Dust and gas in star-forming complexes in NGC 3351, NGC 5055, and NGC 5457

1 Introduction

Star formation is one of the most important processes in the universe. To further elucidate this process, one needs to understand better the components of the interstellar medium (ISM) since these are the interrelations of the ISM components that define a specific mode of the star formation (Ballesteros-Paredes et al. 2020). Four major components of the ISM are ionized gas, atomic gas, molecular gas, and dust. Ionized hydrogen is a source of recombination lines. The radiation of neutral hydrogen at a wavelength of 21 cm is used as a tracer of atomic gas (Kalberla and Kerp 2009). Molecular hydrogen does not radiate under conditions typical of star-forming regions (SFRs), so the distribution of molecular gas has to be judged by observations of other molecules, primarily carbon monoxide (CO) (Bolatto et al. 2013). Continuum observations at wavelengths from few micron to millimeter, as well as observations of emission bands in the near- and mid-infrared (IR) ranges (Galliano et al. 2018) provide an insight about various dust components.

The studies of these ISM components are usually done on a pixel-by-pixel basis. We chose a different approach and consider more or less isolated regions, where the star formation occurs. These regions of dense interstellar matter, which often contain clusters of young massive stars that ionize the surrounding gas, are star-forming complexes (SFCs). In a series of papers (Smirnova et al. 2017a,b, Smirnova and Wiebe 2019, Smirnova et al. 2020, Smirnova and Wiebe 2022), we have considered various relations between the components of the ISM in SFCs of normal and disturbed galaxies.

The results presented in this work complement the studies presented in Smirnova et al. (2017a) and Smirnova and Wiebe (2022). In Smirnova et al. (2017a), we assessed the ISM and star formation parameters mainly from IR data and used emission at 8 and 24 μ m as a dust tracer. We investigated the relationships between various ISM components in 11 galaxies, included in a number of observational surveys, including THINGS (HI-line observations at 21 cm), KINGFISH (far-IR observations at 70, 100, and 160 μ m ; Herschel Space Observatory), SINGS (observations in the near-IR and mid-IR 3.6, 4.5, 5.8, 8.0, and 24 μ m ; Spitzer space telescope), and HERACLES (CO 2–1 line; the IRAM 30-m telescope).

In Smirnova and Wiebe (2022), we studied star formation by comparing the ratio of atomic and molecular gas, considering this ratio as a measure of the star formation efficiency. In that work, we evaluated masses of atomic and molecular hydrogen using fluxes obtained from the THINGS and HERACLES surveys. We also estimated velocity scatter parameters for CO and HI lines and introduced in Smirnova and Wiebe (2019). Four galaxies seen almost face-on were taken for analysis. One of these galaxies, NGC 5457, from the sample has IR data, but it was not included in our study presented in Smirnova et al. (2017a). NGC 5457 is a face-on spiral galaxy that has many satellites, which are the most likely reason for its asymmetric appearance on optical and HI images as well as for the prolonged star formation in this galaxy (Sandage and Bedke 1994). The advantage of this galaxy is that now we have managed to identify more regions in it than in each of the 11 galaxies that were included in the sample (Smirnova et al. 2017a).

Apart from NGC 5457, in this study, we also consider two out of the eight high-metallicity galaxies, considered in Smirnova et al. (2017a), with metallicities taken from Moustakas (2010). These galaxies were not included in Smirnova and Wiebe (2022) due to quite large inclinations (∼55°). However, in this initial study, we needed “nearly ideal” galaxies with small inclinations. In this study, we relax this requirement and consider galaxies with nonideal parameters. One of them, NGC 3351, is a barred spiral galaxy. In the central region of this galaxy, there is a star-forming ring (see Figure 2 in Calzetti et al. (2021)). A number of studies show that active star formation has recently taken place in the center of the galaxy (Elmegreen et al. 1997, Calzetti et al. 2021). NGC 5055 is an isolated spiral galaxy with a warped HI gaseous disk showing recently discovered strong evidence of an ongoing minor merger and recent interaction with a low-mass companion (Chonis et al. 2011).

2 Data

In this study, we mostly consider those observations of NGC 3351, NGC 5055, and NGC 5457, which had not been considered in our previous studies. Parameters of the studied galaxies are listed in Table 1.

For the galaxy NGC 5457, we used archival NIR (3.6, 4.5, 5.8, and 8.0 μ m ) and MIR ( 24 μ m ) observations from the SINGS survey^[1] performed with the Spitzer Space Telescope. FIR observations (70, 100, and 160 μ m ) are taken from the KINGFISH survey^[2]. For the NGC 3351 and NGC 5055 galaxies, we used surveys THINGS^[3] (HI line at 21 cm) and HERACLES^[4] (CO (2-1) line). The selected SFCs in NGC 5055 and NGC 3351 are shown in Figure 1. Even though PHANGS-ALMA data are available for NGC 3351, they cannot be used in our study as they do not cover the entire galaxy. Kuzin and Lisitsin (2022) have shown that for our purposes, PHANGS and HERACLES spectra are in good agreement with each other. The fluxes in the studied ranges were obtained using the aperture photometry procedure described in Khramtsova et al. (2013). The process for calculating atomic and molecular masses is presented in Smirnova and Wiebe (2022).

Figure 1

HI images of NGC 5055 (upper panel) and NGC 3351 (bottom panel) with selected SFCs.

3 Results and discussion

In our previous works, we have identified a number of trends in the parameters of SFCs. Speaking about the dust bands, we saw that fluxes in the near, middle, and far IR correlate well with each other, thus hinting at the common nature of dust particles. We noted that the younger the region, the lower its IR flux. When comparing relations between atomic and molecular gas masses and kinematic properties in extragalactic SFCs, we came to the conclusion that, depending on the spatial location of SFCs, they demonstrate different relationships between these parameters. Regions located in the galactic centers have large fluxes in the CO line, and SFCs located at the periphery produce more emission in the HI line. There is a clear correlation between the mass of molecular hydrogen and the velocity scatter in the sense that the greater the flux in the CO line, the larger the velocity scatter.

Previously we used dust emission at 8, 24, and 160 μ m to select SFCs in the high-metallicity galaxies, and in NGC 5457, SFCs were selected using emission in the CO and HI lines. Therefore, smaller number of SFCs was originally selected in NGC 5457. As a result, these original regions do not stand out from the overall picture.

A general relation of fluxes at 8 and 24 μ m with the total far-IR flux ( 70 + 100 + 160 μ m ) is shown in Figure 2. The fluxes used in Figure 2 were reduced to the same distance of 10 Mpc. The graph also shows the values of the fluxes from SFCs studied by Smirnova et al. (2017a). Coral circles correspond to SFCs in galaxies of high metallicity. Most SFCs from NGC 5457 (black circles) occupy the same region of the diagram as the high-metallicity SFCs from the other star-forming galaxies (the sample from Smirnova et al. (2017a)). Some SFCs from the NGC 5457 galaxy occupy different locations in the diagram. They show substantially lower fluxes at both 8 and 24 μ m and in the total FIR. This dependence is similar to that shown by the SFCs in the ring of the polar ring galaxy NGC 660 (Smirnova et al. 2017b) or the NGC 1512 galaxy (Smirnova et al. 2020). In those works, we suggested that low fluxes in the IR can be an indication of the youth of star formation complexes. In NGC 5457, this may be an effect of observational selection as this is the most nearby galaxy from the current sample, and we have been able to select smaller SFCs in this system.

Figure 2

Comparison of the total FIR flux with the fluxes at 8 μ m (upper) and 24 μ m (bottom) in NGC 5457. For comparison, 8 and 24 μ m fluxes are also shown for high metallicity extragalactic SFRs (coral) in eight galaxies studied by Smirnova et al. (2017a). All the fluxes are reduced to the same distance of 10 Mpc. Distances to the galaxies are taken from the study by Anand (2021).

In Figure 3, we show how the fluxes at 8 and 24 μ m correlate with the emission flux in the CO(2–1) line. In both cases, the correlation is quite significant. In this respect, the CO emission appears to be related to the 8 μ m emission. This may mean that the evolutionary histories of CO molecules and PAH particles in SFCs are similar. It should be noted that some SFRs of the NGC 5457 galaxy have lower fluxes in the CO line than complexes of the high-metallicity galaxies.

Figure 3

Comparison of the CO line flux with the 8 μ m (upper) and 24 μ m (bottom) fluxes. For comparison, the same fluxes are shown for high metallicity extragalactic SFCs (coral) in galaxies studied in Smirnova et al. (2017a).The fluxes are reduced to 10 Mpc. Distances to the galaxies are taken from the study by Anand (2021).

In Figure 4, we show the relation between molecular and atomic gas masses in the considered galaxies. Comparing the obtained masses with the results of Smirnova and Wiebe (2022), we see that these SFCs demonstrate significantly lower masses of atomic and molecular gas than the SFCs from Smirnova and Wiebe (2022). We see a hint of the separation of the SFCs into two groups (the second group includes only two complexes) in the galaxy NGC 5055 (Figure 4, upper). If we divide the SFCs in this galaxy into inner and outer groups, as we did in Smirnova and Wiebe (2022), on the basis of the SFC position relative to r 20 , we see a relationship similar to that in the NGC 628 and NGC 5457 galaxies (Smirnova and Wiebe 2022). The outer SFCs (red circles) have larger atomic hydrogen masses than the inner SFCs (blue circles), while molecular gas masses are comparable in both groups. Thus, M HI / M H 2 ratio in the outer SFCs is larger than in inner complexes. In NGC 3351, we do not see distinctive groups. Its relation between the parameters corresponds to the galaxy NGC 6946 (Smirnova and Wiebe 2022).

Figure 4

Relation between molecular and atomic gas masses for galaxies NGC 5055 (upper) and NGC 3351 (bottom).

Further, we explore the relationship between masses and velocity scatters for CO and HI emission (Figure 5). In both galaxies, most of the dots lie below Δ V CO ∼ 70 km s − 1 , but there are several SFCs in each galaxy with Δ V CO exceeding this value. The same trend was found in the SFCs of the galaxies considered in Smirnova and Wiebe (2022), but in these complexes, more regions with Δ V CO > 100 km s − 1 were observed, and most of the regions have smaller velocity scatters.

Figure 5

Comparison of molecular gas mass with Δ V CO in NGC 5055 (upper panel) and NGC 3351 (bottom panel).

In Figure 6 that shows the relation between the atomic hydrogen mass and the velocity scatter, we do not see any clear correlation. It can be noted in Figure 6 (upper panel) that there are two complexes in NGC 5055 that have Δ V HI in excess of 100 km/s (in Figure 1 they are marked with their respective numbers). The one with a single Gaussian line profile (#26) is an isolated SFC lying at the galaxy periphery. Another complex with the large Δ V HI ∼ 116 km/s (#13), lying closer to the galaxy center, has a more complicated line profile, shows low flux in the HI line and seems to encompass several different SFCs.

Figure 6

Comparison of atomic gas masses with Δ V HI in NGC 5055 (upper panel) and NGC 3351 (bottom panel).

4 Conclusion

In this work, we conclude the analysis presented in our previous works, combining data that have not been considered by us previously.

1. In the galaxies NGC 3351 and NGC 5055, we do not see a correlation between molecular gas mass and a corresponding velocity scatter. There are some SFCs in each galaxy that combine low H 2 mass with high Δ V CO values, some of them being artifacts of the adopted Δ V estimation procedure.

2. The SFCs in NGC 5457 demonstrate a good correlation between fluxes in the IR ranges. Some complexes in the galaxy, while following the general trend as the SFCs on other high-metallicity galaxies, have lower IR fluxes, which may be related to effects of observational selection or the youth of these regions.