Differences In Star Formation Rates Research Articles

The most luminous, merger-free AGNs show only marginal correlation with bar presence

ABSTRACT The role of large-scale bars in the fuelling of active galactic nuclei (AGNs) is still debated, even as evidence mounts that black hole growth in the absence of galaxy mergers cumulatively dominates and may substantially influence disc (i.e. merger-free) galaxy evolution. We investigate whether large-scale galactic bars are a good candidate for merger-free AGN fuelling. Specifically, we combine slit spectroscopy and Hubble Space Telescope imagery to characterize star formation rates (SFRs) and stellar masses of the unambiguously disc-dominated host galaxies of a sample of luminous, Type-1 AGN with 0.02 < z < 0.24. After carefully correcting for AGN signal, we find no clear difference in SFRs between AGN hosts and a stellar mass-matched sample of galaxies lacking an AGN (0.013 < z < 0.19), although this could be due to small sample size (nAGN = 34). We correct for SFR and stellar mass to minimize selection biases, and compare the bar fraction in the two samples. We find that AGNs are marginally (∼1.7σ) more likely to host a bar than inactive galaxies, with AGN hosts having a bar fraction, $f_{\mathrm{bar}}=0.59^{+0.08}_{-0.09}$ and inactive galaxies having a bar fraction, $f_{\mathrm{bar}}=0.44^{+0.08}_{-0.09}$. However, we find no further differences between SFR- and mass-matched AGNs and inactive samples. While bars could potentially trigger AGN activity, they appear to have no further, unique effect on a galaxy’s stellar mass or SFR.

The MOSDEF survey: differences in SFR and metallicity for morphologically selected mergers at z ∼ 2

ABSTRACT We study the properties of 55 morphologically-identified merging galaxy systems at z ∼ 2. These systems are flagged as mergers based on features such as tidal tails, double nuclei, and asymmetry. Our sample is drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey, along with a control sample of isolated galaxies at the same redshift. We consider the relationships between stellar mass, star formation rate (SFR), and gas-phase metallicity for both merging and non-merging systems. In the local universe, merging systems are characterized by an elevated SFR and depressed metallicity compared to isolated systems at a given mass. Our results indicate SFR enhancement and metallicity deficit for merging systems relative to non-merging systems for a fixed stellar mass at z ∼ 2, though larger samples are required to establish these preliminary results with higher statistical significance. In future work, it will be important to establish if the enhanced SFR and depressed metallicity in high-redshift mergers deviate from the ‘fundamental metallicity relation,’ as is observed in mergers in the local universe, and therefore shed light on gas flows during galaxy interactions.

Star formation rate and magnetic fields in spiral galaxies

We have constructed a dynamo model for the magnetic field in spiral galaxies that takes into account the differences in star formation rates in different galaxies. The difficulty in constructing the model is that the star formation rate does not enter directly into the equations of magnetohydrodynamics, which include only the root-mean-square velocity of the interstellar gas, its density, and the half-thickness of the ionized gas disk. We propose a parametrization of these quantities that relates them to the star formation rate and investigate our model in terms of the so-called no-z approximation, which neglects the details of the magnetic field structure in a direction perpendicular to the galactic disk. The influence of the star formation rate on the galactic dynamo is a threshold one. This influence is small at moderate star formation rates and significant only at very high star formation rates. If the starburst intensity reaches some critical level (exceeding that in the Milky Way by an order of magnitude), then the large-scale magnetic field is destroyed and it is restored only after completion of the starburst. We provide a list of galaxies that exhibit a fairly high star formation rate and that can be interesting to study their magnetic fields.

The dependence of galaxy group star formation rates and metallicities on large-scale environment

We construct a sample of 75,863 star forming galaxies with robust metallicity and star formation rate measurements from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), from which we select a clean sample of compact group (CG) galaxies. The CGs are defined to be close configurations of at least 4 galaxies that are otherwise apparently isolated. Our selection results in a sample of 112 spectroscopically identified compact group galaxies, which can be further divided into groups that are either embedded within a larger structure, such as a cluster or large group, or truly isolated systems. The compact groups then serve as a probe into the influence of large scale environment on a galaxy's evolution, while keeping the local density fixed at high values. We find that the star formation rates (SFRs) of star forming galaxies in compact groups are significantly different between isolated and embedded systems. Galaxies in isolated systems show significantly enhanced SFR, relative to a control sample matched in mass and redshift, a trend not seen in the embedded systems. Galaxies in isolated systems exhibit a median SFR enhancement at fixed stellar mass of +0.07 \pm 0.03 dex. These dependences on large scale environment are small in magnitude relative to the apparent influence of local scale effects found in previous studies, but the significance of the difference in SFRs between our two samples constrains the effect of large scale environment to be non-zero. We find no significant change in the gas-phase interstellar metallicity for either the isolated or embedded compact group sample relative to their controls. However, simulated samples that include artificial offsets indicate that we are only sensitive to metallicity changes of log O/H >0.13 dex (at 99% confidence), which is considerably larger than the typical metallicity differences seen in previous environmental studies.

A different approach to galaxy evolution

Abstract The consequences are explored of an observationally established relationship of the star formation rate (SFR) of star-forming galaxies with their stellar mass (M) and cosmic time (t), such that SFR ∝Mt−2.5. It is shown that small systematic differences in SFR dramatically amplify in the course of time: galaxies with above-average SFR run into quasi-exponential mass and SFR growth, while galaxies with below-average SFR avoid such exponential growth and evolve with moderate mass increase. It is argued that galaxies following the first path would enormously overgrow if they keep forming stars all the way to the present, hence should quench star formation and turn passive. By the same token, those instead avoiding the quasi-exponential growth may keep forming stars up to the present. Thus, it is conjectured that this divergent behaviour can help us to understand the origin of the dichotomy between passive, spheroidal galaxies and star-forming, disc galaxies.

STAR FORMATION HISTORY AND CHEMICAL EVOLUTION OF THE SEXTANS DWARF SPHEROIDAL GALAXY

We present the star formation history and chemical evolution of the Sextans dSph dwarf galaxy as a function of galactocentric distance. We derive these from the $VI$ photometry of stars in the $42' \times 28'$ field using the SMART model developed by Yuk & Lee (2007, ApJ, 668, 876) and adopting a closed-box model for chemical evolution. For the adopted age of Sextans 15 Gyr, we find that $>$84% of the stars formed prior to 11 Gyr ago, significant star formation extends from 15 to 11 Gyr ago ($\sim$ 65% of the stars formed 13 to 15 Gyr ago while $\sim$ 25% formed 11 to 13 Gyr ago), detectable star formation continued to at least 8 Gyr ago, the star formation history is more extended in the central regions than the outskirts, and the difference in star formation rates between the central and outer regions is most marked 11 to 13 Gyr ago. Whether blue straggler stars are interpreted as intermediate age main sequence stars affects conclusions regarding the star formation history for times 4 to 8 Gyr ago, but this is at most only a trace population. We find that the metallicity of the stars increased rapidly up to [Fe/H]=--1.6 in the central region and to [Fe/H]=--1.8 in the outer region within the first Gyr, and has varied slowly since then. The abundance ratios of several elements derived in this study are in good agreement with the observational data based on the high resolution spectroscopy in the literature. We conclude that the primary driver for the radial gradient of the stellar population in this galaxy is the star formation history, which self-consistently drives the chemical enrichment history.

Probing the Dust Obscuration in Seyfert Galaxies using Infrared Spectroscopy. II. Implication for the Unification of Seyfert Galaxies

We report near-IR spectroscopic observations of 11 Seyfert galaxies (six Seyfert 1s, one Seyfert 1.9, and four Seyfert 2s) and additional galaxies (four LINERs, two H II, and one type 2 transition) for comparison, obtained using the Gemini twin-channel near-IR camera on the Shane 3 m telescope at Lick Observatory. With the unique design of the Gemini camera, full J and K spectra were taken simultaneously through the same slit. This produced accurate line ratios of hydrogen recombination lines over a large wavelength baseline. For the Seyfert 1s (≤1.5), the line ratios of Paβ/Brγ are not only comparable in both broad- and narrow-line regions but also consistent with case B recombination, indicating little or no reddening in both narrow- and broad-line regions. Seyfert 2 (>1.5) galaxies, however, show substantial reddening toward the narrow-line regions. We compare optical reddening data from the literature and find significant support for the dichotomy between Seyfert 1s and Seyfert 2s, at least in low-luminosity objects. Two different scenarios are explored to explain the observed difference in reddening: a difference in reddening due to an extended dusty torus under active galactic nucleus unification, and a difference due to a different grain size distribution between the two Seyfert types. We also discuss a similar potential difference found in the strength of the 9.7 μm silicate line, along with a possible correlation between the narrow-line reddening and the strength of the silicate absorption line. We also analyzed CO band head absorption features longward of 2.3 μm to look for nonstellar contamination and evidence of recent star formation activity. The CO band head in Seyfert 1s shows heavy contamination from nonstellar radiation, which is correlated with an H - K nuclear color excess. We confirm that the CO spectroscopic indices in both Seyfert types do not show evidence of recent star formation. Taking the nonstellar contamination into account, there is little evidence from the CO index for a difference in star formation rates in the nuclei of Seyfert 1s and Seyfert 2s in our low-luminosity sample.