing only in terms of luminosity-weighted age and metallicity. While the depth of the CO(2,0) band is consistent with the centers of these galaxies having solar or slightly sub-solar luminosity-weighted metallicities, galaxy-to-galaxy variations in the depth of the NaI 2.21µm doublet are found, with NaI2.21µm lines in three galaxies (NGC 4491, NGC 4584, and NGC 4620) that are deeper than those predicted for a solar chemical mixture and a solar-neighborhood mass function. In contrast, the CaI2.26µm triplet shows good galaxy-to-galaxy agreement, but tends to be deeper than the model prediction. Considering the depth of the NaD lines found by Davidge (2018a), the deep NaI2.21µm lines are tentatively attributed to a 1 Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministério da Ciência, Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina).bottom-heavy mass function. This is counter to observed trends between mass function slope and velocity dispersion, and so the possibility of a super-solar [Na/Fe] is also discussed. Two of the three galaxies with deep NaI2.21µm (NGC 4584 and NGC 4620) have Sersic exponents that are consistent with a classical bulge. As for NGC 4491, its central NIR spectrum contains prominent emission lines. The relative strengths of Brγ and H 2 S(1), the presence of [FeII] emission, and the mid-infrared spectral-energy distribution are all consistent with intense recent star formation near the center of that galaxy. The NIR spectrum of NGC 4584 is devoid of line emission in the NIR, suggesting that star formation does not power the emission detected at visible wavelengths from that galaxy. Wavelengths that contain the Ballick-Ramsey C 2 band at 1.76µm are matched by moderately metal-poor E-MILES model spectra with an age of 2 Gyr. The radial age trends in these galaxies are in the opposite sense to those in late-type disk galaxies, and it is concluded that they have evolved in a cluster environment for at least several Gyr. 5 The On-Instrument Wave Front Sensor (OIWFS) is preferred for F2 observations as it does not block NIR light in the science path. However, the OIWFS was inoperable when these data were recorded, and so a Peripheral Wave Front Sensor (PWFS) was used for guiding. The PWFSs are not part of F2, are not cooled, and are not in the instrumental focal plane. As a result, they can physica...

The Near-Infrared Spectrum of the Nuclear Star Cluster: Looking Below the Tip of the Iceberg and Comparisons with Extragalactic Nuclei

“…The CaI2.26µm indices in all four regions are uniformly larger than in the models, which is consistent with what is seen in other galaxies (e.g. Alton et al 2018;Davidge 2020). When light from bright stars is removed from the spectra then the NaI2.21µm indices for Regions 2 and 3 are still 0.4 -0.8 Å deeper than in the 3 Gyr models.…”

“…Having access to spectra that cover a full range of the highly evolved objects that can contribute significantly to the NIR light is also important. For example, the spectra of C stars in the IRTF library may not track the full suite of C star characteristics (Davidge 2020). The IRTF library is being expanded to cover a larger parameter space (Villaume et al 2017), while other independent libraries are also becoming available (e.g.…”

“…Rock et al 2017, and others), suggesting that this index may be influenced by parameters other than age and metallicity. Although deep NaI2.21µm features are most commonly associated with massive spheroidal galaxies, these are also seen in lower mass early-type disk galaxies (Davidge 2020). Differences in [Na/Fe], the nature of the mass function at the lower end of the main sequence, and variations in [C/Fe] (e.g.…”

“…Seth et al 2008). To enable a comparison with a nuclear cluster that is not in a late-type disk galaxy, spectra of the Virgo cluster early-type disk galaxy NGC 4491 (Davidge 2020) is also shown in Figures 7 and 8. NGC 4491 is an SBa(s) galaxy in the Virgo cluster that has little or no on-going star formation in its disk, but has a blue nuclear SED at visible wavelengths and a red nuclear SED in the MIR (Davidge 2018).…”

“…NGC 4491 is an SBa(s) galaxy in the Virgo cluster that has little or no on-going star formation in its disk, but has a blue nuclear SED at visible wavelengths and a red nuclear SED in the MIR (Davidge 2018). It is the only early-type galaxy in the sample discussed by Davidge (2020) that has NIR line emission, and has a near-solar central metallicity (Davidge 2018). Moreover, the NaI2.21µ and CaI2.26µm features in the NGC 4491 spectrum are deeper than predicted by models, as is the case in Regions 2 and 3.…”