Mid-Infrared Colors of Dwarf Galaxies: Young Starbursts Mimicking Active Galactic Nuclei

Abstract

Searching for active galactic nuclei (AGNs) in dwarf galaxies is important for our understanding of the seed black holes that formed in the early universe. Here, we test infrared selection methods for AGN activity at low galaxy masses. Our parent sample consists of ∼18,000 nearby dwarf galaxies (M * < 3 × 10 9 M ☉ , z < 0.055) in the Sloan Digital Sky Survey with significant detections in the first three bands of the AllWISE data release from the Widefield Infrared Survey Explorer (WISE). First, we demonstrate that the majority of optically selected AGNs in dwarf galaxies are not selected as AGNs using WISE infrared color diagnostics and that the infrared emission is dominated by the host galaxies. We then investigate the infrared properties of optically selected star-forming dwarf galaxies, finding that the galaxies with the reddest infrared colors are the most compact, with blue optical colors, young stellar ages, and large specific star formation rates. These results indicate that great care must be taken when selecting AGNs in dwarf galaxies using infrared colors, as star-forming dwarf galaxies are capable of heating dust in such a way that mimics the infrared colors of more luminous AGNs. In particular, a simple -W W 1 2 color cut alone should not be used to select AGNs in dwarf galaxies. With these complications in mind, we present a sample of 41 dwarf galaxies that fall in theWISE infrared color space typically occupied by more luminous AGNs and that are worthy of follow-up observations.

Hard X-Ray-selected AGNs in Low-mass Galaxies from the NuSTAR Serendipitous Survey

“…For the seven NuSTAR low-mass AGN candidates with SDSS photometry, we also calculate  M using the KCORRECT package 24 (Blanton et al 2003) without correcting for the AGN contribution. For our sample, the SED-fitting estimated  M is only slightly lower than the KCORRECT  M by »0.04 dex, which is consistent with previous studies that suggest that the AGN contamination at optical to near-IR wavelengths is generally not significant for local low-luminosity broad-line AGNs (Reines & Volonteri 2015;Hainline et al 2016). In this work, we adopt the  M values estimated using the SED-fitting method described above for the low-mass AGN candidates in our sample.…”

“…With the limited sample size, it is not clear whether the red WISE color at longer wavelengths is a common feature of hard X-ray-selected low-mass AGNs. However, we note that low-mass AGNs selected using other methods, such as the BPT diagram or the presence of the He II λ4686 emission line, do not show as high a fraction of red -W W 2 3 colors (e.g., Figures 3 and 4 of Sartori et al 2015; Figure 1 of Hainline et al 2016) as our sample does, which might simply be due to the higher redshift and luminosities of our sample.…”

“…In particular, recent studies have found that the vast majority of low-mass galaxies with red mid-IR color do not show any sign of optical emission lines powered by AGNs (Satyapal et al 2014(Satyapal et al , 2016Sartori et al 2015;Secrest et al 2015). Several studies have also demonstrated that young, compact starbursts in dwarf galaxies can have mid-IR colors mimicking those of luminous AGNs (e.g., Griffith et al 2011;Izotov et al 2014;Hainline et al 2016). Here we explore whether our hard X-ray-selected low-mass galaxies have mid-IR emission powered by AGN-heated dust to assess the effectiveness of using mid-IR emission to search for obscured AGNs in low-mass galaxies.…”

“…Many studies have utilized Spitzer and WISE observations to show that the distinctive red mid-IR color arising from hot dust heated by SMBH accretion can be used as an effective indicator of intrinsically luminous AGN activity (e.g., Lacy et al 2004;Stern et al 2005Stern et al , 2012Donley et al 2012;Assef et al 2013;Mateos et al 2013). However, AGN identification methods based on red mid-IR colors become more ambiguous for low-luminosity AGNs (e.g., Hainline et al 2016). In particular, recent studies have found that the vast majority of low-mass galaxies with red mid-IR color do not show any sign of optical emission lines powered by AGNs (Satyapal et al 2014(Satyapal et al , 2016Sartori et al 2015;Secrest et al 2015).…”

Composite Spectral Energy Distributions and Infrared–Optical Colors of Type 1 and Type 2 Quasars

“…We also note that simple MIR color techniques are likely to miss the most heavily obscured AGNs, which have MIR colors consistent with star-forming galaxies at similar redshift (e.g., Hainline et al 2016b;Yuan et al 2016).…”