Dust-buried Compact Sources in the Dwarf Galaxy NGC 4449

Abstract

Multiwavelength images from the Hubble Space Telescope covering the wavelength range 0.27–1.6 μm show that the central area of the nearby dwarf galaxy NGC 4449 contains several tens of compact sources that are emitting in the hydrogen recombination line Paβ (1.2818 μm) but are only marginally detected in Hα (0.6563 μm) and undetected at wavelengths λ ≤ 0.55 μm. An analysis of the spectral energy distributions (SEDs) of these sources indicates that they are likely relatively young stellar clusters heavily attenuated by dust. The selection function used to identify the sources prevents meaningful statistical analyses of their age, mass, and dust extinction distributions. However, these cluster candidates have ages ∼5–6 Myr and A V > 6 mag, according to their SED fits, and are extremely compact, with typical deconvolved radii of 1 pc. The dusty clusters are located at the periphery of the dark clouds within the galaxy and appear to be partially embedded. Density and pressure considerations indicate that the H ii regions surrounding these clusters may be stalled, and that pre-supernova (pre-SN) feedback has not been able to clear the clusters of their natal cocoons. These findings are in potential tension with existing models that regulate star formation with pre-SN feedback, since pre-SN feedback acts on short timescales, ≲4 Myr, for a standard stellar initial mass function. The existence of a population of dusty stellar clusters with ages >4 Myr, if confirmed by future observations, paints a more complex picture for the role of stellar feedback in controlling star formation.

Lyman continuum leaker candidates among highly ionised, low-redshift dwarf galaxies selected from He II

“…The overall pattern, however, is the same as in the other objects. The situation we find in the majority of the IMPs is remarkably similar to that in other metal-poor DGs, where this two-stage starburst scenario appears to be linked to superbubbles and galactic outflows, for instance in NGC 1569 (Heckman et al 1995;Vallenari & Bomans 1996;Origlia et al 2001) or NGC 5253 (Monreal-Ibero et al 2010;Calzetti et al 2015). Several LCGs, such as the GPs studied by Amorín et al (2012), have been shown to have a similar SFH.…”

The Timescales of Star Cluster Emergence: The Case of NGC 4449

“…We do not expect that discarding low-mass young clusters will affect our conclusions, which are based on sources with masses 3000 M e (see Section 7). We note that we do not recover any of the sources identified by Calzetti et al (2023); none of the 34 sources selected by those authors show evidence for emission in Paα, although they are sufficiently compact to be indistinguishable from stellar sources and therefore part of the stellar population in this galaxy. After applying both of our selection criteria, we are left with 99 sources (21% of the original sample) that display Paα emission, with >90% of them detected with S/ N > 7 in this line.…”

“…Messa et al (2021) were however restricted by the requirement that sources still need to be detectable at optical wavelengths in order to determine their ages and masses. Calzetti et al (2023), using HST optical and IR images of NGC 4449, attempted to obtain a sample of highly extincted clusters by selecting sources that emit at Paβ but are undetected at optical and shorter wavelengths. The specific selection requirements used in that study resulted in a limited number of sources (34) with a narrow range of properties, which leaves open the question of whether those authors' approach includes potential biases.…”

“…Within a factor ∼3, their masses are comparable to that of the Large Magellanic Cloud (LMC). However, they differ in their SFR surface density, with NGC 4449 having Σ SFR = 0.03 M e yr −1 kpc −2 , or 3 times the SFR surface density of NGC 1313 in the regions targeted by HST (Messa et al 2021;Calzetti et al 2023).…”

“…We leverage the same HST UV to NIR imaging data analyzed in Calzetti et al (2023; Figure 1), but adopt a more general selection process than those authors to recover a larger sample of young star clusters, with a greater diversity of ages, masses, and extinctions. We select our sources based on the presence of ionized gas emission identified from the infrared hydrogen recombination line Paβ (1.282 μm); this ensures that our sources are typically young, 6-7 Myr, since beyond 7 Myr the ionized gas emission is too faint to be detected (Leitherer et al 1999).…”